Blues jr low voltage problem

[Jon Snell]

With no zener and no IC what is the voltage?

[pontiacpete]

With the Zener out of circuit voltage is still low, unbelievable

should i pull the jfet and then d14?

[Jon Snell]

That is very odd.
Nothing else to drag it down. Sanity doubting comes to mind.
This amp I presume is the Mk3, please correct me if I am wrong, Mk series is on the back wooden panel.
It leaves only the capacitor unless the board has become conductive and I have never come across that.

[Jon Snell]

If you have a regulated power supply, can you put 15volts across C34 and monitor the current?

[pontiacpete]

Jon I’m not exactly sure how to determine if it’s a MK3 or what it is. I was going by the layouts only, mind you , not thoroughly. It’s a crème board and on the board it says made in 2001

[pontiacpete]

If you have a regulated power supply, can you put 15volts across C34 and monitor the current?

That might take a while, but i can does this

[Helmholtz]

Just observe negative polarity.

[pontiacpete]

Ok, that was faster than i thought it’d be. Tell me if I’m doing this right. I put +15dc across the cap and measure with an ammeter set to DC or AC?

[Jon Snell]

Meter set to 100mA scale in series with your 15volt external supply. It should read practically zero mA.
With the zener and IC out any voltage up to 20 will be acceptable.

[pontiacpete]

Ok the ammeter reads at about 16.38mA.

[Helmholtz]

Before you start, make sure neither of the output terminals of the power supply is grounded, i.e. connected to safety earth/ground.
(If not, you might use 2 series wired 9V batteries.)
Then connect the positive output of the power supply to the positive end of the C34 (which should be connected to amp ground).
Then insert your DC current meter between the negative output of the power supply and the negative end of the cap.

Also you should disconnect R53 as well as the zener to avoid a current flowing back into the amp's power supply.

[pontiacpete]

Before you start, make sure neither of the output terminals of the power supply is grounded, i.e. connected to safety earth/ground.
(If not, you might use 2 series wired 9V batteries.)
Then connect the positive output of the power supply to the positive end of the C34 (which should be connected to amp ground)..

How does this look for ground? I’ll move the red meter probe

[Jon Snell]

Ok the ammeter reads at about 16.38mA.

16mA to charge a capacitor as that is all that is there? There is nothing left for the IC.
Sounds as though there is something desperately wrong with the board because there is absolutely no current draw from anywhere else.
That's as far as I can go without actually working on it.
Good luck.

[Helmholtz]

As the lab supple has an extra ground terminal I suppose the +/- outputs are floating which is what we need here.

[Helmholtz]

16mA to charge a capacitor as that is all that is there? There is nothing left for the IC.

I don't understand.
Apart from a charging surge, the cap shouldn't draw any continuous current.