My SU-8080 Technics is popping...

[flambart]

Hi, I was listening my SU-8080 Technics, at mid/low volume, when it suddenly started popping and rustling on the left channel.

The noise was really loud, and I've seen the woofer moving very slowly backward and forward. After few seconds, the protection circuitry has cut the signal, so I have to imagine that the unwanted DC current was seriously high.

I've isolated the pre-amp, and I've realized that the problem is limited to the power section, left channel.

This malfunction happened again 2-3 times, in totally unpredictable manner: after a variable time (seconds, or minutes), the noise disappears exactly as it came

Someone other encountered the same problem ? Old electrolytic caps could be the reason, and which ones ? Otherwise, any alternative idea about?

Thank you to everyone in advance , Paolo

 

[ostripper]

Looks like a good old discrete "masterpiece".

"popping/rustling" is usually a semi with a bad junction. The large devices (on the heatsinks) would just quickly blow the fuse . This leaves the small devices in the voltage stage. Before you get the noise , get a can of freeze spray ready ... spray 1 device at a time until the noise stops ... that is the most likely one. Bad Caps will "bulge" and hum will increase , offset will change , or they will just smoke/explode. The random nature of the problem sounds like a small signal device. (had several I repaired do this )

OS

 

[flambart]

Thanx so much for your kind reply.

The strange thing: the last time the problem happened (yesterday) at the very first start, that means with amp & components totally cold.

After few seconds of pops, the amp has kept playing all the day long, with no more problems at all.

I agree with you thinking about small signal transistors: have I to replace all ?

 

[ostripper]

Clean potentiometers , and keep that can of freeze spray on top.
Intermittent's suck.

OS

 

[smartx21]

Bad solder joint?
Quality control on name brand electronics | yProxy? Blog

 

[kapwinnnng]

Clean potentiometers , and keep that can of freeze spray on top.
Intermittent's suck.

OS

How would one go about cleaning the potentiometers? Just some contact cleaner spray or is it more involved?

 

[east electronics]

all capacitors need to be replaced in such aged unit .... half of the problems might be self cured after that

 

[flambart]

Finally I've got it !

During the last wednesday, after months of total oblivion, I've given another chance to my amp...
Well, there was this damn TR301 (a 2SA798A, two PNPs in one 5 pin case) very sensitive to the freeze spray.
I've replaced it, using 2 twin 2SA798 transistors (the original code is not available anymore), and everything sounds nice now!

Thanks a lot to everyone gave me a helping hand

Merry Christmas and Happy New Year to everybody!

Ciao, Paolo

 

[xavirom]

Hi. This is repairing a Technics Su8080. Replace output transistors with MJ15003G / 4G and drives with MJE15030 / 31. It works fine, but I notice some anomalies when the amplifier goes into clipping zone, at 8 ohms, and gets worse at 4 ohms. Any suggestions?
Thank you very much to all.

 

[smartx21]

Power limiter

Could it be anything related to the power limiter? How about adjusting R350 and R351?
The service manual has all the guidance.
https://www.hifiengine.com/manual_library/technics/su-8080.shtml
You could do some careful testing with the limiter disconnected from the circuit. Just wondering.

 

[xavirom]

With or without limiter, looks the same