Blues jr low voltage problem

[pontiacpete]

This fender blues jr had a broken fat switch. One of the low voltage caps c34 was bulging. So i replaced it and the fat switch still didn’t work. Then i finally took a measurement of the voltages and found the negative voltage is around -7 or -8vdc. I replaced the zener diode thinking that’s got to be it, and still no change. Even though the reverb is working with the lacking neg voltage i thought that something might be dragging it down, so I changed the IC. Still no luck. D5 has the required -26vdc. One side of R53 has the -26.8 but the other -8vdc. R53 measures 739 ohms the other one R50 750 ohms. Do you changing R53 would take care of the problem?

[Jon Snell]

Which version of Bj? Mk1, 2. 3 or 4.
I don't think replacing R53 will do it. If it measured OC then yes but it is a resistor and roughly the correct value.
With the IC out, what voltage did you get?
Is C34 the correct polarity and did you use the exact value?

[pontiacpete]

I didn’t think to take a voltage measurement with the IC out. What seems counter intuitive is that r53 is lower in resistance, so i would think that the voltage would be higher. Anyway I had to order some 1 watters. Only have half watt ones. Thanks. Oh and the cap is in right with the correct value

[Jon Snell]

There is only two places the -15v goes to; pin 4 of the IC and R57 on the boost switch.
A 22k won't load it down so it must be something to do with the IC or its supply pin feed.
So, not a lot to go wrong. Have you got +15? if you have a bad ground on the supply, it could be floating and still present +-15volts to the IC but as +22 -8.
Check the 10.8volts feeding the bias. That will tell you if the supply is not balanced.

[g1]

Please post appropriate schematic.

[pontiacpete]

There’s +15v 0n pin 8 and -8v on 4

[pontiacpete]

Rev A seems to fit.

[pontiacpete]

Fender_blues_junior_rev_a.pdf

here it is

[Jon Snell]

Which one is it?

[Helmholtz]

Check C32/33/34 for leakage. What's the voltage at TP33? What does R53 measure?

[Jon Snell]

Check C32/33/34 for leakage. What's the voltage at TP33? What does R53 measure?

Did you read the original post ... I think not.

Edit;
What has the secondary HT voltage got to do with the stated fault

[Helmholtz]

Did you read the original post ... I think not.

Edit;
What has the secondary HT voltage got to do with the stated fault

Well I did, but without a schematic coudn't make much use of it.
Now re-reading it seems to leave C34. - or excessive load current.
Didn't mention /refer to secondary HT voltage.

[pontiacpete]

R53 measures -26.6vdc at the junction with c32. R50 has basically the same voltage at it’s junction with c30, however at other end of r53 there’s -8vdc where as r50 has 15vdc..

[g1]

So something is pulling down the -15V line. Could be the zener, the cap, or anything running off the -15V (which seems to be only U1).

[Jon Snell]

So something is pulling down the -15V line. Could be the zener, the cap, or anything running off the -15V (which seems to be only U1).

And R52, near the Fat switch.
Supply X
Edit; This is why it is so important for the poster to identify the model correctly. We are all just guessing.