Technics SU8080 occasional popping while playing

[elnaldo]

* suspect of everything you did and you don't trust 100%, even pulling too hard from a wire could damage a connection.

If you just desoldered those transistors, the problem should be there. But if you shorted something else while testing the amp, it's hard to say from here. Check the PCB traces and check continuity from your new installed parts to the next point on the PCB.

Make a mental list of things you don't trust and you are not telling us, the problem is usually there.

Try to keep the unit factory original, without replacing more parts so you don't introduce more variables to the problem.

 

[Ian Ramphal]

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

OK. Just checked the voltage on the emitters. The two on the left side towards the front face both read -35.5V. The right side facing the back panel read 166 mV.

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

Two facing front panel read minus 34.5V yes volts. The two facing the rear panel read 166 mV.

 

[ivandezande]

OK. Just checked the voltage on the emitters. The two on the left side towards the front face both read -35.5V. The right side facing the back panel read 166 mV.


Two facing front panel read minus 34.5V yes volts. The two facing the rear panel read 166 mV.

Well there’s your issue. Better start looking as to why.

 

[Ian Ramphal]

Well, after checking those readings on the emitter resistors, I played around with the offset Varialbe pots and have managed to get these general readings - Minus 5mV to Plus 3mV. they fluctuate wildly, but the amp is out of protection mode. The 4 adjustment pots are very sticky and almost impossible to adjust smoothly. I've ordered replacement pots. Do you think it's safe for me to connect some speakers and check to see if changing the 798's for the 992's eliminated the crackels and pops, or should I change the pots first? The new pots won't be here for 2 weeks! Ian

 

[elnaldo]

I,d check the noise with headphones. If those trimpots failed in such way to send 35 V to the output, I wouldn't trust them.

Or, replace the pot with a fixed resistor of approx value for your tests.

 

[Ian Ramphal]

I,d check the noise with headphones. If those trimpots failed in such way to send 35 V to the output, I wouldn't trust them.

Or, replace the pot with a fixed resistor of approx value for your tests.

Pots are coming in 1 week. Turned the unit on Was out of protection) and came back in 30 minutes. The paired and hfe matched 992's are, I assume, thermally protected by a metal cover that keeps everything cozy, so no temp fluctuations should occur within that box. The originals were within that box.

I have two adjustments (actually 4) as per the Service Manual
Step 1 Amp switch set to DC
Tone switch set to "Via Tone"
All readings to be set to as close to zero as possible.

LH channel adjusted to minus 2.0 mV on Pot. # 303
Rh channel adjusted to plus 5.0 mV on Pot. # 304

This is as close as I can get the pots to relatively stabilize.

Step 2 Amp set to Low Cut
Tone switch to Via Tone

LH channel at plus 7.0 mV on Pot. # 301
RH channel set to minus 6.0 on Pot. # 302

Once again, this is as close to zero as I can manage.

The readings (all) fluctuate wildly, for example, set to plus 7 mV it will swing between plus 24 and minus 5. This is typical of all adjustments. The pots are sticky and "jumpy". I've ordered replacements as I think I should be able to get a more stable reading.

Is there anything other than poor Pots. that could be causing these fluctuations?

I'm reluctant to connect to speakers yet.
Any and all input are greatly appreciated.
Regards, Ian

 

[elnaldo]

Fluctuations are normal in many amp designs, it's the circuit correcting for the DC offset. I've seen it may times. Some amps are steady, but some are not, and keep fluctuating up and down all the time.

 

[Ian Ramphal]

Well there’s your issue. Better start looking as to why.

So, I replaced the 2SC1628's with your recommended KSC3503's after tweaking the pinouts. Plugged it in and it was again in protection. Played with the offset pots and it came back to life. Tried to get relatively stable readings and could not. Fluctuations between minus 15 and plus 15 mV's. So I left it running and came back in 4 hours. I managed to get the following very stable readings:

Amp at "DC" and Tone at "Via Tone"
Pot VR303 LH -5.0 mV
Pot VR304 RH -2.6 mV

Amp at "Low Cut" and Tone at "Via Tone"
Pot VR301 LH -7.0 mV
Pot VR 302 RH +2.4 mV

Went to bed and fired it up this morning and guess what? Almost identical stable readings! Even from cold!
But, all the pots are sticky and will be changed. Looked up the Pot Technics p/n EVTS3MA00B54 and on EBAY, found a p/n equivalent that trims Technics SL 1200 turntables (that apparently carry the same technics Pot number). I ordered a few.

I believe that once I replace the pots, I might be able to fine tune those reading close to Zero. Pots arriving in a week. Will keep all appraised. Thanks to everyone so far.
Ian

 

[ivandezande]

So, I replaced the 2SC1628's with your recommended KSC3503's after tweaking the pinouts. Plugged it in and it was again in protection. Played with the offset pots and it came back to life. Tried to get relatively stable readings and could not. Fluctuations between minus 15 and plus 15 mV's. So I left it running and came back in 4 hours. I managed to get the following very stable readings:

Amp at "DC" and Tone at "Via Tone"
Pot VR303 LH -5.0 mV
Pot VR304 RH -2.6 mV

Amp at "Low Cut" and Tone at "Via Tone"
Pot VR301 LH -7.0 mV
Pot VR 302 RH +2.4 mV

Went to bed and fired it up this morning and guess what? Almost identical stable readings! Even from cold!
But, all the pots are sticky and will be changed. Looked up the Pot Technics p/n EVTS3MA00B54 and on EBAY, found a p/n equivalent that trims Technics SL 1200 turntables (that apparently carry the same technics Pot number). I ordered a few.

I believe that once I replace the pots, I might be able to fine tune those reading close to Zero. Pots arriving in a week. Will keep all appraised. Thanks to everyone so far.
Ian

The offset and bias trimmers? I would get new sealed ones instead or you'll just have this issue again. But good to hear.

 

[Ian Ramphal]

The offset and bias trimmers? I would get new sealed ones instead or you'll just have this issue again. But good to hear.

But, if it all works out, I will still never know what may have been the root cause.

 

[ivandezande]

But, if it all works out, I will still never know what may have been the root cause.

And life will go on.

Every unit I restore I don't even bother turning on. I replace what I know needs to be replaced and I work carefully. 95% of the time the unit is working perfectly after the fact. The other 5%, I do more troubleshooting and remember to replace the problem child next time I do that model.

 

[Jimbo40]

And life will go on.

Every unit I restore I don't even bother turning on. I replace what I know needs to be replaced and I work carefully. 95% of the time the unit is working perfectly after the fact. The other 5%, I do more troubleshooting and remember to replace the problem child next time I do that model.

That's an odd methodology.
If you don't even bother to turn it on, how do you know it's condition, or problems, or failures??
Don't you operate the unit at stages of restoration?
If you have made an error you won't know what stage it happened at or even if it was present before you started.
Almost sounds careless, and costly.

 

[ivandezande]

That's an odd methodology.
If you don't even bother to turn it on, how do you know it's condition, or problems, or failures??
Don't you operate the unit at stages of restoration?
If you have made an error you won't know what stage it happened at or even if it was present before you started.
Almost sounds careless, and costly.

Because I’m restoring it which, again, 95% percent of the time fixes everything.
Obviously if the unit has failed badly, I’ll do more digging and testing because things that shouldn’t normally die might have.
If I do run into an issue, I can figure out what section it’s in very quickly. Much better than troubleshooting just to replace something I would anyways….now that would be a waste of time.
I’ve done over 800-900 units in 5 years.

 

[Jimbo40]

Because I’m restoring it which, again, 95% percent of the time fixes everything.
Obviously if the unit has failed badly, I’ll do more digging and testing because things that shouldn’t normally die might have.
If I do run into an issue, I can figure out what section it’s in very quickly. Much better than troubleshooting just to replace something I would anyways….now that would be a waste of time.
I’ve done over 800-900 units in 5 years.

Wow, that's incredible 5 years is about 1800 days with Saturday and Sunday.
Thats 1 unit every 2 days I'm impressed.
It takes me quite a bit longer.
I have to study the schematic, voltages, and lots of putzy stuff, then put together a build list. Possibly order some stuff I don't have.
For me it's not a race to pump out as much gear as possible. When I'm done it's as good as it gets. But it's for my collection and has be be top notch.
I'm talking restoration. I don't rebuild for others.
But I have to say, if I was taking my
McIntosh mc2205
Pioneer sx1980
Yamaha mx 1000m
Soundcraftmen ma5002
Pioneer qx949 in to someone else and they told me it would only take them 2 days to whip it out.
I'd pass and move on to someone who sounds like they give a crap about there workmanship.
I really don't want someone ripping stuff out and hammering it back in.
To fix it sure, even a day or couple hours but not rebuild, restore.
To each there own.
And 1 thing I know for sure is you'll never do a Pioneer quad qx949 in 2 days. That thing has 50 transistors and 8 pounds of cap pack into a sardine can.