Author Topic: Sanyo Plus 75 Amp repair follow-up (Read 2409 times)

Arty30 · « **on:** February 05, 2022, 09:15:24 pm »

Hello, Arthur here, I was on here a few months ago with an excellent example of a 1979 Sanyo Plus series Plus 75 tuner/amplifier. I fixed its faults and replaced 90% of the original electrolytic caps. I have been using it daily since then and couldn't be happier about how it sounds, I have worked on a few newer amps since and none have sounded as good. The issue I'm now facing is that it will randomly have fits if popping/static-y/rumbling sounds, does not matter if there is music playing, my phone is connected/disconnected, or what the volume is set to. the only controls that might have a slight effect is the tone controls. It does this at random times and for random lengths of time, I also don't see a correlation to whether or not it's warmed up. Other times I'll be doing my homework and it will let out one giant pop and scare me to death. I have been putting up with this for a while because I have no clue what could be causing it. I tried to monitor the preamp output with a volt meter and was able to see the needle jump up and down while it was rumbling, so I don't think it's the main amp. I can also see my speakers flexing while it does this. If anyone has any ideas of things to check I would be more than grateful.

Also, If anyone wants a service manual I found one on ElektroTanya: https://elektrotanya.com/sanyo_plus-75_receiver.pdf/download.html.

Thanks.

SilverSolder · « **Reply #1 on:** February 05, 2022, 11:29:36 pm »

Can you provoke the behaviour by knocking the unit? I'm thinking mechanical issues like relay contacts, connectors, etc.

Does the rumbling happen on both channels, or just one of them?

Arty30 · « **Reply #2 on:** February 06, 2022, 01:59:55 am »

It's equal in both channels, I have also tried percussive maintenance with no effect.

NiHaoMike · « **Reply #3 on:** February 06, 2022, 02:07:06 am »

Quote from: Arty30 on February 05, 2022, 09:15:24 pm

I tried to monitor the preamp output with a volt meter and was able to see the needle jump up and down while it was rumbling, so I don't think it's the main amp.

Just replace that with a dual opamp chip?

SilverSolder · « **Reply #4 on:** February 06, 2022, 02:45:17 am »

Hmm, equal in both channels points to something in common between both of them... Power supply? Selector switches? Pots?

Does it matter which input you are using (phono, tape, tuner, monitor, etc.)?

Arty30 · « **Reply #5 on:** February 06, 2022, 03:18:54 am »

power supply and tone section are my main suspects, I did go through the pots and switches with contact cleaner. It doesn't seem mechanical, although I suppose it could be possible for a bad switch to cause issues even while not being moved?, however changing the switches had no noticeable effect, including the mode select (AM FM AUX Phono). when it does it again I'll verify all these ideas. In the meantime I posted an unlisted video to YouTube of the noises, here is the link: https://youtu.be/fuVzlWgpukI.

SilverSolder · « **Reply #6 on:** February 06, 2022, 03:33:47 am »

Bizarre!

Another possibility is a grounding issue... that could also be common to both channels.

SpecialK · « **Reply #7 on:** February 06, 2022, 03:41:39 am »

Audiokarma has a list called Top Ten Worst Transistors. Google this.

OS02 & Q602 in the power amp are 2SA798 which are 5-pin dual package (NPN & PNP attached in the same package for thermal tracking) and reputed to be unreliable. I would hit them alternately with freeze spray and heat and see if they go noisy.

Arty30 · « **Reply #8 on:** February 06, 2022, 05:04:55 pm »

Ok that sounds like a good idea, I did a bit of research and it seems that the best way to go would be a matched pair of transistors like KSA922s thermally coupled. I did see a couple of forum threads describing amps with popping sounds caused by the 2SA798, basically they just said to replace them. also, I only found the 2SA798 to be a dual PNP, is this right?

floobydust · « **Reply #9 on:** February 06, 2022, 05:14:42 pm »

Because the problem is in both channels, I'd go with any circuits with their components common to both.
But first confirm if it's the pre-amp/tone controls or the power amps acting up. The power amps L+R have no common circuits, but STK modules are known to have bad internal connections with the bonding wires. Nothing wrong with tapping (plastic handle end) a screwdriver on the heatsink to flush that out. But for both to fail seems odd.

The pre-amp/tone controls are post-volume control and share common +/-21.7V regulated power so I would monitor those two rails to see if they are causing the problem. Other than that, I did not see common circuitry or feeds for both channels.
Careful your speakers do not get damaged in all this, I would use the headphone jack, or pull the main in/pre-out jumpers to split the two sections.

Arty30 · « **Reply #10 on:** February 06, 2022, 07:36:20 pm »

good point thanks, I have replaced both STK modules since that was the original failure, I believe the speaker output terminals were shorted at one point which is what melted the bond wires of the original STKs, and cooked a couple power supply transistors and diodes. on a similar note I have checked and replaced said power supply transistors, but I may still try to monitor those power rails while it's acting up. I hope the protection circuit is working properly so I won't risk blowing out my JBL P20s which I really like.

SilverSolder · « **Reply #11 on:** February 07, 2022, 02:19:05 pm »

I'm not sure if you have a scope or not... if not, maybe jury rig a headphone or small speaker with a DC blocking capacitor, so you can "listen" to the power rails?

Or, with a small external audio amplifier (and DC blocking cap/resistor to ground on the input), you can "stethoscope" pretty much the whole circuit to find where the noise originates...

Arty30 · « **Reply #12 on:** February 07, 2022, 07:34:11 pm »

I have a scope that I can use, the only roadblock is the nature of the issue, it's very intermittent and random. so I need something that I can have in the background while I'm using the unit normally and hope that it starts making noise that I can see in whatever I'm measuring. That's why I was thinking of hooking a voltmeter up to the 21.7v rail for the tone controls, that way I can just run a wire out of the case and set the meter on top while it's being used. sometimes I can go almost a week before it has an episode of noises, other times I just have to shut it off because it's constant loud rumbling. thanks for those tricks though I'll have to try them out sometime.

Arty30 · « **Reply #13 on:** February 10, 2022, 09:13:00 pm »

The thing had a rumbling fit today, I had an analog volt meter on the +/- 21.7V tone rails and saw no movement at all, it was dead solid at the right voltage. so either there is none or my meter can't detect it. It happened after it was fully warmed up if that means anything

floobydust · « **Reply #14 on:** February 11, 2022, 12:23:05 am »

Again, looking at what is common to both channels:
looks like the pre-amp +B TONE, -B TONE rails are ok but an analog VOM is a bit slow and hard to see smaller changes.
What about the power amp driver stage +46.9V, -47.4V regulators? They get damaged by a shorted STK although the resistors smoke R903/R0910, R916/R911.
I'd check those rails and resistors next. There's nothing left common to both channels other than ground and soldering connections that I can see.
If temperature aggravates things, then I would heat it up with hot air or a hair dryer etc to get it acting up.
See if the tone controls affect the noise (don't crank up bass though) and switches etc.

Arty30 · « **Reply #15 on:** February 11, 2022, 02:32:40 am »

ok thanks, I'll probably get to that this weekend. also you may remember from my first thread of the original repairs that I replaced Q901, 903 & 904 in the power supply section as well as a Zener, clearly there was some sort of overload event and perhaps it baked those resistors as well which I didn't think to check. I recall that the Tone +/- were a couple of rails that were a bit off, only about 3-4v high, that couldn't possibly have damaged anything on the tone board could it?, I wouldn't have expected it to.

Jeff eelcr · « **Reply #16 on:** February 11, 2022, 11:14:20 am »

2SA, 2SB transistors are PNP 2SC, 2SD transistors are NPN, A and C are hi freq B and D are low.
Jeff

SilverSolder · « **Reply #17 on:** February 11, 2022, 11:23:51 pm »

You could leave the scope probing some point of interest (supply rail etc.), at a very slow sweep speed (like several minutes).

Then when you hear the noise, you can look at the scope, it won't have lost the "history". (The trace will get "fat" where the noise is). Assuming the scope can do really slow sweeps, probably has to be digital.

Jeff eelcr · « **Reply #18 on:** February 12, 2022, 08:35:20 pm »

Transistors, diodes even capacitors can cause noise problems. A scope is best to locate the problem start at the input side of the amplifier (input capacitor) and track it from there.
Jeff

Arty30 · « **Reply #19 on:** February 14, 2022, 07:13:38 pm »

I didn't get to much this weekend, but I probed around the power supply, verifying voltages and resistor values. I also heated and cooled parts to see if it made any difference on the scope I was measuring with, it was inconclusive. The only thing I really found and fixed was a cracked solder joint on one of the resistors in series with the +47v rail to the amplifier. It would make some sense for the culprit since it is after all of the controls and wouldn't be seen in the tone rails. I wont keep my hopes up but it has not had any issues yet. then again, it can be like 3 days between outbursts so we'll see.

Arty30 · « **Reply #20 on:** February 17, 2022, 08:28:29 pm »

so far so good, its very likely that it would have made the noise by now. I don't know whether to be frustrated that such a small thing took this long to figure out or glad that the fix was so simple nad not a rabbit hole like I was expecting.

SilverSolder · « **Reply #21 on:** February 17, 2022, 11:22:30 pm »

Maybe it's a winner, winner, chicken dinner?

Arty30 · « **Reply #22 on:** February 19, 2022, 07:15:11 pm »

yeah, I'm going to go ahead and say it's fixed, hooray! I'm glad it wasn't a rabbit hole like I was expecting, thanks for everyone's help.


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Author Topic: Sanyo Plus 75 Amp repair follow-up (Read 2409 times)

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