blueNan

recap oscillation? problem
« on: April 16, 2019, 03:25:40 AM »
Hi, i am almost new to electronics. I have soldering gear and humble soldering chops.

I have recapped a circa 25 years old stereo compressor (Estec Twincomp) using Nichicon KZ capacitors of the same specs, to replace original Thompson capacitors. I did only the audio board.

It was sounding fine at the beginning, just felt that the signal lose a tiny bit of "size" if that makes sense, when passing through the compressor (subtle difference). The task looked easy so i decided to try with expensive capacitors.

Once finished, the compressor exerts a constant hum, which frequency variates (basically making weird noises) if i inject audio. I have examined the board but it looks fine.

So i don't know what to do now... i heard from the previous owner that the designer himself, Sonny Schneid, told him that if he was to build again the compressor he would have used Panasonic FM. So i am tempted to give it a try replacing the Nichicons with FM, but do not know what  to solve this hum problem...




jensenmann

Re: recap oscillation? problem
« Reply #1 on: April 16, 2019, 04:08:58 AM »
Doublecheck if the new caps are installed with the right polarity.
« Last Edit: April 16, 2019, 04:55:52 AM by jensenmann »
Jens
Quote from: PRR
The tubes of course don't care what frequency they distort

blueNan

Re: recap oscillation? problem
« Reply #2 on: April 16, 2019, 07:08:20 AM »
Doublecheck if the new caps are installed with the right polarity.

I double checked, and just did it again now. Polarity is ok.

blueNan

Re: recap oscillation? problem
« Reply #3 on: April 16, 2019, 07:30:59 AM »
Besides the replacement of the 5 x Thompson capacitors per channels with Nichicon KZs, i replaced also 2 capacitors per channel that i assume are related to power the opamps and VCAs, because the designer used here standard Jamicon ones.

For this replacement, i used Panasonic FM. Perhaps the low ESR of the Panasonic FM can be the cause for this oscillation?

I attach a picture of one channel of the audio board with the replacements i did. The controls that govern the VCA are in a physically separated board, connected to this one via ribbon cable.

scott2000

Re: recap oscillation? problem
« Reply #4 on: April 16, 2019, 10:25:01 AM »
So it Did it work well before the upgrades?

Is the new problem on both channels ??

pucho812

Re: recap oscillation? problem
« Reply #5 on: April 16, 2019, 11:07:17 AM »
if the problem is on both channels. look for something common to both channels? start with the power supply.
You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is.

blueNan

Re: recap oscillation? problem
« Reply #6 on: April 17, 2019, 06:16:09 AM »
Yes it worked very well before i messed up with it.   The problem affects both channels. I did not touch the psu, and it has two Philips LQ capacitors supposed to be of the finest quality, according to the info i could find about.

The unit does not pass audio unless i feed a very strong signal, then the hum derives into weird noises rhythmically related to the music i am playing.

I think I will remove one channel's Panasonic FMs and solder the original Jamicon back in place to see if it works... as they are less in number.  Discouraging results for a first recap project!

« Last Edit: April 17, 2019, 06:23:53 AM by blueNan »

scott2000

Re: recap oscillation? problem
« Reply #7 on: April 17, 2019, 06:15:20 PM »
Strange.......

Those weren't some back alley deal caps????

Doesn't make sense....... unless you goofed some trace or bridged  something with solder...,,maybe disturbed something during disassembly/assembly....magnifying glass with bright light is good to check.....etc.
« Last Edit: April 17, 2019, 10:13:31 PM by scott2000 »

pucho812

Re: recap oscillation? problem
« Reply #8 on: April 18, 2019, 12:13:40 AM »


The unit does not pass audio unless i feed a very strong signal, then the hum derives into weird noises rhythmically related to the music i am playing.



again check the power supply. it's not about a recap,yet, it's to confirm it outputs the correct voltages and such...
You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is.

ruffrecords

Re: recap oscillation? problem
« Reply #9 on: April 18, 2019, 04:31:49 AM »
Also check for solder splashes or track bridges. It is unlikely to be the type of electrolytic you have used. More likely something much more simple.

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'


Phrazemaster

Re: recap oscillation? problem
« Reply #10 on: April 19, 2019, 05:41:51 PM »
Simply, if it was working before and now it isn’t, you did it.  (We’ve all been there!)

Take the good suggestions already given - most likely its a solder bridge somewhere. Could you have hit a component with the soldering iron?

I second that unless the capacitors themselves are bad, which is unlikely if you bought from a reputable source, the brand doesn’t matter too much.

If you had an oscilloscope you could trace the signal and discover where it goes bad as an additional step.

Good luck, and report back what you find!
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** * Kablooie!

blueNan

Re: recap oscillation? problem
« Reply #11 on: May 01, 2019, 01:34:28 PM »
Well, after removing the Panasonics FR, things improved (audio started passing), but still distorted and noisy.
So I decided to follow your advice and check the psu:

Removed reservoir caps, they measured perfect, but  checking the VRs, found one rail down to 0v!

Could not find solder bridges - but legs were soldered in parallell to board, and traces were VERY close.

So I scratched between the legs with paper and a screw driver. And the short disappeared!

The VR came back delivering healthy 15v to the rail, and the compressor works as it should. Quite a journey!

Perhaps the movement / bending exerted on the VRs when removing the board caused a short... or an invisible solder bridge. I do not know, but either way you were spot on pointing at a problem in the power supply.

Ultimately it was a great learning experience with a good outcome, thank you all for the heads up!
« Last Edit: May 01, 2019, 01:44:33 PM by blueNan »

scott2000

Re: recap oscillation? problem
« Reply #12 on: May 01, 2019, 04:58:08 PM »
Cool!

Thanks for the update!


 

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