Technics SU8080 occasional popping while playing

[Ian Ramphal]

My SU8080 "pops" loudly every 10-20 seconds from both speakers while playing music. In fact it does this even when there is no input source selected and the vol. control is at zero. selecting "Direct" or "Via Tone" makes no difference to the situation.

I'm trying to teach myself about repairing this amp., as I have yet to find someone here that actually knows what they are up to (I certainly dont1).

I live in St. Catharines, Ontario, Canada, 30 mins from The Niagara Falls/US Border. Can anyone help? Ian

 

[ivandezande]

Both channels?
Has it gotten worse? When did it start?

Could be a leaky transistor or interference from something outside.

 

[Ian Ramphal]

Started about 5 years ago, and I just put it away then. But, I really liked it and want to get it refurbished. every control works, but that popping from both speakers really bothered me. Plus, the pops are not from both speakers at the same time, but are random left and right. Someone mentioned the double transistors (now NLA) 2SA798A as the suspects.

 

[Ian Ramphal]

What I really need is a reliable technician. I'm willing to pay for the repairs, once they actually work.

 

[ivandezande]

Started about 5 years ago, and I just put it away then. But, I really liked it and want to get it refurbished. every control works, but that popping from both speakers really bothered me. Plus, the pops are not from both speakers at the same time, but are random left and right. Someone mentioned the double transistors (now NLA) 2SA798A as the suspects.

For sure a place to start. Easily replaced with a pair of KSA992.

 

[Ian Ramphal]

I was told to use two matched Onsemi KSA992FATA's to replace the 2SA798's.

 

[Ian Ramphal]

Right, I've purchased 100 and found 2 that match exactly. I'm going to jump in and try not to kill myself. All I have is a Cap discharge pen, a cheap transistor tester and a multimeter!

 

[RVT1K]

Look for bulging and potentially leaking electrolytic caps while you're in there.
Consider going over all the solder joints again.

 

[Ian Ramphal]

Look for bulging and potentially leaking electrolytic caps while you're in there.
Consider going over all the solder joints again.

So, I paired the 992's (2 pairs) made sure there were no shorted trans pins and installed. Put my meter across the speaker terminals and got +24Mv on the right channel and 136 on the left. So I removed them and reinstalled the 798's. Basically got the same readings. So I played with the dc - offset pots on both instructions(1) the check with the amp set to DC and again (2) with it set to "lo-cut, and got both within +/- 3-4 Mv. So I was pleased with that. I followed the Service manual all except for 2 instructions -Frist to connect an 8 ohm dummy load to the speaker terminals (I don't have one) and second to short the Main Amp input pins that mate the Pre to the Main. Not sure how to do that either. So, hooked up a pair of speakers and the amp lo longer comes out of protection mode. What a piss off. I need a qualified tech!

 

[ivandezande]

So, I paired the 992's (2 pairs) made sure there were no shorted trans pins and installed. Put my meter across the speaker terminals and got +24Mv on the right channel and 136 on the left. So I removed them and reinstalled the 798's. Basically got the same readings. So I played with the dc - offset pots on both instructions(1) the check with the amp set to DC and again (2) with it set to "lo-cut, and got both within +/- 3-4 Mv. So I was pleased with that. I followed the Service manual all except for 2 instructions -Frist to connect an 8 ohm dummy load to the speaker terminals (I don't have one) and second to short the Main Amp input pins that mate the Pre to the Main. Not sure how to do that either. So, hooked up a pair of speakers and the amp lo longer comes out of protection mode. What a piss off. I need a qualified tech!

Are you sure you put in the transistors correctly? Photos help.

You don’t need a dummy load or shorted inputs.

 

[Ian Ramphal]

Are you sure you put in the transistors correctly? Photos help.

You don’t need a dummy load or shorted inputs.

I think I put them in correctly, and I was able to adjust the DC offset. Pics attached. The one with the paired units looks worse than it actually is and the soldered board also. I plan to build two more pairs and try again.

 

[ivandezande]

I think I put them in correctly, and I was able to adjust the DC offset. Pics attached. The one with the paired units looks worse than it actually is and the soldered board also. I plan to build two more pairs and try again.

I don't know what kind of solder you're using, but it looks awful. Gotta get stuff that has lead in it.
Careful with all the parts switching, you're going to destroy the solder pads.
Gotta find out what else you changed that could put it in protection. Technics can be fussy.

What I do with the KSA992's is hold them, emitters touching, with a pliers, and then solder the leads together at the very top. Snip the extra lead, then bend out the C legs to copy the pinout.

 

[Ian Ramphal]

That's exactly what I did. The back to back 992's were a real pain. I redid them both using new matched 992's just as you said. So much easier. I took out the old 798's and put in the new pairs. Same issue, does not come out of protection. I used a very small manual modeling drill from the pad side to clean the holes out, some very minor pad lifting but extended those a bit and they seem fine. Perhaps they are not. Will probe tomorrow. Point taken on the solder but that's all I can get in the boonies. All I did was replace/change those two components - nothing else - and look where I am now! Strange, when I discharge the main caps, they have a charge? Should they have a charge if the speaker protection relay does not kick in? Does not seem there is anyone within 100 miles that appears qualified to go to up here. I'm about to throw in the towel. What I don't understand is that after I adjusted the offset to almost perfect, all I did was move the unit so I could connect speakers and on power up, it was dead. I should not have been able to do those adjustments if the protection circuit was not operational?
One question, can the main power amp transistors be checked in situ? The last tech replaced one but I assumed they should be replaced as a pair. Ian - PS, thanks for your guidance this far.

 

[ivandezande]

That's exactly what I did. The back to back 992's were a real pain. I redid them both using new matched 992's just as you said. So much easier. I took out the old 798's and put in the new pairs. Same issue, does not come out of protection. I used a very small manual modeling drill from the pad side to clean the holes out, some very minor pad lifting but extended those a bit and they seem fine. Perhaps they are not. Will probe tomorrow. Point taken on the solder but that's all I can get in the boonies. All I did was replace/change those two components - nothing else - and look where I am now! Strange, when I discharge the main caps, they have a charge? Should they have a charge if the speaker protection relay does not kick in? Does not seem there is anyone within 100 miles that appears qualified to go to up here. I'm about to throw in the towel. What I don't understand is that after I adjusted the offset to almost perfect, all I did was move the unit so I could connect speakers and on power up, it was dead. I should not have been able to do those adjustments if the protection circuit was not operational?
One question, can the main power amp transistors be checked in situ? The last tech replaced one but I assumed they should be replaced as a pair. Ian - PS, thanks for your guidance this far.

There’s no bleeder resistors on those caps, no. It’s annoying.
So measuring at an emitter resistor, both channels are about zero?
Yes they can. Yes they need to be replaced in pairs.
An easy troubleshooting step would be comparing the voltages in the service manual of the protection circuit and seeing which are wrong.
Absolutely. I hope we can get this going.

There’s 2SC1628’s in here, real trouble. They’re the ones with the metal tab. Use KSC3503, be mindful of pinout.

I see four fuses. Rail fuses? Try pulling the ones to the amp and see if it comes out of protection.

 

[Ian Ramphal]

There’s no bleeder resistors on those caps, no. It’s annoying.
So measuring at an emitter resistor, both channels are about zero?
Yes they can. Yes they need to be replaced in pairs.
An easy troubleshooting step would be comparing the voltages in the service manual of the protection circuit and seeing which are wrong.
Absolutely. I hope we can get this going.

There’s 2SC1628’s in here, real trouble. They’re the ones with the metal tab. Use KSC3503, be mindful of pinout.

I see four fuses. Rail fuses? Try pulling the ones to the amp and see if it comes out of protection.

Sorry for the delay - housework called!

Ordered the solder 70/30 lead is that high enough?

Forgive my obvious ignorance, but I don't quite understand what you say in "So measuring at an emitter resistor, both channels are about zero?"

I've ordered 10 of the KSC3503's should be here an a few days.

Attached is what I did for the 992's so much simpler to work with.

There are 4 fuses beside the transformers which 2 do I pull?

Grateful for your perseverance, Ian

 

[ivandezande]

View attachment 2758869 View attachment 2758870

Sorry for the delay - housework called!

Ordered the solder 70/30 lead is that high enough?

Forgive my obvious ignorance, but I don't quite understand what you say in "So measuring at an emitter resistor, both channels are about zero?"

I've ordered 10 of the KSC3503's should be here an a few days.

Attached is what I did for the 992's so much simpler to work with.

There are 4 fuses beside the transformers which 2 do I pull?

Grateful for your perseverance, Ian

Yup that’s perfect.
so protection activates when a channel is putting out DC. is either channel putting out more than 50mV?
Never mind on the fuses. Those don’t eliminate channels they’re just for the transformers.

 

[Ian Ramphal]

Yup that’s perfect.
so protection activates when a channel is putting out DC. is either channel putting out more than 50mV?
Never mind on the fuses. Those don’t eliminate channels they’re just for the transformers.

So, when I switch it on, and the relay fails to activate, I get 0 on all speaker terminals so I would have to check somewhere before the relay, correct? Where would I check that? Ian

 

[ivandezande]

So, when I switch it on, and the relay fails to activate, I get 0 on all speaker terminals so I would have to check somewhere before the relay, correct? Where would I check that? Ian

The emitter resistors as mentioned. Or on the input side of the relay.

 

[Ian Ramphal]

The emitter resistors as mentioned. Or on the input side of the relay.

Once again forgive my ignorance, but where are the "Emitter Resistors"? Are they on the board I'm working on? So I'll be measuring for mV with my red lead, what do I use as ground, the Chassis? Ian

 

[ivandezande]

Once again forgive my ignorance, but where are the "Emitter Resistors"? Are they on the board I'm working on? So I'll be measuring for mV with my red lead, what do I use as ground, the Chassis? Ian

They’re the large white resistors on the emitters of the output transistors. Yes, chassis.