Blues jr low voltage problem

[pontiacpete]

The zener has been changed, and U1 also. The cap’s been changed twice, the second one started to dome a bit, and looked like it was getting stressed. R52 measures 22k and has that -8vdc on both ends.

[Jon Snell]

The zener has been changed, and U1 also. , The cap’s been changed twice, the second one started to dome a bit, and looked like it was getting stressed. R52 measures 22k and has that -8vdc on both ends.

That means there is an issue with the capacitor, usually in the wrong way around or wrong voltage rating/old stock etc.
I would remove IC1 then see where you are going by checking pin 4 for -ve 15volts. If the voltage is still -8 then remove the zener and test again.All you have left then is the capacitor or damage on the pcb.
Did you remove the 750R to measure it?

[pontiacpete]

Ok, i forgot mention that the fat switch is now working after changing the zener and cap even though the voltages are low on the negative rail. Also i neglected to say that before all this i had lowered the current draw on the power tubes by replacing r52 22K to a 33K. Usually i install a bias pot here and lower to about 24mA. So if this is the reason for the low negative rail voltage, I never noticed this before, i.e. on all the other blues jrs I’ve worked on.
And finally, I’m working one handed as of the last few days, because of shoulder replacement surgery a week ago..not that that is an excuse
So should I just leave and send this on it’s way?

[Jon Snell]

Adjusting the bias voltage has no bearing on the -ve supply voltage at the zener point.
What ever you think but I would like to complete a full repair.

So at last, it is a Mk3! the only one with R52 bias supply resistor.

[pontiacpete]

If this isn’t a good way to leave it , then I’d like to figure this out. Could it still possibly be 750r resistor that I’m waiting for?

[Jon Snell]

If this isn’t a good way to leave it , then I’d like to figure this out. Could it still possibly be 750r resistor that I’m waiting for?

You will never know without removing the IC and the zener.
My money is on the zener being the wrong value.

[pontiacpete]

The IC I installed is a TL072 and the zener is a 1N4744

[g1]

Did you remove the 750R to measure it?

Could it still possibly be 750r resistor that I’m waiting for?

You tell us.

[mozz]

Easy enough to cut traces with a knife to narrow it down. Easy enough to scrape the coating off and resolder with a tiny jumper to put it back to stock. If that voltage is only going a few places either something is loading it down or your source is collapsing.

[pontiacpete]

I pulled r53 it measures 747 ohms

[Jon Snell]

Remove the IC, measure the voltage, if still low, (which it will be), remove the zener and test again. The only reason for removing the IC is to avoid damaging it with excessive voltage.
Easy and quick to do.

[pontiacpete]

Thanks! But don’t forget I’m doing this with one hand ️

[Jon Snell]

You have succesfully removed them recently.
If you don't, you will not have repaired it properly and will not have gained usesful experience for the future plus you will never know what the problem was.
I like to sleep at night knowing I will not have it in again for the same fault.

[pontiacpete]

I got the IC out and put a socket in place. Still with no IC, also a new zener and 220uf cap and voltage is still low. What to do?

[Jon Snell]

Take the zener out and test again.