Blues Jr III and self repair gone wrong.

[Veltek]

Good day! I am a first time poster here, and I am looking for some guidance. I have scoured the web and cannot find anything that is similar to the issue I am having. So, allow me to backtrace a bit, and give you guys and Idea of what is going on.

I had some crunchy knobs that needed to be cleaned out, and asked my brother (a mechanic by trade) if he had any Contact cleaner I could borrow. He lent me some and I sprayed it all up inside those pots. Got them nice and smooth... until they started to feel loose, and some very stiff too. Well turns out it was the wrong contact cleaner, and it was not safe for plastics, and I melted the shafts. So I ordered some snap-in pots online, looked up all of the resistances, and got everything desoldered, and resoldered. The first time I plugged in the amp, it came on fine, but the volume pot would not work. I still had sound though, a very clean, chimey, almost Deluxe reverb sound, that got overly loud overly quick on the master volume. So I figured I did a poor job soldering, and tried again. This time I got the Volume to work, but the master would cut in and out depending on if it was nudged the wrong way. And over all even the cleans sounded kinda muddy and bad. So I took the amp apart a third time, and worked on my soldered joints. The Master is good again, but the Volume will no longer work! I get very crisp clean tones, but 0 distortion or volume increase from the volume knob. I took it apart a fourth time, and ended with the same result as previously.

All my tubes glow as expected, there is no weird humming or buzzing. any distortion pedals sound like garbage in front of the amp now. I am thinking I either burned the board some where, I messed up one of the solder joints on the PCB mounted Valve socket, I messed up the R8 plate resistor between V1 and V2. or I plain have a bad Volume pot.

Any ideas or suggestions?

[drewl]

Do you have a meter to verify if the pot is still good?
Maybe it got damaged.

Check to see if the solder connections are good and if any of the connecting traces are cracked and that the pot leads are making a good connection.

Did the original pots have four surrounding chassis grounds?
If your new pots don't those connections will have to be made manually with wire.

[Veltek]

Do you have a meter to verify if the pot is still good?
Maybe it got damaged.

Check to see if the solder connections are good and if any of the connecting traces are cracked and that the pot leads are making a good connection.

Did the original pots have four surrounding chassis grounds?
If your new pots don't those connections will have to be made manually with wire.

Hey, I have a multimeter, but I am unsure how to use it, or rather which setting(s) to use? Mind giving me a quick runthrough of what I need to do? (What settings to use mostly. I dont wanna fry the multimeter, Lol)

[Veltek]

So this is what I have found. if I test the amp off, in Ohms, the Volume knob reads 1.006 M ohms (Its a 1M10A pot) so that seems right. I test the wiper in the middle, and it seems functional too. All the rest are about the same, although my bass pot seems to be below its 250k threshold, but the bass seems to be working through the amp, so ill stash that for a later time.

Now when I turn the amp on, and use DC V on the multimeter, all of the pots read 0, or pretty close to it, but the volume pot reads in the 100's.

Also, when I bring the Lead near the Volume, i start picking up the church channel through my speaker, Which i seem to be able to adjust the clarity (and volume to a small degree) with the same Volume pot in question. Does that help any? lol

[Steve A.]

In ordering the pots you do need to get the proper taper (audio, linear or other) for the controls to operate properly.

FWIW the pot cleaner most people use is DeOxit Red from Caig. (They also make a DeOxit Gold which is more of a conditioner/preservative with mixed reviews... I love it but many people don't.)

Steve Ahola

[Dave H]

In ordering the pots you do need to get the proper taper (audio, linear or other) for the controls to operate properly.

The Vol pot says 1M 10A which I assume is 10% Audio (log) taper but the Master is 50k B (linear) so the volume coming up quickly on the master could be normal.

[Dave H]

Now when I turn the amp on, and use DC V on the multimeter, all of the pots read 0, or pretty close to it, but the volume pot reads in the 100's.

Are you measuring with respect to ground (chassis)? All three vol pot legs should measure 0V wrt ground. If your DMM is auto-ranging is that '100s' hundreds of mV or hundreds of volts? If it's hundreds of volts then either C2 is leaky or it could have a solder bridge across the pcb.

[Veltek]

Are you measuring with respect to ground (chassis)? All three vol pot legs should measure 0V wrt ground. If your DMM is auto-ranging is that '100s' hundreds of mV or hundreds of volts? If it's hundreds of volts then either C2 is leaky or it could have a solder bridge across the pcb.

I will check the solder joints again and check for any stray solder.I'll also check the C2 position. I'll get back with you later this evening. Thank you for the reply!

[Veltek]

In ordering the pots you do need to get the proper taper (audio, linear or other) for the controls to operate properly.

FWIW the pot cleaner most people use is DeOxit Red from Caig. (They also make a DeOxit Gold which is more of a conditioner/preservative with mixed reviews... I love it but many people don't.)

Steve Ahola

All the pots are indeed in the correct orientation. I reviewed the schematic heavily before ordering and installing, also I got some Better Contact cleaner lol

[pdf64]

Now when I turn the amp on, and use DC V on the multimeter, all of the pots read 0, or pretty close to it, but the volume pot reads in the 100's.

Sometimes putting the meter probe in the signal path can set the amp oscillating, which may be ultrasonic so you'd not be aware of it.
The oscillation could cause some wacky meter readings, especially if it's a cheap meter.