Sanyo Plus 75 Tuner/Amp Repair

[Arty30]

Hello, my name is Arthur, or Arty. This is my first post here on this forum. I have been working on repairing a broken Sanyo Plus 75 tuner amplifier from 1979. I found the main issue and got it working, it works and sounds beautiful now except for a noticeable 60Hz hum. I'm guessing it has to do with age or its previous failure but have not been able to track it down. The hum is not affected by the volume control and likely not power supply ripple as I checked it with a scope. perhaps some tips on what/where to look would help me narrow down the issue. Thanks.

The PDF of the service manual is too big to attach, this is the link I used: https://elektrotanya.com/sanyo_plus-75_receiver.pdf/download.html
hopefully it works, its not the best clarity but it has every schematic and even a parts list.

[floobydust]

Welcome to the forums!

It's a really nice Sanyo receiver, higher end it's kinda rare. I'm familiar with the STK0070's lol.
Given the age of the unit, I might tend to replace all the electrolytic capacitors because troubleshooting down to the part ends up being more time consuming. I know it's a lot of work, but some low-mid end electrolytics just dry out, especially if in hot spots near power resistors etc.  You can tell if a bunch are all low value like 100uF but measuring 60uF, it usually means other problems will show up. But the receiver might use good Nichicons.

Is the hum in both channels? 60Hz or 120Hz? Hum at 2X mains frequency and common to both channels would be the power supply such as "B+, B- TONE" (note the schematic is missing a dot for +B TONE to Q910 at R909). What was the previous repair?

You could disconnect the pre-out/main in jumper to see if the hum is in the power amp or the tone control section.
Then run a CD player or something with a volume control into the MAIN IN jacks, or just short the MAIN IN jacks out and the power amp should be quiet. You can also use headphones if you are worried about damaging speakers by putting something too loud (level) at MAIN IN.

[Arty30]

Hi, Thanks for your reply. So far I have replaced the electrolytics on the amp board, and checked a few on the PSU board, all of which read accurately or within spec. Though the issue may not be on either of those boards. an it is ridiculously tight for space so I haven't gotten to other boards yet. The hum is identical in both channels and I believe is a 60Hz hum. There seems to be two sources of hum, one very small one in the amplifier which I'm not too worried about right now, and the main elsewhere that goes away when I unplug the pre/main jumpers or short them.

I got this at a thrift store completely silent, it turned on but no sound. it was of course the relay that cuts the output from the amp not turning on. I eventually traced it down to a bad STK0070 which looked like the picture attached, obviously destroyed. so I now had both channels working but still no relay and a bit of distortion. so I just thought heck, throw in the other new STK0070 that I got, and that got me to where I am now. I have no Idea how someone melted the one chip and damaged the other. I took the broken one apart out of curiosity, pretty neat seeing a giant power transistor.

I think Ill go in steps recapping various boards, starting with the power supply since its easiest to access.

[Arty30]

I just measured the hum with an app on my phone and it is actually a 120Hz tone.

[floobydust]

Once I did see a guy short loudspeaker L+ to R+ and it blew both channels on an amp (they were arm wrestling with each other).

Some support parts can get damaged when an STK module fails.
I would measure the power supply (regulator's) output voltages: see if the +/-47VDC is correct (I doubt it survived) and the +/-25.7V is reasonable.
What are the loudspeaker output voltages at rest, they should be under 50mV.
The DC-detect for the protection relay C951 100uF 25V NP gets overvoltaged to 47V when an STK shorts. Especially if the protection relay is false-tripping, I'd replace the part.

This schematic I have for STK0040-II, STK0050-II, STK0060-II, STK0070-II, STK0080-II family, they're all the same except for bias- I show you can test most of the transistors internally using a multimeter on diode-test. Pin 0 sometimes called pin 10. People say there are counterfeit STK's out there but I have no experience with that. This is a pic of an old one I took apart.

[Alex_Baker]

I think it's kind of funny how these days transistors in processors are about 7nm, meanwhile the transistors in those STK modules are 5mm! I know they are completely different transistors , but I still find it hilarious how big they are.

[floobydust]

It's all for power handling, probably 1000x the current of a logic gate, as well as operation to 20x the voltage.
In the 80's I found the STK failures were largely bonding wires breaking off, you would do the tap test on a module and see if they didn't cut out. I must have replaced hundreds of them under warranty.

[andy2000]

I've never been a fan of STK chips amplifiers because they tend to be unreliable.  The internal connections often become intermittent.  They sound fine when they're working though. 

Since the hum goes away when you unplug the jumpers, it must be coming from the preamp.  You could try feeding the output of the preamp into another amplifier to confirm.  You could also test the power amplifier by feeding it from a low level source. 

Does the volume of the hum change when you adjust the volume?  If it doesn't, then it's coming from after the volume control. 

[Arty30]

I did replace C951 as part of recapping the amplifier board. I'm not sure what the proper delay for the relay is on power-up but it may be between 10-15 seconds in mine.

I do recall measuring a couple power rails and seeing about 48v somewhere but Ill go double check them.

Initially the channel that was completely blown up had around 1.2v on the speaker terminals, I was able to adjust it down to below 150mv but not much farther.

I have checked numerous transistors with a component tester and so far none have come up clearly failed. I haven't replaced/found any bad parts except for the power packs (and caps that weren't really bad)

something to note that I don't even know where to begin with is that there are several components (transistors/resistors) that reach flesh burning temperatures,Q907 in particular, even with a big heatsink, was measured roughly at 150 degrees Fahrenheit, along with R909, R917, R971, R972, and other smaller transistors on the PSU board, which branch off to other boards from the PSU. I measured the voltage across the resistors at around 30v, which equates to about 35ma on R909/917 and 20ma on R971/972. not sure if that is excessive.

I'll see if anything changes once I replace caps on the power supply. 

To answer andy2000, no the hum is independent of volume, so it is between the volume control and the amp. Today I looked at the ripple on the +53v rails across the main filter caps and saw a ripple of about 50mv                     

[coppercone2]

Check out Mr Carlsons lab on youtube. He has a guide to finding the hum, for tube circuits he actually found one of the best way (but not the only way he shows) is to use a field probe near the tube inputs. I can't name the video off hand but practically every one of them is relevant. if you watch for a few hours I am sure you will find it

[floobydust]

That Mr. Carlson video is terrifying, he picks up an energized choke 500VDC and moves it around in the chassis to point the flux fringe somewhere quiet. I would not do that 

My guess is to check the PSU +/-47V regulators which are common to both the tone-control section and the power amp front-end.
I would look for shorted Q901, Q904 or burnt current-limit resistors R903, R911, R910, R918 etc.
C763, C863 are 220uF only 25V so the tone-control +/-21V rail must not get too high.

For Q907 +13VDC reg, it should run hot about 91mA 0.9W so many cooked 2SD325 in the world lol. It's like planned obsolescence from Japan, NAD, Sony etc. they all have a hot TO-220 on the board.

Very rare but I had one receiver, I think they shorted a loudspeaker wire (+) to an input RCA jack and it melted a ground trace on the preamp board. It things get really baffling, then check the ground connections, I think the chassis is the central ground point.

[Arty30]

Today I measured every single labelled voltage on the power supply block schematic (wherever there is an arrow pointing to a trace with a voltage stated) they were all good except for the +47v and +25.7v points, both read about 3-5v high. secondly the Tone+ and Tone- are out of spec, instead of +/- 21v I get +25v and -17v. so it almost points to that cluster of components (Q901-903). I think Ill check those transistors but, as things have been going, I think it will be indeterminate, well see how it goes.

[Arty30]

 I just ate my words from the last post, I just tested Q901-903 and found both Q901 and Q903 are bad, Q903 is open circuit, and Q901 reports as two diodes on my tester. hopefully I'm on to something.

[coppercone2]

I wrote that late in the night, I thought for some reason you said 59, not 79, that is past mr calrsons technology focus, I thought it was a tube device for some reason, but he does go over how to do a noise trace on audio and radio circuit to find offending components, some where, I see you wrote tuner, not tube.

[Arty30]

I also found Q904 to be failed. now I just need to track some down.

[floobydust]

The Vreg transistors can be just be hard to find, I would use generic parts and stronger too. Originals are only 50V.
The tall TO-92 parts are even harder to find Q903, Q906. First pass at substitutes:

Q904 2SB514E  PNP 50V 2A 20W hFE 100-200, possible sub KSB596Y D45H11, TIP32A
Q905 2SA999E  PNP 50V 0.2A 0.3W hFE 150-300 TO-92 ECB  {did not look for any}
Q906 2SA1019F PNP 120V 50mA 0.9W hFE 100-560, SC-51 Tall TO-92 ECB, possible sub 2SA1208, KSA1281Y 2SC3939, or TO-126/TO-225 parts BD140-16, 2SB1151Y

Q901 2SD330E  NPN 50V 2A 20W hFE 100-200, possible sub KSD526Y D44H11, TIP31A
Q902 2SC2320E NPN 50V 0.2A 0.3W hFE 150-300 TO-92 ECB  {did not look for any}
Q903 2SC2375F NPN 120V 50mA 0.9W hFE 100-560, SC-51 Tall TO-92 ECB, possible sub 2SC2910, KTC3209, 2SA1533 or TO-126/TO-225 parts BD139-16, KSD1691Y

[Arty30]

yeah, Thanks for the tip, too bad I didn't check here earlier because I already went and found some direct replacements for the three bad ones.

[floobydust]

I would be extra careful fixing the Vreg, with new transistors if one rail doesn't come up it can damage a bunch of stuff.
The error amp (Q903 or Q906) shuts off the pass transistors when the output voltage is high enough.
So I would also check/replace the WZ250 zener diode 25V 1/2W, it might also be damaged when Q903 or Q906 tried to turn off a shorted pass transistor.

[Arty30]

one of those zeners was in fact bad, it split in half as I was removing it, the other one worked although a little out of range, (26.3v), I'll replace both. Is there a certain power-up procedure I could try to make sure there aren't any issues, like ramping up the mains slowly with my variac?

[Arty30]

I think I'm going to go ahead and get proper new/better components even though I already got some identical replacements, I want to do it right the first time.

[Arty30]

ITS FIXED, I finally got it working, Thank you very much for the insight and tips that led me to the final issues. Now it sounds brand new with not even a trace of hum, and voltage rails that are spot on. the power supply still gets warm but at this point that may just be a trait of the regulator circuits being used. Thanks again   ---Arty

[floobydust]

That's great it worked out. It's got some high-end features but uses a lot of parts, so I suspect that is why Sanyo backed off. I remember they made Sear's high-end LXI stereos.