Power Amp IC Replacement on Sharp VZ-3000

[Refugee]

I still think it is oscillating due to a cracked earth plane. This is based on the assumption that an ESR meter has been applied to all capacitors and that a scope has not been used to check for oscillation.

[Lloyd 1985]

I've just noticed that in the photo of the reading of pin 9 your meter is set to AC Volts, not DC. It does seem worrying though that there would be 59V AC on that pin though, surely that would blow things up if there was that much of an AC voltage there? It would certainly make hum through the speakers if it didn't toast everything!

Just a thought I'd like to see what others think, if there is AC voltage present on that pin, could it be worth looking closely at the power supply, especially C701 and C702? It looks like on the circuit diagram these smooth the rectified +25 and -25V, then it goes straight to the amplifier board.

Of course, this could all be a red herring, quirk of the meter?

Regards,
Lloyd.

['LIVEWIRE?']

Do not under any circumstances connect speakers to the set. With + or - 26vDC on pins 6, 7, 10 & 11, the DC resistance of the speech coils will cause a large current(about 5 or 6 Amps) to flow. This will destroy the speaker(s) and may well also destroy the STK, although that is probably faulty already. As Steve has said in post #57, these pins should be at zero DC, or at most a few millivolts. The fusible resistor almost certainly failed due to an internal fault in the IC.

[BillyTheKid]

Originally Posted by Lloyd 1985

I've just noticed that in the photo of the reading of pin 9 your meter is set to AC Volts, not DC. It does seem worrying though that there would be 59V AC on that pin though, surely that would blow things up if there was that much of an AC voltage there? It would certainly make hum through the speakers if it didn't toast everything!

Oh crap! it is on AC! I'll double check it all again tonight just to be sure. I now wish I'd spent a little more on my DMM. I'm going to add some actual labels to it tonight so I don't mix it up again.

I plugged into R417 with my ESR and it thinks it's a capacitor...

However, it does say C=88.7 which is not far off 100?? My DMM displays 1 when I plug it into R417, is it banjaxed? Do I need to take it out of the circuit to test accurately? (trying to limit the amount of my soldering this thing has on it..) I guess I could just pop out one leg.

Originally Posted by LIVEWIRE?

Do not under any circumstances connect speakers to the set. With + or - 26vDC on pins 6, 7, 10 & 11

Roger that! Is R417 the fusible resistor? I guess I'll need to check every thing plugged into the PWB before putting in the new IC when it does arrive.

Originally Posted by Refugee

Looking at the state of the PCB I would be tempted to assume that it is oscillating due to a cracked ground plane between the negative of the main capacitor and pin 9.
Just try two lengths of hook-up wire between pins 3,14 and the negative terminal on the capacitor in the power supply.
A small track in the negative feedback area can also cause oscillation too.

Thanks Refugee, I alligator clipped the pins 3 and 14 to the frame of the unit to see if that reduced or impacted the hum. There was no change. Forgot that you said to plug them into the negative of the capacitor on the PSU. There are a few caps on the PSU, do you know which one I should try? I'd need to solder it down I guess.

Thanks All!

[Refugee]

The location of the negative I was thinking of is the point where the two biggest capacitors in the power supply link together one negative and the other positive with the center tap of the mains transformer. I don't have the diagram in front of me.
It ia like the "home point" of the ground plane.

[BillyTheKid]

Thanks mate, I believe I know where that is! Will give it a shot tonight!

[steve1010uk]

What i would do is to remove the STK completely, clean up the board around the desoldered pins then check all safety resistors etc with the chip out of circuit,
then power up and check the voltages on pins 9 and 5 you should have approx the same voltage on both but pin 9 will be positive with respect to chassis and pin 5 will be negative.

Then fit the new chip but do not yet connect the loudspeakers, switch on and check the voltages at pins 7 6 10 11 they should all be pretty much zero, if so connect the speakers and if you are lucky all should be well,

Steve

[Clydeuk]

I wouldn't use the ESR meter on R417. 88.7uF is not a resistance reading, you're more than likely reading a capacitor that's connected in the circuit.

I can't see the dial of your DMM, but you need to choose one of the resistance settings to read the value of R417. When you say your DMM is displaying 1, is this the same as you get when nothing is connected to the probes, in other words infinity? That would indicate the resistor is open-circuit, though until we're sure what range you have you DMM set to it could just be an error.

Post a picture of your DMM showing the scale and we can advise further.

Have a visual check of R417 as well. If it looks burnt it's certainly going to be open-circuit and would also indicate a fault with the STK.

[Diabolical Artificer]

Some guidance on using DMM's might not go a miss. They can be confusing and need some use to get familiar with em. Forgive me if you know all this, but here goes .... to measure DC (batteries, power supplies, across a resistor to find current) make sure your meter is set to DC, the symbol for DC has two line's - one whole dash with another with gaps in underneath. AC is shown by a wavey line like this ~ .

To measure resistance, (symbol the greek omega) you have to set your meter to the right range. Your meter has I think, 200,2000,20,000 ohm's scale etc, this means on the 2000 setting, your meter will read UP TO 2000 ohms or 2k ohms. What this means is you have have an idea of the value of the resistance you're going to measure, which means you have to read the resistor, see here - http://www.allaboutcircuits.com/text...r-color-codes/ and here - http://www.digikey.com/en/resources/...or-code-4-band . You would have been better getting an autorange meter - see here - https://www.vintage-radio.net/forum/...d.php?t=133000 for some meter recommendations.

If your reading a short circuit (SC for short) [ touch probes together], your meter should change from OL (overload) to 0.00 M/K, or whatever . If you're reading an open circuit it will stay on OL. Set your meter to the highest range - 2000k, put your left hand on the desk or whatever, push the probes into your hand about an inch/2cm apart, what do you get? Should be around 2M (M = mega ohm's ie 1,000,000 ohm) upwards.

One other thing you'll need to get your head round is Ohm's law, this helps you understand what is going on inside all those weird looking devices.

When you measure a resistor on a PCB, your meter won't just read the resistor itself, it will pick up all the other components in the vicinity. So you have to be quick, to take a resistance reading, make sure the thing your testing is OFF, switch your meter to DC volts, put probes each side of the resistor. If you get a reading higher than 0.00v, stop and wait for a while (capacitor's in the circuit are still charged) When you get 0v or even 0.07v say, set meter to ohm's, probes on either side of resistor, quick, take a reading, doesn't have to be bang on. If you see the meter reading going up like a clock, you're reading a capacitor charging up.

Using your ESR meter to measure a resistor in circuit is pointless. ESR means equivalent series resistance. This means that all caps have resistance as well as capacitance in SERIES . See - http://www.allaboutcircuits.com/text...llel-circuits/

You'd really benefit from someone near you showing you the ropes a bit, failing that you'll have to struggle a bit and figure a lot of it out yourself. Your head will hurt. You're doing ok, you have the curiosity and are willing to learn, don't worry if you make a mistake, it's cool, we all do, no matter how long we've been into electronics.

Lastly, the OP's meter looks a cheaper version, which means that it shouldn't read frequencies higher than 5kHz say or be a true RMS type, it's bound to give odd readings.

Andy.

[BillyTheKid]

Hi Guys,
Thanks for the advice Andy, The learning curve is steeper than I thought with this stuff(thought I'd just have to swap out a few belts lol)
I've removed the resistor in question(R417) and the readings seem a bit off... It's 100ohm in the service book bit my reads are no where near that:

I'm getting 256 on my DMM and the same on my ESR.

The bands on the resistor seem to be Violet, black, Violet, Gold.I looked up online which says 700ohms..
Here's a zoomed in pic


Do you guys agree with my readings? Should I bang in a 100ohm resistor as per the manual?

On another note, the new IC arrived yesterday so I got to work soldering out the old IC.

With everything removed I checked the pins and all were 0 except for the 9th pin which was reading 60.7 on the AC setting of the digital multimeter.

I installed the new IC and soldered it in OK.
The results are as follows:
-8.0
-11.6
0
-28
-28.5
-27.6
-27.6
-8.1
28.5
-27.4
-27.4
-28.5
-27.9
0
-11.4
-8.0

The results are pretty much the same.. I very quickly plugged in the speaker and the hum is still there. It seems a bit smoother bit still a bad hum.

So I guess the issue is outside the IC.

I'm gonna go back and re check every capacitor, transistor and resistor in the audio pwb and the power supply. Will also try Refugee's suggestions.

If I can be 100% sure that the power supply is good I think my next move will be to replace the guts with a modern power amp.

[Clydeuk]

Ok. Your resistor should be brown - black - brown, which equals 100 ohms. Sometimes the colours can look slightly different. Your meter is set to the 2000k setting which is wrong. Your meter confirms this is reading 256K ohms which is much too high.

The resistor needs to be replaced with 100 ohms and it should be a fusible type. If you replace with a normal 100 ohm resistor and a fault develops it could overheat and catch fire! Fusible resistors are designed to fail safely in the event of a fault.

You can use a normal resistor just for test purposes though. Worth a try to see if it solves your problem.

[Clydeuk]

By the way, violet, black, violet would equate to 700M ohms which to my knowledge is not a value that exists. No DMM I've ever had has been able to read beyond about 10M ohms.

[Lloyd 1985]

That resistor is toast! Change it then re-test those voltages. I'm still worried about that 60V AC on pin 9, maybe check the 2 caps in the power supply, and that the wiring to it is correct.

Regards,
Lloyd.

[Clydeuk]

I've got a funny feeling the AC voltage reading on pin 9 is a quirk of the DMM. I have a similar meter that someone gave me. Despite being cheap it's fairly accurate but not immune to this kind of problem. It would be worth testing it on a known source of pure DC, say a PP3 or even a car battery, switched to AC and see if it's giving any reading.

I get the feeling that replacing that 100 ohm resistor will solve the problem with this unit. I had a very similar situation on a Pioneer amplifier and replaced the STK only to find it was the same as before, in the case of the Pioneer there was no output due to the output relays not being energised, when I briefly forced a relay closed there was a loud hum though the speaker, the fault was due to an open-circuit fusible resistor.

That resistor doesn't appear to have blown violently. It is possible that the original STK might be perfectly good, though it's not worth removing the new one and refitting it now that it's been replaced.

[Lloyd 1985]

Hmm, I hope it is just a quirk! I must say I have experienced a fusible resistor giving up for no apparent reason in one of my Sony FH-7's, I replaced it and the thing has been working fine since, with no other interventions. I didn't replace it with the correct fusible type either! Must do something about that...

Regards,
Lloyd.

[BillyTheKid]

I dropped in a 100ohm resistor just for testing purposes and unfortunately I'm getting the same results.

I was looking again at the capacitor specs and find it very odd that c708 is 1000mfd while the service manual has 2200mfd.
Could whoever put in that odd resistor have also put in a random capacitor?? Would it be worth trying a 2200 in there?

I have looked at some photos of other vz3000s on Google and there is no difference in size between the c708 and the capacitor it sits next to. Where as mine is much smaller...
What do you guys think?

One other strange thing I notice with this machine is that if I rub my finger along any metal part of the body i feel a vibration when it's plugged into the mains. Is that normal? I had the same with an old laptop an my phone when plugged in to charge.

[Lloyd 1985]

I wouldn't think that capacitor has anything to do with the hum problem, it does seem odd that it is a different value, but looking at it, it looks 'factory fitted', so may be a later mod, or there is another 1000uf further along in the circuit. If any caps were to be responsible for the hum, I'd say C701 and C702.

One other thing that might be worth trying, disconnect CNP401, the audio input to the amplifier board, just in case the hum is being generated elsewhere, it probably isn't though.

Regards,
Lloyd.

[Techman]

4th page and 78 posts!

Isn't this just a simple power supply fault?

Or am I missing something and I haven't read the thread properly (not unknown)?

[Diabolical Artificer]

Hard to tell without being there, is it a 120 hz hum as from iffy smoothing caps or hum from a ground issue? But it would be a good idea to go to first principles and make sure it's ok first. Looking at the schematic there are two 4700u caps , C701,C702. Take a reading with meter on DC volts black probe to ground (you'll need to find a good ground, the -/negative of C701) red probe first to C701 + then C702, then repeat with meter on mV or volts AC. It would be better if this could be done with the rest of the circuit unconnected, but I think the PSU circuitry is part of the main amp PCB.

BTW, Billy, you say when you touch the chassis you can feel vibration, are you sure it's not more of a tingling? Could it be the chassis is slightly live?

Andy.

[Grubhead]

Originally Posted by BillyTheKid

I was looking again at the capacitor specs and find it very odd that c708 is 1000mfd while the service manual has 2200mfd.
Could whoever put in that odd resistor have also put in a random capacitor?? Would it be worth trying a 2200 in there?
I have looked at some photos of other vz3000s on Google and there is no difference in size between the c708 and the capacitor it sits next to. Where as mine is much smaller...

That Capacitor reading on the ESR shows 0.69. But I have a guide for ESR readings on Capacitors for 1000UF which shows for all voltage ratings a fraction of that. For example a 35 volt should have around 0.07.
The guide says that any capacitor with more than double the figure should be looked on as suspicious!
Mind you I am a bit new to ESR readings, just got one of those testers myself after reading this thread. So I will stand corrected if wrong!