WAD Phono II problem

[KennyK]

Hi All,

I'm trying to troubleshoot a problem with my WAD Phono II, which has been long upgraded with various boutique resistors/capacitors, and many PCB tracks have been hard wired for around 15 years

What started as a slight channel imbalance has got worse with the Left channel now virtually inaudible, so I've finally had a poke around with a meter.

I've got a couple of Low voltages on V3 Anode (pin 1) and V4 Cathode (pin 3) which are both stablising at 37v instead of 120v. On power up V3 pin 1 starts at 120, but as the heaters warm up the voltage pulls low. V3 Cathode (pin 3) seems high at 1.6v

The HT seems fine. without the Pre II connected, it's sitting at 270v instead of 250v
Any suggestions where to look?

Thanks
KennyK

[Richard]

Hi Kenny,


Swap in new valves if you have any and check voltages again. If no spares swap V2a/V3a or V2b/V3b with V4a/b and note voltages again to see if the bad voltages move with the valves (valve problem) or stay with the socket position (amp problem).

[KennyK]

Thanks Richard. I will do, and report back

[KennyK]

A quick test of swapping V3 and V4 and the fault seems to be staying with the socket,
I'm not sure if it's V3 anode or grid resistors that's the problem. I'm getting a low voltage at V3 Anode (38v) and high (1.6v) at the grid

[bob orbell]

Kenny, remove all valves, power up and see if the voltage falls ( of course you will see a much larger voltage with the valves removed ) ( be very careful ) if the voltage falls you will know the problem is with board or a component, if the voltage remains constant, fit one valve at at a time ( starting at the front end ) and observe voltages, a falling voltage should point you toward the problem. Bob

[Richard]

Originally Posted by KennyK

A quick test of swapping V3 and V4 and the fault seems to be staying with the socket,
I'm not sure if it's V3 anode or grid resistors that's the problem. I'm getting a low voltage at V3 Anode (38v) and high (1.6v) at the grid

With the valves out you should be able to get correct readings for those and most of the other resistors. If you're not sure if a reading is accurate check what should be the same reading on the other channel. If a cathode resistor reads short suspect its bypass cap.

[KennyK]

with no valves I'm seeing an HT of >330v

The fault seems to be on the LHC

Anode voltages of Both V2 and V3 pin 1 are 250v pin 6 300v

Fitting V2, V2 Anode pin 1 stablises at 140v, pin 6 at 190v
V3 anode pin 6 is 280v pin 1 250v

adding in V3:
Pin 6 anodes are both 180v for V2 and V3

Pin 1 anode for V2 sitting at 120v
pin 1 anode V3 is at 46v

That suggests an issue with R30 (330k)?
Or am I missing something?

[Richard]

Hi Kenny, sorry, I should have said,

"With the valves out you should be able to get correct Ohms readings for those and most of the other resistors..."

Have a check through the resistors with an Ohms meter, with valves out and power off, and the PSU umbilical disconnected.

[KennyK]

Originally Posted by Richard

Hi Kenny, sorry, I should have said,

"With the valves out you should be able to get correct Ohms readings for those and most of the other resistors..."

Seems like all the resistors check out.

Because I didn't make any notes at the time, my C20/21 (PCB numbering)= C16/17 have an extra pair of parallel resistors in series with the 220nF capacitor. It must have been a time constant modification (3.18uS ?)

So likely this capacitor gone bad?

[Richard]

There were 2 or 3 PCB versions and also the 3.18uS mod as you say.


Does your PCB have the version printed on it?


Whatever, yes, if one of the 220pF caps in C20/21 positions (early Phono2 circuit numbers = C16/17 in later Phono2 and early Phono3 PCBs) is failing short it will short the signal to ground and may account for low volume.


They are a bit fragile to heat so change them both carefully and if you can supply further info of your PCB number or whether or not you wish to use the 3.18 mod version let us know. Be careful mto use high enough voltage rating as they could see that 300V HT when the valves are cold at switch on.