View Full Version : Park G25R amp repair help needed

04-09-2012, 01:12 AM
I have Park G25R amp from 90's, designed by Marshall. This amp is humming on clean and drive channel and hum is getting louder if I turn volume up. There is no guitar sound to hear. I hear guitar sound only for about half second, when I turn the amp off. Refers it to something? So far I have checked the pots. They are noisy but working. Also I have checked the bridge rectifier, which should be also ok. Second thing is I think a design flaw, cause it’s always been so. When I switch from drive channel to clean, the amp is making really horrible noise, when reverb is turned on. Without reverb it is making “pop”. Is it possible to add a resistor, which will ground the voltage from switching?

04-09-2012, 03:12 AM
Well you pretty much start with some basic tests...
- Power supply works on the rectifier part but do you get symmetric power supply rails feeding the output chip?
- Two regulators (probably discrete consisting of a dropping resistor and a shunt Zener diode) bring these voltage rails down to about +15V / -15V, powering the preamp. Do these regulator circuits work, does the circuit produce the said symmetric power supply rails?
- OpAmp tests: All inputs and outputs in same DC potential, which should be half of the entire rail-to-rail voltage (about zero volts in amps with bipolar supply, like the G25R). Do these measure ok?

Blues Power
04-09-2012, 11:05 AM
sorry, but arent those disposible. prob not worth the time, $$$, or agrivation

04-09-2012, 11:16 AM
As a Do-It-Yourself job it could be just a 0$ - 10$ repair. I'd rather invest that than throw the thing away entirely.

Taking it to tech for fixing is a different story.

04-09-2012, 11:44 PM
Thanks for your answers. Definitely it is not worth the time (=money), but this was my first amp. I thought that i could give it a try to fix it, cause i have little bit experience on building stompboxes, but however i feel i lack of skills of troubleshooting. Yesterday i checked voltages on IC's. They were on all IC's +17/-17V, except one. This one is mounted on aluminium radiator and had about +23/-23V.

04-10-2012, 03:53 AM
I found schematic of Park G10R, which is smaller amp but i think it's basically the same. On this schematics there are two capacitors 2200uF/25V and they are feeding the output chip. On my amp these are 2200uF/35V. Could it be, that one or both of them is not working and causing the problem? For me it seems, that there must be something wrong with power supply or capacitors, cause at the moment i switch the amp off, guitar sound is briefly heard.


04-10-2012, 07:59 AM
Yesterday i checked voltages on IC's. They were on all IC's +17/-17V, except one. This one is mounted on aluminium radiator and had about +23/-23V.

This is normal. Symmetric 17V supplies for the preamp, symmetric 23V for the power amp chip (the heatsinked IC).

On my amp these are 2200uF/35V. Could it be, that one or both of them is not working and causing the problem?

Likely not since the power supply seems to measure normally. You should be worried if you would have gotten an off-balance reading of the rail voltages. The supply voltages are present, they are in balance and they seem to be what they should be.

The only somewhat alarming thing is that the +-23V seems somewhat low considering the 35V rating of the filter caps. Without schematic voltage references it's hard to say anything specific but there's a slight chance that something is pulling so much current that the voltage sags a lot. You could verify this by isolating the power supply and measuring the voltage without load but I have a hunch it's not entirely doable in the concerned amp. On the other hand, the +-23V could be what the amp is supposed to operate on in the first place.

Verifying ripple voltage amplitude on the rails could be another test method to indicate excessive loading but it's again one of those things that can show way off readings depending on the multimeter and test setup. I'd rather ignore that unless you have a way to measure it accurately.

Did you perform the input/output terminal tests for the opamps?

04-10-2012, 11:42 AM
I measured transformator output AC voltage. This is 18V. Also i measured bridge rectifier output voltage and this is DC 24V. All voltages measured without isolating anything.

I didn't perform input/output terminal tests for opamps, cause i don't know yet how to do it.

04-11-2012, 11:49 PM
I byed some replacement opamps and added sockets to PCB. While soldering one of the sockets i noticed that one soldering point didn't took solder. There was no copper around the hole. So i cleaned the trace and soldered directly there and now the amp is working. It was good feeling to give a new life to this amp. Now i would like to do something against the channel switching noise, which is very ugly when reverb is turned on. Any advices are welcome.

04-13-2012, 07:42 AM
Could be the "pop" bleeding into the reverb effect itself. When signals pop while switching it's usually a DC bias issue, which can be reduced to some extent with resistors that tie coupling capacitors to a steady DC reference, whether its ground or something else. A schematic would in this case be a tremendous help since it's kind of pointless to start guessing design details of the amp.