Fisher Tube Amp KX-90 PS issue

C

CCook

New Member
Weird one.
Just got a KX-90 integrated amp. While checking out the B+ voltages, i noticed a strange thing I had never seen before.
Before tearing into it, I thought I would ask if someone has a simple answer
I fire up the unit. B+ rises slowly to about 390v at the first cap. Then, it starts falling somewhat rapidly below 300v volts. Fuse does not blow.
I turned it off about 2 seconds after the fall starts. Repeated with same result. All tubes test OK, not great but usable.

Only mod I did before was to add a CL-90 between the power switch and the fuse.

My guess is a bad PS capacitor. Any ideas?

Thanks in advance.
 
Register to hide this ad
That, or something causing massive current draw in the output stage. Or maybe a little of both
 
Yeah, I ruled out heavy draw from the output stage since the op stated the drop in B+ occurred nearly immediately after turn on. From what we know, I’m not sure what else it could be……….
 
Turned out to be a bad doubler Cap. Bypassed both of the doubler caps with new units, works fine.

Only issue now is the low voltages on the output cathodes (25v vs 30v) on schematic and the output grids (13v vs 18v on the schematics). Tubes are 7189s, seem to be OK but could the voltages be due to leaking coupling cap or to weak output tubes?

Reading previous posts, I removed R66 (2.2K 3w that bypasses V1, 2, 3 and R65 resistor) and installed three 11v zeners with a 10 ohm resistor. I believe that is what you suggested for the EL 84/7189 amps.

Thanks for your help here.
 
how are the plate and screen voltages? Low on either, especially low screen, will cause this. A shorted cathode bypass cap does this too.
 
Will check again.

B+ voltages at P/S caps/resistors all close to schematic values.

But, given the age, I will put in new cathode bypass cap.

Thanks
 
You must log in or register to reply here.
Back
Top Bottom