Pre-war Television Fault

[Panrock]

The blocking oscillator grid resistor David mentioned is OK and the DC resistances of the little transformer's windings look correct.

But what's happened to C71? See what it does in the earlier circuit diagram. It seems to have been running hot and some goo has emerged around the centre pin. Sure enough, it is now leaky. This is a fake 'vintage' style capacitor originally obtained at a BVWS meet.

I have replaced it with a 1500v-rated part. I'm tired now, so whether it's done the trick I won't be finding out today. But knowing my luck...

Steve

[stevehertz]

Originally Posted by Panrock

The blocking oscillator grid resistor David mentioned is OK and the DC resistances of the little transformer's windings look correct.

But what's happened to C71? See what it does in the earlier circuit diagram. It seems to have been running hot and some goo has emerged around the centre pin. Sure enough, it is now leaky. This is a fake 'vintage' style capacitor originally obtained at a BVWS meet.

I have replaced it with a 1500v-rated part. I'm tired now, so whether it's done the trick I won't be finding out today. But knowing my luck...

Steve

Looks like you're on to something there Steve. At any rate it needed replacing.

[McMurdo]

What's inside that capacitor? Have you cut it open?

[Panrock]

I have now! Take a look inside. This capacitor is leaky, but no - I've switched on again and it's not causing the fault!

Due to the weight of this chassis, it can only really be propped as shown in order to make measurements underneath. Unfortunately, when in this position, the leads going to the tube are not long enough. Some form of safe propping of the tube will have to be devised too. Not a simple matter.

Anyway, I'll start by looking at the waveforms on the scan coils, which are presently accessible.

Steve

[radiomobile]

Looks like a dreaded Rifa capacitor.

[Panrock]

Just run a check for obscure CRT electrode leaks - as you do at this point - nothing found.

Steve

[McMurdo]

For crying out loud, what a way to restuff a cap! Own up!

[Lloyd 1985]

Are there any more of those Rifa’s hiding in there?

[Panrock]

Dunno. It's a bit like deploring the 'wrong sort of concrete' in schools, with the benefit of hindsight.

I don't think any panic is called for, but I will check, particularly looking for any such caps with voltages or spikes across them.

The next job is to check the scan waveforms and presence of video at the tube.

Steve

[FERNSEH]

It's been discussed before. When restoring rare and historic TV sets such as the HMV 904 we don't want to be presented with a chassis full of yellow capacitors.
When ever possible I'll gather up old EMI capacitors and restuff them.

The capacitor C71 in the blocking oscillator grid circuit won't be under too much electrical stress, it's purpose is to hold the negative charge during the scan period. C73 is the sawtooth forming capacitor.
Attachment shows some restuffed EMI capacitors.

DFWB.

[Panrock]

Originally Posted by FERNSEH

.
When ever possible I'll gather up old EMI capacitors and restuff them.

What I aimed to do in this case too.

Originally Posted by FERNSEH

The capacitor C71 in the blocking oscillator grid circuit won't be under too much electrical stress, it's purpose is to hold the negative charge during the scan period. C73 is the sawtooth forming capacitor.

It'll be interesting tomorrow to see what the frame sawtooth voltage on the output actually looks like! I understand that in this case it should closely match the current. Hugo Holden's article may be of interest here.

Steve

[stevehertz]

Originally Posted by FERNSEH

It's been discussed before. When restoring rare and historic TV sets such as the HMV 904 we don't want to be presented with a chassis full of yellow capacitors.


DFWB.

The subject of restuffing capacitors has been discussed before ad infinitum and there's no consensus either way, people have their own opinions. I genuinely 'get' and appreciate that certain people like total originality in sets particularly in such a prized set as a pre war TV. And indeed I myself have restuffed capacitors in the past. For whatever reason (less pernickety - life events (cancer) have changed my outlook?), I would be quite happy with a pre war TV that was well restored using modern, high quality components that have been fitted properly and carefully. It's personal choice, not a given requirement whereby the actions of those who do not restuff are somehow looked down upon.

[Ted Kendall]

For my money, if the external appearance of the unit is preserved and the function of the circuit restored, imitating the appearance of the original components is a fudge too far. Of course, there are odd subtleties to bear in mind, such as the fortuitous effect of the metal-bodied grid capacitors in the Quad II, but outside of a museum piece, the effort is better spent elsewhere.

[Panrock]

A brief check with the scope this afternoon.

The effect seen on the screen is caused by frame timebase running far too slow (though the 'hold' control still works) and there are hairy-looking back emf spikes present - it's a good thing that motor-run capacitor is now in there - it should be able to cope with the abnormal demands. There's also very little video at the tube, riding on a lot of hum.

I point-blank refuse to work on this upturned chassis with the EHT present, so the first thing will be to disable the EHT mains switch - S3! Then I'll remove the harness from the tube, plug that in, and put the tube aside. The faults should then reveal themselves, working with meter and scope. It's good to have some solid faults, present all the time for a change.

Steve

[FERNSEH]

Hi Steve,
the circuit diagram of the frame timebase shows that the oscillator valve is a triode connected tetrode. If my memory serves me well V11 is a Marconi KTZ63 pentode, similar to the American 6J7G.
The Mullard Maintenance Manual informs us to connect the suppressor grid to the cathode when substituting a KTZ63 with a 6J7G. Can't remember where V11 suppressor grid is connected to in the HMV904/5.
A triode connected 6J7 has similar characteristics as the 6C5, a medium Mu triode. G2 and G3 connected to anode.
Is the 8mfd electrolytic capacitor C79 OK?

DFWB.

[radiomobile]

The presence of spikes as well as weak hum ridden video does suggest the possibility of an o/c HT decoupling capacitor.

Re the lethal EHT, for somebody working on early TV's it would be worth considering building a safe high impedance supply delivering around 5kV at a few microamps, perhaps based on one of the cheap (£3-£4) high voltage inverters on eBay. It could be powered by a couple of rechargeable batteries possibly.

[Rich Woods]

Originally Posted by stevehertz

The subject of restuffing capacitors has been discussed before ad infinitum and there's no consensus either way, people have their own opinions. I genuinely 'get' and appreciate that certain people like total originality in sets particularly in such a priced set as a pre war TV. And indeed I myself have restuffed capacitors in the past. For whatever reason (less pernickety - life events (cancer) have changed my outlook?), I would be quite happy with a pre war TV that was well restored using modern, high quality components that have been fitted properly and carefully. It's personal choice, not a given requirement whereby the actions of those who do not restuff are somehow looked down upon.

. Completely agree with this, there is no consensus about restuffing vs just replacing and while I admire the work ethic of those that wish to do it, personally I wouldn’t bother as my only goal would be to get the set working properly. I do feel that folks should resist the temptation to portray what they do as consensus when there really isn’t one.

[Panrock]

The 'video riding on hum' at the tube cathode only looked like that because it was ultra low level and the natural underlying hum of comparable size. What appears to have happened is that C47 in the vision IF has gone short, in the process overheating R25, which has gone high. See the attached diagram.

I have yet to see why the frame speed has gone low, but what is clear is that all the Rifa-stuffed vintage style capacitors I bought in all innocence at a swapmeet will have to go! There are six to replace.

I propose to create new 'originals' stuffed with Vishay 630v-rated axials, end capped and wrapped with several turns of brown paper - to give an appearance similar to FERNSEH's examples. These would then be black labelled in the original style and gloss 'wax' overcoated. David, if you have the correct part numbers used on your labels (for 0.05 and 0.1 uF) please let me have them. It would be nice to get this detail right.

Yes, I'm going to trouble that some may deem unnecessary, but I'm in no hurry - and if these capacitors have got to be replaced, better to do the best-looking restoration job I can. BTW, although I welcome advice on alternative ways to dress these particular capacitors, please don't stray into a general discussion on whether or not recapping is worth it. That's for another thread. Thanks.

Steve

[FERNSEH]

Hi Steve,
no problem. I'll take details and pictures of those EMI capacitors.

DFWB.

[high_vacuum_house]

Just a quick question, Now that the Rifa capacitor has been removed from its paper and wax coat, does it still show that it is leaky.
I was just wandering if the the paper and wax coat applied had somehow absorbed some moisture and was in a fashion "bridging" a leak across the capacitor.

Christopher Capener