jasoncauthron
Active Member
I bought this little KR-5600 in working condition, needing the tuner dial cord restrung. I checked it out briefly before I took it apart to restring it. It had some DC on the output about 65 mV on one side and 15 mV or so on the other. I listened to it for a little bit, tested out all the functions and it sounded pretty good. I didn't think it had ever been worked on before.
Upon taking it apart to restring it, I saw several 1W power resistors that had been hot (which is not unexpected) and had discolored the board below them. That power supply board looked pretty rough, actually. So I figured I'd work it over a little, replace some capacitors and resistors, put new thermal compound under the output transistors, since it seemed to run a little hot, and maybe replace those differential amp transistors at the input to try and lower the offset.
Once I got access to the main amp board, it was apparent that someone had worked on it before. There were several caps removed from the component side of the board and tacked onto the solder side. Q5, the voltage amplification transistor on the left channel had been changed to a TO-220 device with no visible numbers on it, and having a pin configuration that didn't match the original. The drivers had been at least removed and reinstalled, and might or might not be original. But, like I said, the amp worked and sounded okay to me in the brief time that I listened to it.
Since I had it all apart at this point and couldn't easily hook it up to re-evaluate it, I started replacing capacitors and all the parts that I had planned on replacing anyway. Replaced all the electrolytics, replaced the diodes and zeners. I changed both VAS transistors, Q5 and Q6 with KSC2690s and went ahead and swapped out the driver transistors too with the KSA1220/KSC2690's. For the differential inputs, I matched up pairs of KSA992's.
When I put it all back together, I fired it up on a DBT. Bulb dimmed, relay clicked and I felt pretty good about it. I checked the bias by reading mV across the output emitters: Right side was right side was just about right, reading 35mV. Left side was a little higher reading 59mV. I adjusted the left side down, stopping on 30mV to err a little on the low side. Then I checked the offset: Right side now had 10-14mV (had been 65mV :thmbsp Left side still read 15. Or at least that's what I thought until I tried to hook up a speaker and realized I was testing on the wrong speaker terminal. I'm 90% sure I had done that the first time too, but regardless, I actually had in excess of 200 mV on that channel... I hooked up a speaker anyway and verified that it worked. Still sounded okay, actually, though I didn't listen long and kept the volume low.
Now, of course, I don't know for certain that I didn't "cause" the problem, but I'm pretty sure that it was actually like that before I started working on it, especially seeing that that channel had been operated on before. The outputs are original and test fine. 200mV is too much though isn't it, indicating that something is actually wrong? I started checking my work, double checking cap polarity, solder joints, etc. No problems found, so I started checking voltages. This is a simple amp, but I can't figure out what's going on in the left channel. Here's what I measured, relative to chassis ground:
Q1: b) .003V c) -35.0V e) .485V b) 0.08 c) -35.0 e) .652
Q3: b) -.09V c) -35.9V e) .485V b) 0.08 c) -36.2 e) .652
Q5 b) -34.7V c) -1.38V e) -35.3V b) -34.7 c) -1.18 e) -35.1
Q7 b) .238V c) -1.38V e) .870V b) .485 c) -1.18 e) 1.13
Q9 b) .442V c) -.242V e) .868V b) .662 c) -0.014 e) 1.12
Q11 b) -.23V c) .448V e) -.240V b) .008 c) .662 e) -.014
Q13 b) -8.8V c) -.24V e) -1.38V b) -.690 c) -.015 e) -1.18
Q15 b) -.24V c) -8.8V e) -.24V b) -.035 c) -.69 e) -.015
Q17 b).875V c) 37.7V e) .32V b) 1.14 c) 37.2 e) .585
Q19 b) -1.38V c) -37.7V e) -.832V b) -1.19 c) -37.5 e) -.636
Q21 b) .32V c) 37.7V e) -.230V b) .59 c) 37.7 e) .010
Q23 b) -.83V c) -37.8V e) -.240 V b) -.635 c) -37.5 e) -.033
I think this is something simple and it may be obvious to someone who knows more than me. If anyone would be willing to take a look, here's a shot of the relevant portion of the schematic. Thanks!
Jason
Upon taking it apart to restring it, I saw several 1W power resistors that had been hot (which is not unexpected) and had discolored the board below them. That power supply board looked pretty rough, actually. So I figured I'd work it over a little, replace some capacitors and resistors, put new thermal compound under the output transistors, since it seemed to run a little hot, and maybe replace those differential amp transistors at the input to try and lower the offset.
Once I got access to the main amp board, it was apparent that someone had worked on it before. There were several caps removed from the component side of the board and tacked onto the solder side. Q5, the voltage amplification transistor on the left channel had been changed to a TO-220 device with no visible numbers on it, and having a pin configuration that didn't match the original. The drivers had been at least removed and reinstalled, and might or might not be original. But, like I said, the amp worked and sounded okay to me in the brief time that I listened to it.
Since I had it all apart at this point and couldn't easily hook it up to re-evaluate it, I started replacing capacitors and all the parts that I had planned on replacing anyway. Replaced all the electrolytics, replaced the diodes and zeners. I changed both VAS transistors, Q5 and Q6 with KSC2690s and went ahead and swapped out the driver transistors too with the KSA1220/KSC2690's. For the differential inputs, I matched up pairs of KSA992's.
When I put it all back together, I fired it up on a DBT. Bulb dimmed, relay clicked and I felt pretty good about it. I checked the bias by reading mV across the output emitters: Right side was right side was just about right, reading 35mV. Left side was a little higher reading 59mV. I adjusted the left side down, stopping on 30mV to err a little on the low side. Then I checked the offset: Right side now had 10-14mV (had been 65mV :thmbsp Left side still read 15. Or at least that's what I thought until I tried to hook up a speaker and realized I was testing on the wrong speaker terminal. I'm 90% sure I had done that the first time too, but regardless, I actually had in excess of 200 mV on that channel... I hooked up a speaker anyway and verified that it worked. Still sounded okay, actually, though I didn't listen long and kept the volume low.
Now, of course, I don't know for certain that I didn't "cause" the problem, but I'm pretty sure that it was actually like that before I started working on it, especially seeing that that channel had been operated on before. The outputs are original and test fine. 200mV is too much though isn't it, indicating that something is actually wrong? I started checking my work, double checking cap polarity, solder joints, etc. No problems found, so I started checking voltages. This is a simple amp, but I can't figure out what's going on in the left channel. Here's what I measured, relative to chassis ground:
Q1: b) .003V c) -35.0V e) .485V b) 0.08 c) -35.0 e) .652
Q3: b) -.09V c) -35.9V e) .485V b) 0.08 c) -36.2 e) .652
Q5 b) -34.7V c) -1.38V e) -35.3V b) -34.7 c) -1.18 e) -35.1
Q7 b) .238V c) -1.38V e) .870V b) .485 c) -1.18 e) 1.13
Q9 b) .442V c) -.242V e) .868V b) .662 c) -0.014 e) 1.12
Q11 b) -.23V c) .448V e) -.240V b) .008 c) .662 e) -.014
Q13 b) -8.8V c) -.24V e) -1.38V b) -.690 c) -.015 e) -1.18
Q15 b) -.24V c) -8.8V e) -.24V b) -.035 c) -.69 e) -.015
Q17 b).875V c) 37.7V e) .32V b) 1.14 c) 37.2 e) .585
Q19 b) -1.38V c) -37.7V e) -.832V b) -1.19 c) -37.5 e) -.636
Q21 b) .32V c) 37.7V e) -.230V b) .59 c) 37.7 e) .010
Q23 b) -.83V c) -37.8V e) -.240 V b) -.635 c) -37.5 e) -.033
I think this is something simple and it may be obvious to someone who knows more than me. If anyone would be willing to take a look, here's a shot of the relevant portion of the schematic. Thanks!
Jason
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