I managed to get my hands on a "fixer-upper" Pathfinder 15R with a couple of issues that required some resoldering and an op-amp swap (yes! An op-amp swap that actually fixed a problem!).

I was going to go germanium-clipping-diode on the Pathfinder but then thought that I was going to try the famous LED mod instead. The latter being an obvious misnomer since there are no LEDs post-mod.

I ran the amp through an external speaker with an attenuator. So I was able to max the gain while maintaining the volume at room temperature.

Anyway, I was surprised at the end result of the mod. The sound was pretty amazing: a very nice crunch with no fuzz/fizz. Wow!

The problem, I realized, was that you could only really get this nice sound if the gain controls were somewhat past 2 o'clock (with my Tele). Which made for a very loud amp if I were not using the attenuator.

So in typical Bluesky6 fashion, I changed the volume circuit:
1. Swapped out the linear taper volume pot for a similar value audio pot
2. Swapped out the 4.7K series resistor to the volume pot for a 470K. I know: it's high, but it was one value that gave me room volume with the volume pot set at 9 o'clock while the gain pot was at max
3. For good measure, I put in a 4.7nF tone bypass cap. I may yet swap that out for a slightly larger value

Once I had done this, I had reduced the main signal so much with respect to the reverb return signal that the latter basically overpowered the main signal. So I swapped one of the reverb resistors from 4.7K to 100K (after experimentation with different values).

The end result is a pretty simple amp with a very nice tone at reasonable volume.

The attached pic shows the changes I made. Apologies for the crude mouse-drawn tone bypass cap.

I started looking at my new/used Marshall MG15CDR last night.

Nothing wrong with it. So I haven't quite figured out what to do with it. Perhaps socket the op-amps and do some swapping.

Picture of the innards attached. The schematics can be found at amparchives.

The thing at the bottom below the PCB is the reverb unit. It looks rather fragile: hanging from some springs.

The power amp is different from the one on the schematic. This one's an LM1875 which I guess is an equivalent. I like the big heatsink.

I'm taking suggestions for mods/not-mods/experiments

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

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