Solved - Repairing Peterson P100G Guitar Amp - Preamp Noise

[Whoops]

Hello,
I'm trying to repair a Peterson P100G guitar amplifier.
It's a Solid state amplifier,  it is famous in the Jazz circuit as the Polytone.
I can't find the schematics for this amp, and the company is long gone.

I made a video of the noise in the amplifier, please check it out:

https://www.youtube.com/watch?v=x9JXBEyagZM&feature=youtu.be


The Problem
-There's some noise s that randomly blow.
The noise is independent of the guitar being plugged in or not.
It doesn't matter if the guitar has the volume cut or not .

I guess the problem is from the preamp because if I plug anything into the Power amp only the noise disappears. Also, I'm using an Audio probe at the moment, and the last preamp opamp's output just before the Power amp has the noise there, so it should be before that.

I've heard these type of noises before in failing transistors and in Condenser mic capsules

If you have any tips please let me know something

 

[CJ]

process of elimination.

you have ruled out the pwr amp already,

next, figure out if it could be mechanical by rapping on the side of the amp with your palm.

if no additional noise by doing this, then spray the preamp in/power amp out jacks with deoxit or alcohol or contact cleaner,

if no luck, then it is probably a noisy transistor or opamp,

short out pins 1 and 7 with a .1 uf cap to  gnd with clip leads and see if the noise stops,

work your way back towards the input jack, see if you can narrow down the region and then change  out parts, opamps are cheap, so the shotgun approach can save time trying to isolate the exact component.

twist all the controls to see if it is a noisy pot.

check all gnd points, including the pwr cord gnd

tighten all hardware, in/out jacks, pots, pc board standoffs,

 

[Whoops]

SOLVED

Thank you so much for your input CJ
I also thought it was a bad transistor/opamp, and maybe a cold or cracked solder joint.
Then problem was related to the last one.

This PCB is really bad, also the layout and implementation is quite horrible.

What happens is that this is a really crude double sided PCB without through hole contact between top and bottom pads, so all the components that have top and bottom pads should be well soldered on each side and is the component leg that is connecting the top and bottom pad.

I tried to Audio Probe the circuit, but this amplifier is a mess to service, the preamp section is divided into 3 pcbs with circuit sections going back and forth between the pcb's though wires (some of them have to be well soldered also in both sides of the pcb). Also it's quite difficult to take the pcb's out of the chassis.

I went then for the Chopstick method of tapping each component and found one 1M resistor that when tapped make the amp silent or bring back the "farting/blowing" noise. One of the resistor legs had pad on top and bottom of the pcb and when I applied some heat and solder to the pads the noise disappeared completely.
I replaced the resistor anyway, although it measured fine.

For the new resistor (Blue Metal Film in the photo) I soldered top and bottom of pcb and even let the legs be a little bit bigger to solder on top of the track.

I then re-did/retouched all the solder joints in the pcb's

I did also some restorations and improvements in some of the parts.

There's not a lot of info on these amplifiers so I will post some tips for servicing this amp (in the next post), it serves for future reference , as the amp is such a mess that I'm sure there will be more people in the future trying to repair one

 

[Whoops]

TIPS for Servicing Peterson P100G Guitar Amp

Problems:
- Amp will have problems in bad contacts in components or wires that are joining top and both tracks of the PCB
- Really difficult to service, components have no names silkscreened on the pcb
- 3x Power supply 5W resistors get really hot, they burn the PCB and are next to the Filter capacitors that have a shorter life or higher failure rate when exposed to heat
- Wires everywhere preamp layout is very confusing

TIPS
If you servicing a Peterson G100S amp here are my recommendations:

1) Replace all the Power Supply Electrolytic capacitors, they were exposed to excess heat and after many years they are probably under-performing

2) Replace all the Pots. These amp has some really low quality pots, replace all of them for New , good quality pots. There are 14 pots in total. 4x 22K Log (Gain and Master both channels) and 10x 10K Log

3) there's 3x 5watts resistors in the PSU. (2x820r and 1x 3,3K), those resistors get really hot and are next to the Filter capacitors also they burn the PCB.  Replace them for 10 watts resistors , will dissipate less heat, and solder then on the underside of the pcb

here are some pictures:

The 3 resistors

Burned pcb

I soldered on the pcb underside

less heat on the Filter Caps

https://i.imgur.com/Lw9hZXF.jpg

4) Re-solder all the solder joints. Apply solder to all top and bottom pcb pads (components and wires also)

5) If you have static/antenna noise while no guitar is connected at the input, check if the Cliff input jack is shorting signal to ground when so jack inserted. If not, replace the input jack (or jacks)

6) If everything seems fine, Amp turns on, LEDs light up, you have the correct voltages on the preamp (+15V and -15V), but you have no output sound. The problem might be the Output relay.
When you turn the Amp on you should hear a mechanical click inside the amp, thast the Relay actuating, when you turn off the amp you should hear another click from the Relay

7) if Headphone output doesn't work, probably it's a burned resistor. There's a 3W resistor soldered under the headphone output jack, really difficult to reach or see. That resistor attenuates the speaker signal so it can be more moderate for headphones., check if that resistor is burned, if so replace it. I don't know the original value, I took a burned one and measured 270r, but I can't be certain that was the originals resistor value. I replaced it with 330r

Bye Bye Peterson P100G, hope to never see you again