WAD Phono II problem

[Richard]

Hi Bob,
No, I do mean C10/11 0.1uF 250V coupling caps.

[KennyK]

the 0.1uF cap seems to have been the culprit. All voltages seem to be close to the correct readings after swapping it. Thanks everyone for the help getting my knowledge back to where it was. This is what happens when you don't do things regularly, things seep out of the brain!

I'm now chasing an audio signal issue, but I'm confident I'll find that shortly.

I probably need to scrap the PCB and start again with a fresh one, have emailed an enquiry to WD

[KennyK]

Originally Posted by 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

Yes, I was going by the diagram numbering in an attempt avoid confusion during troubleshooting , thanks for that Bob. it is really confusing.

[Richard]

Tht's good news Kenny and yes there were a few changes in the early days of Phono II. Thanks too for Bob's comments, I was just relying on Kenny knowing (or asking!) what I was on about, whether or not the actual numbers I was using were the ones he had

[KennyK]

Well... I'm a bit of an idiot.

The first capacitor I replaced (C16 or C20 on the PCB), I accidentally substituted a 220nF instead of a 220pF. I've put the original back in and now I have both channels working again! I need better glasses to read the circuit diagrams.

[Richard]

Ah yes I bet that killed the sound all right

An interesting thought ocurred from your mention of (C16 or C20 on the PCB),

does your Phono II have cathode bypass caps C10/11 and C18/19 fitted or marked on the pcb?

I didn't think any kits with them went out but some may. The drawing I attached a couple of posts ago of the Issue I pcb shows C18/19 but they are the numbers later used for the output caps.

I've attached the June 2000 and Aug 2000 circuits with these changes on in case anyone following thinks we're going barmy.

Those circuits also show Bob's line of thought which I understand but the build instrcutions show Issue I as being the later Aug 2000 circuit.

The reason for asking is if yours is very early and does have cathode bypass caps a failing one could account for your original voltages problem.

[Richard]

A way to identify a June 2000 pcb would be if it has C20/21/22/23 printed on it as these numbers were not used on later pcbs.

If Kenny does actually have a June 2000 pcb it would be quite easy to fit the wrong value components if populating or repairing it using a later diagram.

[KennyK]

Originally Posted by Richard

Ah yes I bet that killed the sound all right

An interesting thought ocurred from your mention of (C16 or C20 on the PCB),

does your Phono II have cathode bypass caps C10/11 and C18/19 fitted or marked on the pcb?

I didn't think any kits with them went out but some may. The drawing I attached a couple of posts ago of the Issue I pcb shows C18/19 but they are the numbers later used for the output caps.

I've attached the June 2000 and Aug 2000 circuits with these changes on in case anyone following thinks we're going barmy.

Those circuits also show Bob's line of thought which I understand but the build instrcutions show Issue I as being the later Aug 2000 circuit.

The reason for asking is if yours is very early and does have cathode bypass caps a failing one could account for your original voltages problem.

Yes, It most certainly did kill the sound! The error was strictly down to me not reading the (later) diagram correctly and just assuming that it was nF not pF! I simply wasn't paying attention, and ordered a bag of 10! fortunately the original one is good.

I seem to have an issue 1 PCB with spaces for C10/11 and C18/19 but there was an addendum sheet with instructions to ignore these positions. I'd never seen the circuit with the bypass caps in the circuit. I think I got mine around 2005.
Out of interest, what effect would including them have? (Or, why were they removed in a revision)

Good to know that a failing bypass cap could have that effect as well. I think, the clue was in the fact that the anode voltage was correct initially, but as the valve started to conduct, the leaking coupling capacitor allowed the grid of the other stage of V3 to see a DC voltage. when turning off the psu, the faulty channel would briefly get louder again as that voltage disappeared.

[Richard]

Hi Kenny, so does your pcb have spaces marked for C20/21/22/23 printed on it?


If so then it must have been made originally for the first circuit and I'd go over it carefully to be sure you actually have the correct parts in the correct circuit places.


I fitted the first stage caps to mine and they gave it a little more gain. In theory they also change the valve output impedance a fraction and would also influence the corner frequencies of the correction but it is very small.


So you'd notice more gain and probably it didn't need that gain and the lower distortion from the degenerative fb from not having cathode bypass caps was thought better though I didn't notice any extra distortion with mine from fitting them.


Yes hard to say but if a cathode bypass cap were fitted and was failing by passing dc it could cause problems. If you do have them fitted (old circuit) certainly swap them for new at this age.

[bob orbell]

Richard, my first Phono II issue 1 has C10/11 and C18/19 with, NOT USED stamped over the four capacitor positions, these were the cathode bypass capacitors and it was found that they shifted the RIAA equalization, a later PCB had the tracks still in place but, no numbers and no through holes, later still the PCB was changed to the present one and all capacitor markings numbers were changed, I think most resistor numbers were changed after the first stage, but the circuit was almost identical. Bob