Grid coupler change?

[Danndad]

I have a Ultra FM950 that needs a lot of its hunts caps replacing due to them being cracked and falling to pieces.
The grid coupler is a 30nf rectangular Erie mica type(I think), would members replace this type of cap or leave it in? It is fairly close to the UL84 output valve.
   


Danndad.

[ppppenguin]

You said 30nF. Can't read the markings but nF wasn't used back then. Is it marked 0.03uF?

It should be OK but I'd be tempted to replace regardless. You can meaure the grid voltage on the UL84. If it's not negative with respect to chassis then the cap is definitely faulty. Or it can be checked with a megger.

The green snubber cap on the on/off switch is interesting. Marked BS415 and 3kV. Obviously a safety rated part in the days before X and Y rated caps.

[Danndad]

Thank you for the reply Jeffrey.
I don't have anything like a megger here to test components.
I have had to replace the ul84 with another as I had around +650mv on the grid, a replacement ul84 has brought that down to around +70mv , I still have a small pos voltage on the grid so I will replace the cap.
A polyester cap will be the replacement but it will be surrounded by heat from the rectifier and output valves plus the heat from the dropper resistor, not sure what the lifespan of it will be.
The green cap across the mains is staying .


Danndad.

[PerdioPal]

For me it's a no brainer, even though micas are very reliable I would replace. Modern components are so much better.

[ppppenguin]

The +650mV on the grid of the UL84 screams "change the capacitor". Which is better done before replacing the valve.

[Freya]

I repaired one of these last year for someone, the PCB around the UL84 had become conductive with similarities of what your getting.

[Murphyv310]

Hi.
I'd agree with Stephen. These boards are often worse than the caps for leakage, sometimes bad enough to render the set to the bin. Those Erie 0.03uf caps are generally good.

A question for the OP. What are you using for reading the 70mv on the grid, DVM or AVO analogue meter etc, don't forget the ohms per volt on an AVO as the actual voltage could be higher.

[Murphyv310]

The +650mV on the grid of the UL84 screams "change the capacitor". Which is better done before replacing the valve.

That voltage was prior to replacing the UL84 Jeffrey it's now an acceptable 70mv

[Mike Watterson]

I agree, more likely leakage on PCB. Clean it with toothbrsh and maybe IPA or WD40.
Also check cathode capacitor isn't leaky. Dried out simply reduces gain, won't affect DC.

[Danndad]

Sorry for the delay in replying.
The grid coupler has been changed as have quite a few resistors and caps and the board cleaned as best I can
I thought this had cured the pos voltage on the grid but after 10 mins the voltage was up to + 225mv measured with the DMM, no idea why I had the lower reading of +70mv last time I tried.
Looking like the board is bad.

Danndad