I've recently recapped my Silver SP5050/SA5050 combo, which is almost the same as the HK725/770. There's 2 things that I haven't been able to work out though that I'd appreciate help on.
I'm using a known good CD player as the audio source.
1. Every time I turn the amp(s) on and play music, there is crackly distortion for about a minute, both channels impacted. This is not caused by pots being turned, and it goes away I guess once something warms up. But if I play music for 20 minutes so everything is warm, turn it off and then turn back on straight away, the same distortion is there for a minute. I've replaced all the electrolytics plus a couple of transistors, but wondering what that behaviour could be a symptom of?
2. The left channel for the "A" speakers doesn't work. The LED power display comes on, showing a signal for both channels, and the "B" speakers work fine. As far as I can see, there is one relay for A and another relay for B, so on that basis I guess there is some sort of fault on the actual speaker terminals or the board they mount on. Is that correct?
I'm using a known good CD player as the audio source.
1. Every time I turn the amp(s) on and play music, there is crackly distortion for about a minute, both channels impacted. This is not caused by pots being turned, and it goes away I guess once something warms up. But if I play music for 20 minutes so everything is warm, turn it off and then turn back on straight away, the same distortion is there for a minute. I've replaced all the electrolytics plus a couple of transistors, but wondering what that behaviour could be a symptom of?
2. The left channel for the "A" speakers doesn't work. The LED power display comes on, showing a signal for both channels, and the "B" speakers work fine. As far as I can see, there is one relay for A and another relay for B, so on that basis I guess there is some sort of fault on the actual speaker terminals or the board they mount on. Is that correct?
I've answered my own questions.
The first issue was the pre-amp, seems there is a relay that was commonly used back then and which commonly causes trouble like this. It's a Fujitsu 221D024, I can't easily find a replacement so will try cleaning it and see what happens.
The second issue was on the speaker terminal board, which is double sided. Dry joint on the component side, was actually a bit tricky to fix, but works now.
I now have another problem with the amp cutting out when hot - just for a second or two, then comes back, but it is not an issue at low volumes. Not quite sure how to troubleshoot that and haven't isolated it to the pre-amp or power amp yet.
The first issue was the pre-amp, seems there is a relay that was commonly used back then and which commonly causes trouble like this. It's a Fujitsu 221D024, I can't easily find a replacement so will try cleaning it and see what happens.
The second issue was on the speaker terminal board, which is double sided. Dry joint on the component side, was actually a bit tricky to fix, but works now.
I now have another problem with the amp cutting out when hot - just for a second or two, then comes back, but it is not an issue at low volumes. Not quite sure how to troubleshoot that and haven't isolated it to the pre-amp or power amp yet.
Ok, so some more testing done, and it's the power amp that is cutting out. When i turn the volume up, after just a few minutes, the relays will starting clicking off for a few seconds, then back on. If I turn the volume down it will happily play with no issues.
Hard to be certain, but it also sounds slightly distorted. I've set the bias to a total of 24mV across both test points, there was conflicting info in these threads, wondering if that could be part of the issue?
Hard to be certain, but it also sounds slightly distorted. I've set the bias to a total of 24mV across both test points, there was conflicting info in these threads, wondering if that could be part of the issue?
From the web, I've attached a service manual for benefit of members.
This design has flat gain to DC, so it will be sensitive to offset trim. I surmise the problem channel is more erratic compared to the better channel? Do you have a scope to better observe fast variations? You observe the 70mV output offset with no input to the PA?
Good luck!
This design has flat gain to DC, so it will be sensitive to offset trim. I surmise the problem channel is more erratic compared to the better channel? Do you have a scope to better observe fast variations? You observe the 70mV output offset with no input to the PA?
Good luck!
Yes. I set the offset to correct values a while back and then went chasing some issues with the pre-amp. With that solved, the only remaining problem is that both channels will regularly cut out for a second or two at high volumes.
At low volume it seems ok. The relays are switching in and out so I assumed it is something to do with the protection circuit and had a closer look at the offset.
I only have a multimeter, but on one channel it is a lot less stable and moves up and down over time. The trimpot is also very sensitive, moving it by 5 degrees can make the offset change by 200mV.
So I will replace the trimpot, but wondering if there is anything else I should be looking at.
At low volume it seems ok. The relays are switching in and out so I assumed it is something to do with the protection circuit and had a closer look at the offset.
I only have a multimeter, but on one channel it is a lot less stable and moves up and down over time. The trimpot is also very sensitive, moving it by 5 degrees can make the offset change by 200mV.
So I will replace the trimpot, but wondering if there is anything else I should be looking at.
From the schematic, the amp seems to have a DC gain of about 18.4 V/V. I estimate the the offset pot should produce about +/- 72mV at full rotations at the wiper terminal. In turn, this should yield about 2.65V peak-to-peak adjustment range observed at amp output. I would think that 200mV adjustment would correspond to roughly 8% of the adjustment range of the trimmer. Are the 20V Zeners (D402, D404) regulating properly? Do both channels behave similarly?
Another interesting clue would look at DC across R406. It will reveal net base bias current drawn by Q402 and Q406. Ideally it will be 0mV. Compare with the other channel for insight. If adjacent range is badly centered, the base currents of Q402 and Q406 may be badly mismatched. Let us know what you observe in each channel.
Another interesting clue would look at DC across R406. It will reveal net base bias current drawn by Q402 and Q406. Ideally it will be 0mV. Compare with the other channel for insight. If adjacent range is badly centered, the base currents of Q402 and Q406 may be badly mismatched. Let us know what you observe in each channel.
