Troubleshooting sherwood RV-6010R surround reciever (PSU?)

[amelbye]

Hey,

I finally got the courage to open up my old Sherwood RV-6010R reciever. I know it has a few issues, but wanted a real challenge. The front speakers has gone completely silent. Checking signal continutity it seems fine until it reaches the main power transistors which doesn't seem to add any gain whatsoever.

So I started by checking voltages. My main concern is that there is a significant amount of AC on the B+ line, which supplies the big power transistors with power. Is this normal?

The DC voltages are as expected, but I also measure almost 70VAC on B+ when CNT602 is disconnected (about 104 when it's connected)

Is this normal?

capacitors C606 and C607 are missing, I guess to cut costs, should I put them in?

I tried also removing C614 and C613, and lifted the "output" legs on the positive half of the bridge rectifier.

On the negative half I get 35VAC in on each input and 0VAC out and -32VDC out, like I'd expect.

on the positive diodes I get 35VAC both before and after the diode (when only one leg in circuit) along with +16V at the output. If I turn the diode around, I get 0VAC, and the expected -16V.

but why does the diode let through AC in one direction and not the other direction? Is this normal? I thought diodes were meant to allow current in only one direction?

[amelbye]

Re: Troubleshooting sherwood RV-6010R surround reciever (PSU?)

here's the PSU schematic

[amelbye]

Re: Troubleshooting sherwood RV-6010R surround reciever (PSU?)

I've done some more measurements, I started with the power in my flat:

I have 150V between ground and either phase, and I have 239V phase to phase.

I had expected 230V phase to phase, and 115V ground to phase (as according to Norwegian standard) but I guess I'm not too far off.

The amplifier I'm trying to fix does not have a ground connection, but gets its ground point from a center tap in the transformer. I measured for any differences between chassis ground and the ground in my flat, and they're the same.

I've attatched an image of the PSU board, with measurements on it. These measurements are with everything connected and switched on.

Can please someone tell me whether it is normal to have a significant amount of AC on one output of a rectifier bridge?? Thanks

[kaboom]

Re: Troubleshooting sherwood RV-6010R surround reciever (PSU?)

You're not measuring 'AC' with CNT602 open- that's the pulsating DC straight from the rectifier. Accurate readings of B+/B- will only be obtained with the two big filter caps (C521/C522) in the circuit, as in CNT602 connected. Are you sure you're measuring across each cap seperately when you check the voltage? 100+ volts on 80V caps would make 'em explode... Sounds like you're measuring across the positive terminal on C521 to the negative on C522. Don't do this. Go + to - on C521, then + to - on C522.

C606 and C607 are to quiet the diodes. They bypass RFI and/or switching noise. You should be OK leaving them out.

Are you sure you have 'split-phase' power? Usually, you've got a single 240V winding in the dist TX, grounded at one end. Or a 416Y/240, grounded at the wye point, yielding 240V phase to ground and nearly zero phase to ground.

Your 'ground' may be floating. Measure from both sides of the recep to a copper water pipe. Also from the 'ground' to water pipe.

Check the power transistors for shorts and emitter resistors for opens.

[amelbye]

Re: Troubleshooting sherwood RV-6010R surround reciever (PSU?)

You're not measuring 'AC' with CNT602 open- that's the pulsating DC straight from the rectifier. Accurate readings of B+/B- will only be obtained with the two big filter caps (C521/C522) in the circuit, as in CNT602 connected. Are you sure you're measuring across each cap seperately when you check the voltage? 100+ volts on 80V caps would make 'em explode... Sounds like you're measuring across the positive terminal on C521 to the negative on C522. Don't do this. Go + to - on C521, then + to - on C522.

When measuring the voltage I measured from the positive terminal on C521 to ground, and from the negative on C522 to ground.

I just came across a tread describing how some cheap multimeters are unable to measure AC correctly if the waveform is not sine, and I suspect that this may be the case here. I'll check it on my scope, but won't get around to it until after the weekend I reckon...

Are you sure you have 'split-phase' power? Usually, you've got a single 240V winding in the dist TX, grounded at one end. Or a 416Y/240, grounded at the wye point, yielding 240V phase to ground and nearly zero phase to ground.

Your 'ground' may be floating. Measure from both sides of the recep to a copper water pipe. Also from the 'ground' to water pipe.

I believe we have split phase here in Norway, our electrical system is not entirely the same as most of Europe. I'll try the measurements you said anyway

Check the power transistors for shorts and emitter resistors for opens.

In the amp you mean? I believe the power transistors are shorted, as they have no gain whatsoever. I can trace a signal from input to output, it just doesn't get amplified. I'd like to make sure they get clean power before replacing them though

[amelbye]

Re: Troubleshooting sherwood RV-6010R surround reciever (PSU?)

I got my scope out this morning, and measured the B+ and B- again, and it appears that they're fine!

I get about 0.1V of ripple on them both, at about 100Hz, which makes sense.

The huge AC voltages reported by my multimeter are not there!

So my multimeter can not be used for measuring AC unless it's a pure sine wave. Lesson learnt.

--------------
I also measured by house power in relation to the kitchen tap. I got 75V from either phase to water. I was expecting a higher voltage, but then I realised the water tubes are plastic, so I guess 75V is normal. Ground potential is 0V.
--------------

I guess my next step would be to remove the power transistors from the circuit and test them. By emitter resistors, do you mean resistors connected to the emitter on the power transistors?

Sorry if I'm asking stupid questions. Electronics is purely a hobby for me, and I'm learning as I go. Thanks for your help so far

[j2000]

Re: Troubleshooting sherwood RV-6010R surround reciever (PSU?)

Thanks for the pic and PS schematic bro...
I got this Sherwood with problem , this sherwood cant power on...just "click" sound from relay, the power amplifier final (Transistor and STK) is Ok.
Still find it...now..

[amelbye]

Re: Troubleshooting sherwood RV-6010R surround reciever (PSU?)

Posted here is only a fraction of the schematic. If you want the whole thing, send me an email. amelbye (you know what to put here) gmail dottety com

I gave up on mine, by the way, when the radio board caught fire :p

[burman2k]

Re: Troubleshooting sherwood RV-6010R surround reciever (PSU?)

My A/V receiver audio output went "dead" during the summer and I now have some "holiday" time coming up to work on it. I checked the headphone and the amp works ok. The main power supply output section is also OK (+/- 70Vdc). I also checked all 8 fuses and they are all fine.

I noticed there are two Hybrid-IC amps: STK 4026-II (1x25W) and STK-4122-II (2x15W). Probably one of them must be for the headphone. I checked the AM/FM tuner and it is working fine also (where I was able to hear the stations).

Next I needed to check the pre-amps and amps and I need some help. I noticed the Pre-Amp <-> Power Amp jumpers at the back of the unit. Should I disconnect the jumpers to check each stage?

Any help you can provide is greatly appreciated.

[amelbye]

Re: Troubleshooting sherwood RV-6010R surround reciever (PSU?)

hey burman:

The two two Hybrid-IC amps are for powering the the surround and center speakers.

My memory might decieve me, but I believe the headphone amp is simply the main power section padded down by resistors.

If I'm right about this, it means that your problem is probably in the main speaker selection circuit, or in the speaker protection circuit, or possibly some part of the power section is blown.

The first thing to do is to check all voltages. I believe you'll need them all for the speaker protection/selection stuff to work.

Then, like you say, connect a signal source, remove the preamp->poweramp jumpers and see if you have signal coming out of the pre. Most likely you do, and you can at least leave the preamp alone.

Next thing would be to feed signal to the power amp, and see if it reaches the outputs of the main power transistors (the 4 big ones connected to the heatsink)

That was about as far as I got before the tuner board caught fire :p
I still have mine though, and I'd be willing to send you parts if you have a use for them.

I got your email, and I'll send you the schematics over the weekend

[Dilum]

Re: Troubleshooting sherwood RV-6010R surround reciever (PSU?)

I also need full service manual.having trouble with power on intermittant