GS1200 amplifier help needed

[Paul Adsett]

The track 2 channel of my version 2 GS1200 has failed. Track 1 is still working fine. By susbstituting a power supply module from another GS1200 it appears that the problem is somewhere within the big 2-board module mounted with the two level meters. Here are the symptoms:

1. Both level meters are hard over to max+ all the time, even when in standby mode.

2, Track 1 (left) is fine all the time through earphone sockets and AUX left output socket, even though the level meter is always hard over max all the time.

3. Track 2 (right) extremely weak, or no sound, from earphone and AUX right socket.

4. If I press the record button (red light comes on) and level meter drops back to a normal zero. This is true fro both track 2 and track 2.

5.If I switch to optical sound, both level meters drop to a normal zero

I have already replaced the STK 439 STEREO AMPLIFIER IC on the main board, but this had no improvement.

If anyone has any ideas where to go from here. any help much appreciated.

[Lee Mannering]

Hi Paul. Sounds a bit like a Capacitor issue but difficult to come up with a firm answer from a distance.
Sometimes we get a clue hearing a crack before hand.

[Lance Alspaugh]

Hi Paul,
This problem happened to us some time ago however the problem prevented both tracks from being audible instead of just the one for you. The unit that has the VU meters, audio knobs, recording buttons and all the side panel rear adjustments was defective. We found pushing in or putting forward pressure on the Tone/Bass knobs brought the channels back to life after some static. However it was an ongoing problem that ultimately required replacing the VU/Audio unit in its entirety. Another more unlikely possibility could be one of the wires that connects to the magnetic sound head has become disconnected, in your case the balance track, and since they are small they can be deceptive. Hope this may help or you find the actual solution.

[Graham Ritchie]

Paul

I wonder if you have disturbed a old dry joint on the board.

Looking at the age of these projectors, it would not surprise me.

[Burton Sundquist]

Paul, If unable to sort out on your own it might be time to ask Leon Norris. He has brought many GS-1200's back to life.

[Phil Murat]

Hello Paul,

First of all, Happy new Year to the Forum !!

I have sent you a message.

To generate a Sin wave signal you can use this on line (Basic use is FREE):

https://www.wavtones.com/generateurdefonction.php

It is essential to make a "Test Print" (1000 Hz is fine, medium band is from 200Hz to 2000Hz) on a sain machine to make a comparison.

[Paul Adsett]

Lee mentioned about a possible capacitor issue. So exactly how do you test for a failed capacitor? Can you do it directly on the board, or does the capacitor have to be removed from the board to test it?

[Phil Murat]

I use a small device call ESR Tester, you can find it for few money.
It is not possible to test a capacitor when installed , you have to remove it........... So , it is better to install a new cap in that case.
ESR Tester give you 2 values : ESR plus Percentage of Capacity.
Each capacitor has an ESR Value depending on its technology, its Capacity, its Voltage limit.
You can find Cross References Chart for capacitors on the Web.

ESR = Equivalent Serie Resistance

If you replace a capacitor on Channel 2 , it is recommended to replace its "brother" on Channel 1

[Simon McConway]

An ESR meter allows you to test a capacitor in situ without removing. That’s the whole advantage of using one.

[Lee Mannering]

Between £10 up to £600.

[Paul Adsett]

Thanks so much to everyone who posted suggestions for solution to my GS1200 amplifier problem.
Fortunately the problem has now been fixed. I called Leon Norris and he suggested removing the amplifier module and spraying all the amplifiers microswitches with electrical contact/lubricant. I did this, including the mono/stereo switch and the mag/opt switch. After exercising the switches for several minutes, the stereo sound was fully restored. It is my belief that the actual culprit switch was the sliding MAG/OPT switch, which I thought was clean, but apparently not so.
So what originally seemed to be a potentially complex repair job was in fact really simple. Yes, those pesky switches again!
Anyway, thanks to Leon my GS is now back 100% with glorious stereo sound.

[Lee Mannering]

Phew good news Paul, Elmo share the same glitches as Eumig and others. Switches are getting a bit ancient now.
Glad you sorted it and also thanks to Leon.

[Paul Adsett]

Well after a week of flawless running with glorious stereo sound the problem is now back! This time no amount of exercising the switches will fix the problem. I have separated the two boards in the amplifier module and removed the bridge board. Very surprised to find that both the bridge board and the contacts on the circuit board are gold plated! Everything looked in immaculate condition, but I sprayed the bridge board with contact cleaner anyway. All to no avail, the problem is still there.
Here is what is happening. When I turn on the main power switch the meters both peg fully over to the right and stay there. If I switch from MAG to OPT the meters swing back to the normal zero, but swing back again to fully hard right as soon as I switch back to MAG.
Despite this, sound playback is fine on track 1, but track 2 is all hum.
Has anyone seen this problem before? I am wondering if it might be associated with a problem in the muting circuit ( just a wild guess) since all this is occurring while in standby mode.
Any suggestions greatly appreciated.

[Patrick Steemers]

That’s bad luck... I sprayed my GS1200 and solved clicks and pops and static crackle during playback. But the recording is still unreliable. With drop outs and sometimes hum... any suggestions?