I went out and ordered several cheap used amps from GC. Cheaper than Eb*y and cheaper shipping. Goal was to try out different brands of amps and their "tone".

One Crate came in missing half the screws that hold down the speaker with the rest not tightened down. The chassis was also loosely screwed on. Sigh. Waiting to hear what GC has to say before I decide what to do with it.

A Fender Frontman Reverb Amp aka 15R looked fine and tight but had this BUZZ on power up. More buzz when I turned on reverb. Otherwise, it worked fine, if a little loud.

Touching the "ground" points didn't help. Tomorrow, I'll follow one online suggestion to connect a wire between a signal ground (at one of the jacks) to the handle hold down. If that doesn't work, that's yet another fun project! Whoopee!

My Behringer GM108 had a similar problem until I resoldered some of the power supply components (caps etc).

Can't wait for the weekend to start digging in... :duh

I just ordered several sets of op-amps (2132, 2134 and tl072). One option I have for the amp is to use it primarily with headphones (due to aforementioned non-deaf spouse). So step one will be to see if I can reduce the noise level.

I have also been researching power transformers. Again, with the goal of reducing the volume, I was thinking of using a 10-0-10 transformer. This will "naturally" provide a lower voltage power supply to the op-amps without the voltage drop resistors. It will also reduce the max power of the audio amp. Or allow me to use a lower wattage and voltage transplant like a TDA7052A or the Velleman 3W amp kit.

I discovered several sets of transformers from Hammond that would just slot in to the existing mount holes.

Guess what: the compatible transformers range from 10 to 12VA power rating.

If the OEM transformer is of the same or similar capacity, that means the amp in factory configuration is really too underpowered to claim a 10W output, considering you really don't have 100% efficiency.

More thoughts to follow.

Roly, thanks for the pointers on the diodes. I'll start my experimentation with an green LED and may play with other types of diodes in series...

Picture of the innards.

I don't like the incoming AC having to go through the main PCB.

The power amp on a separate PCB with a long 5-wire pigtail also looks nasty. The schematic Roly posted indicates a ferrite core but not on the amp that I have.

The big positive is the modability of the board. I have a Behringer GM108 (now for sale) which I wanted to mod but gave up on the idea because of all the SMT components.

Quote from: BigPolishJimmy on April 09, 2014, 10:07:26 AM
I've been curious about the pathfinder 10.  How do you like it so far?  Obviously you're looking to mod it, but is that more from dissatisfaction where it is lacking, or more from wanting to customize it to your liking?

To be honest, I spent about 20 minutes playing through the amp before taking it apart. While the cleans were okay (I have a Fender FM65R as a comparison...!), I didn't like the drive channel, even using my external 1x12 cab. Hence the thought of reducing the gain on the 2nd op-amp. But that's my personal preference: I like a dry compressed crunch without too much fuzz.

I did find that, like some others, using the gain as some kind of volume control worked pretty well.

Roly, thanks for the suggestions and the schematic.

I'll look into replacing the leds, either with a single one of different color or with a couple of germanium diodes. There's a blog article where someone used a pair of 1N34As.

The volume control pot is a B50K which I assume means that it's linear as opposed to audio taper. An alternative to replacing it (since it's probably hard to get an exact physical replacement) is to replace R18 with a large value.

My problem is that of too much volume (hearing sensitive spouse and all that) 

Other mods I'm thinking of:
1. Move the AC circuit from the main PCB and use a fuse holder instead. Will mean drilling a hole in the chassis. This will make removing the PCB easier for tweaks.
2. Remove all the original audio amp components and use one of those PSU+AMP TDA2030 boards from Eb*y. These boards include power rectifier and amp. The question is of available space...
3. Putting in sockets for the op-amps to try out the lower noise ones

I'm going to try to reduce the noise level as much as possible so that if I don't do the speaker out mod, I can use headphones without the hiss. I've a Vox AC30 Amplug and all I hear (with gain >2) is hiss.

Just got myself a Pathfinder 10 (the Pathfinder 15/R are going for way too much money for a mod platform).

After playing with it a little bit, I found that at high gain/distortion, there is a lot of speaker buzz on the little 6.5". No buzz with an external 12" cab. So I'm thinking of putting in a Jensen MOD6 to see if things improve. The Eminence 620h is tempting but costs more than the price of the amp...!

Question for those who have tried both the 15/R and 10: is it worth tweaking the 10 to recreate the pre-amp of the 15/R tone-wise? From the schematics, the two seem to have very similar gain and tone stages.

Other mods I'm thinking of:
- Change the gain resistor of the second stage to reduce the gain when the drive switch is on
- Use an analog taper pot for volume
- Zeners to "control" the supply voltage for the op-amps
- Remove/short the headphone resistors and use the headphone jack as a speaker out jack