Power Amp IC Replacement on Sharp VZ-3000

[BillyTheKid]

Originally Posted by Clydeuk

Before you put it away, here's a test. Disconnect it from the mains, then with your meter set to measure resistance, measure between pin 8 and pin 9 of the STK. You should read 100 ohms or less. If you get anything higher than that then either (a) your 100 ohm resistor R417 has failed again or (b) there is a break in the printed track from one end of R417 to pin 8 (we know there is +28v on pin 9 so that track must be ok).

You can also measure the voltage drop across R417, select DC range on the meter, power up and put a probe either side of the resistor.

There is no way you can have over 30 volts across that 100 ohm resistor without it going up in smoke. Give it a test and report back.

Thanks Clyde! Will do!

[Lloyd 1985]

I know what you mean about the way these look, I like them too for the fact they are space saving, where most record players lying flat need a fairly deep shelf to stand on. There were some made by Mitsubishi too, which someone told me were better units than the Sharp's! Also, if you are looking for just a turntable, there is a Mitsubishi that it just a vertical standing record player, no amplifier, tuner or cassette deck getting in your way! I think they only play one side though, so you have to turn over part way through, unlike the Sharp models.

Best of luck with the house move!

Regards,
Lloyd.

[Clydeuk]

The other one, are you sure you've fitted 100 ohms for R417? It should be brown, black, brown (4th band will probably be gold but isn't that important at the moment).

We really need to find out what's happening here as there should be very little difference between pins 8 and 9. 100 ohms isn't quite a piece of wire but it's not that far off!

[ben]

I wonder if the ground connection is somehow compromised and is the source of the noise/ripple/hum. For all we know it could be 'floating' /at some voltage above ground due to a broken pcb trace or lead off, etc.

Another thing which I have had cause this hum is when the voltage selector is incorrectly set or transformer incorrectly wired, but I suppose this would have shown itself in the form of heat or other damage by now.

[Refugee]

I get the impression that this set is to be used with a pair of valve mono-blocks and if that is the case then an extra connector needs to be added to divert the input to the STK blocks to the external amplifiers.
I did a flat style music centre with dead output stages by simply lifting a couple of links to clear the short from the power supply and adding two 1/4 inch jacks to the back to feed the valve amps. It worked fine and if it needs to be converted back again the board is still in there with the duff chip there for the number to be taken for a replacement to be obtained.

[BillyTheKid]

Oh MY God!!!!!

The hum is gone!

I had put in a 100 K ohm resistor!

Once I put in a 100 ohm resistor and turned it back on the pins were reading 0 where they should. so I plugged in the speaker and nothing but silence!!!

I'm Not getting any sound from the radio just yet but probably have mis plugged something!

Thanks so much guys! Will report back on my progress

[Clydeuk]

I'm glad you persevered with this one, it was worth the effort. I take it you're now getting sound from the record player as well as the silence?

You will have gained some good experience from this, and the end result has been worthwhile.

[Lloyd 1985]

That's fantastic news!!

I must say, I have done daft things like that with resistors too... At least you found it in the end! I thought there was something a bit odd that nothing really changed when you swapped out that resistor. Well done for sticking with it and not giving up!

Daft thought, If there isn't sound at all from the tuner on any waveband, then a way to see if it's doing anything at all would be to switch to FM, make sure you have a good aerial and try tuning across the band, see if the 'stereo' indicator lights when it's found a station then at least you know there is power getting to the radio circuit and it's receiving ok.

Regards,
Lloyd.

[Grubhead]

So glad the hum has gone

[Diabolical Artificer]

"I had put in a 100 K ohm resistor!" I've done that a couple of times that I can remember, the last as a sense resistor on a fixed bias valve OP stage when I should have put in a 10 ohm R! It's a good feeling when you fix something. Doesn't last, something else will go wrong. : ) Welcome to the roller coaster world of fixing electronic device's.

Well done for sticking with it, let's hope the rest works.

Andy.

[BillyTheKid]

Hi Guys,

Still no sound, but I think the audio amp and circuit are nothing to to with it. There's just no signal going in.
Here's what the pins read as now:

Both outputs 6&7 and 10&11 are zero. Power is coming in correctly.

I switch on the radio and tune it in to a strong signal worth volume up and still nothing on both channel's


I'm not getting any volts from the input cables. Maybe I could put a head phone wire on to the input and are see if it amplifiers that?

Do you guys agree that the IC and board are ok? I guess I should start another thread on debugging the no audio from what seems to be a mostly working tape and and radio section...

[Lloyd 1985]

Those voltages look a lot better

Right, at least we know the radio is receiving something (the signal meter is a nice touch!), is there also no sound on FM Mono, MW and LW? I'm going to assume you have good sound from tape and phono?

If not then you could inject audio direct into the amplifier via CNP401, that will tell you if the amplifier board is 100% working, if not then more troubleshooting!

Regards,
Lloyd.

[Grubhead]

You have sprayed the switches with Servisol? Those push button switches can cut off the sound completely. I have had many Radiogram owners tell me the thing is dead. And I went in with my Servisol and one minute later perfect sound.

[julie_m]

Both the above two posts are sound (no pun intended) advice. The voltage readings certainly look healthy enough for it to be worth risking connecting speakers; if in doubt, a 4.7 Ω, 0.5 watt resistor wired in series with each speaker will act as a fuse in the event of any potentially-damaging problem such as DC on the output or HF oscillation (but note, it will also blow if you turn the volume up too much).

If you can get sound by introducing a signal directly via CNP401, then suspect the input switching arrangements. Those sort of interlocked push buttons are a known problem. They have "self-cleaning" contacts that can go bad from disuse, requiring a good dose of contact cleaner and several cycles of switching action to restore operation.

[BillyTheKid]

Hi All,

Thanks for the suggestions! I've tried some DP 60 contact cleaner in there and no joy. All the inputs(radio, Tape and record) were working perfectly fine before I stripped it down.

I'll try a direct input on CNP401 and hopefully that will produce some sound and validate my work so far.

Back when I was messing around with a few things I did take out a transistor on the Power Supply called Q701. I forgot it was out and powered it on and quickly saw that it was out so turned it off promptly. I then put back in Q701 and think from around that time don't recall hearing any output. Maybe I poorly soldered this back in. I'll pop it out again, test and put it back in.
Here's what it says in the Service book for this transistor:




Do you think the above not working could kill the audio but still allow the rest of the functions such as tape and radio etc work?

Thanks!

[Lloyd 1985]

You could see if Q701 working by checking for 15.5V on pin 3 of CNS203, or the positive leg of C708. Also check for 25V either side of R701, it's one of those fusible resistors again. Follow the diagram carefully and make sure you have plugged all the connectors into the right sockets.

Regards,
Lloyd.

[WaveyDipole]

A while back I mentioned removing fuses F401 and F402 to protect the speakers and circuit. Forgive me for stating what might seem obvious, but if you had previously removed them, have you put them back in?

Q701 seems to supply a regulated 15.5v to the tape recorder. You might want to verify that you are getting +15.5v and -15.5v as indicated to pin 3 and pin 5 of connector CNS203 as Lloyd has already explained. However, this should not affect the power amp supply which is taken directly from the bridge rectifier.

BTW, is that +10v on pin 13 correct? A positive voltage here does not make sense although everything else seems about right.

[BillyTheKid]

Hi All Thanks for the tips!

I tried all the above and everything reads correctly. This makes me think the fault is still in the Amp section. I'm confident that the Power supply and the inputs are still good. I'm getting signal from the input cable(15V on each cable) I put signal onto the pins from a headphone cables for about a second I did hear some music but it was gone shortly after.

I'm going to desolder every single thing off the amp board and test. I hope this will highlight a problem. If not I'll swap out the amp with a new one and come back to fixing the original amp down the line once I've moved house.

I'll keep you posted!

[Grubhead]

If the power amp is good, what about the pre-amp? Does it go into an IC? I once had a Sony amp in that everytime anything went on the power amp it took out the IC, due to it being a static sensitive type, probably one of the early type.

[Clydeuk]

Ideally you need a signal injector/tracer of some kind. Something that will allow you to inject a signal at the inputs of the amplifier, and also something that can give you an output to test if the preamp stages are working. I don't have the circuit but I'm not sure why you would be measuring 15 volts at an amplifier's input? Surely this shouldn't be much more than 1 or 2 volts and would be a variable frequency depending on the input source (music), such that you won't get a meaningful reading on a multimeter. You would need a scope to measure the input signal.

I don't want you wasting money on more parts, but is it possible either the new STK is faulty or has been damaged (possibly when that capacitor went pop)? Have you still got the original one intact? Is it worth refitting it since the problem was an open resistor (not visibly blown)?