wd hd3 problem

[Princo]

yes, i thought failed cap C6 at first, so swapped, the fault remained on the left channel, (right channel fine), so it seems the caps are fine, (what i dont understand is why the voltages should be correct when the cap is removed??).

I will check the other components as you suggest.

Thanks

Jim

[Princo]

I checked again solder joints, all ok.

Removed C5 and checked voltages, same fault..

I keep coming back to pin 9, where can -45v come from?

[Richard]

A large negative voltage on Pin 9 will shut off the valve and stop the current flow through it. That would cause no/low flow through the cathode 750R and the low V reading at cathode Pin 7 but I can't really see where the negative voltage would come from.



It’s easy to get a small construction fault or even mis-read the meter. I’d go back to basics and get a magnifying glass over all the tracks checking for solder whiskers or poor joints. Check all caps for correct +/- orientation. Many of the circuit connections are pcb tracks but check any wire links and connections you have fitted. Check valve pin numbers and mark pin 1 and 9 on both so you probe the correct ones.


Clip the black neg multimeter lead onto the neg of C3 and meter all the voltages and resistances with the red probe.

[bob orbell]

Jim, you have PM. Bob

[bob orbell]

Jim sent me the CD via email to have a look at and a note of his build, I may be wrong but it appears he is not making this on a PCB but hard wireing point to point. So Jim, if I am correct, you need to look at your asembly again as something is wrong, but what ? . Bob

[Princo]

I pulled it apart and built it completely again.

Same issue, L chan strange voltage -45v at pin 9, I swapped the output transformers, same problem.

If i disconnect or short the feedback winding, problem disappears, pin 9 0v as it should be and pin 7 14v as Pin 3 is slightly higher at 4v (i guess as there is no feedback), all other voltages as they should be.
All this is only applicable to the L chan, the R chan is fine and no matter what i do i cant replicate the problem on the R chan.

It is such a simple circuit, i must have done something fundamentally wrong when combining the two channels and power supply, but what?

[Richard]

Originally Posted by Princo

If i disconnect or short the feedback winding, problem disappears, pin 9 0v as it should be and pin 7 14v as Pin 3 is slightly higher at 4v (i guess as there is no feedback), all other voltages as they should be.
All this is only applicable to the L chan, the R chan is fine and no matter what i do i cant replicate the problem on the R chan.

Interesting as the fb resistor R9/16 1k is also the cathode resistor for the triode stage so disconnecting it would mean Pin3 cathode should then be floating (not DC connected) at all and that first stage not working at all. The pentode stage is then just idling correctly. So the pentode stage seems to be fine and connecting the triode stage to it is causing the issue. Something amiss in the triode section or its connection to the pentode (Pin1 to Pin9).

[Princo]

Thanks Richard,

The other strange thing is when i disconnect C6 the fault disappears, i soon as I connect i get -45v again at pin 9, i already tried a different (new cap) and same result.

The only conclusion i can come to is i have made an error in the wiring somewhere, does the combined schematic i attached in the previous post look ok, i've checked a few times and cant find anything obvious.

A common power supply and earth couldn't cause some problem?

Other strange thing is the R chan is a carbon copy of the L chan, yet R chan is perfectly fine.

[Richard]

I would now disconnect C6 after the cap and before Pin9. Leave Pin 9 connected ground via 1M.


Switch on and both stages should be able to idle independently from each other, so check voltages on both triode and pentode stages.


Also check for any voltage present on the end of disconnected C6 which will show if it is electrically leaky (switch to mV range) should be zero with a plastic cap and only 100V on the other end.

[Princo]

The measured voltages with the cap C6 disconnected from pin 9/R8 are:

R Chan Pins L Chan

1 110v / 105v
2 0v / 0v
3 0.9v / 0.9v
4 H 6.3ac/ H
5 H / H
6 195v / 196v
7 14.8v / 15v
8 196v / 196v
9 0v / 0v

There is very small voltage passing C6 fluctuating between +100mA to -50mA, this is a new cap.

All as they should be.