Can't set Bias on a KA-7100
Having a problem with an ineffective bias trimmer on a Kenwood KA-7100.
Got a KA-7100 to restore and after initial DBT test and a little de-oxit I tried setting the trimmers for center voltage and bias before the restoration. The Left channel trimmers would not respond. The amp played, the left side wasn't blown, so I didn't worry about it. I was planning on a full recap, new trimmers, etc. anyway, so I put it in the repair pile.
Time to rebuild, so I changed all electrolytics and all diodes, put in new transistors for the relay circuit and voltage regulators and the new Bourns trimmers. I measured the old pots and set the new replacements similar. The amp passed every DBT test during rewiring, and the relay would always click on. At full AC power I tried setting the bias and no luck, there was no response at all while turning the Left Bias trimmer. The restoration included changing every transistor in the channel (except the Darlington pair), so I did that, hoping to fix the bias issue with a new driver at Q13. Nope. The center voltage trimmer sets to 0, but the bias pot - nothing. Took it out, tested it, it seems fine. Put it back in, it doesn't change the voltage at the emitter resistors. It measures 0 across the control points. And the Right side works fine.
Maybe there are clues in these noticeable voltage readings:
Q13 E -176mv (yes, neg) (s/m= 1.15v)
C -.89v (s/m= -1.15v)
B -.68v (yes,neg) (s/m= .6v)
Q7 E 43v (s/m= 44.5v)
C 7.47v (s/m= 5.3v)
B 42.7v (s/m= 44v)
Q9 E 43v (s/m= 44.5v)
C -176mv (s/m= 1.15v)
B 42.9v (s/m= 44v)
I have checked the outputs, the emitter resistors, the diodes, and other resistors involved in this circuit, even the thermal dot. All of the new left channel transistors were checked, compared, rechecked for orientation, and DBT tested, and remember, the amp plays. Actually sounds great. But I can't set Bias on the left.
Something's amiss somewhere, but where do I look now?
Help!?! and thanks...
Having a problem with an ineffective bias trimmer on a Kenwood KA-7100.
Got a KA-7100 to restore and after initial DBT test and a little de-oxit I tried setting the trimmers for center voltage and bias before the restoration. The Left channel trimmers would not respond. The amp played, the left side wasn't blown, so I didn't worry about it. I was planning on a full recap, new trimmers, etc. anyway, so I put it in the repair pile.
Time to rebuild, so I changed all electrolytics and all diodes, put in new transistors for the relay circuit and voltage regulators and the new Bourns trimmers. I measured the old pots and set the new replacements similar. The amp passed every DBT test during rewiring, and the relay would always click on. At full AC power I tried setting the bias and no luck, there was no response at all while turning the Left Bias trimmer. The restoration included changing every transistor in the channel (except the Darlington pair), so I did that, hoping to fix the bias issue with a new driver at Q13. Nope. The center voltage trimmer sets to 0, but the bias pot - nothing. Took it out, tested it, it seems fine. Put it back in, it doesn't change the voltage at the emitter resistors. It measures 0 across the control points. And the Right side works fine.
Maybe there are clues in these noticeable voltage readings:
Q13 E -176mv (yes, neg) (s/m= 1.15v)
C -.89v (s/m= -1.15v)
B -.68v (yes,neg) (s/m= .6v)
Q7 E 43v (s/m= 44.5v)
C 7.47v (s/m= 5.3v)
B 42.7v (s/m= 44v)
Q9 E 43v (s/m= 44.5v)
C -176mv (s/m= 1.15v)
B 42.9v (s/m= 44v)
I have checked the outputs, the emitter resistors, the diodes, and other resistors involved in this circuit, even the thermal dot. All of the new left channel transistors were checked, compared, rechecked for orientation, and DBT tested, and remember, the amp plays. Actually sounds great. But I can't set Bias on the left.
Something's amiss somewhere, but where do I look now?
Help!?! and thanks...