Help! Humming from Adcom GTP500 CD out

[jdtla]

Hi:
I'm hoping someone can give me some advice. I have an Adcom GTP-500 II preamp, the CD Out now makes a humming noise. I've tried different amps and cd players with the same results. Tape out, Video out work fine.

I'm comfortable working on electrical equipment, so any advice/guidance would be very much appreciated.

Thank you!

 

[TLS Guy]

Hi:
I'm hoping someone can give me some advice. I have an Adcom GTP-500 II preamp, the CD Out now makes a humming noise. I've tried different amps and cd players with the same results. Tape out, Video out work fine.

I'm comfortable working on electrical equipment, so any advice/guidance would be very much appreciated.

Thank you!

What do you mean by CD out? I think you mean CD in, tape in, and video in.

Questions.

Is it both channels?

What happens if nothing is plugged into the CD input?

If it hums with nothing plugged in does it still hum with shorting plugs on the input? I see the unit comes with shorting plugs in those inputs.

Have you made sure record CD is not activated? Do you have anything plugged into the tape outs? Make sure the shorting plugs did not get inserted into a tape out.

What test gear do you own?

Do you have, or can you get a circuit of the unit or service manual? I could not locate one on the ADCOM site.

 

[jdtla]

Thank you, and you're right: CD input (long day)

Is it both channels?
-Yes

What happens if nothing is plugged into the CD input?
-It stops humming

Have you made sure record CD is not activated?
-Yes

Do you have anything plugged into the tape outs?
-No

What test gear do you own?
-I used an old Yamaha AX-900U 130x130w. That works fine when I eliminate the preamp.
-I used 2 different cd players, an Onkyo and a Phillips. Humming with both.
-Also tried it with an Adcom GFA5500 200x200w. Same result.

Do you have, or can you get a circuit of the unit or service manual?
-No I don't and I'm not sure where I'd get one.

Thank you for your quick response. Hope we can figure this out...

 

[TLS Guy]

Thank you, and you're right: CD input (long day)

Is it both channels?
-Yes

What happens if nothing is plugged into the CD input?
-It stops humming

Have you made sure record CD is not activated?
-Yes

Do you have anything plugged into the tape outs?
-No

What test gear do you own?
-I used an old Yamaha AX-900U 130x130w. That works fine when I eliminate the preamp.
-I used 2 different cd players, an Onkyo and a Phillips. Humming with both.
-Also tried it with an Adcom GFA5500 200x200w. Same result.

Do you have, or can you get a circuit of the unit or service manual?
-No I don't and I'm not sure where I'd get one.

Thank you for your quick response. Hope we can figure this out...

I really doubt there is anything seriously wrong with the ADCOM preamp. It would be most unlikely for both channels to develop that fault at once. Also it does not hum if there is nothing connected.

I think this is a grounding problem at that input, since this problem does not occur on the other inputs, or a switching problem

Are you using the same cable, when you connect to the other inputs?

Now you need a multimeter to do the next check. You need to check the resistance of the ground side of the CD inputs to chassis and compare it to the others.

To go beyond that you need.

A clear circuit diagram of the unit, and if possible a service manual.

The ability to read and understand a circuit, and understand how the sections of the circuit work.

A FET VOM

An oscilloscope, preferably two channel.

An audio signal generator.

A Variac is also a very good idea and makes service work a lot easier and safer for the technician and the device being repaired.

I suspect this problem is in the connection of the input jack itself to the chassis, or in the switching circuit.

I have no idea of the theory of operation of the input circuits on that unit, I need a circuit diagram for that.

Call Adcom service.

You are not equipped to open the case. Stay out of the unit.

 

[jdtla]

Thank you again for your help.

Here's what I've done so far. Grounded one end of a wire to the chassis and touched the rca jack with the other end and the hum stopped. So you're right there's a ground problem with the jack. What can do to fix that?

Thank you again!

 

[TLS Guy]

Thank you again for your help.

Here's what I've done so far. Grounded one end of a wire to the chassis and touched the rca jack with the other end and the hum stopped. So you're right there's a ground problem with the jack. What can do to fix that?

Thank you again!

Chalk up another one for the chump. This is a problem you will easily fix yourself.

How are those RCA jacks fixed to the chassis?

Also how our the outer grounds of those jacks fixed to the base?

If you can take pictures it will help me.

Can you solder, and do have a soldering iron? Depending on the problem there may well be some soldering involved.

Let me know the answers to these questions and I will tell you what to do.

 

[jdtla]

I definitely chalk one up to you! (although not 'chump').

Here's some pix, I hope this can answer any other questions, and yes I can solder.

Here's to moving another step closer, and again, thank you!!

 

[TLS Guy]

I definitely chalk one up to you! (although not 'chump').

Here's some pix, I hope this can answer any other questions, and yes I can solder.

Here's to moving another step closer, and again, thank you!!

If you don't have multimeter, go out and get one. If you are going to get involved in this area, I strongly advise ponying up for a high impedance amplfied meter. That will save you blowing as much gear up.

Any how this is what I feared, these are board mounted jacks. From the pictures it looks as if these RCA jacks are a series of two pairs of four jacks. I think that the center strap is the ground strap. I think either it has a poor connection to the board, or more likely there is a fracture in the printed circuit board.

This is what I would do. I would avoid taking out the board, and I doubt the chassis gives you access to the under side. Correct?

Now identify the ground plane of the board.

Test with your meter for continuity from the RCA screens to the ground strap.

Then check for continuity from the ground strap to the board ground plane of the board.

If the latter is the problem, and I bet it is, solder a jumper cable from that ground strap to the closest adjacent one. If the interruption is from that ground strap to the board ground plane, that will solve it.

 

[jdtla]

Hi again:

You're right about the board mounted pair of jacks.

So....I do have access to the underside and the board ground plane is separated and from the soldered wire.

How would you do it then? Is it possible to re-solder the wire underneath so that it is in contact with the ground plane? Or...

I would think so, however, since you've been my 'guru' here, I'm going to wait for your advice!

Thank you!

 

[TLS Guy]

Hi again:

You're right about the board mounted pair of jacks.

So....I do have access to the underside and the board ground plane is separated and from the soldered wire.

How would you do it then? Is it possible to re-solder the wire underneath so that it is in contact with the ground plane? Or...

I would think so, however, since you've been my 'guru' here, I'm going to wait for your advice!

Thank you!

Yes! Solder it back to the ground plane. Just make sure there no tension. If there is, get a piece of copper hook up wire, solder that to the ground plane and then the hook up wire to the ground wire. Check that your solder does not spread, and that there are no shorts created on the board. Check you new joint with your continuity meter. Be neat and careful.

You should be sorted now. Good luck!

 

[jdtla]

I'm going to do that later this evening and I'll let you know. Right now I've got to take my 14 year old to a school dance....

This is a fantastic website/forum, helpful and knowledgeable people such as yourself are the reason!!

Thank you again...

 

[jdtla]

Hi:

So here's what I've done, and still not working. Did exactly what you said, re-soldered to the ground plane, no meter reading. I also soldered a jumper wire to another ground source, no reading. What am I missing? Or, what do you think at this point?

Thank you...

 

[jdtla]

Do you think it's the jack itself? I wrapped a wire around the phono jack, which it's paired with, then wrapped it around the cd jack, then once again I'm getting the correct reading. Baffled....

 

[TLS Guy]

Hi:

So here's what I've done, and still not working. Did exactly what you said, re-soldered to the ground plane, no meter reading. I also soldered a jumper wire to another ground source, no reading. What am I missing? Or, what do you think at this point?

Thank you...

What do you mean by no meter reading? Do you mean that you don't have a meter or there is no continuity? If you don't have a meter, I can't go any further with this. This is going to be a simple problem, but we need hard data, to work on. You can't go fixing equipment without instruments, but I'm pretty sure this one can be analysed with a test meter.

 

[TLS Guy]

Do you think it's the jack itself? I wrapped a wire around the phono jack, which it's paired with, then wrapped it around the cd jack, then once again I'm getting the correct reading. Baffled....

Well what is the reading from the jack to the link, does it show continuity? Is the link from the jack showing continuity to the ground plane of the board, not just where you soldered it? Did I correctly identify the ground link from your pictures?

What are the meter readings from those four RCA sockets in that group to the ground link? How to the readings compare to another set of four?

If there is no continuity from the RCA CD sockets to the ground link, you will have to remove that set of four, find out why, and see if there is a way to fix it, without replacing that group of four.

 

[jdtla]

Sorry I wasn't clear in the first post. I'm using a digital multimeter, and by no reading I meant no continuity. All the other jacks had consistant continuity readings.

I ended up removing the pin jack, and BINGO! One of the gold pins that come out of the back of the pin jack and and through the circuit board was broken at the elbow. So I'm going to try to fix that break.

Hopefully one last question. How would you repair that break (replacing the jack is not an option) in a way that would make it very solid and strong?

Thank you.

 

[TLS Guy]

Sorry I wasn't clear in the first post. I'm using a digital multimeter, and by no reading I meant no continuity. All the other jacks had consistant continuity readings.

I ended up removing the pin jack, and BINGO! One of the gold pins that come out of the back of the pin jack and and through the circuit board was broken at the elbow. So I'm going to try to fix that break.

Hopefully one last question. How would you repair that break (replacing the jack is not an option) in a way that would make it very solid and strong?

Thank you.

Well now we have it! Can you post a picture of the broken broken board?

It is hard to advise you without knowing how it looks.

 

[jdtla]

Here you go. One pin right below one of the the jacks broke off. Not sure what to do to. They're all one piece, coming out of the jack and bending 90 degrees down and through the board. Hmmmmm....

 

[jdtla]

I think these pictures should tell you more. If you need them larger let me know.

 

[TLS Guy]

Here you go. One pin right below one of the the jacks broke off. Not sure what to do to. They're all one piece, coming out of the jack and bending 90 degrees down and through the board. Hmmmmm....

View attachment 5669

View attachment 5670

View attachment 5671

I would not recommend a bodge with glue.

I would get four of these.

http://www.radioshack.com/product/index.jsp?productId=2103234&cp=&pg=6&sr=1&origkw=rca+plug&kw=rca+plug&parentPage=search

Mount them in the four holes. Connect them to the main PC board with hook up wire.

I don't see any other option if you can't replace the broken part.