Mesa Studio .22 troubleshooting

[Cordwangle]

Thanks for having the same problem as a cal .22+ I'm working on.......lol

I'm trying to find the cause of some problems with this Studio .22 Caliber thing.

SCHEMATIC:
http://www.webphix.com/schematic%20h...ogie_22cal.pdf

Symptoms:
-Loud hiss, especially in the clean mode.
-All the pots located in the preamp are very static-ey when rotated, the volume control is the worst.
-The volume control gets louder up to 3 on the dial and then gets progressively quieter as it's turned to 10.
-The tone controls do very little if anything in the clean mode.
-In the distortion mode the bass and to a lesser degree the middle control act more as volume controls that work backwards, with the most gain (massive gain) at zero.

Troubleshooting thus far:
I see no visible signs of trouble.
I plugged into the return jack on the back and the power section checks out fine.
Reverb is fine.
I subbed a new pot in place of the volume and the static continued, but to a lesser degree. That and the fact that the non-preamp pots are without issues suggests that the problem is not with the pots themselves.
I bypassed the tone stack and if I recall correctly the problems disappeared, but I'm not sure that the V2a plate resistor isn't the problem.
Problems appear in both the clean and distortion modes, so that would indicate the problem is somewhere in the vicinity of V2/tone stack and not anywhere in front of V2.

TO SUM UP: the problem seems to be in the V2/tones stack area. I've not run into a problem like this before. And I'm not proper repair tech. But my best guess is that it's either the V2a plate resistor or a cap. I have no idea how to check a cap to see whether it's bad - can someone advise on that?

I haven't removed the circuit board yet.

So there it is. Can you guys help me figure this out?

Will replace some caps in the tone-stack & get back; cheers, Paul

[soundguruman]

Will replace some caps in the tone-stack & get back; cheers, Paul

Everything you "think" it is : WRONG. Randomly replacing caps and other parts will get you no-where.

WELCOME to: "The Arc of Technology."

BIG chance that something on the board is arcing, or burning through the fiberglass
CLEAN all your controls and jacks, with Deoxit D 100, don't use cheap spray cleaners, like WD 40.

There are two main things:
1. Resistors laying on top of circuit tracks. LIFT THEM UP OFF THE TRACKS!!!
LIFT the resistors off the tracks, so there is a space between each resistor and the track. 1/8" space would be ideal.
The resistors arc and burn, down to the tracks. This forms a burn spot under the resistor. LIFTING the the resistors, one at a time, will reveal the burned spot.
When the resistors burn down to the tracks, it causes crackling, popping noises. This is high voltage arching, usually between the plate and the heater or plate and grid.
This problem is often misdiagnosed as: "loose soldering connections." Even by factory techs. Re-soldering WILL NOT fix the problem.

SEE the 3.3 meg resistor connected to the input jack? That resistor is sitting on top of a high voltage track. LIFT that resistor, and see if the bottom of it is burned. MAKE SURE there is a space, under that resistor, when you re-install it.

2. Arching, burning through fiberglass:
It is common for the plate track (high voltage) to arc through the fiberglass to the heater, cathode, or grid track.
This forms a carbon thread through the fiberglass. The carbon thread IS an extra resistor in the circuit.
This carbon thread will typically measure 2.2 Meg ohms, approximately...
The carbon must be CUT, DRILLED, REMOVED from the board, COMPLETELY. The traces that are arcing must be disconnected and BYPASSED.
A. Take ALL the parts out of the circuit stage. B. MEASURE with a meg ohm meter, for leakage between the traces on the board. C. Measure to find leakage between plate and heater, plate and grid, or plate and cathode tracks.
B. Bypass the tracks that are leaking into each other. CUT the traces and bypass them, with wires.

* You CANNOT measure this leakage through the fiberglass, without a MEG OHM scale meter. The range of the meter should be 2-10 meg ohms. A normal "consumer grade" meter, with 1K scale WILL NOT detect the carbon burns in the fiberglass. Measure these traces, with all the parts OUT of the circuit, CAREFULLY.
** LOOK closely at the board with a lighted high power magnifier. LOOK for BLACK SPOTS, signs of burning, between closely spaced circuit tracks. SHINE a flashlight through the board, to LOOK for burned spots.
*** You CANNOT "clean" carbon off the board with "spray cleaner." Once the burning has occurred, the carbon must be removed entirely. CUT, SCRAPE, DRILL, until the carbon is COMPLETELY gone, with NO trace left anywhere.

TECH TIP: More than 80% of the Boogie amps I have fixed in the passed 35 years have some type of arching, burning, on the circuit board. FACT!!!
SWITCHES: that change between pentode and triode, or change the power level: These switches MELT. Check carefully that the plastic switches are not damaged or melted.
WIRES that connect from the board to the switches, or wires that connect to the tube sockets: INSPECT the wires for burned spots, or signs of carbon build up on the outside of the wire.

HOWEVER once you find and correct (bypass) your burned spots, the amp will work just fine, maybe for several more years. I have 100% success rate fixing Boogie amps. BUT, it takes waaaaaaaaaaaaaaaaaaaaaaaay more time to find and correct the problems, compared to other amplifiers. Mesa Amps are VERY time consuming to work on, most techs won't go near them. (and I can understand why!)

[woodyc]

The Boogie Board • View topic - Lead Drive mod Studio 22+ Here it is,this one says to use a 1meg pot,you can use either,but a 500k will give you close to stock when turned up full,the 1meg may give too much drive when turned up full.Real easy mod,3 pieces of wire and a pot.

A guy brought me a Studio .22+ last week along with a handful of printouts of Boogie Board messages, and said he had to have this mod.

I tried my best to talk him out of it, but he was insistent.

Its true that the inability to change the gain of V1 in this amp does impact your ability to get a good sound out of the lead channel, but...

1) its not all that easy.
Getting the board in a position where you can work on it requires bending leads you shouldn't be stressing, and/or desoldering stuff from the board. The desoldering is not all that easy because in some cases the wires are big, and in all cases the traces are wide and on both sides. Even the original joints on those boards aren't that good -- in some cases the solder stayed on one side and didn't flow thru. It just takes a lot of heat. Since the board I worked on was old and I didn't want to risk lifting a trace, I just used enough heat to get stuff out and used a small drill to reopen the solder tabs. Also the plastic standoffs near the tubes were brittle and discolored from heat, and broke, so they had to be replaced. I got it done without compromising the board or the extant wiring at all, but it was tedious and took a couple hours.

2) the mod, as described, is not effective.
The problem is that when you switch in V1, you are also switching in a downstream divider (the lead master.) If you don't also mod that divider, you only have a very limited ability to use your new pot. You could turn up the clean master to compensate, but that messes up your ability to get a good clean sound. It would probably suffice to substitute a higher value for the lead master pot, but I haven't tried it -- I just did exactly what the guy wanted.

[soundguruman]

A guy brought me a Studio .22+ last week along with a handful of printouts of Boogie Board messages, and said he had to have this mod.

I tried my best to talk him out of it, but he was insistent.

Its true that the inability to change the gain of V1 in this amp does impact your ability to get a good sound out of the lead channel, but...

1) its not all that easy.
Getting the board in a position where you can work on it requires bending leads you shouldn't be stressing, and/or desoldering stuff from the board. The desoldering is not all that easy because in some cases the wires are big, and in all cases the traces are wide and on both sides. Even the original joints on those boards aren't that good -- in some cases the solder stayed on one side and didn't flow thru. It just takes a lot of heat. Since the board I worked on was old and I didn't want to risk lifting a trace, I just used enough heat to get stuff out and used a small drill to reopen the solder tabs. Also the plastic standoffs near the tubes were brittle and discolored from heat, and broke, so they had to be replaced. I got it done without compromising the board or the extant wiring at all, but it was tedious and took a couple hours.

2) the mod, as described, is not effective.
The problem is that when you switch in V1, you are also switching in a downstream divider (the lead master.) If you don't also mod that divider, you only have a very limited ability to use your new pot. You could turn up the clean master to compensate, but that messes up your ability to get a good clean sound. It would probably suffice to substitute a higher value for the lead master pot, but I haven't tried it -- I just did exactly what the guy wanted.

Hahahahahahah! Hell on earth...

[Tom Phillips]

...The desoldering is not all that easy because in some cases the wires are big, and in all cases the traces are wide and on both sides. Even the original joints on those boards aren't that good -- in some cases the solder stayed on one side and didn't flow thru. It just takes a lot of heat. Since the board I worked on was old and I didn't want to risk lifting a trace, I just used enough heat to get stuff out and used a small drill to reopen the solder tabs...

Note that the PC board in that amp uses plated through holes. The solder should wick through to both sides if the proper amount of heat is used and the lead/wire diameter is correct for the hole size. In addition to wicking through during installation, plated through holes allow one to work from only one side and remove the solder from both sides with suction or solder wick. If you drill out the hole you may be removing all the original metal that connects the two sides together. If so, you will loose the ability to solder / de-solder from one side.

Also notice that there are some solder filled holes on the Mesa PCBs that don't have any components or wire installed in them. These are via holes that connect top traces to bottom traces forming part of the printed wiring circuit. Their integrity depends on the conductor that coats the inside diameter of the hole and allows the solder to flow through from one side to the other. Some PCBs don't even have solder in those via holes.

Regards,
Tom

[woodyc]

...If you drill out the hole you may be removing all the original metal that connects the two sides together. If so, you will loose the ability to solder / de-solder from one side...

Sorry I didn't mention that. I thought it should be obvious to anybody. The drill was .032", big enough to get a wire thru but much smaller than the hole so there is zero damage of any kind to the board. I think it was a really good solution in this case but as Tim suggests, you have to be precise. Here's my drill...

[DaveCoatesAudio]

....TECH TIP: More than 80% of the Boogie amps I have fixed in the passed 35 years have some type of arching, burning, on the circuit board. FACT!!!

Spot on soundguruman. I've just had a Studio 22 in for repair making crackling and frying sounds and the problem was breakdown of the fibreglass PCB between R212 and the ground plane. The traces run very close to each other for having 160v between them and after a bit scraping away of burnt PCB, it works fine again.

[J M Fahey]

[g1]

You know what they say about that broken clock.