I'd just loosen that set screw and remove the remnants of the broken knob. Make sure the pot nuts are tight before replacing. Unless the shafts are broken, I think the pots are probably fine, unless they're cutting out or something when you turn them.

I've got a couple beat-up traynor knobs. You can have them for the cost of shipping if you want.

Those little electric fans can squeal and wail like hell when the motor is starting to fail. They're easy to replace, though.

Nate Dort wrote: Sat Mar 23, 2024 9:03 pm
Bought the mic on a whim without any of the necessary accessories, like the power supply and 6-pin cable or shockmount.
Found a Sterling/Groove Tubes PSM-1 tube mic power supply with a broken 7-pin connector, so I swapped it for a 6 pin. Converted one of my star-quad mic cables from 3-pin to 6-pin, which worked because the mic only uses 5 of the 6 pins (6.3 V DC heater, 120 V DC B+, Audio +, Audio -, GND).
I had to tweak the power supply slightly, as it uses an LM317 in a Constant Current configuration to generate the heater voltage. The LM317 used in this way is less noisy than when it's used in the more common Constant Voltage mode. The PSM-1 was originally used with a mic that had a 6205 miniature tube, which only draws 150 mA, so the LM317 circuit was scaled for that. The tube in the AT4060 is a 6922, which requires 365 mA. I initially powered it up and could only get about 2 V on the heater, even after adjusting the trimpot (VR11). It just didn't have enough range.

PSM-1 schematic:

I've seen a number of modern low to mid-range tube mic PSUs regulating the B+ with a zener diode at the last stage, in parallel with the output. Just one more stage of RC filtering after the zener drops the noise on that B+ rail by a lot, I've found. I wonder why they do that. It's not cheaper.

Ampeg SVT-II (non Pro):

I've been wanting to use this to record with, but the fan was so damn loud that it was bleeding into the other mics in the room, mostly the drum overheads.

It comes with an 80mm 120VAC fan, which are substantially noisier in general than DC fans, due to the way their motors are wound. I found a 15V rail easily accessed from the back of the graphic EQ PCB, so I removed the old fan and mounted a super quiet 12VDC fan intended for PC cases (Noctua NF-A8 FLX). I had to add some rubber spacers under the fan, as it was slightly thinner than the original, but that also helps isolate it from the case and reduce vibration noise. Also added a cable clamp I found in the garage, and used an existing screw to secure it.
It was still a little louder than I would have liked when hooked directly to 15V, so I added one of the current-limiting in-line resistor adapters it comes with to slow the fan down slightly. Still seems like it's moving about the same amount of air as the original, but is maybe 30 dB quieter. I can barely hear it now, and it's completely drowned out by the slight 120 Hz hum of the power transformer and residual noise coming from the speakers once I take the amp out of standby. Well worth the $15 cost.

Related -

A friend has a JHS Clover. Recently, the only sound it is making is a good, solid "Whooosh...." sort of a sound.

He got in touch with JHS, and didn't get much of anything back from the company.

Anyone ran across this? Any chance it is a reasonably simple fix?

numberthirty wrote: Fri May 10, 2024 5:44 pm Related -

A friend has a JHS Clover. Recently, the only sound it is making is a good, solid "Whooosh...." sort of a sound.

He got in touch with JHS, and didn't get much of anything back from the company.

Anyone ran across this? Any chance it is a reasonably simple fix?

Could be anything really, but typically connectors, stomp switches, and pots are the first things to fail in a pedal. Basically anything that requires a human to touch it.

The right way to diagnose this would be to put a signal into it and start tracing it through the circuit with an oscilloscope. Might find an open solder joint somewhere, or maybe a failed component.

The brute-force method would be to resolder every solder point inside the box and see if that fixes anything.

Depending on the era of JHS pedal a repair could be more trouble than its worth. A lot of them now are all surface mount components with everything mounted to a PCB. Switches, jacks, etc. If its something solved easily by reflowing solder joints, that's okay, but replacing components could be a drag depending on one's tech abilities.

GK 800RB:



Build date of April 1984. Second-oldest one I've ever worked on. Somebody blew up the 100W section at some point, and attempted to rebuild it. A lot of the components were replaced (poorly). They had stuck some SK3007 germanium transistors in place of the MPSA56 transistors for some reason.
They also had a replacement toroidal transformer and new bottom plate. I confirmed that the primary side of the original EI transformer was open, so I did some surgery and exposed the internal fuse. That was open, but the windings still measured OK, so I replaced that fuse. I happened to have some replacements from another transformer repair I did a while back. Taped it back up and confirmed that the power supply was working again.



I cleaned up some dodgy solder joints on the output board, replaced a couple old electrolytics while I had access, and tried to bring it up slowly on the variac. It was pulling way too much current, so I started probing the output transistors and found that the 100 W section was showing a short between the main power transistors and ground. Turns out whoever replaced those transistors didn't put a mica insulator under one of them. I don't have any on hand, so I'll need to wait for some to arrive.

That new bottom plate won't fit this old chassis. It's about an inch too deep. They must have changed the dimensions of these things at some point. I'm missing one of the plates, so I'll have to make a new one out of aluminum. I'll sell that new transformer and plate to somebody who can use it.

Wurlitzer Electric Piano 206a/200a

This is the non-vibrato student model I chopped to a portable size a decade ago, but I only this week added the vibrato components that were missing from the amplifier board. I had a couple pads lift on the PCB and I had to update a couple connectors (thanks FM Nate Dort for the advice) but it now has a working vibrato and a line out. It seems like the speaker outputs don’t work and I wonder if it’s an issue with the power transistors. There again, I also don’t have the internal speakers now either so who cares, I guess, I’ll just use a line out to an amp.

Nate Dort wrote: Thu May 16, 2024 1:14 pm GK 800RB:

I'm missing one of the plates, so I'll have to make a new one out of aluminum.

Got a $5 aluminum baking sheet and cut a new plate with a jigsaw, then sprayed it with a couple coats of black. It's about half the thickness of the original, but it's good enough for the bottom.

I swapped a bunch of the LM353 opamps in the preamp section with some TLE2072s. Lower noise, so way less hiss at idle. I set the bias, tested both 100 W and 300 W sections with a dummy load for a while, then played through it a bit today at volume. Sounds like an 800RB should. Might hold onto this one for a while. Its the only solid-state amp I currently own.