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Dynaco SCA-80Q distortion on one side

Ron Joiner

Ron Joiner

New Member
I have had this amp a while and decided to eliminate the hum by replacing the C9 and the C11, which I have completed. All the voltages are within specs. No more hum, great!

However, I am getting distortion on one side which may have been there before I started the upgrade (perhaps I didn't notice) but it is certainly pronounced though not so much at low volume. I have switched the speakers around to eliminate those as the problem but the distortion is definitely on one side and manifests itself as a very distorted bass sound at higher volume The other side sounds great.

I am now wondering if it is a problem in the pre-amp stage, the balance pot which is very stiff or the C7s which I have not replaced. I have not done anything with the pots yet but I have some DeOxit on order.

P.S. I am not that tech savvy!

Ron
 
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Start with cleaning the pots and switches. Also check all the connections for cold solder joints. Many of these were built from kits, and sometimes not quite as well built as the factory built units.

Beyond that pretty tough to advise on what could be causing your distortion. At 40+ years old, all the electrolytics are past their useful life. But you could replace them all and still have a distortion problem. Best to troubleshoot the problem first before large scale cap replacement, but hard to do without test equipment.
 
Do the output transistors on both channels stay cool when the amp is idling, or do the ones on the bad channel heat up? The SCA-80Q's power amp section is biased in Class B, so the outputs should stay cool at idle because they aren't carrying any current. I worked on one SCA-80Q that had distortion on one channel and found that the output transistors on that channel got warm even when the amp wasn't passing signal. That channel was oscillating in the ultrasonic range because one of the electrolytic signal caps on the driver board had failed open. I replaced all four (two per channel) and the heat and distortion problems were cured.
 
Dr Tinear said:
Do the output transistors on both channels stay cool when the amp is idling, or do the ones on the bad channel heat up? The SCA-80Q's power amp section is biased in Class B, so the outputs should stay cool at idle because they aren't carrying any current. I worked on one SCA-80Q that had distortion on one channel and found that the output transistors on that channel got warm even when the amp wasn't passing signal. That channel was oscillating in the ultrasonic range because one of the electrolytic signal caps on the driver board had failed open. I replaced all four (two per channel) and the heat and distortion problems were cured.
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Thanks TPettanti Cleaning the pots is the most cost effective solution.and one that I will try once I get some DeOxit.

As for the output transistors I will look into that. Assuming that is the case are they obtainable, what is the cost of replacement and how much of a job is it to get in there to effect the repair?
 
I have no effective way of measuring the heat output of the transistors beyond putting my hand over them / touching the heat sinks but there is no discernible difference between the sides at idle.
 
Ron Joiner said:
I have no effective way of measuring the heat output of the transistors beyond putting my hand over them / touching the heat sinks but there is no discernible difference between the sides at idle.
Click to expand...

I did a not so scientific test of both sides by measuring the temperature on the top of both heat sinks at idle. One side is 76.5F and the other is 70.7F. Is that enough to make a difference?
 
Ron Joiner said:
I did a not so scientific test of both sides by measuring the temperature on the top of both heat sinks at idle. One side is 76.5F and the other is 70.7F. Is that enough to make a difference?
Click to expand...

Probably not. The one I worked on that had an oscillating power amp channel had a much greater temperature differential than that. The output devices on the good channel were close to room temperature; the ones on the bad channel nearly burned my fingers when I brushed against them.

Do you have a copy of the manual or schematic? You can isolate the problem to the preamp section or amp section by swapping the wires that run from the preamp boards to the amp's driver boards. After the swap, the left preamp board will drive the right amp channel and vice versa. If the problem changes sides, it's a preamp issue; if it stays put, it's an amp issue.
 
Thanks I will check that. The speakers caps are the original ones (C7). Could they be causing the distortion?

Ron
 
Just want to clarify the distortion issue. At low volume there is no speaker distortion but if I turn it up I get distortion on the right speaker only. Too timid to swap wires from pre-amp to power amp. Anything else I can check?
 
When you have the distortion, try working all the controls and switches and see if it changes or goes away. Sometimes it takes more that one application of cleaner.

Other than that not much else you can do. The suggestion from Dr Tinear about swapping wires between preamp and amp boards is a good way to isolate the problem. It's only two wire to swap. PC-17 #19 to PC18 #12. Cross over those two wires so the Left pre-amp goes to the Right amp board, and vice versa. But you'd still need some technical troubleshooting skills to get beyond that.

Any chance you have a local Hi-Fi repair shop that would give you an affordable quote to repair it?

I like the SCA-80Q and think they sound sweet when working correctly. I bought mine from CL for cheap with a known blown channel. Fixed the bad channel first (output transistor) and then replaced every electrolytic cap in it. The big caps had a mfg. date from 1969, so around 45+ years old.

Unfortunately if you are not doing the repairs yourself, it will be easy to tie up more money than it will ever be worth.
 
I live in such a small town that even getting one can of Deoxit is a real challenge. However you may have a point, cleaning is required. The pots are very, very stiff.

I am already a good way through replacing the caps; C9 and C11 are done. I have the C7 caps for the speakers and will do those next. Not that the speaker caps are an issue but what the hell, I have them so in they go. I will take a second look at the wire swap.

No repair shop locally but likely in a larger centre an hour from here.

I know that I may be putting in more than its worth but for sentimental reasons (I built the kit in 1974) I want to see how far I can getting without spending a fortune.

Thanks

Ron
 
In 1969 my brother and I both bought a SCA-80Q kit. Both amps tested to spec and were great sounding amps. One thing they cheapened on were switches and pots. They needed constant cleaning with contact cleaner. Try that first.
 
Deoxit did the job by cleaning up the pots but not the distortion. The distortion turned out to be a bad 35V Cap on the PC-18 (AMP) left channel. Now runs like a champ.

Ron
 
That's good news. Enjoy it for awhile, but consider replacing all the electrolytic caps on PC-17 and PC-18. I'm usually not a proponent of large scale cap replacement. But in the case of these 45 years old Dynacos, the caps are living on borrowed time. The ones that haven't failed are probably at 2X the original uF spec.

I took the easy path and ordered cap kits on that auction site.

Just do one board at a time and confirm it works before moving on to the next one.
 
Yes, I think you are correct but I think I will wait until they fail. Might even be years or if I am ambitious enough I can tackle one board at a time as you suggest.

Ron
 
run at low vol. on and off a hour or two let cool, even idle if you don't want to listen... caps can reform. I have amps of that era and old caps that came back, aside from the deox apps if nec, etc cleaning rca connectors, dust outs.. are still performing clean.
 
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