Need help with '73 superbass problem..........almost no signal and weird sound artefacts

[Rej]

Hi all, having problems with a superbass my friend owns. It has worked well for years and just suddenly the other day the signal became almost non existent other than a weird farty abrupt sound to the notes when played hard.
Bear in mind i'm a player not a tech but have some knowledge and history of successful repairs.
Here's some findings so far -

All valves seem fine , bias steady at -56v on all pin 5's, no redplating or excessive hum.......
No burnt smell, no burnt components
screen grid resistors all read approx 1k
Can't see any loose or bad solder joints anywhere via magnifiying glass
i/p and o/p jacks all ok and everything cleaned by me
fuses all good and correct value
I piggy backed 50uf 450v axial caps over all the big filter caps in case any were gone and it made no difference to symptoms
checked speaker leads my friend uses , all good and operational

*Now, and I don't know whether this will give any clues but I checked impedance with power off from pin 3 of each group of two power valves to end of HT fuse and got around 36 ohms one side and a little more the other side.

Do any of you experienced guys have any ideas as to what might be going on?

many thanks in advance ! Rob

 

[Purgasound]

Piggybacking caps over caps that may be shorted wouldn't make any difference.
If the electrolytic caps are original my first order of business would be to replace them along with the two in the bias supply. It will save time later. I won't troubleshoot an amplifier with 50 year old electrolytic capacitors in place. Removing them from the circuit to test takes more time than replacement and it's never a question of "if" they go bad but "when". There are some maniacs who like to push them as far as they'll go but there isn't any tangible or tonal benefit to keeping old worn out parts in place. I just think the techs that like to keep them have never had one explode in their face... yet.

Start there then you would want to check the voltage on all the tube pins. Sounds like you may have a problem earlier in the circuit. A loose preamp tube socket can cause a similar issue. You could either have a signal loss problem or power loss problem in the preamp.

 

[Pete Farrington]

bias steady at -56v on all pin 5's, no redplating or excessive hum...

What anode or cathode current are the output valves passing at idle?

Is the issue affected by setting presence full CW and full CCW?

 

[TassieViking]

I just think the techs that like to keep them have never had one explode in their face... yet.

That reminded me how we used to wire low voltage electro caps into the light switches in the bosses office when I was an apprentice a few years ago ( late 1970's to 1980's)

He never needed coffee to get started in the mornings.

 

[Rej]

What anode or cathode current are the output valves passing at idle?

Is the issue affected by setting presence full CW and full CCW?

Hi Pete, I don't have any 1ohm sensing resistors and was investigating at my friends home so only had basic tools present. Certainly moving the presence control made not difference to anything. Re current measurement I am not equipped for that at present but the bias voltage readings were stable with EL34's in the amp and also when they were unplugged and taken out of the amp........another post reply has me thinking that there may be something going on earlier in the circuit around the preamp..........I won't see amp till next week but will take voltage measurements on the ecc83's then..........thanks for your interest Rob