Hughes & Kettner

[Valvehead]

yes--thats the 4 i tested

i just called them 1 thru 4 , sorry

[J M Fahey]

Multimeter on diode test mode shows voltage across the junction.
Clearly 0V across one (or a very low value such as 30mV) means a shorted transistor.
Too sleepy now (4 am here) but tomorrow will suggest something.
I hope .

[Steve Conner]

Sure, but if the transistor was shorted it would read "0" in both directions. Getting 0 in one polarity and 0.4 in the other is impossible, as there is no way of making a rectifying junction with zero forward voltage. I suspect a slip of the probe or some leftover DC voltage.

I doubt the output transistors are blown anyway. If they were, there would be a huge DC offset and the thing wouldn't run, it would pop fuses as soon as you turned it on. Look for the usual like bad connections, dried up small electrolytics etc.

[J M Fahey]

Agree. Getting wrong or inaccurate measurements when "repairing online" is a constant possibility.
If *I* had gotten such a "perfect diode " measurement, I would have re-read within 5 seconds, after all test probes are in my hands; but through mail, will have to ask for re-measurement and wait.
Oh well, better than nothing
Back to sleep, only 4 hours so far.

[Valvehead]

ok, i will look over the pcb again with my magnefier.....im glad i read this , as I was just about to order 4 new output trans. Damn things are $8.50 EACH ...

i doubt there was a slip of probes--- i took these measurements several times....

i will drain all the caps i can to ground and measure again

[Valvehead]

pcb looks ok

i re-tested the 4 output transistors , with a different meter , set on "diode" , and got :


From pin 1 / 2 to case(ground) :

trans 1 ...... 1.5v / 0.4v __ swap leads : 0.6v / open

trans 2 ....... 0.4v / open __ swap leads : open / 0.6v

trans 3 ...... 0.5v/ 1.5v __ swap leads: open / 0.6v

trans 4 ....... 0.4v / open __ swap leads: open/ 0.6v



measuring in each direction (reversing the leads), from pin to pin :

trans 1 ___ .6v/ 1.6v

trans 2 ____ open / 1.6v

trans 3 ____ 0.6v / 1.6v

trans 4 ____ open / 1.6v


I hope this makes sense

[Valvehead]

UPDATE

UPDATE---

I ended up replacing the four output transistors. They are now NTE 249/250 .

Same exact problem -
Right channel has no output--yet lights up the LEFT input led ...why the left ??

the left channel seems to work fine , but seems like its more like 40 watts instead of 100w . Maybe its just me--im used to the roar of a 100w TUBE amp ..

I do have +15 and -15v at all the IC chips . I do have 39v going to the output transistors. none of the larger resistors are bad , the input jacks are new.
Ive looked over the pcb joints with a microscope 3 times now --and the output jack board --checked the output jacks and cleaned them.


Im at a dead end --- any more ideas before i throw in the towel ??

[52 Bill]

UPDATE---

I ended up replacing the four output transistors. They are now NTE 249/250 .

Same exact problem -
Right channel has no output--yet lights up the LEFT input led ...why the left ??

the left channel seems to work fine , but seems like its more like 40 watts instead of 100w . Maybe its just me--im used to the roar of a 100w TUBE amp ..

I do have +15 and -15v at all the IC chips . I do have 39v going to the output transistors. none of the larger resistors are bad , the input jacks are new.
Ive looked over the pcb joints with a microscope 3 times now --and the output jack board --checked the output jacks and cleaned them.


Im at a dead end --- any more ideas before i throw in the towel ??

If neither right channel led is lighting up, the signal is not getting past the input stage. The signal is monitored at the output of IC 2 and if the led doesn't light up then the signal is not passing thru the chip. Or it may not even be getting into the chip, if the volume control is bad. If you switch to bridge mode and input a signal into the right input jack do the left side leds still light up?

The left side leds light up because the inputs are connected together when there is no plug in the left side input, so the amp works in mono mode.

As for the volume level, what are you driving the input with? This is a power amp and it needs a bigger signal to drive it to full power.

[olddawg]

Does the amp have a "Bridge mode" switch? That might explain your indicators if it is in bridged position. Btw, it is usually frustrating and expensive to just start shotgunning parts. I doubt if your outputs were bad. Usually in an old power amp, if you aren't blowing fuses and drawing current, outputs are not blown. Those 22 ohm 5 watt resistors sure do sound like .22 ohm output emitter resistors. I would check the schematic. In all reality you probably have a dry cap or an open low value resistor somewhere. But even a simple problem is difficult without the proper tools and background. Use your meter to check your old output transistors one at a time out of circuit. If they measure good put them back in.

[Valvehead]

Thanks Bill-- driving it with a tube amp--the fx send. it also has a level switch-

bridge mode doesnt work--- neither input led's light up

[Valvehead]

yes Dawg--- there is a bridge switch

[Valvehead]

ok-- update--- no progress ....

just replaced the right pot, also checked all parts between right input jack and IC2 . all is normal , have the 15v at the diodes ,

all grounds are there

[Valvehead]

More ----

checked components from IC2 pin 6 ( after) all are good , r24 , c14 , c15 , r31 , r30 .

if this helps -- I have 0v at IC2 pin7 , and 0V at IC2 pin1

i have +/- 16v at IC2 pins 4 and 8

[MarkusBass]

How did you check that there is not input on right power amp? A common failure is similar amp is shorted muting FET transistor on the input of the amp (T6 in your case). Have you chekced it? You can just desolder it temporarily.

Mark

[Valvehead]

Something interesting-----

at R9 , i get 0v on both ends

on D1 + i get 0v and on neg ........ i get open and the Left LED lights up

on D2 , i get open and left led liights on the + , and on the neg i get 0v .