UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Television and Video

Notices

Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc.

Closed Thread
 
Thread Tools
Old 15th Apr 2015, 9:50 am   #41
wd40addict
Octode
 
wd40addict's Avatar
 
Join Date: May 2009
Location: Stevenage, Herts. UK.
Posts: 1,515
Default Re: HMV 904 prewar television restoration

Excellent write up - well done!

Faults left by previous repairers are much harder to find than 'real' faults. Took me ages to find an unsoldered wire left by a repairer to my Ekco TMB272. Was the trapped aerial wire in your set another pre-existing fault?

Even in the 70s manufacturers didn't get resistor voltage ratings correct: my Solartron Oscilloscope was getting progressively dimmer and it turned out to be a chain of high value resistors with a couple of kV across them. All of them had gone high and one had finally gone open circuit. They all looked like bog standard carbon jobs, I think the designer just thought 'I need a X meg resistor' without thinking about voltage. I replaced them with a chain of 500v rated parts recalculated to spread the voltage evenly and well within the rating.

One questions: Who are AJ Electronics?

Regards,

Paul
wd40addict is offline  
Old 16th Apr 2015, 3:07 am   #42
Catkins
Pentode
 
Catkins's Avatar
 
Join Date: Feb 2015
Location: Chepstow, Monmouthshire, UK.
Posts: 234
Default Re: HMV 904 prewar television restoration

Quote:
Originally Posted by wd40addict View Post
One questions: Who are AJ Electronics?
Argh another typo, I meant F&T AI electrolytic capacitors. They were some replacement capacitors with screw mounts, suitable for replacing chassis mounted reservoir and smoothing capacitors. I got them from the BVWS stall at an event at the British Vintage Wireless and Televsion Museum (BVW&TM) in 2012.

I found this thread here: https://www.vintage-radio.net/forum/...ad.php?t=75350
Catkins is offline  
Old 17th Apr 2015, 7:47 am   #43
Catkins
Pentode
 
Catkins's Avatar
 
Join Date: Feb 2015
Location: Chepstow, Monmouthshire, UK.
Posts: 234
Default Re: HMV 904 prewar television restoration

Hi Guys,

Back again I've finally got around to the point of switching on the television ...

On the 14th of January, about two weeks after getting the radio working, and finishing all the remaining issues of EHT capacitors, bleeder resistors, and double checking everything, it was after months of work, finally time to test the television.

This was really the moment of discovery in terms of whether I'd ever get the television to work. Previously to my surprise considering the damp storage conditions of the set (and the open circuit audio output transformer due to corrosion), the EHT transformer had proved on a multimeter test to read the correct ohmage. The CRT filament had also proved not to be open circuit.

But, this was the real test, and lots could go wrong, the dreaded green spot on the EHT transformer and no emission on the CRT.

As I was uncertain about the condition of the EHT transformer, I slowly brought the set up on a variac, over a couple of hours, giving it plenty of time, and also measuring the EHT voltage and the temperature of the EHT transformer to spot any overheating.

Photo 1 shows the a photo of the set at this time. The set is on its side leaving the EHT transformer exposed, to connect a multimeter to the EHT output (hands free), and allowing the temperature to be checked with an infra-red temperature gun. This is the only time I've operated the set like this with the EHT circuits exposed, and I took no chances, and I covered all the exposed points at EHT potential.

After a couple of hours increasing the input voltage to max voltage, the EHT transformer showed no signs of overheating, and the EHT output voltage was as expected. Time to connect the CRT, but first a check of the filament/cathode/grid/anode voltages on the plug, all correct (this wasn't the time to blindly connect the plug, and discover I'd miswired the anode to filament ).

I was rewarded with a large diffuse spot at the centre of the screen (~30%).

Mixed emotions. The large diffuse spot showed focus, frame/line deflection wasn't working, and probably no signal (I had connected the standards converter), though it was obviously impossible to tell this on the CRT, there was nothing on the speaker.

But, it proved the EHT was working, and the CRT was working, with good emission. So I was happy because the worst (and most difficult to fix) problems hadn't occurred.

Next step was tracking down the problems with the focus and frame/line deflection. As the focus circuit was the simplest, this seemed the first thing to look at.

I tested the HT at the input to the focus coil (magnetic focus), and discovered 0 HT volts! Subsequent backwards tracing with the volt meter showed no HT volts from choke 1 in the power stage (this only feeds the tv circuits, and hence wasn't tested by the radio). This proved to be an internal break in one of the wires from the choke. I also discovered one of the resistors in the focus circuit had gone way of range.

I fixed these problems and retested. Of course the failure in the television circuit HT could have explained the lack of deflection/signal but no such luck.

I was rewarded this time with nothing. Absolutely nothing on the CRT screen. Panic stations as I feared the CRT had stopped working. But no, the gun was lit and everything was correct. I finally decided with no frame/line deflection the now focused beam was hitting an ion burn in the centre of the screen (no ion burn was discernable, so it had to be very small). Subsequently this proved to correct.

So next step was fixing the frame/line deflection. As an obvious sanity check before getting bogged down in the timebase circuits (looking for a fault that wasn't there), I checked the frame/line deflection coils. These proved correct.

So the problem was in the timebase circuits. I hadn't replaced the waxy capacitors in the set (preferirng to wait until switch on to avoid unnecessarily replacing components), and these were the obvious suspects along with resistors going out of spec (the timebase oscillators are obviously very sensitive to capacitors/resistors going out of spec).

On a spot check of randomly selected waxies in the timebase circuits, they were all leaky and out of spec. A more detailed check showed all but one waxy was out of spec and leaky, and four resistors were too.

I restuffed the waxies and replaced the resistors. Photo 2 shows a photo of the restuffed waxies. The waxies were fairly easy to restuff, a slit along the length of the waxy enabled the card to be slowly pulled apart wihout creasing to about 1 centimetre wide which was just sufficient to pull out the insides. If the waxy showed signs of tearing or cracking, a couple of minutes heating (I used a fan heater), was sufficient to loosen it and the contents up. The contents were replaced with a modern capacitor and filled up with hot glue.

Once replaced, I retested the set. But first I checked the output of the frame/line deflection circuits for correctness and linearity on an oscilloscope (the output was taken across a 100 ohm resistor inserted into the deflection circuits to convert varying current into varing voltage for the oscilloscope).

Photo 3 shows the oscilloscope output traces, with the framebase output on top, and the line base output on the bottom. The framebase output showed good linearity, the linebase output showed good linearity too, but the oscillator wriggle at the start suggested an unresolved problem with the line output damping circuit (I had replaced the waxy and checked the resistors). Twiddling the line and frame hold controls showed the oscillator frequencies changing as expected and so they worked. It was time to reconnect the CRT. As often happens, I subsequently found the lack of damping in the line output circuit was simply due to having set the form control too low,

The output of the CRT was as shown in Photo 4. The frame and line base circuits worked in so far as I could get a "box" on the CRT, but it was impossible to get a raster as expected because the output was completely out of focus.

This left me completely stumped for a while. I had checked every component in the focus circuit individually and (having replaced all the failures) they were all correct. One explanation for the lack of focus was the dreaded soft CRT, but, there was no evidence for that looking at the gun.

I checked the HT voltage at the focus coil and discovered it was way too low. Given all the components checked out correct individually, the only explanation was a partial short to earth somewhere in the focus circuit. Checking the circuit against earth showed a 1K resistance, when the resistance should have been 6.5K through the focus coil. Disconnecting the focus coil had no effect.

Splitting the circuit into two, and then into two again (split into two, discover which half is still shorted, then split that half into two, and discover which of that half is still shorted, and repeat until you have a single component) showed quite unexpectedly the 1K ohm short to earth was in the focus control pot.

This subsequently was shown to be that the centre tap had developed a partial short to the case through its insulator. Spraying the insulator with lubricant fixed the short, but only for a couple of days before the lubricant evaporated.

I finally took the pot apart, drilling out the centre tap rivot, and putting a thin piece of transparent insulating plastic between the case and the failed insulator. This fixed the problem. Photo 5 shows the pot disassembled, and reassembled.

Sorry out of time again
Attached Thumbnails
Click image for larger version

Name:	1.JPG
Views:	462
Size:	199.8 KB
ID:	107109   Click image for larger version

Name:	2.JPG
Views:	458
Size:	155.0 KB
ID:	107110   Click image for larger version

Name:	7.JPG
Views:	413
Size:	133.6 KB
ID:	107115   Click image for larger version

Name:	5.jpg
Views:	459
Size:	93.9 KB
ID:	107116   Click image for larger version

Name:	8.JPG
Views:	420
Size:	177.0 KB
ID:	107117  

Catkins is offline  
Old 18th Apr 2015, 8:46 am   #44
Hunts smoothing bomb
Octode
 
Hunts smoothing bomb's Avatar
 
Join Date: Oct 2005
Location: Wimborne, Dorset, UK.
Posts: 1,407
Default Re: HMV 904 prewar television restoration

Excellent work, really enjoying this write up!

Cheers
__________________
Lee
Hunts smoothing bomb is offline  
Old 18th Apr 2015, 1:03 pm   #45
line sync
Heptode
 
Join Date: Sep 2009
Location: Kinver, Staffordshire, UK.
Posts: 628
Default Re: HMV 904 prewar television restoration

Yes I`m really enjoying it too.
That was a good find with the focus pot and just shows what you sometimes have to do to fix the fault , you cannot just order a new pot the same type as that one.
Great write up , please keep it going.

Robin
line sync is offline  
Old 19th Apr 2015, 4:44 am   #46
Catkins
Pentode
 
Catkins's Avatar
 
Join Date: Feb 2015
Location: Chepstow, Monmouthshire, UK.
Posts: 234
Default Re: HMV 904 prewar television restoration

Quote:
Originally Posted by Catkins View Post
As often happens, I subsequently found the lack of damping in the line output circuit was simply due to having set the form control too low
A small correction to my earlier post. I found the form control was set too high, rather than too low. The amount of damping in the circuit is inversely proportional to the resistance, the higher the resistance the less damping. Of course damping circuits consisting of a capacitor and resistance across the line deflection coils/lopt secondary was only employed on mains derived EHT sets. Later designs using flyback derived EHT used booster/efficiency diodes and circuits which did the damping.

The other thing I forgot to mention is the frame deflection trace on the oscilloscope has good linearity except for a dip at the start of the frame deflection. When I finally got signal output working, I discovered the top of the picture was compressed.

But that's for the next write-up which hopefully will be tomorrow (Sunday).
Catkins is offline  
Old 20th Apr 2015, 3:54 am   #47
Catkins
Pentode
 
Catkins's Avatar
 
Join Date: Feb 2015
Location: Chepstow, Monmouthshire, UK.
Posts: 234
Default Re: HMV 904 prewar television restoration

Hi,

Sorry I didn't get chance to continue the write up today. I connected up the television to the oscilloscope to get some more traces, and literally hours "disappeared" tinkering
Catkins is offline  
Old 22nd Apr 2015, 6:35 am   #48
Catkins
Pentode
 
Catkins's Avatar
 
Join Date: Feb 2015
Location: Chepstow, Monmouthshire, UK.
Posts: 234
Default Re: HMV 904 prewar television restoration

Hi Guys,

Sorry for the delay, but I got distracted tinkering on the weekend, and I've come down with a nasty cold too This may be a bit shorter than normal, hopefully normal service will be resumed soon

After fixing the focus control, I went straight onto looking at the next fault - even without focus it was obvious there was no signal.

All of the valves in the signal circuit (RF, IF and video detector) were NOS, and checked out OK on the valve tester, so I decided to look for other more obvious faults before suspecting them.

As I'd had such a high waxy failure rate in the timebase circuit, I did a spot check of the waxies in the signal circuit, and found they were, as expected, all bad. A more thorough check showed they were all leaky and out of spec. So, restuffing was the next thing on the list, although they were unlikely to be the ultimate fault (as there was no signal at all), I wasn't going to get very far without restuffing them.

Two weeks of evening work and the waxies were restuffed, and then resoldered into the set over the next couple of evenings. Photo 1 shows another exciting picture of restuffed waxies This time I remembered to take a "before restuffing" photo of a waxy, showing the condition when taken out of the set. All the waxies were pockmarked with mould on the underside of the waxies, which is why I had no issue in replacing them all.

Time to retest? Not yet, the total lack of signal suggested a more serious fault, and the most obvious suspect was the video oscillator. I carefully checked my repair of the video oscillator coil and went through the circuit, and discovered R10 (50.8Kohms) in the HT feed of the oscillator anode was more than twice the correct value. This easily explained the lack of oscillation, though, curiously, this was also used by the radio circuit, which did get oscillation. Clearly the radio circuit is less demanding of correct voltage.

Retesting, I was rewarded with the test card picture in photo 3. As the CRT is mounted on the top of the case, the picture is upside-down.

For the first working picture in +60 years, I was happy with the result. Though obviously there were still many problems to be solved. There is top of the frame cramping (as already suggested by the frame trace on the oscilloscope in the previous post), and there was very poor sync separation, cogging/pulling on blacks (it looked more like this than pulling on whites, but, it was difficult to tell), and generally rather noisey.

Much of the noise proved to be simply due to not having bothered to put the screening cans on the valves, but it didn't solve it all. It obviously didn't solve any of the other problems too.

The next stage was to look at the sync separation problem. For this I tended to suspect a problem with the sync circuit rather than poor high frequency response in the IF circuit, simply because the sync separation was so bad. If it was poor high frequency response in the IF/vision circuit I would have suspected pulling on whites (failure to respond to the "front porch") and nothing else.

But, unfortunately, that will be the next post ....
Attached Thumbnails
Click image for larger version

Name:	_DSC_0831.jpg
Views:	355
Size:	128.7 KB
ID:	107299   Click image for larger version

Name:	_DSC_0844.JPG
Views:	344
Size:	150.1 KB
ID:	107300   Click image for larger version

Name:	_DSC_0878.jpg
Views:	467
Size:	25.0 KB
ID:	107301  
Catkins is offline  
Old 24th Apr 2015, 7:48 am   #49
Catkins
Pentode
 
Catkins's Avatar
 
Join Date: Feb 2015
Location: Chepstow, Monmouthshire, UK.
Posts: 234
Default Re: HMV 904 prewar television restoration

The previous post finished with a suspected problem in the sync circuit. My next step was to work out how it worked.

The sync circuit seems to be fairly peculiar to early EMI sets and consists of a “pulse diode” and a “limiter valve”. The pulse diode separates the sync pulses by only conducting on picture and by not conducting on sync pulses, this is done by having the diode cathode sit in the vision detector anode circuit, and biasing the diode anode/cathode appropriately. The sync pulses are “delivered” to the limiter valve by applying a voltage to the grid via R75 and C61 (connected to the pulse diode cathode), this input is short circuited when the diode is conducting on picture. The limiter valve is operating as an “amplitude filter” with anode/screen volts of 30&40. “Under these conditions the anode current cuts off for only a few volts negative grid bias, but rises steeply as the bias is reduced. Saturation sets in at about -1 - 0 grid volts, and the current tends to stay at a constant value as the grid potential becomes more positive" (*). In effect the limiter valve delivers sync pulses to the frame/line oscillators but limits the max value of the sync pulse by use of the flat slope in the “amplitude filter” response curve.

(*) I found this description of the amplitude filter in “Television Receiving Equipment” W.T.Cocking, 1940.

Having got that out of the way, it was time to see where the problem lay. Obviously, given the above, the two crucial elements are the biasing of the valves (e.g. so the pulse diode doesn't stop conducting on picture), and the grid voltage on the limiter (e.g. C61 is correct).

C61 was the easiest thing to check, and it would explain the problems, so I checked that first. To my total surprise it was out of spec and leaky. It was a surprise because although it was the only non-restuffed capacitor left in the timebase/sync circuit, I hadn't replaced it because it already was a replacement waxy capacitor (the only one), it was in good physical condition, and it had previously tested perfect (exact 0.23uF reading and non-leaky). I had tried very hard to find something wrong with it (thinking this is too good to be true), but it was perfect.

On removing the capacitor completely from the set, I discovered how I had been deceived. The circuit diagram specified it as a 0.23uF capacitor, but I discovered it was a 0.1uF capacitor. So when it had read a “perfect” 0.23uF previously it was out of spec all along and likely to fail.

Photo 1 shows the capacitor, with the date stamp of “NOV 54”, which is the newest date I found on any of the replacement components. I restuffed and replaced.

OK, so that quite probably explained the sync problems, time to retest? Well I still had the cause of the top of frame cramping to discover. That is often caused by HT problems in the framebase circuit, and as the set of resistors which drove the HT for the framebase circuit, was also involved in the HT supply to the pulse diode anode, it seemed a good time to check them all, killing two birds with one stone.

This set of resistors is a tangle of resistors in parallel and series with each other and across the sync, width and height pots, and they all contribute to the ultimate HT. As a quick check I tested them in circuit, with a guestimate of the expected parallel/series resistance at each point, and even with a likely terrible guestimate, the resistance around the height/width pots seemed wrong.

Time to strip it down again, and test individually again.... and this time I found the height pot was open-circuit at the negative end!

Now at this point I thought my run of failures with the pots had gone beyond a joke. This was the fourth pot I had found broken in some way (the previous ones were the width pot, the line/frame hold pot and the focus pot). I also realised I had probably never tested the pot, because unlike the width and line/frame hold pots it wasn't obviously broken. So it was like the focus pot, something I didn't discover until testing.

I took the pot apart, and sure enough the resistor track was open-circuit for about 10% of the way from one of the connections. The other 90% was OK, and curiously it read the correct value for the pot (0.25Mohms), so either it had gone high, or it was always high (but within tolerance). I thought it would be difficult (but not impossible) to find a replacement, but the fact the remaining 90% read the correct value gave me an idea . If I could bridge the open-circuit part of the resistor track with conductive material, it should fix the pot. As a quick test I covered it in a thick layer of graphite using a pencil, and it worked. Obviously the graphite probably wasn't very durable, and anyway considering HT was going across it, it might cause problems. But a more durable and more conductive material should be OK. Searching on the web I discovered conductive silver paint, very durable and highly conductive (pure silver formulation). A couple of layers of that did the trick.

Photo 2 shows the fixed pot on the right, compared with the previously fixed width pot on the left. When I took it apart I also discovered, like the width pot, it had been forced but this time although the retaining metal clip was buckled, it didn't break it.

Replacing that, and retesting I got the test card shown in Photo 3. A significant improvement, both the sync problems and the frame cramping were fixed. The height pot open-circuit was certainly the cause of the frame cramping, but it is probably difficult to say if it also caused any of the sync problems (over and above the C61 capacitor).

But, the job wasn't completely done, you can notice the occasional line tearing. This is shown more clearly in a particularly bad example in Photo 4, with the contrast turned down.

There is also evidence of folding on the left hand of the screen.

(incidently, the pictures are the correct way up this time because I've temporarily swapped the deflection coil connections).
Attached Thumbnails
Click image for larger version

Name:	_DSC_0893.JPG
Views:	323
Size:	172.2 KB
ID:	107378   Click image for larger version

Name:	_DSC_0898.JPG
Views:	375
Size:	157.7 KB
ID:	107379   Click image for larger version

Name:	_DSC_0927.JPG
Views:	446
Size:	100.1 KB
ID:	107380   Click image for larger version

Name:	_DSC_0923.JPG
Views:	454
Size:	94.8 KB
ID:	107381  
Catkins is offline  
Old 24th Apr 2015, 7:59 am   #50
Freya
Octode
 
Freya's Avatar
 
Join Date: Jan 2009
Location: Falmouth, Cornwall, UK.
Posts: 1,987
Default Re: HMV 904 prewar television restoration

Great write up, the progression through the restore is very interesting. glad you shared it with us.
__________________
Stephen
_________"It`s only an old telly" ___
Freya is online now  
Old 24th Apr 2015, 9:45 am   #51
SteveCG
Nonode
 
Join Date: Jun 2009
Location: Herefordshire, UK.
Posts: 2,495
Default Re: HMV 904 prewar television restoration

I agree with Freya, the write up is a fascinating read.

Have you any idea where the set may have been before you got it? I was just wondering how good the set would have been in terms of 'fringe' operation.
SteveCG is offline  
Old 25th Apr 2015, 4:34 am   #52
Catkins
Pentode
 
Catkins's Avatar
 
Join Date: Feb 2015
Location: Chepstow, Monmouthshire, UK.
Posts: 234
Default Re: HMV 904 prewar television restoration

Quote:
Originally Posted by SteveCG View Post
Have you any idea where the set may have been before you got it? I was just wondering how good the set would have been in terms of 'fringe' operation.
Sorry, I don't know anything about the history of the set, beyond the fact I bought/collected it from someone in Bournemouth. Bournemouth is obviously outside of the transmission range of AP, but I don't know where it was before then.

You bring up an interesting point about fringe operation, as the picture shows a small amount of ringing. I always assumed the ringing was most likely due to alignment drift (I've not realigned the set, that's one of the todo items to see if it cures the ringing). But, introducting a small amount of ringing was aparently intentionally done in areas of fringe reception as it sharpened the image. Perhaps this set was used in a fringe area, and the ringing was intentionally introduced.

I forgot to mention in the last post, at this point I made a video recording of the set displaying television, and I put the video up here https://youtu.be/MffGCNQvHmk if anyone's interested. It was recorded on a mobile phone and the phone constantly goes in and out of focus (if anyone wonders what's causing the lack of focus).
Catkins is offline  
Old 25th Apr 2015, 10:59 am   #53
SteveCG
Nonode
 
Join Date: Jun 2009
Location: Herefordshire, UK.
Posts: 2,495
Default Re: HMV 904 prewar television restoration

Fringe reception could cause problems with line sync locking due to the sensitivity of 'direct' line sync designs to noise and interference. In later years 'flywheel' sync was used in 'fringe' versions of TVs and even later it became standard. It is one of the reasons that a RF pre-amp with a better noise figure than that of the set could make such a difference. As an increase in S/N not only gave full contrasted pictures but also greatly improved the line sync timing, giving straight verticals compared to ragged verticals.

Given the quite high cosmic background radio noise at 45 Mc/s a move to a 4 element aerial from, say, a dipole could also make a noticeable difference for similar reasons.

'Flywheel' also helped when the reception location had a lot of car ignition interference around.

As for the service area of AP - well my pre 405 line shutdown experience was that signals at 45 Mc/s had a habit of going a long way. So a well situated reception location in or just outside Bournemouth may not seem impossible.

When you've got the HMV working to your satisfaction it could be an interesting experiment to compare its performance on a weak signal with a more modern 405 line TV.

Last edited by SteveCG; 25th Apr 2015 at 11:09 am. Reason: correcting a sentence
SteveCG is offline  
Old 25th Apr 2015, 1:27 pm   #54
sexton_mallard
Hexode
 
sexton_mallard's Avatar
 
Join Date: Sep 2013
Location: Ellesmere, Shropshire, UK & Co. Cork, Ireland.
Posts: 499
Default Re: HMV 904 prewar television restoration

Great write up . I think I can hear the faint 10KHz whistle from your set running on your video.

Last edited by sexton_mallard; 25th Apr 2015 at 1:29 pm. Reason: fix a typo
sexton_mallard is online now  
Old 28th Apr 2015, 4:48 am   #55
Catkins
Pentode
 
Catkins's Avatar
 
Join Date: Feb 2015
Location: Chepstow, Monmouthshire, UK.
Posts: 234
Default Re: HMV 904 prewar television restoration

Sorry for the lack of posting. I have hit a major stepback in the restoration, basically the lack of focus fault has come back.

I have again tracked the problem down to the focus control. It has again developed a short circuit to earth around the same connection. Stripping it down shows HT has been arcing from the connection to the case (see photo).

I could fix it again, but .... this is now looking like a symptom (of another fault) rather than a cause. The obvious conclusion is the circuit is drawing more current than designed.

Everything checks out OK in the focus circuit, except one thing which is now becoming suspicious. The ohmage of the focus coil is 6.9Kohms, whereas the circuit diagram shows it as 9320 ohms.

Of course I've always known about this, but the focus circuit incorporates two 1500ohm resistors with screw taps designed to short out one or both. The clear intention is these can to used to adjust overall resistance to get correct focus (for variabilities in the focus coil).

I get correct focus with both resistances shorted out.

But, of course reducing the resistance increases the overall current flow, which shows more current is needed in the focus coil to get focus.

I'm left wondering whether the focus coil is in fact partially shorted. Which would explain the low ohmage.

I will compare it with my brother's focus coil for his Marconi 706 tomorrow. I would do it now except for the fact he's asleep and probably wouldn't be too pleased with me rummaging in his room looking for it.

Has anyone experienced focus coil shorts? Does this sound like this could have happened here?

Does anyone have the readings for their HMV 904/Marconi 706 (and perhaps the 7" models too) focus coils handy to compare, to see if it is indeed too low?
Attached Thumbnails
Click image for larger version

Name:	_DSC_1061.JPG
Views:	265
Size:	116.0 KB
ID:	107555  
Catkins is offline  
Old 28th Apr 2015, 11:46 am   #56
Jac
Heptode
 
Jac's Avatar
 
Join Date: Jun 2006
Location: Eindhoven, Netherlands.
Posts: 640
Default Re: HMV 904 prewar television restoration

Hi Catkins,

Sorry for the delay, but my sets are not easily accessible due to space problems (don't we have them all?)
I managed to do some measurements for you on unrestored sets in situ:

Marconi 706:
RDC of focus coil is 6,58 kΩ.
series and parallel inductance measurements:
@100Hz: Lp=120H - Q=2,11 Ls=98H
@1kHz: Lp=85H – Q=1,04 Ls=44H

HMV 905:
RDC of focus coil is 7,08 kΩ.
series and parallel inductance measurements:
@100Hz: Lp=106H - Q=2,04 Ls=85H
@1kHz: Lp=80H – Q=1,03 Ls=42H

The focus knob is marked “tone” in both cases, so unfortunately not the original ones.
The focus pot of the 706 has no influence at all (open circuit?) and the pot of the 905 is probably open in the extreme settings…

So most likely there is not an immediate reason for concern of the focus coil in your set (I hope so anyway – otherwise I have 2 defective ones).
If you are somehow able to make an inductance measurement, that could give some certainty. An inductance value around the 100H mark should be fine.

I read in the EMI service manual that there are 2 different types of focus coil possible, but could not find specs of them. Only values of the different adjustment resistors. Maybe the value of these could be a pointer to which focus coil you have.

Hope this helps.

Jac
Jac is offline  
Old 29th Apr 2015, 6:40 am   #57
Catkins
Pentode
 
Catkins's Avatar
 
Join Date: Feb 2015
Location: Chepstow, Monmouthshire, UK.
Posts: 234
Default Re: HMV 904 prewar television restoration

Quote:
Originally Posted by Jac View Post
Hi Catkins,
Marconi 706:
RDC of focus coil is 6,58 kΩ.

HMV 905:
RDC of focus coil is 7,08 kΩ.

So most likely there is not an immediate reason for concern of the focus coil in your set (I hope so anyway – otherwise I have 2 defective ones).
Thank-you very much for your quick response

I can confirm that my brother's Marconi 706 focus coil has a resistance of 6.44 Kohm.

So all of the focus coils seem to roughly match each other.

I tend to blame a late night and tiredness to explain myself becoming prey to worst fears. I'm sure we've all been there.

I have fixed the focus pot again. I think the underlying issue is I didn't fix the initial problem, and hence it reoccurred. But I have not tested it and so cannot say for sure yet. So I will leave things for later.

Quote:
Originally Posted by Jac View Post
If you are somehow able to make an inductance measurement, that could give some certainty. An inductance value around the 100H mark should be fine.
Out of curiousity, what equipment do you need to do that?
Catkins is offline  
Old 29th Apr 2015, 7:27 am   #58
Jac
Heptode
 
Jac's Avatar
 
Join Date: Jun 2006
Location: Eindhoven, Netherlands.
Posts: 640
Default Re: HMV 904 prewar television restoration

I used a DER EE DE-5000.

Jac
Attached Thumbnails
Click image for larger version

Name:	61TUg08eUTL._SL1500_.jpg
Views:	183
Size:	26.3 KB
ID:	107615  
Jac is offline  
Old 29th Apr 2015, 12:04 pm   #59
FERNSEH
Dekatron
 
FERNSEH's Avatar
 
Join Date: Mar 2005
Location: Gateshead, Tyne and Wear, UK.
Posts: 7,444
Default Re: HMV 904 prewar television restoration

A year earlier in 1937 HMV introduced the model 903. Like the 904 this receiver employed a vision anode bend detector and like the 904 the detector valve was the MS4B, an RF tetrode. For some reason this rather old fashioned valve (even for 1937) must have been the best choice the the detector function. The sync separator is described as the limiter type and again is similar to the arrangement found in the 904. This type of sync separator would continue to be in use right up to the end of the valve era in the 1970s. The sync separator valve conducts on the positive tips of the sync pulses and is cut-off for the remainder of the video waveform.

Note the biasing arrangement of the video detector, R33, R34 and R35.

Compared with the 903 the HMV 904 is a much better TV receiver. The 904 has an RF amplifier and the IF amplifer valves are the high slope RF beam tetrodes type KTZ41. 12.5Ma/v, that's impressive for 1938.

DFWB.
Attached Thumbnails
Click image for larger version

Name:	903_0081.jpg
Views:	157
Size:	51.0 KB
ID:	107622   Click image for larger version

Name:	903_0082.jpg
Views:	180
Size:	88.4 KB
ID:	107623   Click image for larger version

Name:	903_0083.jpg
Views:	190
Size:	70.3 KB
ID:	107624  
FERNSEH is offline  
Old 5th May 2015, 5:08 am   #60
SpiggyTopes
Retired Dormant Member
 
Join Date: Jul 2010
Location: Sintra, Portugal
Posts: 50
Default Re: HMV 904 prewar television restoration

Hi there!

I suspect there are plenty of lurkers like me ardently following your restoration, any news?

Cheers.
SpiggyTopes is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 5:09 pm.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.