Technics SA-303L amp

[barrywebb]

Hi, I have a Technics SA-303L amplifier that has lost its left channel has anyone any ideas were I should look?

Barry

[Diabolical Artificer]

Start with the obvious like check/clean switches and sockets, give it a close visual inspection, check for burnt R's, leaky caps, dry joints, etc, then check OP Q's for shorts. If no blown fuses or parts you'll need to put a sinewave in the IP, then scope the amp to see what's what. Or you can stick a signal in just before the OP section, if you get sound the OP is OK, so the pre amp section is duff.

Andy.

[Edward Huggins]

Andy had just offered a very succinct, yet 100% thorough, series of suggestions.....
The only thing is - what is the experience level of the OP?

[barrywebb]

Thanks for the reply's in still at novice level and dont have a scope i can clean the switchs and look for dry joints and solder well.
Ill have a good look over it and see what i can find.
Barry

[Goldie99]

Before digging too deep, I'd check the following:
- speaker fuses on back of the receiver.
- swap speakers (and wiring) to confirm it's a receiver issue, and not speaker / cable related.
- if those haven't helped, check the dc-offsets at the receiver speaker connections (should be ideally zero, but up to ca. 50 mV DC is fine. Volts would not be good).

Alan

[Diabolical Artificer]

"The only thing is - what is the experience level of the OP? " Good point Edward, should have taken that into account. Alan makes a good point too, re swopping the speakers over.

If you don't have a scope Barry, you can still do a lot with a DMM or analogue meter. If the speakers are ok and it is your amp, the first thing to do is check fuses and what have you, then check for short circuits, after that check voltages. If you have a good working channel, you have something to compare the non working one with.

One last thing, if you can take some good pictures of the circuit boards and post here, that would help us help you, we have a lot of experienced eagle eyed members on the forum.

Andy.

[barrywebb]

Thanks for all your support .
I have checked the speaker output and the is no left channel with speakers or headphones.
The front of the unit when i turn up the volume is showing leds only on the right channel but not left .
I have also tried putting a mini disc player though it and it only plays one channel with or without headphones.
I have also cleaned all the switches with contact cleaner but its still the same i have also looked closely for any dry joints but cant find any all looks good.
Any more help would be much appreciated.
Barry.

[BulgingCap]

It is worth noting for future faults that the very first test to do is to check the DC offset on the output connections (easy and takes 5 seconds). Second is to find some El Cheapo speakers, or load resistors, before any fiddling about is done. I have just repaired a high end amp that had burned out a Harbeth speaker; the owner had swapped the speakers and burned out the other unit too. Very expensive fault finding technique !
BC

[Diabolical Artificer]

So, left channel kaput. Check voltages, start at the power supply, then make sure the left channel has power. Some pics here would help us help you, I looked but couldn't find a schematic, so we are flying blind.

A list may help - 1) Disconnect speakers, check board, are there any fuses blown?
2) if no, check you have 30v or whatever on the big caps
3) Follow wires from big caps to amp board, do you have 30v or whatever on both channels?

Andy.

[Goldie99]

Originally Posted by Diabolical Artificer

...I looked but couldn't find a schematic, so we are flying blind.

Andy - the hifiengine.com site has the manual / schematic for the SA-303, which I assume is either the same or very similar. The 'L' may just be a different area designation, or cosmetic finish, or...?

https://www.hifiengine.com/manual_li...s/sa-303.shtml

Would still be good to check status of the left speaker fuse - the speaker, headphone, and power led indication, all come after the fuse (if it's the same as the 'base' SA-303 model).

Alan

[barrywebb]

Hi both fuses are fine ,There is no output from the left speaker and no output on the left from headphones.
There are a set of indicator lights on the front of the unit and only the right side lights up nothing on the left side.
I will test for the 30v on the big caps and get back.
Thanks so much for all the great help so far.
Barry

[barrywebb]

Hi tested both big caps for voltage with the power on and i am getting 40volts on both.
Barry

[ben]

This photo from t'internet suggest that there are STK XXXX output ICs. My money is on one having blown, especially if the supply is OK. They also handle the preamp and VU outs, which would probably explain the combined symptoms. Have seen this several times over the years with these amps. But check for open circuit resistors first, just in case.

[PJL]

From the service document it uses STK1039 for the power amp and AN7060 for preamp, and a bunch of other IC's for the phono and LED drive. The LED drive is taken from the power amp output.

The DC voltages for each pin of the IC's is provided on the circuit diagram so it should be possible to narrow down the fault. I assume the 'Speaker Circuit Protection Fuses' have been checked?

[ben]

Originally Posted by PJL

From the service document it uses STK1039 for the power amp and AN7060 for preamp, and a bunch of other IC's for the phono and LED drive. The LED drive is taken from the power amp output.

That makes things a bit more hopeful then. On later units, which I was thinking of, there was far more integration - on some early 80s Technics and Sanyo amps, there's basically one big STK module and a handful of resistors and caps, and that's your lot! I'm sure some had no protection whatsoever at the o/p.

Getting back on topic, it sounds like there's a missing voltage somewhere. Any signs of corrosion on the PCB print? Or perhaps a fine crack if it has had a knock at some time.

[Diabolical Artificer]

The SA-303 from pics I saw is an early tuner Alan, the pic Ben linked to is a tuner amp, how about some pics Barry so we know what it is were looking at? Regarding the L, I got bit by a Marantz amp, a 10 something A or other, it was nothing like an earlier similarly numbered amp.

Good, we have have power, no fuses blown so hopefully no destructive shorts, what to do next is tricky till we know for certain it has amp chips, but as mentioned earlier in my replys, basically you have a preamp and output section, could be either.

Andy.

[Goldie99]

There was a SA-303L sold on eBay a couple of months ago (the pics are still available on eBay if you search sold items). It's definitely a tuner-amp, and it looks identical (externally) to the SA-303 service manual images as far as I can see.

Would be good to know if it's actually that unit we're talking about - it had some significant damage to some of the controls that could be relevant here.

Alan

[vidjoman]

I think the L refers to having Long Wave radio as well as MW & FM.

[wd40addict]

At least some STK modules have no output protection. I once had to change one on a friend's Sanyo unit following accidental shorting of a loudspeaker output. Afterwards I fitted fuses inline with the loudspeaker outputs to prevent a recurrence.

I also once met one in another friend's Technics amp which had burnt out the rather nice AR speakers it was connected to due to massive DC offset. It also destroyed my own headphones before I realised what was happening

From this you may have gathered I'm no great lover of STK modules and amps using them would fall under my do not resuscitate list.

[boxdoctor]

Vidjoman (#18) is correct in his assumption. That unit has a longwave band facility.
I would suggest to the O.P. that he has a really good look at the soldered joints on IC603. They may appear Ok, but dry joints on these can be easily missed. Reflow them all was my normal approach, even if they looked perfect. R625 has been known to go o/c without a clear cause, and Q601/603 may fail spontaneously. If IC603 does prove to be at fault, the two transistors mentioned should be replaced, even if they test good.
Initially, the use of a lamp limiter (50 watt) may save you from blowing a new IC603 if you have to replace one, and it does not resolve the problem.
I also am no lover of the cursed "STK"s. Penny-pinching again by the manufacturers...Tony.