Polytone problem...

[stefano]

Hi, i have an old Polytone Minibrute IV ('80) that has been sitting for 10 years or so. It was in working condition back then.
When i tried to turn it on it made a terrible noise (thunderstorm..) after few tries it suddenly started working as it should, but after few minutes than that awful noise reappeared.
Do you have any idea of what's going on with it?
Thanks
Stefano

[Jazz P Bass]

My first thought is there is a bug in it.

Second thought is the power supply capacitors may be dried out.
In that case you must replace them.

Third thought is that you must troubleshoot the problem.

[Chuck H]

IME the two most common problems that occur in tube amps that sit for a long time are contact oxidation and power supply capacitor failure.

Tubes behave very badly when only some of the pins are actually connected. Oxide forms with age on tube pins, sockets, switches, jacks, etc. You can buy contact cleaner at any electronics supply store. Even Radio Shack and Fry's.

The capacitors used in the power supply have a limited life because of the materials they are made from. These capacitors actually age faster when the amp is not powered up for an extended time.

I'll guess you have one or both problems. You shouldn't power up the amp again before cleaning all the contacts. If the problem persists you should have it looked at by a pro or repost any continuing symptoms here. But don't leave it powered up unnecessarily if problems persist. Failing components can take out other, more expensive parts when they go.

[52 Bill]

IME the two most common problems that occur in tube amps that sit for a long time are contact oxidation and power supply capacitor failure.

Chuck, while this is true, the amp in question is pure solid state.

Stefano, welcome to the place. It might help if you could describe the noise a little bit. Is it a loud hum or a static sound or ??? Is the noise changed by any of the controls? If you give the amp a good Enzo thump with your fist, can you make the noise start or stop? If the amp has reverb, then be sure to turn down the reverb control before thumping the amp.

If the noise comes and goes, then you have an intermittent problem that may be as simple as a loose Molex connector or as difficult as a failing component. How capable are you when it comes to electronics?

[Chuck H]

Chuck, while this is true, the amp in question is pure solid state.

Ha! Love it. Thank you for alerting me.

I'll still go with either dirty contacts or bad electrolytics as the most likely possibilities.

[olddawg]

Without seeing the schematic. On a discrete SS amp, I would suspect a diff amp pair of transistors that have had the beta change on one or both on the input to the power amp stage. Freeze mist helps in tracking this stuff down. Make sure you have the speaker disconnected so you don't damage it.

[J M Fahey]

Don't overthink it.

Amp 10 years in storage, now making thunderstorm noises at random?

DIRTY pots/jacks/contacts.

Throw in some oxidation and corrosion in general.

Never ever saw a transistor changing Hfe , even less on its own.

Ane even if diff pair ones were grossly unmatched, it would mean increased offset.

Say, from 45 mV to 120mV.

[olddawg]

Don't overthink it.

Amp 10 years in storage, now making thunderstorm noises at random?

DIRTY pots/jacks/contacts.

Throw in some oxidation and corrosion in general.

Never ever saw a transistor changing Hfe , even less on its own.

Ane even if diff pair ones were grossly unmatched, it would mean increased offset.

Say, from 45 mV to 120mV.

Juan, I've literally seen it hundreds of times in old and new amps. I have a Fender 212R on my bench right now with a bad diff pair. It is more common in old amps though. It doesn't take much of a mismatch to cause a thunderous sounding intermitant DC offset. (Why I said disconnect the speaker.) He did say he had the problem before he put it away? Tickling the pair with freeze will usual identify a diff problem. By all means, clean the pots and jackets with Caig D and resolder any bad looking connections. But if it still crackles loudly at random, check the diffs if it has them. It can be two discrete transistors or a 5 leg dual package. The duals are more stable.

[gbono]

Juan, I've literally seen it hundreds of times in old and new amps. I have a Fender 212R on my bench right now with a bad diff pair. It is more common in old amps though. It doesn't take much of a mismatch to cause a thunderous sounding intermitant DC offset. (Why I said disconnect the speaker.) He did say he had the problem before he put it away? Tickling the pair with freeze will usual identify a diff problem. By all means, clean the pots and jackets with Caig D and resolder any bad looking connections. But if it still crackles loudly at random, check the diffs if it has them. It can be two discrete transistors or a 5 leg dual package. The duals are more stable.

I wonder where OEMs were getting matched discrete BJTs in seperate packages? I worked for Fairchild in the 70's and we never matched transistors for anyone. If you want matching in a BJT or FET it is either hybrid assembly (adjacent die from a wafer attached on the same lead frame) or monlithic topology. semiconductor processing has improved over the years and you will find a decent distribution on DC HFE and VBE within a given date code but if you are looking for 10mV matching with discrete devices you will be hard pressed to get there.

[olddawg]

I wonder where OEMs were getting matched discrete BJTs in seperate packages? I worked for Fairchild in the 70's and we never matched transistors for anyone. If you want matching in a BJT or FET it is either hybrid assembly (adjacent die from a wafer attached on the same lead frame) or monlithic topology. semiconductor processing has improved over the years and you will find a decent distribution on DC HFE and VBE within a given date code but if you are looking for 10mV matching with discrete devices you will be hard pressed to get there.

Back in the old days beta matched pairs were marked with a red (PNP) or white (NPN) dot of paint. Nowadays if you just order the part by generic number it will be fine. In the old days, I used to have to go through a pile sometimes, measure the hfe, and group them in pairs of the most identical ones. I was always told that the 5 lead dual packages were for temperature stability. (They were 6 leads internally with the emitters tied together of course). I always used the duals where I could.

[52 Bill]

I remember the duals in some higher end hifi amps, but I can't think of any guitar amps that used them.

I never saw a transistor that changed gain either until recently, or at least I never actually documented it before. I had a Hartke bass amp that wouldn't come out of protect at start up. I checked for dc offset on the output line and there was about 4 vdc there that couldn't be removed with the offset control. I didn't have any 2sc3200 transistors, so I swapped two from another section of the power amp and that fixed it. Just to document it, I removed all 4 transistors and checked their hfe. The two that came out of the differential pair were both down to about 12, the two from the other section of the circuit were around 250.

I put in an order for the replacement transistors and waited. Apparently the 2sc3200 are now obsolete and I had to find an alternative replacement. In the mean time I sent the working amp back with the owner until I could get some new transistors. Well the amp came back in, again not coming out of protect. I immediately pulled the differential pair transistors and checked the gain. One was down to 12 and the other was still around 250. I have replaced all 4 transistors and the amp is running again. I'll have to see if there is any further problems with the new transistors. But this is the first time I've ever seen that happen before.

[olddawg]

I remember the duals in some higher end hifi amps, but I can't think of any guitar amps that used them.

I never saw a transistor that changed gain either until recently, or at least I never actually documented it before. I had a Hartke bass amp that wouldn't come out of protect at start up. I checked for dc offset on the output line and there was about 4 vdc there that couldn't be removed with the offset control. I didn't have any 2sc3200 transistors, so I swapped two from another section of the power amp and that fixed it. Just to document it, I removed all 4 transistors and checked their hfe. The two that came out of the differential pair were both down to about 12, the two from the other section of the circuit were around 250.

I put in an order for the replacement transistors and waited. Apparently the 2sc3200 are now obsolete and I had to find an alternative replacement. In the mean time I sent the working amp back with the owner until I could get some new transistors. Well the amp came back in, again not coming out of protect. I immediately pulled the differential pair transistors and checked the gain. One was down to 12 and the other was still around 250. I have replaced all 4 transistors and the amp is running again. I'll have to see if there is any further problems with the new transistors. But this is the first time I've ever seen that happen before.

The classic problem is an amp that will pop in and out of protection. I've seen customers remove protection relay covers and put then on sideways over the contacts so they would be permanently closed. One guy that did this I told him he was lucky he didn't blow all of his speakers. It was a friend of mine and he then showed me 6 pairs of blown speakers in his garage. But you are correct Bill, it is more common in hifi amps, especially old Pioneers. I don't mean to sound like SGM, but back in the day I repaired 10-12 or more amps a day in a busy shop as an authorized tech from Akai to Zenith. But an amp is an amp. I do know a lot more about SS repair than tube stuff. Always have.

[stefano]

Chuck, while this is true, the amp in question is pure solid state.

Stefano, welcome to the place. It might help if you could describe the noise a little bit. Is it a loud hum or a static sound or ??? Is the noise changed by any of the controls? If you give the amp a good Enzo thump with your fist, can you make the noise start or stop? If the amp has reverb, then be sure to turn down the reverb control before thumping the amp.

If the noise comes and goes, then you have an intermittent problem that may be as simple as a loose Molex connector or as difficult as a failing component. How capable are you when it comes to electronics?

WOW, thanks everybody for helping me out. Sorry it took so long to answer..
The noise is indistinct thunders-loud noise..no humm or background buzzz, just random noise. The amp sounds fine, but after few minutes of use it starts to thunder..maybe one or two isolate thunders and then continuos noise. A couple of times the noise started immediately when i turned it on. Volume-reverb-distortion were always turned down when i switched the amp on, and when it's making noise nothing really changes if i turn the knobs.

I don't know anything about electronic, but i think i'll be able to screw in new capacitors.
I opened the amp and took one out. Here's what's written on it:

MEPCO/ELECTRA
3186BE732U050ALA1
86F566L
7300UF - 10 + 75%
50 VDC 65 SURGE
85 C MAX AMBIENT
362 8105 56699

I looked for similar replacement parts online and i found something that should work, but the part code is a bit different (from the pictures and the specs it seems exactly the same as mine).
Here's what is written on the replacement:

MEPCO/ELECTRA
3042177
3186BE732U050AMA2
7300UF - 10 + 75%
50 VDC 65 SURGE
85 C MAX AMBIENT
362 8105 56699

Should i go ahead and buy the new capacitors that i found?

Do you have some suggestions about how to clean the electronics in this amp without damaging anything?
Thanks a lot!
Stefano

[52 Bill]

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.

[stefano]

Ok, i'll try DeoxIt and see what happens
Thanks a lot!