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 9th Apr 2015, 3:24 pm   #21
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

Overcome with guilt after making that outrageous statement suggesting I might chop up the HMV 904 I'd better fix it instead, and also the Marconi 707 next to it. Both sets need attention.

DFWB.
Attached Thumbnails
Click image for larger version

Name:	904_0046.jpg
Views:	507
Size:	70.2 KB
ID:	106768  
FERNSEH is offline  
Old 9th Apr 2015, 6:50 pm   #22
Heatercathodeshort
Dekatron
 
Heatercathodeshort's Avatar
 
Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
Posts: 9,145
Default Re: HMV 904 prewar television restoration

Quote:
Originally Posted by FERNSEH View Post
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.
You know it's not good for you to visit David. It takes you far too long to get over it.

Hi John,
I think they were just left over stock but it was surprising how many 900's survived into the early 60's. There was also a 6/7. Some data sheets give this as aluminised but I don't think it was.
Regards, John.
Heatercathodeshort is offline  
Old 9th Apr 2015, 8:42 pm   #23
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

I'm sure there was adverts for unsold HMV 904 sets in the Wireless World after the closedown of the television service in 1939. The sets were offered without the television valves. The buyer would become the owner of a first class radio set. Exceptional performance on short waves.

DFWB.
FERNSEH is offline  
Old 9th Apr 2015, 9:46 pm   #24
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

Quote:
Originally Posted by Heatercathodeshort View Post
There was also a 6/7. Some data sheets give this as aluminised but I don't think it was.
6/7. Hi John, that was the 12 twelve inch 57 degree "onion tube" used in the HMV 902A which is the direct viewing version of the 902 TV/radiogram. The 902A was first set to employ the KT44 line output valve.
The 904 got away with using the KT63 audio valve in the line output stage.
The deflection angle of the 3/1 CRT is only 40 degrees or so.

DFWB.
FERNSEH is offline  
Old 9th Apr 2015, 9:57 pm   #25
Catkins
Pentode
 
Catkins's Avatar
 
Join Date: Feb 2015
Location: Chepstow, Monmouthshire, UK.
Posts: 234
Default

Quote:
Originally Posted by Heatercathodeshort View Post
My guess is that it is a late production 3/1 from EMI.
Yes, I think you're right, the comments about the label and plug being original seem conclusive. Plus the tube doesn't have the tell tale kink that Mark Pirate mentioned.

Quote:
Originally Posted by FERNSEH View Post
Overcome with guilt after making that outrageous statement suggesting I might chop up the HMV 904 I'd better fix it instead, and also the Marconi 707 next to it. Both sets need attention.
Very nice to both the 5" and 7" versions. I hadn't realised until I saw your photograph how much larger (in all dimensions) the 7" model was. I assume the chassis is the same size, so it's going be less cramped inside? (there isn't much room in the 904).
Catkins is offline  
Old 9th Apr 2015, 11:40 pm   #26
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

When I return to the shop tomorrow I'll turn the two sets around to show the internals. I don't think there are any electrical or mechanical differences between the 5 and 7 inch CRT sets.
The 5 inch model is a difficult set to do any servicing on because of the high component count on what is a compacted chassis. An engineering marvel for 1938.

DFWB.
FERNSEH is offline  
Old 10th Apr 2015, 4:16 pm   #27
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

Rear view pictures of the Marconi 707 and HMV 904. The cabinet of the Marconi is about two and half inches wider than the HMV.
The 7" CRT Emiscope CRT in the Marconi was rebuilt by in 1996 by Display Tubes Ltd.of Uxbridge. Are they still business?

There is no firm in the UK wiling to attempt CRT rebuilding. However, a firm called Moores Glassworks was the subject of discussion here: http://forum.radios-tv.co.uk/viewtopic.php?f=5&t=10883

DFWB.
Attached Thumbnails
Click image for larger version

Name:	904_0051.jpg
Views:	457
Size:	98.7 KB
ID:	106801   Click image for larger version

Name:	904_0052.jpg
Views:	447
Size:	88.2 KB
ID:	106802  
FERNSEH is offline  
Old 10th Apr 2015, 7:23 pm   #28
Freya
Octode
 
Freya's Avatar
 
Join Date: Jan 2009
Location: Falmouth, Cornwall, UK.
Posts: 1,987
Default Re: HMV 904 prewar television restoration

Not to go off topic with this but i wonder if this is too bigger leap; "ET Enterprises Limited is a new company which has taken over the business of Electron Tubes Ltd, Uxbridge"
et-enterprises.com
__________________
Stephen
_________"It`s only an old telly" ___
Freya is offline  
Old 10th Apr 2015, 8:15 pm   #29
Jac
Heptode
 
Jac's Avatar
 
Join Date: Jun 2006
Location: Eindhoven, Netherlands.
Posts: 640
Default Re: HMV 904 prewar television restoration

Hello Catkins,

Thank you for this very interesting write up!

Jac
Jac is offline  
Old 10th Apr 2015, 9:19 pm   #30
PJL
Dekatron
 
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 5,997
Default Re: HMV 904 prewar television restoration

HMV are not known for miniaturisation and you have to wonder what influenced the decision to make them so compact? Perhaps copying USA sets or maybe the Baird Televisor?
PJL is offline  
Old 13th Apr 2015, 4:33 am   #31
Catkins
Pentode
 
Catkins's Avatar
 
Join Date: Feb 2015
Location: Chepstow, Monmouthshire, UK.
Posts: 234
Default Re: HMV 904 prewar television restoration

Hi Guys

Thanks for all your great comments so far

To continue from the last post. At this point, I'd done all the pre-switch on work, except for the bleeder resistors (which were open circuit), the restuffing of the dual EHT capacitor unit, and a sanity check of the timebase circuits (see later). The audio output transformer had been found to be open circuit (secondary winding), and this had been sent off to Steve Barker for rewinding, and he did a first class job.

But this was Christmas Eve, and I decided I could delay this work, and go straight to testing the radio (with the EHT transformer primary physically disconnected for safety). Hopefully this would give me a nice Christmas present ...

In the event I manged to get everything ready for switch on of the radio on Boxing day. As this was probably the first time the power transformer had been run for ~60 years, and the power section had been completely rewired (and elsewhere), I brought it up slowly over an hour on a variac, checking for transformer overheating, signs of shorting, and any other issues.

After an hour input was at full voltage, HT was correct, filament voltages were correct etc. Time to see if the radio worked. To my surprise no stations on any waveband, but it was not completely dead, clicks on waveband changes, slight hum on the speaker, and the finger test on the grid of MHD4 produced a nice burr, so the audio output was at least working to an extent. Going backwards through the circuits showed the problem was in the HF amplifier, right at the very start of stage 1, the aerial feed wire was partially caught underneath the oscillator coil can, splitting the rubber, and shorting it to earth.

Once this was fixed, I got stations on medium wave, but only at the top end of the scale (Smooth radio from Bristol, and Absolute radio). Nothing on long wave (or short wave). Injecting RF signals into the set from a signal generator showed all the bands were offset to a wide extent (~50%), long wave wave appeared dead because it was offset to a region which doesn't have any stations anymore. Obviously, there were lots of probable causes, but the fact the fault was across the bands tended to pin-point misalignment of the tuned/IF circuits, rather than faults in individual aerial/oscillator coils etc. The fact there was an audible click on the speaker when the ganged variable capacitor was at exactly half gang, and then nothing, suggested this was the first thing to check.

I disconnected this, and sure enough it was faulty, giving the wrong values on a capacitance test, and nothing after half gang. I cleaned it with switch cleaner, no improvement, soaked it in water/washing up liquid, no improvement. I finally cleaned it with acetone, and this fixed the problems.

In the meantime, after visiting my parents over Christmas I found the missing 5 capacitor Dublier block I'd been looking for (for over two years), in a contemporary wrecked HMV radio (rusty and full of woodworm). My parents visit car boot sales, and buy radios matching the descriptions of sets we're looking for. Normally they don't find anything of use, but this time they did.

Photo 1 is a composite of 4 photos (hence maximising the amount of photos I can attach, sneaky). This shows three views of the capacitor can as found in the radio. Like most radios of this age, the electrolytics had been replaced, but unually for the time (late 50s/early 60s judging by the replacement capacitors), the servicer had taken the time to restuff it, rather than throwing it away. But as can be seen the can suffered quite badly in the restuffing, getting a seam split, badly bent, and a lot of the paint had flaked off. The can was fixed, sanded down and repainted, and then restuffed again.

Photo 1 (just to fill the space) also shows a photo of the variable capacitor before cleaning.

The test of the radio showed the power, HF, shared audio/vision IF and audio output stages were working. It was time to finish off the work to test the television.

The next stage was the EHT capacitors. Photo 2 shows the underside of the set at purchase time. The huge Radiospares 6000Volt EHT capacitor added after the set's working life can clearly be seen dangling from the bottom of the chassis. Given this, it was no surprise to find one of the EHT capacitors was open circuit. Also given the Radiospares capacitor is very close to the video oscillator coil, I suspect this was when/why the video oscillator coil got broken.

This meant the dual EHT capacitor can had to be restuffed. Photo 3 shows a composite of 4 photos of the restuffing, in the upper left side, the capacitor is shown with the bottom removed showing the tightly wound capcitor coil. I had thought this was going to be a nightmare to remove given my previous experience with the rock hard metal electrolytic cans. To my surprise, after soaking in lubricant, it pulled out in one piece. The next two photos (in the composite) show stages of the restuffing. The last photo to fill up the space shows some of the restuffed paper cased electrolytics.

The presence of the Radiospares EHT capacitor got me thinking. It is perhaps always useful to try and determine what was the original fault. The fact that there was a badly replaced EHT capacitor (clearly not by a professional repairman), tends to suggest failure of EHT was the original fault.

But, given the Radiospares capacitor should have fixed the EHT fault, and the EHT fault (following my premise), was the original fault, why did the set get abandoned, and why did the width and line/frame hold pots get broken? They don't break themselves. The EHT circuit in a mains derived EHT television is completely separate from the timebases, and failure in the EHT should not have caused failure in the timebases. So something else had happened.

Of course after ~80 years it is completely normal to find multiple faults in multiple circuits, but this looked like two separate contemporary faults occuring at the same time (EHT and timebases). What is the likelihood of that? Possible but suspicious all the same.

Given the timebase circuits showed work had been done on them, and the general lack of knowledge/expertise shown, I wondered whether the timebase circuits had been modified, causing the problems which led to the breaking of the width and line/frame hold pots. I decided it was necessary to carefully sanity check the timebase circuits against the circuit diagram before switch on.

This wasn't a check of the individual components (that would come later), but merely that the wiring was correct. Sure enough I found the timebase circuit had been subtly but stupidly miswired, so that it would never have worked. A wire that should have been soldered to C78, had instead been soldered to the adjacent R44. This connected the negative side of the width/height/sync pots and the HT feed for the limiter, line/frame oscillator valves to the grid of the limiter valve. Given the voltages at these points are critical for correct sync separation, and oscillation, it's not surprising the pots got broken trying to get correct width, line/frame hold. It's surprising it worked at all.

My guess is the unknown person blindly attacked the set (attacked being the operative word), trying to find the fault, and by the time the original fault was found he'd left a trail of mistakes, which meant it was now broken beyond repair.

I had hoped to get further in the repair description this time, but, I've again ran out of time.

I've attached photo 4 which shows the television sometime after first switch on. Next time will describe the work leading up to that, and hopefully beyond (to infinity and beyond perhaps to quote Buzz Lightyear's non sequitur).
Attached Thumbnails
Click image for larger version

Name:	4.jpg
Views:	426
Size:	158.8 KB
ID:	106928   Click image for larger version

Name:	1.jpg
Views:	427
Size:	190.9 KB
ID:	106929   Click image for larger version

Name:	2.jpg
Views:	410
Size:	196.2 KB
ID:	106930   Click image for larger version

Name:	5.jpg
Views:	419
Size:	93.9 KB
ID:	106931  
Catkins is offline  
Old 14th Apr 2015, 2:25 pm   #32
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
This wasn't a check of the individual components (that would come later), but merely that the wiring was correct. Sure enough I found the timebase circuit had been subtly but stupidly miswired, so that it would never have worked. A wire that should have been soldered to C78, had instead been soldered to the adjacent R44..
A correction, it was soldered to the adjacent capacitor C61 (my memoy played a slight trick on me). So it was the other side of the R44/C61 junction, and connected to C61 and the cathode of the sync valve. Same general effect though.

Photo 1 shows a photo I took at the time, the yellow wire soldered to the brown waxy capacitor on the right, should have been soldered to the partially disconnected electrolytic capacitor on the left. The partial desoldering of the electrolytic was me to test it previously.
Attached Thumbnails
Click image for larger version

Name:	6.jpg
Views:	313
Size:	44.3 KB
ID:	106996  
Catkins is offline  
Old 14th Apr 2015, 8:38 pm   #33
mark pirate
Dekatron
 
mark pirate's Avatar
 
Join Date: Mar 2008
Location: Worthing, West Sussex, UK
Posts: 5,185
Default Re: HMV 904 prewar television restoration

I am enjoying reading your write up
It seems that you owe a debt of gratitude to the incompetent repairer, when the set had been given up on it probably had very few hours use as a TV.

as we know, the television service was switched off at the start of the war.
The set would have not been used as a TV during this time.
If the 'repair' had been carried out in the late 40's, the CRT would have seen little use, hence why it has such good emission.

Mark
mark pirate is offline  
Old 14th Apr 2015, 9:17 pm   #34
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 mark pirate View Post
I am enjoying reading your write up.
It seems that you owe a debt of gratitude to the incompetent repairer, when the set had been given up on it probably had very few hours use as a TV.
Ha, that's one way to look at it, every cloud has a silver lining. The TV had had a good service history up to that point, and if it had been fixed properly then, maybe I'd now how a failed CRT to contend with

If anyone's got the impression I have any enmity towards the unknown repairer, then I don't really. It's all just part of the process of the repair description. This was my first television repair, and all the faults made it the more challenging and interesting. What would the fun have been if I had to replace a couple of electrolytics and that was that?
Catkins is offline  
Old 14th Apr 2015, 10:17 pm   #35
HamishBoxer
Dekatron
 
HamishBoxer's Avatar
 
Join Date: Aug 2007
Location: W.Butterwick, near Doncaster UK.
Posts: 8,923
Default Re: HMV 904 prewar television restoration

If only life was that simple.
__________________
G8JET BVWS Archivist and Member V.M.A.R.S
HamishBoxer is offline  
Old 14th Apr 2015, 10:50 pm   #36
mark pirate
Dekatron
 
mark pirate's Avatar
 
Join Date: Mar 2008
Location: Worthing, West Sussex, UK
Posts: 5,185
Default Re: HMV 904 prewar television restoration

Quote:
all the faults made it the more challenging and interesting. What would the fun have been if I had to replace a couple of electrolytics?
Very true. if you enjoy a challenge, try an HMV 1807 next!
Unlike the previous offerings from EMI, this was a truly dreadful design throughout. but makes a great restoration project.

Certainly the most challenging set I have restored so far!

Mark
mark pirate is offline  
Old 15th Apr 2015, 12:04 am   #37
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 mark pirate View Post
Very true. if you enjoy a challenge, try an HMV 1807 next!
Unlike the previous offerings from EMI, this was a truly dreadful design throughout. but makes a great restoration project.
Yes, I remember reading the "HMV 1807/Marconi VT53DA 10" table 1949" writeup from Heatercathodeshort (https://www.vintage-radio.net/forum/...ad.php?t=83897) from 2012. In fact I had only recently got the HMV 904 then, and that article gave me a lot of encouragement.
Catkins is offline  
Old 15th Apr 2015, 6:13 am   #38
Catkins
Pentode
 
Catkins's Avatar
 
Join Date: Feb 2015
Location: Chepstow, Monmouthshire, UK.
Posts: 234
Default Re: HMV 904 prewar television restoration

Hi,

This post should finally get the tedious pre-switch on television work out of the way

The final thing to fix was the bleeder resistors, there are 5x1M resistors in the bleeder chain, and 2 proved to be open circuit, and the rest read significantly higher than rated. As all of them needed replacing, but I didn't want to spoil the look of the set, I decided to keep them in place, and add a set of new resistors hidden underneath them.

The immediate thing was what to replace them with? The 5x1M resistors drop a total of 2,500Volts@1.25W, each dropping 500V@0.25W. Looking for replacements, I discovered all readily available modern resistors are based on metal film technology, and most are rated at a max continuous voltage of 350V (some claim 500V, but even so I wasn't happy with the lack of margin). There are so called high voltage metal film resistors, but, not a lot of people seem to sell them.

I finally found a seller with 5.1M resistors, peak 10KV, 1W. But 1W wasn't sufficient and I didn't like the idea of a single resistor in the bleeder chain But, a brief calculation showed 4 of them in parallel&series would give the right resistance, with each resistor dropping 1250V@0.3W. But fitting them under the existing resistors proved not to give a lot of clearance because they were quite fat.

In the end I used 10 470K 2W resistors (350V) in series with a 220K resistor. Giving about a 235V drop@0.125W.

Photo 1 shows the underside and the view from the top of the replaced resistors.

Hmm, quite a lot of detail on the bleeder chain replacement But this was the first time I 've had to replace bleeder resistors and so this was one of the areas which concerned me most (for obvious reasons).
Attached Thumbnails
Click image for larger version

Name:	3.jpg
Views:	352
Size:	176.8 KB
ID:	107017  
Catkins is offline  
Old 15th Apr 2015, 8:54 am   #39
mark pirate
Dekatron
 
mark pirate's Avatar
 
Join Date: Mar 2008
Location: Worthing, West Sussex, UK
Posts: 5,185
Default Re: HMV 904 prewar television restoration

That is a neat solution, the other option would be to make hollow copies of the original resistors and hide the new resistors inside.

Mark
mark pirate is offline  
Old 15th Apr 2015, 9:27 am   #40
Panrock
Nonode
 
Panrock's Avatar
 
Join Date: Feb 2005
Location: Worcestershire, UK.
Posts: 2,525
Default Re: HMV 904 prewar television restoration

Quote:
Originally Posted by mark pirate View Post
...the other option would be to make hollow copies of the original resistors and hide the new resistors inside.
Not recommended. I did something like this on my pre-war Marconi 702 and later one of them failed. Even resistors with modest dissipation need to radiate heat...

Catkins, what an impressive project you've got there!

Steve
__________________
https://www.radiocraft.co.uk
Panrock is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 3:17 am.


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.