Odd issue

[rgccarter]

Another post from me..

Been enjoying the WD88 very much - not made any mods yet apart from HT supply caps C12-13.

Noticed a strange thing yesterday however. When powering down with an audio signal the left channel turns off straight away. However, the right speaker fades out slowly over several seconds - which I would expect as normal behaviour whilst capacitors discharge?


1st Q - Am I right in expecting one channel turning 'off' immediately isn't expected behaviour?

I have had the bonnet up testing voltages, wiring, continuity etc, and nothing seems to be a miss. Also swapped around valves and temporarily swapped out some of the larger vaue caps (C7, C8 & C9) on the driver and output boards for similar values , but still the same behaviour.

Can't say whether this happened before I swapped out C12/C13 or not.

2nd Q - any pointers of what could be the cuplrit?

Not sure if it's actually affecting the sound, although I'm now telling myself the right channel sounds slightly louder and bass slightly more fuller

[Richard]

Hi, Both channels should display the same behaviour as they are from a common PSU.

Can we clarify which amp you have? Is it WD88VA? Is it a power amp only or is it an integrated with switched relay input?

I’d investigate the inputs if it has relays. Thoughts are a relay may be staying connected when powered off. Try using a different pair of input phonos.

[rgccarter]

it's a Single Input intregrated WD88VA without switched relay input

[bob orbell]

I suspect that you have a whisker of solder or wire that is shorting, but not enough to completely stop the amp. from working, I just tried my WA88 and with music playing both channels diminish equally at about 8 seconds. Do you have any discharge resistors fitted anywhere ? or even a by-pass capacitor that may be the culprit, check both channels for equal resistance, a bit time consuming, but might show something. Bob

[Richard]

The HT is bled by the divider at R25/R26 but that would affect both channels equally.
If you've not changed them I'd replace the 100uF 500V C9 caps as there is one per channel. Other than that check for a fault in construction or a wrong component value by comparing both channels as Bob says.

[rgccarter]

OK, done some poking around with a multimeter and found a few rogue resitances, some of these maybe within tolerance?
N.B. They have the right colour codes but measure differently.



I think R4B reading at 2.3M rather than 470k is probably problematic

Anything else on there jumpout as an issue?

[Richard]

Hi, If your readings are accurate then they need changing. (Perhaps remove that R4B at 2.3M and check it out of the amp if you are in any doubt.)

I’m not sure it will affect the switch-off music-dying times though and suggest you also swap out all the electrolytic capacitors as good practice if you don’t know the hours the amp has run as they may have dried up.

R1, R4 and R5 face high voltage when running and much higher at switch on. They may see in excess of 400V until the valves heat and conduct (no current is drawn so no voltage is dropped) and may have broken down particularly if they are small size carbon film.

I would swap those for 2W metal oxide which have a higher overload voltage rating. RS have them as 220K 470K and 1M for not a lot of money.

[rgccarter]

Thanks very helpful. Replacements arrived today.

Looking at the physical boards R1 and R4A look to be carbon 3w going by the size, so I'm guessing these have been replaced some point before.

[rgccarter]

So happy bunny, fixed

Replaced resistors as mentioned except r1 as they read OK when out and were already metail 3W.

Also c9, c4, c7, c8.

I also dropped my ECC83 on hard kitchen floor so that is new as well now.

So one/some of those was the cuplrit..

Thanks for the help

[Richard]

Hi, Excellent, well done, have fun
R1, R4 and R5 will all actually be dissipating less than a 1/4W each so 2W will be fine power-wise.
2W and 3W metal oxide have the same voltage rating and are the most robust of the film types.
The circuit shows R1 as 2W and R4 as 1W. I don’t know what yours was actually built with, but the 2W MO should sort the problem