Power Amp IC Replacement on Sharp VZ-3000

[BillyTheKid]

Originally Posted by Grubhead

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!

Oh my god! I had assumed that the ESR reading was normalized. With that in mind my caps are indeed not in good shape! I really hope this is the case! Here's a look up that came with my home made ESR:

If the above is true, a lot of the caps are questionable. I'll go back and re re review everything again.

Originally Posted by Grubhead

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?

Thanks Andy, I'll give this a shot when I get home and update here. Definitely a vibration more than tingling. I think I've become a human multi meter to make up for my crappy one I'll try test it with the DMM and ground to a proper ground in my house.

Originally Posted by 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.

I ordered 105c replacements of c701&2 and will swap them out as soon as they arrive to be on the safe side. I think I already plugged out the audio in on the audio power amp and the hum was still there.



I really hope it's these capacitors! I'll be gutted if I can't get this thing to work and I think I'm at the end of the road it it's not the caps.

Thanks again for all the help!

[WaveyDipole]

To protect both speakers and integrated circuits I would be tempted to remove F401 and F402 until the reason for the incorrect voltage readings has been ascertained.

I was curious about the reading of zero volts across the speakers, but actually -27v on the output of the IC. I can only assume that the reading is zero volts when the speakers are connected because the low impedance is dragging the output down to zero volts. I was also intrigued by the reading of 56vAC on pin 9. While this might well be a quirk with the DMM, I did wonder whether ground (0v) was carrying some AC leakage, perhaps via a leaky C717 or C701? If ground were for some reason swinging from 0v to -27v, then might this explain 56vAC and 27vDC on pin 9 and as well as possibly negative voltage readings on the output and other parts of the IC as the biasing would be unbalanced? It would also cause a buzz in the speaker regardless of volume setting.

I did wonder whether the transformer centre tap (black wire) was properly connected to chasis/ground/0v. However, if there was a dry joint, corrosion orbreak somewhere, then I suspect that R707 would tend to drag ground towards -27v and throw of the DC voltage readings? Whatever the case, the leakage current appears to be tollerable for the present in that it does not appear to have caused severe damage, but it is probably worthwhile to replace both C701 and C702 to rule them out.

[Grubhead]

I have just done a rummage in my 1000uf cap box and tested a few at random. None were higher than 0.12. Several came in at 0.08 figure. So your 0.69 must be out!

To the technical guys on the forum. Just as a matter of fact how important is the Vloss measurement to a capacitor? I had one at 3.0%. Would that be critical?

[Diabolical Artificer]

Well spotted Grubhead. My ESR meter shows good, iffy, bad, I only have to get the chart out now and again.

That's good thinking too WD.

We seem to be getting somewhere, hope so.

Andy.

[BillyTheKid]

Hi All,
As I wait for the new caps to arrive, I decided to get to work removing the old caps.

Unfortunately, each cap I removed now reads perfectly healthy on the ESR meter

I guess replacing the caps is a good thing regardless. Although I'm not so sure anymore that this was the issue.

I'll take more photos of the inerds with the caps out and upload.

Cheers,
Billy

[Lloyd 1985]

I think it's always best to check caps out of circuit, to make sure there isn't any other components interfering with the results.

Maybe a good idea to check the wiring against the circuit diagram to make sure it's connected correctly and none of it's come adrift. Maybe lasso test the bridge rectifier for shorts too.

If this was on my bench I'd have it hooked up to my bench power supply to rule out the set's own power supply!

Regards,
Lloyd.

[Clydeuk]

Each to their own, but personally I would not remove any capacitors until I had replacements; then I would replace them one or two at a time. It is very easy to mess up and make a mistake, then you're into the realm of the man made fault that's very difficult for others to help with. Obviously if you can follow a circuit diagram then it helps.

We've been here before with people diving in removing all the capacitors then getting in a real mess.

Sometimes capacitor ESR readings can improve after the heat of the soldering iron has got to them. They should return to reading poor once cooled down though.

[BillyTheKid]

Hi All,

Indeed the Capacitors were giving kinder ESR readings as they were warm from the desoldering. I re-checked them this morning, C716 - 2200/25v looks very ill(Also someone has marked it with a black marker so maybe the tech had this in mind for swapping out) c702 4700/35v is also on the Very Low ESR side. So replacing these maybe will help! (I really hope so!)

I've attached a PDF of the readings I carried out this morning if anyone wants to take a peek.
VZCapacity.pdf

Will lasso test the Bridge Rectifier once I've googled it a bit.

If the above doesn't fix, I'll get to work on understanding how to read a wiring diagram and keep pluging away at the thing.

This is probably a silly question but would it be possible to feed the power through something like this:
https://www.amazon.co.uk/Yeeco-Regul...drop+down+volt
Would it clean out the bad stuff if I put it in between the Power supply and the Audio Board? Or is there another device that could clean it?

[steve1010uk]

Hi could you carefully remeasure all voltages on the power amp ic again, use meter on dc range connect negative meter probe to earth by which i mean somewhere like the junction of where the positive terminal of C410 connects to the negative terminal of C409, carefully measure each pin starting with pin 1 and let us know the voltage neg or pos on each pin,
do not connect the speakers during this test, also if you can disconnect the input to the board at CNP 401 that would be good,

[Diabolical Artificer]

ESR = Equivalant series resistance, which means to all intents and purposes the cap has a resistance in series with its capacitance. What happens is that as a cap ages or gets warm from being near a heat source, the chemical dilectric deteriorates and the ESR goes HIGHER. We're talking about resistance to AC here. A cap can also go "leaky". It can leak the dilectric which shows as brownish gunge. it can also go leaky in an electrical sense in that it no longer acts as a near perfect blocker of DC current.

A capacitor is a complex device (it has capacitance, resistance and inductance - put simply. The last two change with frequency as do electronic circuits, there's a lot going on. You have a very steep learning curve, but seem willing to learn.

Here's - https://www.youtube.com/watch?v=dqkT6hF0O3E a bloke showing a simple test for a BR, you use your diode checker on your DMM (the setting is shown by an arrow head sticking in a wall).

I wouldn't bother getting the buck boost convertor, it will only make your DC "dirtier" There is no off the shelf device that I know of that will wash your electrons and clean them. What you're hearing if the hum problem is down to your PSU, is a high AC content on top of your "clean" DC. It's called ripple. Most power supplies have it, but at a low level and amplifier designers do some clever stuff with the OP (output) circuitry that rejects the ripple - CMRR. More stuff to learn about. : )

Lloyd in an earlier post was refering to connecting to a linear variable power supply. These have a big mains tfmr (transformer) in them. See here for more info on power rectification, smoothing etc. He's describes stuff in a good way - https://www.youtube.com/watch?v=O3Cvhnhv0dM . He's mainly talking about guitar valve amplification in his videos, but the same principles apply.

Hope that helps, Andy.

[BillyTheKid]

Cheers Andy, I'll give these videos a watch. It looks like I'm going to have to become some kind of electrical expert to get this thing to work properly...

I popped in the fresh Caps and turned it back on. Still humming. Then one of the Caps exploded! I pretty sure I had the polarity correct and the cap i swapped out was 10uF 50 same as the one I took out. Anyways, I cleaned up the mess and put back the original cap as it didn't measure too bad at all on the ESR. Here's what happened to one that blew up:


@Steve, I did as you said and tested all the pins with the Inputs plugged out and no speaker. Here's how my DMM and when I grounded:

The readings came back as follows:

1. -08.0
2. -11.8
3. 0.00
4. -27.4
5. -28.3
6. -26.9
7. -27.0
8. -08.0
9. 28.3
10. -26.8
11. -26.8
12. -28.3
13. -27.2
14. 0.00
15. -11.7
16. -08.0

My new auto ranging DMM also arrived so I'll have a look to see if things are same on that and have a look to see if this one also picks up 60 DCV on the 9th pin.

Question) Do you guys reckon I could plug out/remove the wires used for the other voltage settings just to rule them out? Or would that put things out of whack?

Cheers

[Clydeuk]

Wasn't it 60 volts AC you measured on pin 9?

I still fail to see how you can have 28 volts DC on pin 9 and minus 8 volts on pin 8 when there's only a 100 ohm resistor between the 2 pins.

The exploding capacitor must have been installed with the wrong polarity. It is easily done and something I've done a few times over the years.

[Lloyd 1985]

Ouch! Bit of a mess...

That negative voltage on pin 8 is because either the replacement 100 ohm has gone O/C again, or the original part has been re-fitted.

Without actually having the thing in front of me, I'm a little stumped as to what else to suggest!

Regards,
Lloyd.

[Diabolical Artificer]

Cor, you really blew the a$%e out of that cap. We've all connected caps the wrong way, it's best to triple check your work and use a lamp limiter on start up.Still it's a lesson learnt.

At first I thought pic one was taken as it blew, as the caps either side look like they're on their way to mars. It looks like though that you've mounted em too high up, they want to be flush with the board really.

It is a tricky one to diagnose from afar, it really wants popping on a scope. I'd try putting a post in the "Repair/services wanted" section Billy, see if there's someone in London who can have a look at it for you. I think someone mentioned there's a "repair club" in London, might be worth a visit.

Failing that put it away until you learn more and then have another go, I do this as I'm still learning.

Andy.

[steve1010uk]

As Lloyd says you need to check that 100 ohm resistor between pins 9 and 10 , you have far too many - volts on all those pins and not enough +

aim for zero (or nearly zero) volts on pins 7 and 16 before connecting the speakers,
double check all the work you have done so far, where did you get the replacement chip from ? are you sure it's a good one and not some counterfeit Chinese thing

[BillyTheKid]

Originally Posted by Clydeuk

Wasn't it 60 volts AC you measured on pin 9?

Sorry! Yes it was 50 V AC(apologies I keep flipping it in my mind) I'll go back over the reading with my new(better) DMM.

Agree with you Guys, I think I've gone as far down the rabbit hole as I can with my current ability and equipment.
I'm going to pack up the VZ3000 for now and try get my 2 VZ3500s in a sell-able shape and pull back some of the money I've fed into this so far.
Plus I'm moving home soon in a few weeks and have been told that only one of these is allowed to come with me

In the mean time, I spent about 3 hours last night watching videos on power supply and how to troubleshoot it. I'm getting a much strong gut feel for the thing so will came back around to this thing. wIll check if there's a repair club in London, that sounds like fun! I'm pretty sure they won't let me in when they see me carrying the VZ3000..

As I intend to plug this thing into the back of my main amp and have no intention to use the actual speakers with it or the cassette/Radio I'm thinking of just powering the Turn table and taking the audio out from there. Not sure if that's feasible but once I know how to read the schematics I'll check it out.

Thanks again so much for all the help guys!

[Edward Huggins]

So you are going to abandon any attempt to use its Tuner, Cassette Deck and Speakers?

If this be so - surely there are far, far, better ways of playing vinyl than just using this unit as a (compromised) source for your records!

[Grubhead]

I tend to use a cheap pair of headphones when testing amps. You can pick up problems better and you don't have to mess around with speaker wires, which you can short, moving it around.

[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.

[BillyTheKid]

Originally Posted by Edward Huggins

So you are going to abandon any attempt to use its Tuner, Cassette Deck and Speakers?

If this be so - surely there are far, far, better ways of playing vinyl than just using this unit as a (compromised) source for your records!

I Like the way this thing looks, it has a very thin depth so will fit nice on my shelf and like that it can play both sides seamlessly. I also enjoy seeing the record go around on it. It would also be nice to put in a Bluetooth receiver like this: http://store3.sure-electronics.com/2...x-tpa3110-1751
Plus I'd rather use my own speakers and valve amp. I'm not that into cassettes, and Radio is due to be switched off soon.

Originally Posted by steve1010uk

As Lloyd says you need to check that 100 ohm resistor between pins 9 and 10 , you have far too many - volts on all those pins and not enough +

aim for zero (or nearly zero) volts on pins 7 and 16 before connecting the speakers,
double check all the work you have done so far, where did you get the replacement chip from ? are you sure it's a good one and not some counterfeit Chinese thing

Agreed! I think it may have been that previous DMM, I'll re run all of these again when I get home tonight.

Got the replacement chip from amazon so I'm confident it's legit. I also read somewhere that the letters on the back of the fake ones have Blue writing poorly stamped on it. Mine wasn't blue and was professionally printed on the back.

Agree that I need to go back and check everything. I now think I know why my ESR readings were so incorrect when in circuit. I hadn't zeroed my prongs first.

I'm going to check out the diodes on the bridge rectifier as per Lloyd's suggestion to see if they are letting AC through.

I've also ordered a digital oscilloscope which should arrive mid next week!

Happy Weekend Guys!