Technics SA-202 Stereo Receiver issues
 

Hello everyone, today I picked up for $20 a Technics SA-202 which has an audio problem, the left channel works fine, but the right channel blows fuses, and I'm thinking someting about the right channel of the amplifier section is shorting out somehow but not sure what.

Its a capacitive Coupled Amplifier system, so I'm wondering if the capacitor for the right channel is shorted out or something. It uses two STK Chips to power the amplifier stage and 2 6800 MFD 35V electrolytics for the capacitive coupling stage of the amplifier.

What are some of the things that can go wrong with these stereo receivers that can cause a speaker protection fuse to blow on one channel?

Any help in this issue would be appreciated.

Thanks.

Almost always the STK is shorted. You can do resistance checks to
ground & compare them to the good channel.
BEWARE ! there are lots of bootleg STK's out there & they are crap.
Get them from a trusted distributor that gets them from Sanyo or
has packaged NOS from the mfgrs that used them.

73 Zeno:smoke:
LFOD !

Ok, how do I tell which one is the right and which one is the left channel? Because the left channel is the one that's still good the right one is the one that's bad.

OK so I measured both STK Chips and they don't measure opened or shorted resistance wise, so I'm suspecting that its something else that's causing the right channel speaker protection fuse to blow on this thing.

I noticed that there was a bulging capacitor in the vacinity of one of the STK Chips, C622 which is a 220 MFD 25 VDC Electrolytic capacitor, could that cause the problem I'm having, rather than the STK Chips?

I've also ordered 2 6800 MFD 35V capacitors for the capacitive coupled circuit just in case.

Leaky caps can most assuredly cause all sorts of problems.

OK, that's what I thought, I'll see if I can get another one ordered to replace the one that's problematic in the circuit, and see where that takes us with this stereo receiver.
I downloaded the service manual and the owners manual from HiFi Engine.
I looked up the part C622 but I wasn't able to find out what exactly that part did in the amplifier circuit, but I'm pretty sure that's the cause of the blowing fuses in the Right channel speaker protector fuse block.

So I replaced C622 (I happened to have a capacitor rolling around in my parts stash for that value) but its still blowing fuses, so I'm thinking it may very well be a bad STK0029 chip after all, of course I wouldn't be too surprised if that bad capacitor that was in there wasn't responsible for taking that STK chip out in the first place.

Anyways looking on ebay I found several sellers on there that seemed like they were selling non-knockoff STK0029 chips on there for prices between $7.99 and $20 so I may check into one of those.

The nice part is that on this stereo they only utilize six of the 10 pins of the STK Chips so it will be fairly easy to get it out of there and replace it.

EDIT: Was doing some voltage checks on the STK chips and the one for the right channel had the two inner most pins that are utilized pins 3 and 8 measured 0 volts on them which I take means its shorted.

So I'll be ordering an STK0029 chip from eBay to replace the bad one, unless someone has a good reputable supplier that they've used in the past that they would like to recommend to me to use to source a new STK0029 Chip from.

Of interest
https://www.youtube.com/watch?v=EZJjqpDaSZs
He also has other great videos.

73 Zeno:smoke:
LFOD !

The leaky cap could have caused the failure of other parts.

OK So my replacement capacitors came yesterday (Sunday) to replace the original Capacitive coupling circuit capacitors in the amplifier section of this receiver and I went to try and test the leads of the STK chips to try and find the one that was the bad one so I could remove it from the circuit so I could replace the the old one with my (hopefully non-counterfeit) STK Chip I ordered online the other day which should be arriving later today in the mail.

I noticed that this receiver doesn't seem to have any sort of ground that I can connect to with the negative lead for my DMM so I can test the old STK chips to find the bad chip (whenever I try to use what I think is a good grounding surface to connect to, the DMM doesn't give me any readings of any sort on any of the STK Chips all I get are "0.00" readings on both chips which I know isn't right because I know one of those chips is still good yet, so there shouldn't be a "0.00" reading on both of them, just one of them).

So where's the grounding point in this receiver? I'm asking because it doesn't seem to have any sort of grounding point in it (the metal chassis frame that the circuit board and the outside case panels attach to don't seem to attach to anything that can provide a ground surface, and there isn't anything else metallic in the receiver that's attaches to any sort of grounding surface either).

And All of my readings I was getting was done in Volts DC.
I also tried to do some resistance readings on the STK Chips as per Zeno's advice but the readings were all over the place and wouldn't settle down on one number, the values jumped from anywhere between 1.5kOhms to 15MOhms but it wouldn't settle down on any specific value in between those figures it would just keep jumping around.
As for the Voltage values went when I could get a reading from them (when I was able to get a ground connection for my negative lead for my multimeter) the values I got for the good pins were usually around -1.0 to -2.0 mV on some pins and some pins gave me about -4.0 mV, but the bad pins gave me 0.0 V readings.

Any ideas as to what's going on here?

Post the amp part of the schematic if you can. At this
point you will go in circles.
For a ground use the shielding in the tuner section.
STK's use either one + voltage to run or both a + & -.
voltage. If it uses +&- the output will have zero VDC. If the
STK shorts it will have DC on it & either blow the fuse or
trigger off the speaker relay if it has one. Any other bulged caps
should be changed. Also change the 220mfd in the other channel as
it came from the same lot & ran under the same conditions.

BTW there must be 1,000 fix it videos for STK's on Utube. The
good ones will walk you through troubleshooting. You may even find your
unit. Be VERY happy this is not a high powered amp with discrete
components. I wouldnt touch them with a ten foot pole !

73 Zeno:smoke:
LFOD !

Ok, thanks, I'll look into it.

OK so here's what I have so far:

I replaced the 2 6800 MFD 35V coupling caps in the amplifier circuit (the capacitive coupling capacitors) and I hooked the stereo back up and turned it on and the speaker protection fuse for the right channel isn't blowing anymore but I still have no audio coming out of the right channel speaker, so I'm not sure what's going on there.

I tested the STK Chips specifically the one for the right channel and it seems to be testing normally (its not testing blown, as its putting out the same voltages and resistance readings as the left channel STK Chip which is the good one).
So now I'm kind of confused, what else would be causing the the no audio output on the right channel if its not the STK Chip being shorted?
I still plan on replacing the right channel STK Chip just in case but the symptoms the stereo is having currently is throwing me for a loop.

I would of posted a picture of the schematics for the amplifier stage for this receiver but its a part of a larger service manual that is in PDF format which I can't just get the amplifier stage schematics out of.

This schematic might be useful, but it is not high resolution:

https://elektrotanya.com/cgi-bin/dow...ics_sa-202.pdf

Looks to me as if the 6800 mf caps are in the power supply (c701 & c702) and the output from the STK chips is direct coupled to the speakers.

jr

I got it working! I replaced the old STK Chip with the new one I got in the mail today that I ordered off eBay (I managed to actually get a real STK Chip and not a knockoff one), and it works now.

Although I ended up losing a speaker in the process, I blew out one of my Realistic Minimus 1 speakers by accident when I put a 4 amp fuse into the speaker protection fuse socket rather than the 3 amp it called for which the 4 amp fuse didn't blow at all when I tried out the stereo after replacing the bulging capacitor to see if that fixed the stereo or not (which it didn't fix it).

I didn't realize I had another 3 amp fuse yet in my stash which is why I used the 4 amp fuse because I had a 2 amp fuse but I didn't think the 2 amp fuse would of worked because it was smaller than the 3 amps it called for, so then as a result the receiver passed some DC current into my speaker and blew it out.
So now the speaker doesn't play at full volume anymore and it sounds like someone's singing into a tin can when its playing, so I guess lesson learned, I could probably get a new speakers for it but it probably won't sound the same.

Theres a link to the video of the stereo working below.

https://photos.google.com/photo/AF1Q...8qR9q88s0nkeh4

Pardon the poor audio quality, i used the camera on my phone which doesn't do a very good job at reproducing audio.