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Old 14th Oct 2020, 6:15 am   #41
Electronpusher0
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Default Re: Restoration of a 1937 first generation pre-war television (HMV 900)

Wow, that's pretty amazing, thanks for sharing the restoration.

Peter
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Old 14th Oct 2020, 7:22 am   #42
peter_scott
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Default Re: Restoration of a 1937 first generation pre-war television (HMV 900)

+1

Peter
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Old 14th Oct 2020, 7:56 am   #43
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Default Re: Restoration of a 1937 first generation pre-war television (HMV 900)

Fascinating stuff!

Good to see the EHT transformer is CE marked!
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Old 14th Oct 2020, 4:04 pm   #44
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Default Re: Restoration of a 1937 first generation pre-war television (HMV 900)

Hi Catkins,
That's great work on the PSU.
How were you able to get to the point of being able to polish the chassis? Wast the rust only a thin film without any pitting?

The open circuited EHT transformer was no great loss as I suspect that if it had been intact then it would have only lasted a few hours before it started to break down anyway.

Some data on GPO heat coils:-

Heat Coil Green 5.5 ohm, trip in 60 seconds at 0.5A
Heat Coil Brown 0.7 ohm, trip in 60 seconds at 1.5A
Heat Coil Violet 8.2 ohms, trip in 120 seconds at 0.38A
Heat Coil White 20 ohms, trip in 30 seconds at 0.3A
Heat Coil Yellow 0.55 ohms, trip in 60 seconds at 2.0A

Cheers
Andy
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Old 14th Oct 2020, 6:37 pm   #45
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Default Re: Restoration of a 1937 first generation pre-war television (HMV 900)

Thanks Andy, I've never found it easy to get information on heat coils. My power supply is supposed to use a brown heat coil but the green one I have been using seems to be quite tolerant.

Later EMI information reduced the spec.

Peter
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Old 15th Oct 2020, 4:49 am   #46
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Default Re: Restoration of a 1937 first generation pre-war television (HMV 900)

Quote:
Originally Posted by beery View Post
Hi Catkins,
That's great work on the PSU.
How were you able to get to the point of being able to polish the chassis? Was the rust only a thin film without any pitting?
Hi Andy,

I used Hammerite Rust Remover gel to get off the rust and most of the corrosion. That will typically bring it back to clean metal (with repeated applications if it is quite thick and fine wire-wool or a tooth brush) but it will be quite dark or stained. I then repeatedly polish with Auto-Sol metal polish. With enough effort it will produce a good result.

As to thin film or pitting is concerned, it was a mixed-bag. Some parts had only a thin film, the most exposed parts had some areas of pitting, and some parts were somewhere in the middle (perhaps 45%, 40%, 15%). After removal of corrosion the difference is some parts will be smooth but stained (surface corrosion), some parts will be rough (the corrosion has started to eat into the plating), and some parts will be pitted (the corrosion has bitten a bit deeper).

This may sound obvious, but, the condition and plating quality determines how much re-polishing you can do. Rough finish can usually be polished back to a good finish without going through the plating, and much pitting can be polished out to a certain extent. It all depends on how thick or good the plating originally was.

This is why I mentioned I found it to be heavy metalwork with a good level of plating. Due to this I found the corrosion (even in the more exposed places) hadn't yet destroyed the plating, and to my surprise, it was possible to get back to a good finish. In other circumstances corrosion has ended up going back to the bare metal with no plating remaining (that was the case for much of the metalwork in the Murphy A56V, which is why I ended up replating it).
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Old 22nd Oct 2020, 5:58 am   #47
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Default Re: Restoration of a 1937 first generation pre-war television (HMV 900)

To continue the restoration...

After restoring the PSU, the next chassis I worked on was the CRT enclosure. This was for two reasons, firstly it showed quite a degree of corrosion at floor level like the PSU, and secondly the CRT enclosure contains the EHT bleeder chain/capacitors/focus-circuit, and so it was a logical continuation of the work on the PSU.

I'll let the photographs do most of the talking.

Photo 1 shows the lower half of the CRT enclosure. You should be able to see that there is a thickish layer of white silt covering the bottom, and underneath that is a layer of rust. If you look at the large green capacitor, you should also be able to see that the top half is disconnected (the large connector ringed in black). The resistor which should lie between the two capacitor connections is dangling off the lower connector. This means the upper part of the capacitor has likely failed at some point, and it has been taken out of circuit.

The brown paxolin board at the back of the EHT enclosure holds the EHT bleeder resistors and focus circuit resistors (electrostatic focussing hence no focus magnet around the CRT neck, only deflection coils).

Photo 2 is a composite photo, which shows on the top the rotten state of the cotton wiring at floor level. The rubber EHT wiring was found to be in good condition and not perished. The lower photo in Photo 2 shows the removed paxolin board and the various connections to it.

Photo 3 is a composite photo showing both sides of the paxolin board, and the EHT bleeder chain resistors/focus-circuit resistors. Three of these were found to be open-circuit or very high.

The paxolin board was stripped down, the corrosion removed and generally cleaned. The failed resistors were replaced by high voltage (10kV) resistors. Due to the high voltage drop across each resistor, you should always replace using high voltage types.

Photo 4 shows the new cotton wiring bundles, and Photo 5 shows the replaced wiring and resistors.
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Old 22nd Oct 2020, 6:45 am   #48
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Default Re: Restoration of a 1937 first generation pre-war television (HMV 900)

Continuing...

The next stage was the capacitors, there are two capacitor blocks in the EHT enclosure, a large green capacitor block which contains two .1 uf 5000 volt capacitors, which are the EHT reservoir/smoothing capacitors, and a smaller grey 1 uf 1500 volt capacitor in the focus circuit.

As seen in Photos 1 and 2, they had some body corrosion, and it was already evident one of the capacitors in the green capacitor block had probably failed at some time.

I have a capacitor analyser which can test for leakage/reform up to a maximum of 600 volts. Connecting that up to each of the capacitors, showed they were all extremely leaky at 600 volts. Neither side of the green capacitor block performed any better than the other either. I reformed each of the capacitors over about a week (10 hours each day), and the leakage currents slowly reduced to an acceptable level for 600 volts. Obviously, it was going to be entirely pot-luck whether they'd work at the higher EHT voltages, but, I thought it was at least worth a try and not write them off too soon.

The dangling EHT resistor was found to be open-circuit and this was replaced too. The capacitors were cleaned up and the corrosion treated and removed.

That left the CRT itself, which was found to need no work, and the bottom of CRT enclosure. This was rust treated and repainted.

Photo 3 shows the empty CRT enclosure after treating/repainting the bottom.

Photo 4 shows everything put back.
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Old 22nd Oct 2020, 9:47 am   #49
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Default Re: Restoration of a 1937 first generation pre-war television (HMV 900)

Good progress. I hope your big caps continue their healing.

Peter
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Old 22nd Oct 2020, 10:42 am   #50
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Default Re: Restoration of a 1937 first generation pre-war television (HMV 900)

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Good progress. I hope your big caps continue their healing.

Peter
One out of three eventually worked But that is quite a way in the future as to where I've got to!
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Old 22nd Oct 2020, 10:24 pm   #51
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Default Re: Restoration of a 1937 first generation pre-war television (HMV 900)

Quote:
Originally Posted by Catkins View Post
Continuing...

The next stage was the capacitors
Hi, interesting about the high voltage caps, if they were to fail what would you use as replacements? Are they still made? I tried to get some 0.1 3kV recently but didn't find any.

Graham.
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Old 23rd Oct 2020, 7:13 pm   #52
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Default Re: Restoration of a 1937 first generation pre-war television (HMV 900)

Hi Catkins, I'm a little puzzled as to how these caps reformed as they appear to be non polarised types. Also being sealed ther should have been no way for moisture to escape.

I have quite a few paper in oil types (which need testing but are normally good) if you are in need of any

Ed
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Old 23rd Oct 2020, 8:47 pm   #53
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Default Re: Restoration of a 1937 first generation pre-war television (HMV 900)

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Hi, interesting about the high voltage caps, if they were to fail what would you use as replacements? Are they still made? I tried to get some 0.1 3kV recently but didn't find any.

Graham.
They are still made, but, often you can't get exact specifications, and so you have go for something close (e.g. higher voltage) or use multiple to get the capacitance.

I often find the nearest supplier is in the far east too.
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Old 23rd Oct 2020, 8:52 pm   #54
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Default Re: Restoration of a 1937 first generation pre-war television (HMV 900)

Quote:
Originally Posted by Ed_Dinning View Post
Hi Catkins, I'm a little puzzled as to how these caps reformed as they appear to be non polarised types. Also being sealed ther should have been no way for moisture to escape.
Ed
Hi Ed,
You've beaten me too it.
I was wondering the same thing.
Maybe any trapped moisture was moving about or being split into oxygen and hydrogen to reduce the leakage current. Or if some how the oil has decomposed, passing a current through it alters it chemically in such a way as to restore its insulating characteristics again. Either way, I'm not surprised it wasn't particularly successful.

Cheers
Andy
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Old 23rd Oct 2020, 8:54 pm   #55
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Default Re: Restoration of a 1937 first generation pre-war television (HMV 900)

Quote:
Originally Posted by Ed_Dinning View Post
Hi Catkins, I'm a little puzzled as to how these caps reformed as they appear to be non polarised types. Also being sealed there should have been no way for moisture to escape.
They are polarised , and the leakage current went down which as far as I was concerned indicated they were reforming. As to why they failed being sealed I have no idea.

Dealing with that was a couple of months ago, and so I've got replacements. Thanks
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Old 23rd Oct 2020, 11:13 pm   #56
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Default Re: Restoration of a 1937 first generation pre-war television (HMV 900)

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They are polarised , and the leakage current went down which as far as I was concerned indicated they were reforming. As to why they failed being sealed I have no idea.

Well I thought the grey one was polarised at the time I tried to reform it (as one of the terminals was painted red) But later cutting it open it was found to be paper-in-oil. So there wasn't any point in reforming it.

I try to write these posts up from the point of view of what I thought at the time, even if I now know different.
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Old 24th Oct 2020, 12:31 am   #57
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Default Re: Restoration of a 1937 first generation pre-war television (HMV 900)

Even when sealed, some moisture ingress is unavoidable. Applying a voltage to a leaky capacitor will heat it up a bit, sothe moisture might be absorbed in the oil or even vent out a little bit.

A good sanity check is that electrolytics above roughly 500-600V are mostly not economic to manufacture, so a paper cap was to be expected.
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Old 24th Oct 2020, 4:43 am   #58
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Default Re: Restoration of a 1937 first generation pre-war television (HMV 900)

To continue the restoration...

I'll try and get the pre-switch on work out of the way as soon as possible. What remains is the sync/trf chassis, the sound/radio chassis and the loudspeaker. After this post most of the work was just restuffing electrolytics/waxy-capacitors and so it should be done fairly quickly.

Photos 1 & 2 show the top and bottom of the trf/sync chassis. As should be seen, it is in good condition, the rust areas on the top chassis is only surface rust. The exception is the bottom most panel (on the left of photo 2), which is pretty filthy, and has areas of deep penetration of rust. Again this follows the pattern anything towards floor level is more heavily corroded.

I removed the panel (see Photo 3), and treated it, and cleaned the electronics.

Photos 4 & 5 are the result. The electrolytic has been restuffed.
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Old 24th Oct 2020, 7:01 am   #59
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Default Re: Restoration of a 1937 first generation pre-war television (HMV 900)

To finish the trf/sync chassis...

All that remains is to restuff the electrolytics and the waxy capacitors. In photo 2 of the previous post you should be able to see that the electrolytics were completely replaced in the 1946 refurbishment after the resumption of television post-war. The original electrolytics were metal tubular types mounted on the underside of the chassis, these have been replaced by paper electrolytics (made by TCC). As I feel the 1946 refurbishment is an important part of the television service history, I chose to restuff those, rather than try to recreate an original 1930s look.

There was an additional pale green electrolytic capacitor block mounted on top of the chassis. In the refurbishment of 1946, many of these televisions have had this replaced by a paxolin board with capacitors mounted on that. In this set the capacitor block has been replaced by an original replacement type dated DEC/46. See photo 1 of the previous post for a picture of the capacitor block on the chassis, and photo 1 of this post for a close up, and the original contents.

I restuffed this capacitor block, with a paxolin board with capacitors mounted on it. Photo 2 shows the board, and it in place in the emptied out block.

The waxy capacitors are a combination of originals and presumably 1946 replacements (though of course some could be later depending on how long the television was in use post-war, but all are of the same look/make). Again I chose to restuff these.

Photo 3 is a combination photo with the top photo showing a mixture of paper electrolytics and waxy capacitors as removed from the set. The lower photo shows a mixture of capacitors restuffed and rewaxed.

Photo 4 shows two views of capacitors in the process of unstuffing, and restuffing. The process of restuffing which I do is to first remove any external wax with a heat gun, this also heats the wax impregnated paper, allowing one side to be uncrimped without tearing. After reheating to soften any wax internally, the contents can then be pulled out (top photo). A modern capacitor (which will be smaller) can then be soldered onto the capacitor ends (lower photo), inserted into the capacitor body, and after heating with a heat gun, the one side can be recrimped. The capacitor is then dipped in hot wax to recreate the original look.

Photo 5 shows the underside of the chassis with the restuffed capacitors. If you compare it with photo 2 in the previous post, you should see that the original unrestored look has been preserved.
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Old 24th Oct 2020, 8:01 am   #60
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Wow, that's pretty amazing, thanks for sharing the restoration.

Peter
Thank-you for your kind comment I hope you find the rest of the restoration interesting!
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