Polytone problem...

[stefano]

While you might want to replace the filter caps at some point, I would suggest that you fix the problem with the amp first and then do some upgrades. If the main filter caps are bad, the usual symptom is a constant low frequency hum. The noise that you describe sound more like oxidized contacts on the many plug in connectors inside the amp. It could also be other things, but cleaning the connectors will not cost you much more than a can of cleaner and a little time.

I would suggest that you get a can of DeoxIt spray and use it to clean off all of the metal contacts like the main molex connector that connects the preamp chassis with the power amp chassis, and the RCA plug that connects the signal from the preamp to the power amp input, etc. Unplug each of the different connectors and spray a little of the cleaner on the metal contacts and then plug and unplug the connectors to burnish the contacts clean.

Do this to all of the connectors and see if the noise goes away. You might want to spray a little of the DeoxIt into each of the pots and work the knobs back and forth a few times to clean them up as well.

Ok, i took some parts out with the intention of remove dust and cleaning and i noticed something obviously wrong. Could it be the cause of the problem? (the last photo is how the protective plate looked underneath the circuit board)
Thanks for your help
Stefano

[stefano]

Ok, cleaned everything with compressed air and deoxit and it seems to behave much better.
Thanks everybody, hopefully i won't be back for other problems..

[Chuck H]

it seems to behave much better.

That's not the strongest elation I've heard for a repair We usually get something more like "IT"S FIXED! "

I know you're listening with a critical ear right now and likely staying a little apprehensive. If there is still a woosh sound you can try freeze spray on the amps as olddawg suggested. Old transistors do sometimes get noisy with age and the freeze is the easiest way to ID a noisy one IME. I've only done a couple of SS repairs and this worked for me on one to identify problems.

[stefano]

Ok, i'm back. Yesterday i left the amp on during the afternoon for few hours and listened to music trough it. It was mostly good, it made few pops every once in a while. I turned it on in the evening and the popping-crankling thundering noise was there...turn it off, waited a couple of minute, tried again and it was ok. Left it on for a while and it made some popping sounds here and there.
This morning same thing, i turned it on and made a lot of noise..same thing, turned it off, tried again, the noise was still there but after few seconds it was gone, i listened to music trough it and it was good except for a couple of pops. I turned it off after one hour, waited few minutes and powered it again and the noise was there, i punched the top of it a couple of times (quite strongly) and the noise stopped.
Turned it off, turned it on again and it was ok. At that point i punched it a couple of times to see if i could make it start to producing the noise again and it kind of worked..
I know, i should bring it to a technician, but i really don't have money to spend on it..any advice?

[52 Bill]

...i punched the top of it a couple of times (quite strongly) and the noise stopped.

Then it would appear that you have a loose connection or a bad solder joint inside the amp. What sorts of soldering skills do you have?

If you take the back panel off of the amp you can tap on the different sections of the circuitry with an insulated stick and see if you can localize the problem to either the preamp or the power amp. The preamp is the chassis that is mounted to the top panel with the controls, the power amp is the chassis that is bolted to the bottom of the case below the speaker.

Do this carefully and don't hit things hard enough to break them, just enough to make the noise start or stop. Hopefully you will be able to find the area if not the actual part that is loose. If the problem is in the power amp, you will need to remove the speaker to get the power amp out of the case.

Let us know what you find.

[stefano]

I tried to tap with a wooden stick everything i could tap inside, moved all the cables-connections but nothing happened, amp quiet.
I moved it to a chair while it was on and it popped while i was moving it, but then still quiet. I hitted it with my hands few times on the top of the cabinet and it started.
Could it be related to the power capacitors? Could something be wrong with the power outlet in my building?

I also noticed that it has been repaired in the past, here you can see pictures of the repair that has been done:

http://i122.photobucket.com/albums/o...f/IMG_1683.jpg
http://i122.photobucket.com/albums/o...f/IMG_1684.jpg
http://i122.photobucket.com/albums/o...f/IMG_1686.jpg
http://i122.photobucket.com/albums/o...f/IMG_1687.jpg
http://i122.photobucket.com/albums/o...f/IMG_1688.jpg

Ok, any suggestion for a good repair guy in NYC?
Thanks

[52 Bill]

I can't tell, but it looks like there is an burned arc mark on the back side of the board where it sparked to the back of the preamp chassis. Is there a cardboard insulator between the board and the chassis?

Did you try tapping around the power amp?

[Chuck H]

With the above situation it might be a good idea to CLEAN EVERYTHING AGAIN. Yup. Stubborn dirt. That or possibly a cold solder joint (though you didn't detect it with poking). My money is on a bad mechanical connection. I'd clean all the mechanical contacts again.

[stefano]

I can't tell, but it looks like there is an burned arc mark on the back side of the board where it sparked to the back of the preamp chassis. Is there a cardboard insulator between the board and the chassis?

Did you try tapping around the power amp?

No nothing between the board and the aluminium, the mark on the aluminium is right where the repair has been made. Should i try to put something in between the board and the chassis?

I tried to tap around everything on the power unit, but i could't reach the components under the power unit chassis.

Another thing, could it be a loose ground connection? Where should the ground wire be attached?

Thanks!

[stefano]

Ok, found something else that was wrong. I took out the accutronics reverb unit and one of the two springs was missing (it broke off), i found the missing spring beween the cabinet and the insulation on the bottom of the amp. Could it be part of my problem?
Here's a picture:
http://i122.photobucket.com/albums/o...f/IMG_1729.jpg

[The Dude]

Is the noise still there if you turn the reverb all the way down?

[stefano]

Is the noise still there if you turn the reverb all the way down?

Yes

[The Dude]

Likely a separate problem then. I would try freeze spray as others have mentioned to try and narrow down the noise problem.

[stefano]

With the above situation it might be a good idea to CLEAN EVERYTHING AGAIN. Yup. Stubborn dirt. That or possibly a cold solder joint (though you didn't detect it with poking). My money is on a bad mechanical connection. I'd clean all the mechanical contacts again.

Ok, cleaned everything again and got rid of the faulty reverb box. I left the amp on for few hours a day in the past couple of days and it did good. I turned it on this afternoon and after 30 min it started again, it started soft and got louder. I turned it on-off few times but the noise was still there..then i gave the cabinet a light thump and it stopped. I tried to poke around with a wooden stick again but nothing happened. Now it's still on and it's behaving good..

[52 Bill]

..then i gave the cabinet a light thump and it stopped. I tried to poke around with a wooden stick again but nothing happened. Now it's still on and it's behaving good..

Did you get to the power amp to see if anything was loose there?