WAD Phono II problem

[KennyK]

Thanks Richard

My board is WAD PHONO II Issue 1


I've had to bypass a number of tracks as and when I was doing mods previously (that PCB seem fragile), including some PIO caps in the PCB C22/23 position (=C18/19) as well as a number of resistor mods.

The Phono has worked faultlessly for a number of years since the last modifications which included an extra switch on the PSU to turn off the Phono. From memory I think it only switches off the heaters, so I'm guessing that everything would be getting stressed with a 'cold' HT ?

One channel has gradually dropped in volume over time. interestingly when turning off, the channel volume increases briefly before it finally dies down again.

I'm fairly sure I have the 3.18uS mod and I'd like to keep that

[Richard]

With the heaters off the valves wouldn't be drawing current so all the voltages would be full HTV around 250VDC and that would also be across C16/17.

You'll decide how much of it you want to rebuild, Matthew would be the best supplier for the PCB and correct parts and could advise what is now available. The last equivalent PCB was PhonoII Issue II which was the same circuit as your Issue I.

Re the 3.18 mod you can include that again. It wasn't standard on that PCB (it was a mod as per the FAQ) and became standard on Phono3S.

[KennyK]

Thanks for the advice. I think if I can get the existing board going with swapping out the 220nF caps I'll do that first, with a view to completing a new board over the next few months.

I'm not sure if I have anything suitable, I'll need to have a look to see what I can find online

[KennyK]

I've swapped out the capacitor, and also taken the 3.18uS mod out of circuit and it's still the same. It seems to be something to do with V3. Still dropping 200v across R30 (330k), and only around 5 volts across R32 (180k)
so there's still 30-40v across C17 (220nf) I get high readings without V3 in circuit.

is the 330k looking the most likely culprit?

[bob orbell]

Can you post a picture of your Phono Kenny, it may help us. Bob

[KennyK]

I'll get some pictures later this evening.

All the resistors around V3a are reading ok, and TP2 at the anode of V3a only pulls low when the Valve is in situ.

So I'm trying to think logically. I don't have TP voltages for the grid of V3a, but if the resistors are checking out, then it's looking like there must be some dc leakage reaching the grid? C11 (0.1uF) presumably should be blocking any DC , so if there's voltages wrt ground at the R24/R221 network of 1M/316K would that point to a leakage which is allowing current to flow through V3a and cause the voltage drop? Or am I completely wide of the mark?

[Richard]

Originally Posted by KennyK

I'll get some pictures later this evening.

All the resistors around V3a are reading ok, and TP2 at the anode of V3a only pulls low when the Valve is in situ.

So I'm trying to think logically. I don't have TP voltages for the grid of V3a, but if the resistors are checking out, then it's looking like there must be some dc leakage reaching the grid? C11 (0.1uF) presumably should be blocking any DC , so if there's voltages wrt ground at the R24/R221 network of 1M/316K would that point to a leakage which is allowing current to flow through V3a and cause the voltage drop? Or am I completely wide of the mark?

Hi Kenny, the grid of V3a should be 0V wrt signal ground. Any dc should be fully blocked by C10/11. Are there paper caps at C10/11? They may leak electrically. If so change for plastic film.

[KennyK]

Thanks Richard

I'll have another poke this evening. I'm a bit rusty, not having looked at things for a while. I have a funny feeling it is a paper cap that I fitted during the mods.

So, even with V3 removed, if C11 is leaking, I would stlll see a voltage wrt signal ground at the junction of R22/24. If it's good it should be 0v?

[Richard]

Originally Posted by KennyK

So, even with V3 removed, if C11 is leaking, I would stlll see a voltage wrt signal ground at the junction of R22/24. If it's good it should be 0v?

Correct, or put another way, if you remove V2 and V3 then meter grid pin 2 wrt signal ground it should be 0V with power on. If there is any voltage on pin 2 it will have leaked via C10/11.

[bob orbell]

A note about series II issue one PCB, C10/11/18 and 19 are not used, I think Richard means C12/13, please tell me to **** off if I am wrong but, I don't think I am. Bob