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Sansui R-70 Troubleshooting

Bungalow Dyl

Bungalow Dyl

New Member
Hi, I'd like to introduce myself first of all. I'm an avionics technician apprentice from Newfoundland, Canada's most easternly island province. I recently graduated college and moved west to the prairies. I have this old Sansui R-70 reciever and a pair of Audio Research AR 176A speakers.
The issue I have is that while the highs and mids sound fairly clear, the bass is very distorted and muddy. It behaves like this when the source is FM radio or my turntable, however it sounds fine through headphones. I am blaming the reciever because both speakers are behaving the same way. I opened up the recievers case and noticed one component labled ZD204 (Zener Diode?) that looked damaged. I couldn't see one of its legs for a brown stain that was on the circuit board. I've attached the best picture I could take of it.
Also, here is website that will give you a PDF file of the the R-70 reciever, however I have yet to find ZD204 written anywhere. If anybody has any experience with an issue like this or can assure me that this may be causing my issue, I would appreciate some advice. I have Neil Young's On The Beach record to listen to and I can't handle the anticipation haha.
Sincerely, a lost Newfie
http://elektrotanya.com/sansui_r30_r50_r70.pdf/download.html20160202_200118.jpg
 
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Hi - you need a copy of the service manual - try the following link, you need the 'alt' version, and you'll need to register (free).

http://www.hifiengine.com/manual_library/sansui/r-70.shtml

My fault finding skills are not good enough yet, but ZD204 is shown as a 15V zener - it's setting a -15V supply voltage (top left corner of the schematic on page 12). I can't say if it's your problem or not, but I suspect probably not - a quick voltage measurement across it should tell you.
 
That's glue on the diode, and it wouldn't normally be a problem, but this is Sansui glue which ages and becomes conductive and corrosive. In any reconditioning of vintage Sansui all this glue has to go - it can be removed with Acetone based cleaners.

However your issue sounds (unusually) like bad main PSU filter capacitors, but I wouldn't replace them without being able to measure them and be sure they were the culprit.
 
Hyperion said:
That's glue on the diode, and it wouldn't normally be a problem, but this is Sansui glue which ages and becomes conductive and corrosive. In any reconditioning of vintage Sansui all this glue has to go - it can be removed with Acetone based cleaners.

However your issue sounds (unusually) like bad main PSU filter capacitors, but I wouldn't replace them without being able to measure them and be sure they were the culprit.
Click to expand...

I'm skeptical that it's glue, this is the only component on all the circuit boards that has any of this stain.
I appreciate the input though!
 
Look at the blue capacitor next to it, it has the same stuff around the base - it's a different colour because the diode will get quite warm and has 'cooked ' the glue and turned it a darker brown. It is used to stabilise bigger components so their weight doesn't rip them out of the PCB during transit, the manufacturing workers were a bit generous with it and it gets everywhere - I'm telling you, it is glue - no question.

Do a search on AK for "Sansui Glue" - you'll soon get the idea.
 
From the schematic - the headphone signals are essentially identical to the speaker outputs, just dropped through a couple of 22R resistors - so, if your bass is undistorted through headphones, but distorted via speakers, it suggests maybe the speaker switches need a good clean ?
 
Doesn't this amp have a STK in the output stage? It's probably blown. They tend to be.

Headphones can easily be driven without it, and so can your tweeters.
 
A power amplifier built inside an IC:

Buena-calidad-IGBT-IPM-GTR-POWER-Module-STK412-150-SANYO-SANYO-HYB.jpg_350x350.jpg


BTW, the thing you see seems to be glue, overheated and turned dark. But follow the mark on the PCB, it's the same glue holding the larger capacitor.
 
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