I picked up this SX-880 over the summer and overall it’s in nice shape. Unfortunetly it won’t come out of protection. I have been pushing myself to troubleshoot this one on my own and I have made some good progress, but it seems like I’m going to need some advice on this one if I ever hope to listen to it.
When I first got this unit I powered it up on the DBT and the bulb stayed dim but no relay click. I checked the offset and bias, and discovered that the bias could not be adjusted up from zero. I also discovered that Q36 in the protection circuit was shorted. I replaced it with KSC1845, but the unit stayed in protection.
Next, based on lots of previous threads on this site about the SX-880, I reflowed the power supply transistors which are known to unsolder themselves. Still no luck.
I did some more measuring of the voltages throughout the unit and I discovered that the regulated supply voltages (+41.5/-41.5) were missing. I was able to track the missing regulated supplies to a shorted Q28 which I replaced with KSC2383. After changing Q28 the supplies returned and I was able to adjust the bias in both channels to 30mv. I adjusted the offset in one channel down to 0mv, but I could not get the other channel below 200mv. I tried cleaning all of the controls with D5 to see if that would help bring the offset down, but I was still unable to get it below 200mv until I pulled one end of the “P” jumper wire.
With the “P” jumper pulled, I now have the bias and offset adjusted properly in both channels. Unfortunetly the protection issue remains and I cant seem to track it down on my own. Time to ask AK for some help here.
I’m wondering if Q35 might be bad?
Q35 (PA3004 on DBT while unit in protection)
1 = 13.25v
3 = .548v
4 = 95mv
5 = 0v
6 = -12.6v
7 = 9vAC
8 = 1.15v
When I first got this unit I powered it up on the DBT and the bulb stayed dim but no relay click. I checked the offset and bias, and discovered that the bias could not be adjusted up from zero. I also discovered that Q36 in the protection circuit was shorted. I replaced it with KSC1845, but the unit stayed in protection.
Next, based on lots of previous threads on this site about the SX-880, I reflowed the power supply transistors which are known to unsolder themselves. Still no luck.
I did some more measuring of the voltages throughout the unit and I discovered that the regulated supply voltages (+41.5/-41.5) were missing. I was able to track the missing regulated supplies to a shorted Q28 which I replaced with KSC2383. After changing Q28 the supplies returned and I was able to adjust the bias in both channels to 30mv. I adjusted the offset in one channel down to 0mv, but I could not get the other channel below 200mv. I tried cleaning all of the controls with D5 to see if that would help bring the offset down, but I was still unable to get it below 200mv until I pulled one end of the “P” jumper wire.
With the “P” jumper pulled, I now have the bias and offset adjusted properly in both channels. Unfortunetly the protection issue remains and I cant seem to track it down on my own. Time to ask AK for some help here.
I’m wondering if Q35 might be bad?
Q35 (PA3004 on DBT while unit in protection)
1 = 13.25v
3 = .548v
4 = 95mv
5 = 0v
6 = -12.6v
7 = 9vAC
8 = 1.15v
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