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Old 7th Apr 2015, 5:43 am   #1
Catkins
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Default HMV 904 prewar television restoration

Hi Guys

Back in 2012 I was lucky enough to purchase a HMV 904 television (1938), which I intended to restore to working operation. I spent an initial 6 months working on it, got disheartened due to the faults uncovered/work to be done, and then temporarily put it aside. This turned out to be for two years! Becoming annoyed with myself I went back to it in September last year with fresh determination, and I have spent nearly every weekend/evening since then working on it.

My brother (Draenog on the forum here), has been insisting for some time I post here about my progress. So here it is Happily I think I have made good progress, but, because this is my first post which I believe needs to be vetted, I will be brief here and let the pictures do much of the talking. I will/can post follow ups going into more detail about the myriad faults uncovered, and my stages in fixing them.

To start from the beginning. When purchasing the set, I looked at the chassis in the case, and saw it had been kept in extremely damp conditions and was hence extremely rusty and corroded. On removing the chassis I discovered my first impressions were if anything rather optimistic. Everything was rusted and corroded, and any "delicate" metal, like the springs on the tuning cord, had completely rusted away. It was in worse condition than any set I had seen online. However, for some odd environmental reason, the underside of the chasis was in much better condition (though not perfect). If someone had shown me photographs of the topside and underside, I would have believed them to be from different sets.

I tested the filament of the CRT (Emiscope 3/1), and the only slight consolation was the filament appeared to be intact (although you only really know if it works when when you apply the correct filament/EHT voltage etc.).

Leaving the corrosion aside, I checked the set over for obvious faults, and discovered the set had been serviced professionally many times, leaving a total mess of layers of indifferent work, mainly in the power section but elsewhere in the vision section (timebases etc) too, and the original wiring plan was obscured, perhaps the last power section overhaul replacing the 5 electrolytic capacitor Dublier can on top of the chasis (it was missing) with Hunts electrolytic capacitors jury rigged into the bottom of the chassis.

Worse was the fact that having gone wrong again, and having evidently become uneconomical to repair professionally, a bodger had been at it. The EHT section was totally dismantled, and a huge Radiospares 1M ohm 6000Volt electrolytic capacitor was dangling from wires. The timebase section was partially dismantled too, and the CRT cathode bypass capacitor was missing also.

Even worse was the video oscilator coil had been unscrewed, the porcelian base broken, and the soldered centre tap broken off. The width pot had been forced, and was rotating freely, and the dual ganged line/frame hold pot had the outer gang shaft broken in two at a weak point (where there was a grove for the surclip).

But apart from that, it looked OK (joke).

Subsequent dismantling of the top of the chasis brought up further faults, not least the finding that the audio output transformer was open circuit (secondary winding).

But, this first post is already longer than I anticipated.

To wrap up, there are 5 photos attached. Unfortunately I forgot to take a photo of the set as found, but photo 1 is from early on, when I had partially dismantled the top of the chasis but before any restoration had been started. Photos 2, 3, and 4 are views of the set from now, showing the restoration done, and that this had been done in a sympathetic way trying to preserve the look of the set (you'll notice the 5 capacitor Dublier can is there, after two years of looking I finally found a circa 1938 radio with the can in it last Christmas).

Photo 5 is test card C, showing the current progress in getting the television to work. There are still issues (ringing mainly), but, I'm happy with the results so far.
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Old 7th Apr 2015, 8:48 am   #2
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Default Re: HMV 904 prewar television restoration

Hi Catkins,

Welcome to the forum.

It certainly sounds like you've had more than your fair share a tricky issues to solve with your 904 but the result looks very good both over and under the chassis. Still looks like a genuine original set and produces a very respectable test card image.

Well done!

Peter
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Old 7th Apr 2015, 9:26 am   #3
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Default Re: HMV 904 prewar television restoration

Hi Catkins,

That's an excellent write up and restoration. Hope to hear more from you soon.

Cheers,
Brian
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Old 7th Apr 2015, 10:35 am   #4
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Default Re: HMV 904 prewar television restoration

Dont think that post needs much vetting - a superb write up.

Please post further and how you overcame all the faults and snags etc.
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Old 7th Apr 2015, 11:29 am   #5
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Default Re: HMV 904 prewar television restoration

Hi,

Excellent write up and what appears to be a darn good job.
Looking forward to hearing more about the set and more photos too.

Cheers,
Marc.
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Old 7th Apr 2015, 6:54 pm   #6
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Default Re: HMV 904 prewar television restoration

Great work, I can understand how you felt having to deal with all the problems the set threw at you, but I am glad you stuck with it
The CRT looks nice and bright, so a very worthwhile restoration.

Mark
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Old 7th Apr 2015, 8:37 pm   #7
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Default Re: HMV 904 prewar television restoration

You have a pretty good CRT there for its age.

Peter
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Old 7th Apr 2015, 10:21 pm   #8
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Default Re: HMV 904 prewar television restoration

Super job so far, that crt looks extremely lively. You clearly have more perseverance than I do as you have had a very steep hill to climb to get here! Well done that man!

Cheers
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Old 8th Apr 2015, 4:57 am   #9
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Default Re: HMV 904 prewar television restoration

Hi

Thanks for all your very nice comments

A number of people have commented on the brightness of the CRT. Yes, it is very bright, this is perhaps more surprising given that the television was in use through-out the 40s and at least up to November 1954, according to the date stamps on replacement components.

This has led me to wonder whether during that time the CRT was re-gunned. Comparing it with my brother's (Draenog) Emiscope 3/1 from his Marconi 706 (filament unfortunately open circuit) there are some obvious differences.

Photo 1 shows the two CRTs side by side, with the CRT from the 706 on the bottom. The bottom most CRT has the typical air evacuation point on the top side, whereas the 904 CRT has a circle of rough glass. The 904 has the conventional evacuation point on the bottom of the tube inside the plug (photo 2). As can be seen from photo 3, the guns are also different.

Back in 2012 doing some web searching brought up previous speculation whether later or alternatively sourced 3/1 tubes had rear evacuation points, but due to lack of evidence I don't think this has ever been solved.

I have always tended to think if the CRT always had a rear evacuation point, then there should be no reason for the rough glass on the top side. The rough glass is perhaps the clearest sign it originally had a side evacuation point. But then you think why bother to remove it anyway, so you tend to go around in circles, unless you assume breaking the evacuation point was how the vacuum was reduced.

Has anyone else seen Emiscope 3/1s with rear evacuation points? Available information on the internet tends to suggest there are examples of the larger Emiscope 3/2 with rear evacuation, but, again I have a contemporary Marconi sales brochure from September 1938, which pictures an Emiscope 3/2 with side evacuation (photo 4).

Has anyone any idea on how many of the surviving working pre-war CRTs that have been re-gunned? This might make an interesting piece of information?

For someone that wasn't around in the 50s/60s (well just for the 60s), it is always difficult to get a feel for how often televisions were repaired and then repaired, as oposed to our throw away culture now. Was re-gunning a common practice? I'm old enough to remember the local town TV repair man in the late 70s/early 80s, but it was clear even then due to the increased reliability of sets it was a doomed profession (but one that appealed to me). Would the larger repair shop have been able to re-gun CRTs, or would it have been the job of a handful of specialist companies in the UK? I still find it incredible that in the space of 40 years we've gone from that to zero capability to repair CRTs
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Old 8th Apr 2015, 8:00 am   #10
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Default Re: HMV 904 prewar television restoration

Re -gunning was common practice by specialist companys.
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Old 8th Apr 2015, 8:03 am   #11
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Default Re: HMV 904 prewar television restoration

Re gunning tubes was very common practice in the 60's and 70's, probably in the late 50's as well, not sure about earlier. Some firms would re-gun others would rebuild, or that's what the adverts claimed, both B&W and colour.
It was a mixed bag, some firms much better than others, the best ones I saw were the colour rebuilds from Mullard, I could not tell them from new.
Somehow I could always tell a third party rebuild both B&W and colour from the OEM, perhaps I did not search hard enough and there were equally good third party rebuilders out there.
With not doing rental we mainly replaced CRT's with new Mullard or Cathodeian( think that name is correct). Mullard were good B&W tubes in the 60's 70's, rarely failed in service life, the ones we replaced were usually Mazda with an O/C cathode, I think that was the fault. The Mazda CRT's did seem to give a brighter more contrasty picture though when new.

Nice job on the refurb.
Frank
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Old 8th Apr 2015, 8:33 am   #12
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Default Re: HMV 904 prewar television restoration

I have four early 50's sets fitted with rebuilt CRT's, all have good emission and produce a good picture.

There were quite a few tube rebuilders around in the 50's & 60's, a look through Practical Television magazines of the period turns up dozens of company's offering rebuilds, often requiring the old tube to be exchanged.

My rebuilt tubes carry the following names: Re-view, Sabrina, Nu-view, Suffolk tubes and Lawson.

The one thing they all have in common is the tell tale 'Kink' in the neck where the new gun was welded to the old glass.

Some tubes were fully rebuilt, just using the old glass. others were just re-gunned.

Now RACS in france are no more, there is no facility left to carry out rebuilds, the Early television foundation in the USA now has most of the equipment from RACS, and intends to get set up to rebuild CRT's at some point.

Mark
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Old 8th Apr 2015, 8:52 am   #13
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Default Re: HMV 904 prewar television restoration

I don't think this CRT has been rebuilt. The original EMI label is still intact and this would not have survived the regunning process.
CRT rebuilding did not become popular until the later 50's and by that time I cannot imagine a company rebuilding such a difficult tube [Pyrex] The standard Mullard and Mazda 9" tubes had been dropped by most rebuilders by this time. There would have also been the problem of obtaining a gun assembly.
EMI as far as my records show, did not offer a reconditioning service for these early tubes and when the limited new stocks were exhausted, that was it! The receivers would then have been 15 years old and well obsolete television wise.
It is possible that a new EMI tube was fitted as they had a limited new stock of 3/1 and 3/2 tubes into the 50's. The 3/3 was obsolete in 1946 replaced by the TA10 and a special fitting kit. There were no post war replacements for 5" and 7" tubes. The 12" 6/6 could still be obtained in 1960.
The base also looks original and would not have been refitted by a rebuilder who more likely would have fitted say a simple octal with mod sheet.
My guess is that it is a late production 3/1 from EMI. It is certainly bright with no Ion burn, a problem that these tubes suffered from very badly.
Great job! Regards, John.
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Old 8th Apr 2015, 11:55 am   #14
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Default Re: HMV 904 prewar television restoration

Quote:
Originally Posted by Heatercathodeshort View Post
The 12" 6/6 could still be obtained in 1960.
Hi John. That is surprising to read. Why would it still have been available do you think? Running repairs to first generation televisions, or was there some other use?

John.
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Old 9th Apr 2015, 7:06 am   #15
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Default Re: HMV 904 prewar television restoration

Hi

A number of people have asked for more info and pictures, and I promised more info on the restoration process. My first post wrote up my first observations on getting the set. I think it is easier to continue in a chronological order, and so the next stage is the long slog to first switch on, the process of making the set safe so it didn't blow myself or the set up.

Perhaps 60-70% of the time was spent doing this (I first switched it on last Christmas), but it was by far the most tedious and boring part of the process. So this may make this the most boring and tedious post, apologies

After the initial inspection, the question was what to do now. The obvious answer was dealing with the rust/corrosion, the electrolytics, and rewiring.

Deciding what to do with the rust/corrosion was a difficult decision, with radios previously I'd never attempted to repair something this rusted/corroded, and hence I was happy to always leave it "as is" to keep originality. To be blunt I'd never dealt with anything previously so rusted I didn't want to turn it on.

Browsing on the web I saw previous examples of restoration where they'd completely stripped the components, and sand-blasted and replated the chassis. I personally thought this destroyed a lot of the originality (there's originally in the construction as well as the components, and this is all lost on a complete strip down). Personally I thought a strip down was for the ease of the restorer rather than anything else, and against originality.

A careful inspection of the underside of the chassis showed it was generally in good order, and a strip down of that to sand-blast/replate was destructive, a carefull check of all components and wiring in place would work. This left the top of the chassis which needed complete restoration. So I decided on a hybrid approach, a stage by stage strip down of the components on the top of the chassis for checking/cleaning, leaving the chassis free for rust removal with available rust removal products. Of course this was going to be a long process, rust removal products only work on a limited area at one time, and require repeated application if the rust is deep.

It took about two months to remove the rust this way, and it was very tedious. Each small area took about 4 applications, over four days with a tooth brush. But in the end I think it was worth it

Photo 1 shows the process about 2/3rds done, the right side has been cleaned and reassembled, the left has been cleaned, and was reassembled before moving on to the middle part, which hasn't yet been disassembled and cleaned. The insistance on reassembly is because I've been "burnt" before when I've disassembled too much, got distracted for some weeks, and then realised I'd forgotten how to put it back together (photographs only help so much).

The process with the electrolytics was standard. All 18 electrolytics obviously proved to be bad (leaky, open circuit, or shorted). Of the 18 electrolytics, six were original, the other 11 having been replaced in the 40s/50s, and one missing. The six originals were restuffed. Four of the replacements were deemed sufficiently "original" to be re-stuffed too (two from 1949, two from 1950, and this also preserved the service history of the set, which I think is important too)). This included two of the four chassis mounted cans which were (co-incidently) just large enough to fit aound modern AJ Electronics screw mounted cans. The other two chassis mounted cans were replaced with AJ Electronics cans, which look a bit too short, and I'm still looking for 30s vintage cans to replace them (these were placed at the front of the set, and hence will be invisible when the chassis is in the case, looking from the back).

The restuffing of the metal capacitors was quite tedious, they all proved to have dried out to a rock hard consistency, and no amount of heating, or soaking in lubricant would allow the insides to be pulled out, and so eventually they all had to be drilled out piece by piece. Photo 2 shows a capcitor cut open at the end and drilled out, before restuffing.

This left 5 Hunts capacitors which replaced the lost 5 capacitor Dublier can mounted on the chassis. These were definately not going to be restuffed. With the lack of the Dublier can, I constructed a replacement temporary can using a project box of the approximate size, hoping to find a replacement Dublier can to put it in (which I eventually did). This is shown in photo 3. For the missing electrolytic I found a vintage 30s electrolytic of the right size and look, and restuffed that.

This left the rewiring, all the wire on the surface of the chassis needed replacing, and this was replaced with silicone wiring at the time of disassembly/reassembly. All the EHT wiring was perished and need replacing (and would have been anyway for safety). I initially bought car ignition wire which turned out to be graphite cored and useless, but found "old fashioned" ignition wire from a specialist vintage car site. The wiring underside was checked, and being a mixture of cotton and rubber, found the cotton wire was OK, but approx 30% of the rubber wiring needed replacement.

The two pots (width pot, and dual ganged frame/line hold pot) were then repaired. The width pot proved to be fairly easy to repair, the metal clip attaching the shaft to the pot brush proving to have been broken, and the paxolin middle of the brush shattered. This was reassembled glued back into place, and the metal clip bent back into shape and glued.

The dual ganged pot proved to be much more difficult to repair, The outer shaft had been broken in two at a weak point (a grove for a circlip), and the amount of surface metal at the join proved to be too small to glue with any strength. Various makes of "max strength" epoxy were tried, with steadily increasing hardening time (the strength supposedly getting stronger), but a determined twist at max/min gang proved easy to break the bond achieved. Finally I went back to the old fashioned approach of soldering it, filling the groove with solder for extra stength. Of course the shaft assembly conducted heat too readily, but I finally got sufficient heat using the blowtorch function of a butane soldering iron. The solder join was then wrapped in a layer of chemical metal for further strength, and to replace the function of the circlip in tensioning the pot.

Photo 4 shows the pot disassembled with the broken shaft, and photo 5 shows the pot reassembled, with it's ring of chemical metal.

But I'm out of photo space and time (it is way past my bedtime).
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Old 9th Apr 2015, 11:52 am   #16
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Default Re: HMV 904 prewar television restoration

Quote:
Originally Posted by Catkins View Post
So this may make this the most boring and tedious post, apologies
Nope, not for me

Quote:
Originally Posted by Catkins View Post
Browsing on the web I saw previous examples of restoration where they'd completely stripped the components, and sand-blasted and replated the chassis. I personally thought this destroyed a lot of the originality (there's originally in the construction as well as the components, and this is all lost on a complete strip down). Personally I thought a strip down was for the ease of the restorer rather than anything else, and against originality.
I've often seen this in the excellent write-ups in the BVWS Bulletin articles too. The results are spectacular, with an as-new appearance, and I have a lot of respect for the people who take the bull by the horns and do things in this way.

But like you, I think this can eradicate a lot of the "character" from an old set.

Nick.
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Old 9th Apr 2015, 11:59 am   #17
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Default Re: HMV 904 prewar television restoration

The CRT in my 904 has an ion burn and now the EHT transformer has burnt out. What a pity so now I'll just have to smash it up for spares.

DFWB.
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Old 9th Apr 2015, 12:17 pm   #18
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Default Re: HMV 904 prewar television restoration

...You've been spending too much time with Mr Wakely!
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Old 9th Apr 2015, 12:55 pm   #19
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Default Re: HMV 904 prewar television restoration

Really enjoying the write up and photos.
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Old 9th Apr 2015, 1:48 pm   #20
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Default Re: HMV 904 prewar television restoration

This is brilliant job that has been done and of course write up.Keep it going.
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