technics amp SU-Z200 low volume problem
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From: b on 18 May 2010 18:04 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.
From: Franc Zabkar on 20 May 2010 01:07 On Wed, 19 May 2010 13:44:20 -0700 (PDT), b <reverend_rogers(a)yahoo.com> put finger to keyboard and composed: >It's looking as if the IC is shot, and it seesm to be NLA..... http://www.bdent.com/search/part.jsp?partnum=STK4152II - Franc Zabkar -- Please remove one 'i' from my address when replying by email.
From: William R. Walsh on 20 May 2010 09:40 Hi! > 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. http://www.youtube.com/watch?v=K0cObJ7o8vE (SA-929) http://www.youtube.com/watch?v=sgdTnBU0_N0 (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): http://john.ccac.rwth-aachen.de:8000/misc/rr100rep/ I've never tried it, nor is that my page. William
From: b on 21 May 2010 06:28 On 20 mayo, 15:40, "William R. Walsh" <wm_wa...(a)hotmail.com> wrote: > Hi! > > > 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 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 : http://www.datasheetcatalog.org/datasheets2/10/106635_1.pdf 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..... -B
From: William R. Walsh on 21 May 2010 09:45
Hi! > 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. 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. > 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. 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? > Might this work as a drop in replacement? the output would only > be 15 watts, but better than nothing..... 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. If you try it, please do post back! William |