I've just found this technics amp in a skip, I'd say mid 80s vintage., along with the rest of the system! :-)
The amp only outputs at very low volume - and then for a short while. Also if you push it up more than 3, it cuts out and needs a power cycle. Both channels are affected. I noticed R701, 702 (470R)and D701,
702 (Bzv85c) get very hot and there is evidence of some scorching on the pcb. Might change those diodes.
It uses an STK 4152 II, pretty typical design of the period.
The headphone o/p works normally.
Any suggestions before I go on a cap change oin the o/p stage? anyone have a schematic? cheers B.
took a look at that, thanks. Unfortunately there is nothing connected to the mute pin (6) on this amp, so that ruled out a relatively easy line to pursue. I've changed most of the caps around the subcircuits of the IC to no avail. It's looking as if the IC is shot, and it seesm to be NLA.....
It's a common problem. In a lot of these 80s Technics amplifiers, the Sanyo power amplifier IC goes bad. I've come to believe that they are somewhat "failure prone". Interestingly enough, it seems that other designs using these modules were more reliable--I have some Aiwa "all in one" stereo systems and a little Pioneer receiver that use the Sanyo module. None of these run as hot as do the Technics receivers...perhaps another data point?
Despite being played hard at times, these units have never caused any trouble. Of course, most of the time I happened to come across such older Technics gear, it had already been treated badly.
There is a more sinister problem--the Sanyo module integrates speaker protection through electronic means. The idea is that if one of the final transistors breaks, this electronic protection circuit will break the path between module and speaker so that your speaker does not become exposed to a power supply rail and burn out. Unfortunately, the protection circuit is usually destroyed and the speaker burns out.
I thought all of the Technics amplifiers that used these hybrid modules relied on the electronic protection. After searching for a long time to find a nice one in working condition, I discovered an SA-929 receiver (also based on a house numbered Sanyo integrated amplifier module) that has an outboard DC protection relay. This should save the speakers if anything goes wrong.
I also found that the relay itself had pitted contacts, resulting in intermittent audio output from one speaker or another. Cleaning the relay contacts resolved this problem, but in the process I found bad solder joints that were aggravated by my opening the relay.
formatting link
(SA-929)
formatting link
(SA-929 repairs)
I have seen at least one other report that suggested bad solder on the protection relay's coil contacts was to blame for intermittent audio.
If you have enough room and another partially broken part, you might try this for a fix (different amp, similar module):
I think the reason could be the supply voltage. Seems they can take anything from 27 to 40-odd volts, which might explain the varied reliability.
I too was surprised at the lack of protection visbile in this amp. there's only a resistor separating the o/p from the speakers! ironiocally in this case, the protection seems to kick in far too early - muting the o/p even when the sound is barely audible.
i just took a look in my spres box and found an STK 4121 II which, according to a quick look at the dateasheet here :
formatting link
has the same pinout as my 4152 ii
Might this work as a drop in replacement? the output would only be 15 watts, but better than nothing.....
Hmm...that is a thought. It might also explain why these little receivers run so darn hot. Interestingly, mine seems to run hottest when idle.
Be glad it does, but please don't use it with valuable speakers or depend upon it to work correctly. All too often it...doesn't. The result is smoke from your speakers.
That is why I was so surprised that the SA-929 receiver I found had an honest to goodness relay. What motivated Technics to use one there when they didn't use one in the other models I've seen?
As they're all pin compatible, I think you can pretty much swap them at will as long as the voltage ranges are the same for each part. You probably couldn't get away with a bigger module (it would surely overtax the power supply) but a smaller one ought to work, especially if not driven hard or into inefficient speakers. The page that I linked to suggests that sometimes a larger module was used at reduced voltage and current levels.
You'd be surprised what "just" fifteen watts worth of output power will do in terms of volume output.
Just soldered in the other ic and guess what? same problem! a tricky one this, as there aren't that many things on the pcb common to both channels. will keep looking...
I wouldn't think that the replacement module would be broken in exactly the same way as the first, unless I were doing the work. (At times, my life is a comedy.) I suppose that something could be loading down the power supply or the output of the amplifier module. I'm not sure what it would be.
That's what I am beginning to suspect, as it is pretty much the only common thing left! A few days back, I did also notice that as I pushed the vol. up by the small amount I could before it muted, that the sound appeared to be a *bit* distorted (as in overloaded type distortion).... or did I imagine it?!
I've had to 'park' this one for now as other projects are piling up, but will probably return to it before long. watch this space....
Really though, it's likely the problem is just somewhere else. Replacing the amp module did gain valuable information though, so it's not a total loss. That's how I look at these situations anyway...
Same here. At the end of the day this thing cost me nothing and is worth little -I'm doing this out of curiosity, so if I learn something from it then so much the better. ISTR the replacement module was taken from something working (a samsung midi system or something), but I could be mistaken (it was at least 6 years back) so will keep an open mind on that front
Any electrolytic capacitors? Have you tried replacing them - did not see anything relating to that so far in this thread...
John :-#)#
--
(Please post followups or tech inquiries to the newsgroup)
John's Jukes Ltd. 2343 Main St., Vancouver, BC, Canada V5T 3C9
Call (604)872-5757 or Fax 872-2010 (Pinballs, Jukes, Video Games)
www.flippers.com
"Old pinballers never die, they just flip out."
I changed all the ones near the ic, except the two large filter caps in the bridge rectifier and one 50v non polar cap which I have no spare for. There are a few I haven't changed in the phono preamp and headphone amp as they don't appear to be directly related to the problem.
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.