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6th Mar 2021, 6:20 pm | #1 |
Octode
Join Date: Feb 2010
Location: Leicester, UK.
Posts: 1,431
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Marconi VC76DA
Hi all,
Some of you may recall this set from a few years back, I got some way with the repairs and then it came to a stop after the sound output transformer was getting hot and I lost the enthusiasm to carry on. Fast forward to now and I decided to raid my spares of valves and get the ones all out of the set and test them all, the sound output valve had various shorts so this was the fault there. I made up a good tested set and put them in the receiver as well as changing some more capacitors. I've now got the set working enough to investigate the further faults. The current one being that there is no line whistle from the set, not even the faintest of whimpers. I've checked the voltages of the valves as follows and it seems as though there are some differences to the data. B36 Data: Pin 5: 230v Actual: 240.6v Pin 2: 100/203v Actual: 117.8v KT33C Data: Anode: 200v Actual: 232v Screen: 230v Actual: 239v Cathode: 18v Actual: 23v KT36 Data: Anode: 209-263v Actual: 206v Screen: 145v Actual: 206v Cathode: 14v Actual:27v Can anyone advise me on what to check further? Some of the voltages seem to vary a lot from the data so presumably something isn't right somewhere. Cheers Bren |
6th Mar 2021, 7:46 pm | #2 |
Heptode
Join Date: Jun 2006
Location: Ware, Hertfordshire, UK.
Posts: 988
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Re: Marconi VC76DA
Hi Bren,
Thankfully the line output transformers on these sets are good. In fact they are about the only good thing about the set. It looks like the line oscillator is not running. Check the resistance of the windings of the blocking oscillator transformer. Other than that, suspect the B36. It is hard to find a good one, so best to substitute with a 12SN7 from a US manufacturer such as RCA or Sylvania. Cheers Andy
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6th Mar 2021, 7:50 pm | #3 |
Dekatron
Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
Posts: 9,145
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Re: Marconi VC76DA
Check the voltage or better still monitor the waveform on pin 5 of the KT36. It should be around 20v negative if it is being driven. Scope the output from the B36 anode. The B36 valve can be troublesome. Check the continuity of the windings of the blocking oscillator transformer. The line output transformer appears to be 100% reliable. They rarely if ever fail on this model but you never know. J .Sorry Andy! Amazing snap! John.
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6th Mar 2021, 8:14 pm | #4 |
Heptode
Join Date: Jun 2006
Location: Ware, Hertfordshire, UK.
Posts: 988
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Re: Marconi VC76DA
No worries. Its a good sign when there is consensus. You know these sets more than me. I've never restored one.
I once bought an HMV 1807, no back, a bit of woodworm and something odd about the tube mask. It turned out to have MW22-7 bodged in with a heater transformer screwed to the inside of the cabinet. Oh and the MW22-7 was flat as a pancake too! Why did I buy it? I broke it up of course and passed the chassis on to someone that wanted it. I've never been tempted to buy another one since. Cheers Andy
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6th Mar 2021, 8:19 pm | #5 |
Octode
Join Date: Feb 2010
Location: Leicester, UK.
Posts: 1,431
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Re: Marconi VC76DA
Thank you both, I will give these a check. Just as a side note I noticed the 4.7k supplying HT to the SG of the KT36 to be lower than stated, it actually measured 1.5k. I've got a spare so just strung one in for now. I also have a tested good B36 so I'll also fit that as well as check the blocking oscillator transformer. I've checked continuity of the LOPT and all is good there.
Luckily this set has a 12" Mullard CRT and I've always wanted to get one of these troublesome sets going, I'm sure they perform fairly well especially given a treatment of modern components. |
6th Mar 2021, 8:59 pm | #6 |
Octode
Join Date: Feb 2010
Location: Leicester, UK.
Posts: 1,431
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Re: Marconi VC76DA
I've replaced the B36 with another and also checked the continuity of all the transformers in the set, everything checks out as it should. There is still nothing when powering up, a close listen and I can hear some form of oscillation coming from around the LOPT and an occasional very quiet high pitched whistle. It's almost as though it wants to work but it's not quite there.
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6th Mar 2021, 8:59 pm | #7 | ||
Dekatron
Join Date: Mar 2008
Location: Worthing, West Sussex, UK
Posts: 5,185
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Re: Marconi VC76DA
Quote:
Quote:
My restored sets actually work quite well, without all the well documented instability, due no doubt to the fact that decent modern caps have replaced the awful original EMI items! Watch out for the Z77 valves, these are another cause of grief with these sets, I have replaced many with the good old Mullard EF91 or 6AM6. Do check the slider pots, they are a common problem in these sets. Mark |
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6th Mar 2021, 9:09 pm | #8 |
Octode
Join Date: Feb 2010
Location: Leicester, UK.
Posts: 1,431
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Re: Marconi VC76DA
Thanks Mark, all good pointers and I'll give those sliders a check, iirc I tested them the first time round and they were okay. I've replaced a few Z77 with EF91s, I have quite a few. Out of all the original Z77s I have only a handful were actually any good, the rest had very bad H/C shorts or just almost dead. The one for the sound output had a heater, cathode and grid short.
Strange but I would like to get my hands on a set with the original 10 inch emiscope screen, I know they're very poor but I'd like one just to see how it looks in person. |
6th Mar 2021, 9:27 pm | #9 | |
Dekatron
Join Date: Mar 2008
Location: Worthing, West Sussex, UK
Posts: 5,185
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Re: Marconi VC76DA
Quote:
You can see how good my VRC74DA performs here: https://www.vintage-radio.net/forum/...d.php?t=113547 I think your set should be fine, as it has a Mullard tube, I have a spare Mullard to fit my HMV 2811, when the Emiscope chucks in the towel! Mark |
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6th Mar 2021, 10:41 pm | #10 |
Octode
Join Date: Feb 2010
Location: Leicester, UK.
Posts: 1,431
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Re: Marconi VC76DA
Thanks Mark, I'll give them a read as I'm sure they'll also provide useful information too. I've checked the voltage of pin 5 of the KT36 and it is actually +15v. I've scoped this pin as can be seen in the first photo, the second and third shows pin 2 of the B36 that is supplying the transformer at different time divisions. As can be seen in all photos there seems to be a lot of oscillations/harmonics, any thoughts on where to proceed next?
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7th Mar 2021, 12:05 am | #11 |
Dekatron
Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
Posts: 9,145
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Re: Marconi VC76DA
That is very wrong! Check the coupling capacitor from the B36 to pin 5 of the KT36. It is probably very leaky. J.
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7th Mar 2021, 12:59 am | #12 |
Octode
Join Date: Feb 2010
Location: Leicester, UK.
Posts: 1,431
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Re: Marconi VC76DA
Thanks John, I'll give it a check tomorrow and report back. I've also been double checking the circuit to the schematic, just making sure no mistakes have been made or any ancient repairs/bodges. This had a BIII tuner fitted and also the 50uF capacitor that sits on top of the chassis near to the LOPT was missing when I first purchased the set.
I've found a problem with the vertical hold slider, it's a good job I've checked again. At one end there is a lot of loose resistance wire, someone has obviously tried to make it good by unravelling some and they also strung more wire around this resistor tag to the height VR tag, there should be a link between these two anyway but this has been cut and a loop of unsoldered wire just wrapped around them. I think I have a couple of these sliders somewhere so I'll take a look and hopefully change it for a good one. |
7th Mar 2021, 9:52 am | #13 |
Dekatron
Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
Posts: 9,145
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Re: Marconi VC76DA
Regarding the lack of line timebase operation, check all the components including the focus control in the grid pin 5 and cathode pin 8 of the KT36. J.
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7th Mar 2021, 10:45 am | #14 | |
Dekatron
Join Date: Mar 2008
Location: Worthing, West Sussex, UK
Posts: 5,185
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Re: Marconi VC76DA
Quote:
I replaced three of mine, as all intermittent or O/C. I remember that I had no EHT unless a combination of these sliders were at their end stops. As John says, check the focus pot, this can cause no or little EHT. Mark |
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7th Mar 2021, 11:07 am | #15 |
Dekatron
Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
Posts: 9,145
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Re: Marconi VC76DA
Be relieved that you have the version with the Mullard tube. I doubt if many decent 3/16 10" or 12" 3/18 tubes exist today.
The getter is usually OK pointing to cathode poisoning when pumping. They also developed an O/C heater without warning. The VC76DA is the superhet channel 4 Sutton Coldfield model. It has a slight edge over the 1807/VT53DA series but is still sensitive to mains input variations. Most of the nasty presets broke due to them requiring constant twiddling during the power shedding experienced in the 1940s/50s. John. |
7th Mar 2021, 1:18 pm | #16 |
Octode
Join Date: Feb 2010
Location: Leicester, UK.
Posts: 1,431
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Re: Marconi VC76DA
I've now replaced the faulty vertical hold control with a good one and had a look at C48 coupling capacitor, this is actually a new one but I disconnected one end and strung another over just in case. The set is still the same, today I'm getting 12.7v on pin 5 of the KT36, on the other end of C48 going to T2 secondary (grid of B36) I'm getting -15v. There does seem to be more groaning/low frequency noise from the line stages since changing the vert hold control but nothing else has changed from the last post.
I've also checked all components around the cathode and grid of the KT36 and everything seems to be fine here. |
7th Mar 2021, 2:20 pm | #17 |
Octode
Join Date: Feb 2010
Location: Leicester, UK.
Posts: 1,431
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Re: Marconi VC76DA
Another update and another low resistor found, R43 which is connected to the vertical hold slider was measuring only a few hundred ohms and not the 10k it should be, I've replaced this and the sound from the line stages is now louder than before but still low frequency that changes pitch when adjusting the sliders.
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7th Mar 2021, 3:03 pm | #18 |
Dekatron
Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
Posts: 9,145
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Re: Marconi VC76DA
If the B36/12SN7 is definitely OK it might be worth checking the KT36. You may not have one of these but if you have a PL38 it might be worth giving it a try to see if it livens up. It is not an equivalent but it has the same base connections and is designed for line output duty so it should do something. J.
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7th Mar 2021, 3:21 pm | #19 |
Octode
Join Date: Feb 2010
Location: Leicester, UK.
Posts: 1,431
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Re: Marconi VC76DA
Thanks John, I actually have three KT36 and tested them all, two were very poor and one gives about 42mA on an avo tester, the avo data book states it should give 60 mA. This is the one I've been using in the set but I could source another and give it a try. Voltages around the B36 are not too far off from the Marconi data so I presume it is working to some degree.
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7th Mar 2021, 3:51 pm | #20 |
Octode
Join Date: Feb 2010
Location: Leicester, UK.
Posts: 1,431
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Re: Marconi VC76DA
Just to update the thread with a few more scope traces, the first showing the first anode of the B36 on pin 5, photo 2 showing the second anode of the B36 on pin 2 (supplying T3). Photo 3 is from the secondary of T2 taken before C48 and the last photo is of the grid (pin 5) on the KT36. I think these are looking somewhat better than yesterday.
Thanks to everyone who has helped so far, I've often been lucky with line stages of sets before and any faults are usually easily rectified. I suppose with this sort of set it's to be expected really. |