PYE R33 Over Voltage.

[ms660]

It sounds similar to a floating grid fault.

Lawrence.

[Kentode]

Umm, tried looking for floating grid but it was a load of ballcocks!

I'll sleep on it for now but tomorrow it may be sleeping with the fishes.

[ms660]

I tried.

Lawrence.

[Kentode]

I'm sorry Lawrence, that wasn't a joke at your expense! I googled floating grid and got lots of advertising for grids that float on water. My sincere apologies.

Wikipedia had some articles on triodes and tetrodes but I struggled with them, the frustration is starting to get to me.

What started out eight years ago as an exciting new hobby has turned into something I'm finding hard to enjoy, hence the six month gap.

Fresh eyes tomorrow and another readup on floating grid's...

[ms660]

By floating grid I was meaning no DC path from the control grid to the cathode.

Lawrence.

[Kentode]

Thank you, I'll do some research.

[Al (astral highway)]

Originally Posted by Kentode

Just to clarify, C7 is now a 100pf 630v with no DC on the chassis side and 75 volts DC on the hot side, measured by analogue meter.

Hi Ken,

I feel for you and your frustration. Just one Q, why did you replace C7 in the first place? I may have missed something here but these low values are beyond suspicion.

[Gabe001]

Ken, there was a recent thread where someone had a fault in a valve and purchased an eBay replacement which was also faulty.

As long as you're happy with the voltage at the valve pins (and current draw if this can be calculated), I would return the valve you've bought as it's probably faulty too, and get another one.

The photosensitivity is probably a red herring; as one of the moderators has mentioned it's a known phenomenon and from my very basic physics knowledge it's not hard to imagine that photons of light energy may excite electrons in the valve in some way or another

Is your heater voltage ok?

[AD360 Rob]

Originally Posted by Kentode

Yes Graham, shining an led torch onto v1 makes the signal Pop up and the pulsing disappears. It's so unbelievable that I've made a video, but putting it on utube and hyperlinking is beyond me as l only have a smart phone.

PS the voltages are all acceptable.

You say an LED torch shone on the valve causes the fault to clear. Have you tried a standard incandescent bulb torch? I'm thinking the LED drive circuitry within the torch may be affecting the circuit. You might find that if the valve IS sensitive to outside electrical interference that a metal screen might help.

[Al (astral highway)]

Yes I was going to add that we could see this whole interaction between Ken and the valve as a quantum experiment ...

[murphyv310]

Hi
I'm doubting photo sensitivity. It's more like hand capacitance that's the issue, pointing to instability. Make sure the decouplers in the frequency changer and IF stages have been replaced with the correct values and they are placed exactly the same way as the manufacturer did, leads need to be short.
Low value PF wax caps are actually very reliable and never do a blanket swap out of them, it can cause all sorts of troubles.

[Kentode]

Ahah, Trevor may have the answer! I'll remove C7, test and replace it without the long leads.

I'll reply to you all after breakfast.

[murphyv310]

Hi.

I'd not say the leads are too long. Have you replaced any decoupling capacitors in the frequency changer and IF?

[Al (astral highway)]

Hi Ken , as I also said in post #47 — but why did you replace it in the first place? That might give us a insight into your thinking. And you have a silver mica or similar type, or the original? I’m not sure what type you have there but it isn’t silver mica.

Also I can see you replaced a resistor - what was the wrong there?

[Gabe001]

I think you need to replace the silver mica with a silver mica Ken, as the others have said.

[paulsherwin]

Nobody seems to have mentioned that these sets use a PCB. PCB based valve radios of this era often develop 'interesting' faults as the material degrades and becomes conductive. It can be difficult or even impossible to fix them.

[murphyv310]

Very good Point Paul.
These PCBs are badly effected by damp conditions and can become conductive leading to all sorts of issues.
I do believe thinking about it that it could well be the issue.

[Kentode]

Okay, minor improvements.

I replaced C7 flat on the PCB and put a NOS UCH81 in. Started out working but after10 minutes the audio began to have a slight warble as if the old fault is making its way back.

C7 was replaced due to an error on my part, using a DMM instead of an analogue meter.

I wonder if I've cooked C30 when I was removing C7?

The DC path from control grid to cathode - is that via C7, C11 and L3?

Cricklewood Electronics don't do mica caps and it looks like I'd have to realign the set, which I have no confidence in, so I might put this to one side for now.

Thanks to everyone for the support...

[Station X]

Originally Posted by Kentode

The DC path from control grid to cathode - is that via C7, C11 and L3?

You cannot have a DC path through capacitors unless they're leaking very badly electrically, I don't mean leaking electrolyte.

V1 oscillator gird is grounded via R3.

V1 mixer grid is grounded via R1, R5 and R4.

Check the resistors.

Check that the valve pins are clean and making good electrical contact with the valveholder sockets.

[Kentode]

Of course! That's an indication of how confused l am.