40 year old Technics Amp repair - troubleshooting circuit

R!f@@

Joined Apr 2, 2009
9,751
Hey, some thing is wrong.
What is your DMM GND point on the PCB ?
Can you show me where you connect your DMM -ve terminal.

Hi @R!f@@
Have you ever seen anything like this before?
A few, but not if someone messed with it.
A working unit will not have ground issues unless a screw is missing or wire oxidation is there.
 
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R!f@@

Joined Apr 2, 2009
9,751
Hi @R!f@@

chassis: -19.4V
RCA GND: -19.6V
GND terminal: -19.6V <-------
J3 T2: -20.5V
J1 T4: -19.4V
J2 T2/4: -19.3V/-19.2V
J4 T2/5: -19.6V/-19.6V
What is tht point. !!! ?
GND terminal should be used as your meter -ve

I asked for 6 readings and you provided 7.
Pls explain.
 

Thread Starter

sparkyjf

Joined Feb 13, 2020
13
Hi @R!f@@

Ok I think you nailed it - let me explain the readings and what I have found.

So first of all that "7th" reading - on the rear panel of the SU-V3 is a ground terminal (next to the RCA jacks). The mains cable on the UK model is only a 2 wire cable, so live and neutral - no earth. The "ground terminal" is the external earth connection for if you want to earth the chassis of your amplifier. It's on the same part of the PCB as the RCA jacks so I thought it would be a good idea to check it.

Anyway, after your feedback last night I started tracing the PCB out. I've gotten so used to multi-layer PCB's with lots of ground plane, I've become a bit rusty with single-layer designs such as this. Anyway, it turns out that what I called "0V" (the mid-point of C601 and C602) makes it's way across the CPU through a series of wire links, and in a couple of cases even wires that are simply routed over the PCB to get 0V across to the various parts of the circuit.

I traced out the ground plane that is used for the RCA jacks, external ground terminal and 5 pin DIN plug because I was curious to see how it got back to the smoothing caps. It turns out that they ran a single track (no bigger than any signal tracks!) from the corner near the external ground connector to the metal can that forms the body of switch S3 (rec selector). They then used the metal can of S3 to get the 0V across to another part of the PCB, where it finds it's way across and back to the smoothing caps.

You can't see this trace on the copy of the service manual that I have - some of the print is rather faded but on the PCB I could see the track, and I realised that on my PCB it was discoloured. I gently wiped away the discolouration which turned out to be the green PCB coating. The coating just flaked away, revealing the copper track, and in one place the track was broken and had lifted from the PCB. Having found this, I immediately soldered a wire over the track, from one of the pins on the external ground connector to the can of S3. I haven't had time to test yet as my lunch break has just finished and I don't have speakers in my workshop, but the amp now powers on and the speaker protection relay clicks on after a short delay which is what I'd expect.

Subject to a quick test, I think you nailed it with the ground problem - I just needed to find the break. I'll update this thread later with a photo of the fix and confirmation of whether everything is working as it should, but this is a huge leap forwards!

Thank you!
 

R!f@@

Joined Apr 2, 2009
9,751
And that is why I asked you to solder a wire at filter caps 0V line.
The chassis is not reliable at times.
My very first attempt to fix amps is to check the caps 0V line has continuity connection to the chassis.
And sometimes I use the main speaker ground output as my 0V reference. This method requires tht amp is NEVER in bridge mode.

I think you should be OK now.
But do check for other oxidized traces.
After fixing use UV solder mask paint or some clear acrylic spray to cover those scraped areas.
 

Thread Starter

sparkyjf

Joined Feb 13, 2020
13
And that is why I asked you to solder a wire at filter caps 0V line.
The chassis is not reliable at times.
My very first attempt to fix amps is to check the caps 0V line has continuity connection to the chassis.
And sometimes I use the main speaker ground output as my 0V reference. This method requires tht amp is NEVER in bridge mode.

I think you should be OK now.
But do check for other oxidized traces.
After fixing use UV solder mask paint or some clear acrylic spray to cover those scraped areas.
Just to confirm it's all working perfectly again. I do need to take the time to go round it and check the other connections, but both channels are crystal clear. Photo of the fix attached - not elegant but it works.

Thank you for all your help, I've learned a lot!
 

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R!f@@

Joined Apr 2, 2009
9,751
Get your soldering iron out and link all the Grounds together with wire links...
Umm ! that could lead to an issue as some grounds are from point to point grounding to minimize signal and biasing issues.
Adding wires over existing tracks will lead to additional current and signal paths. Might lead to issues when driven to the limit of the amp

OP needs to replace or complete broken tracks as in the PCB layout. These little things are what makes certain amps what they are.
 

R!f@@

Joined Apr 2, 2009
9,751
Congratulations! Well done fellas. I knew @R!f@@ would be the man to call.
Now, where were you two years ago when we needed you to help fix an SU-VX800?
Thank you Chips. I can't take all the credit.

As for where I am, I was occupied with life.
Got a new job. I over see building of electrical panels, Gensets troubleshooting and MV transformer and RMU switch gear installation, MV cable termination and laying and all sort of stuff related to electrical installation.
We are the one dealing with Volvo Penta marine engines and khonaysser Motors. DSE programing, Electrical fault rectification in Resorts and rural islands.
They keep me well occupied. I hardly get time to sit at my PC.
In the back of my mind I always wanted to log in to AAC. But alas.
I will try to be around in the future cause at times I need help.
Besides I got good friends here.
And then there is the baby.
 
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