Peto Scott TV169 Restoration.

[Jac Janssen]

Concerning the resistors i.s.o. the EY51 filaments:

The TV I'm working on at the moment has only one EY51 (it is an 8 kV can), and the two positions where no EY51 is present each have an 82 Ohm resistor in place of the (not present) filament.

Hope this helps a bit.

I tried a few resistor values (including omitting them) and it did not make a lot of difference.

Jac

[Murphyv310]

Hi.
C13 is technically in series with C14 but the voltages are not pure DC or even balanced. As you stated C13 is lower rated than C14 & 15. I suspect the caps are rated at 1kv each and that would give 8kv for C14 & 15 and 4kv for C13.
I know in other triples the caps are only around 20% higher rated than the drive pulse volts, hence going for 10kv caps.

[Murphyv310]

Here is the circuit of the EHT unit.

[Jac Janssen]

Hi Trevor,

The capacitors between connection "1" of the assy and the +25kV connection are in series. If you draw little or no output current, the capacitors are charged and these two together have 25 kV (minus 350 V) over them.
I cannot see how 10 kV capacitors would be sufficient. Whatever the voltage distribution over them, I don't think it is good.
The 10 kV rating for the higher value capacitor (the one connected to the +350 V) is fine.
I think the voltage across the capacitor connected to the 25 kV conn is twice as high as the +350V capacitor. If indeed this is the case, the capacitor to the 25 kV conn gets 2/3 of the voltage = 16.4 kV.

On the other hand, capacitors can usually withstand a much higher voltage than their rated voltage.
I bought some (very) cheap Chinese 2 kV ceramic capacitors for decoupling use, and I tested them as high as my supply would go, i.e. 3.5 kV, and there was no problem at all. During longer use and with pulses this might be different.

So you might get away with it.

Just my 2 cents (or 2 p).

Jac

[Murphyv310]

Thanks Jac.
I do understand your reasoning and yes it makes total sense. Certainly in triplers in other sets I always pondered why the capacitors were seemingly under rated.
The problem is the next step up is 15kv rated, so I either go for lower rated caps with higher value and connect them in series, or go for the 15kv caps which are bulky and may be awkward to install.

I think today I'll fill the can with oil and not solder it and just run it at rated HT on a soak test and see if it fails. To be fair Im not certain why it just stopped, I was still getting drive from the EL38 but EHT just faded as if the set was switched off, it pointed to a valve failure.
We shall see.

[ppppenguin]

On the other hand, capacitors can usually withstand a much higher voltage than their rated voltage.
I bought some (very) cheap Chinese 2 kV ceramic capacitors for decoupling use, and I tested them as high as my supply would go, i.e. 3.5 kV, and there was no problem at all. During longer use and with pulses this might be different.

I have some 1n and 10n ceramics with no voltage rating marked on them. The 10n parts are about 14mm diameter discs, marked Erie .01 Z AX. The 1n parts are about 7.5mm diamter, marked 1K MAO

I felt that these would be reasonably regarded as 500V parts. They withstood 5kV (the limit of my PSU) for a few minutes. As Jac says, will they cope with long term HV and/or pulses?

[Murphyv310]

Thanks Jeffrey.
Let's be honest, who will watch a projection set of that vintage every day for hours on end?
Obviously I want a good degree of reliability. So I'll run it today as I've said, any hint of an issue I'll fit the new caps when they arrive and soak test again in oil without soldering the can.
Ive also got a liter of transformer oil coming, it's shell Diala B which I'll use after I'm happy with the transformer and its resoldered together.

[ppppenguin]

Trevor, like Jac, I think you'll be OK with the caps you're using. They are probably a lot better than the originals.

But I had a doorknob EHT capacitor fail recently. It was probably over 50 years old but had been run well within ratings and showed no signs of distress.

[peter scott]

I had a doorknob EHT capacitor fail recently.

If you had left the door a jar the police might have charged you.

Peter (Oops! wrong thread.)

[Murphyv310]

Hi.
Well changing the capacitors actually turned into a nightmare. Before I did I repaired my EHT meter. One of 300meg resistors was way high as I had a couple in My junk drawer this cured the issue, funny as tried replacements years ago. I ran the EHT can for a good three hours at 330v and 25kv, I nearly decided to leave the original caps in place and I wish I did.
The new caps came today so I bit the bullet. After replacing the caps there was nothing, traced to no HT on the EL38 anode which turned out to be a broken very fine wire from the transformer. Oh dear what a job, it doesn't help with my eyesight but even another pair of eyes told me no way! After about another hour I managed to do a repair. Fortunately I had cleaned the oil off the surrounding area and then seal the joint with a tiny dab of epoxy resin. Glad to say it's working out of can. I'll try it again tomorrow in oil at full HT.
I'll also add some packing for the new caps to stiffen how they are sitting.