Have a Citation 22 where DC offset in the right channel is difficult to master. When turned on, DC speeds up to 1 volt to slowly creep down for many hours and when readjusting, i can keep it around 0. When changing the program source, it clicks into the right speaker at the same time as my DVM shows up to 1 volt and then drops slooooowly. The same result at the next switch-on and program change. Bias is OK according to the "book", so I guess the problem is on the pre-driver card, maybe the differential transistors in the right channel have given up? The question then is, which on the schedule are these transistors now? It is quite possible that there is something else that makes the DC offset go bananas when turned on, suggestions for sources of error are received with the greatest gratitude!
There might be other causes more likely than a semiconductor fail...eg
- dirt, dust, grime on a high impedance area of the PCB causing current to flow where it shouldn't
- an electrolytic capacitor in or near the signal path that is failing with low parallel resistance (or an electrolytic leaking conductive goo)
I would say clean the board overall and replace C431 just in case that is it.
Also, is idle current right or close?
Also, is idle current right or close?
I had an apt-1 where the beta of the input pair had degraded and caused an offset. In the case of the apt, the resistance to ground was different between the feedback side and the input side. The increase in base current from the degraded beta's did it. Changing out the input pair fixed the problem. This was after 40 years of use or so and beta can degrade over time.
I'd also be suspicious of C401, chiefly since it's easiest to think about. 😉
If I'm seeing correctly, the power stage has flat gain at DC =1+R433/(R423+R425)--- or about 50 V/V. So if you're observing about 1VDC at the output, 20mV at C401/R403 would be problematic. Careful check with a meter would reveal that. If C40 is innocent, you could compare with the good channel for differences. Voltages at R403, Q405 base would be good starting points.
Good luck.
If I'm seeing correctly, the power stage has flat gain at DC =1+R433/(R423+R425)--- or about 50 V/V. So if you're observing about 1VDC at the output, 20mV at C401/R403 would be problematic. Careful check with a meter would reveal that. If C40 is innocent, you could compare with the good channel for differences. Voltages at R403, Q405 base would be good starting points.
Good luck.
THE JUDGE, you say
maybe the differential transistors in the right channel have given up? The question then is, which on the schedule are these transistors now?
A differential transistor has its two emitters tied directly together, with no capacitors or resistors across that junction. They can be united within the same encapsulation (it will have 5 pins) or they can also be found with the same configuration with two separate conventional 3-pin TRs, but the emitters are united by means of the PCB.
I haven't seen anything like that in the attached circuit, but my eyesight isn't that great....
http://www.interfacebus.com/Transistor-Differential-Amplifier-Circuit-Description.html
maybe the differential transistors in the right channel have given up? The question then is, which on the schedule are these transistors now?
A differential transistor has its two emitters tied directly together, with no capacitors or resistors across that junction. They can be united within the same encapsulation (it will have 5 pins) or they can also be found with the same configuration with two separate conventional 3-pin TRs, but the emitters are united by means of the PCB.
I haven't seen anything like that in the attached circuit, but my eyesight isn't that great....
http://www.interfacebus.com/Transistor-Differential-Amplifier-Circuit-Description.html
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Hit the suspect transistor with freeze spray while you are measuring the offset. It will reveal if it is a problem.
Selector switch, clean and lube with spray, dirt in the mechanism can be a source of issues, springs may not be at full strength.
A proper cleaning is always beneficial, particularly if used in a closed room, with smokers inside, or as here, lots of airborne dust.
A proper cleaning is always beneficial, particularly if used in a closed room, with smokers inside, or as here, lots of airborne dust.
There are symmetrical diff pairs Q403/Q405 and Q411/Q457. If the freeze spray affects the offset when sprayed on those transistors, I'd suspect the input pairs.
To elaborate a bit, there will be temperature induced shifts if the differential pairs (eg. Q403/Q405) experience unequal cold-shot test sprays. Try to apply comparable experiments between the channels.
There's a fair amount of emitter degeneration that will moderate temperature sensitivity (roughly 1/4 V across R411, R413, R499, R501). Between-channel differences might be insightful.
There's a fair amount of emitter degeneration that will moderate temperature sensitivity (roughly 1/4 V across R411, R413, R499, R501). Between-channel differences might be insightful.
Thanks all for your responds.
I measured the C402 and got - 0.72 volt on + side and 0 volt on - side. Then i measured the C401 and it was 0 volt on both + and - side.
I removed C402 and adjusted dc offset to 0 and switched off. Then i switched on i measured 12 mV at output.
Then i installed another new C402 and got - 0.25 volt on + side of C402 at switch on and after a couple of hours 0.001 volt.
Later i found lots of glue around the legs of Q402 and removed most of it and now dc offset is around 100mV at switch on and falls faster than before.
I would like to replace Q402 but were to find a 2SK364 or equivalent?
I measured the C402 and got - 0.72 volt on + side and 0 volt on - side. Then i measured the C401 and it was 0 volt on both + and - side.
I removed C402 and adjusted dc offset to 0 and switched off. Then i switched on i measured 12 mV at output.
Then i installed another new C402 and got - 0.25 volt on + side of C402 at switch on and after a couple of hours 0.001 volt.
Later i found lots of glue around the legs of Q402 and removed most of it and now dc offset is around 100mV at switch on and falls faster than before.
I would like to replace Q402 but were to find a 2SK364 or equivalent?
Not all of the schematic is visible. Can you "zoom out" just a bit for a more complete view? Thanks!
Follow up:
I have now replaced the Q402 with a new correct 2SK364, DC offset quickly moves to 0 in both channels after turn on and no more "snapping" changing program sources.
Problem solved!
Thank you all whom responded.
I have now replaced the Q402 with a new correct 2SK364, DC offset quickly moves to 0 in both channels after turn on and no more "snapping" changing program sources.
Problem solved!
Thank you all whom responded.
