Mesa Boogie .50 Cal + hum problem

[Chevy]

Hi Guys,

Having a hum issue with this amp... 60 cycle stuff that's pretty annoying. I've just changed the power supply filter caps, but the problem persists. I've replaced V2 and V3 with new tubes today, and no change in the hum issue.

I've decided to trace the 60cycle signal on a scope, back from the Master volume, as the hum goes away completely when the Master is turned down to zero. The hum follows the Master volume knob setting, and is there even with the Gain knob (the first volume knob on this amp) at zero. So, this seems to eliminate V1 and V2a completely from being part of the problem. Scoping the power supplies at the top of each plate R...., this shows that V2B, V3A & V3B plate supplies are totally clean at the most sensitive scope setting.
The most hum signal appears at the plate of V3B, the driver for the reverb section. There is a little 60Hz at the grid of V3B, maybe .004V p-p, if my scope is accurate at that level. At the plate of V3B, I see about 200mV p-p.

So, it seems that the 60HZ first appears somewhere between V2B and V3B. It would seem the tubes are at fault, but changing them didn't fix it. (I also see some 60Hz on the bias supply, but with both grids getting the exact same signal, they should zero out, right? Also, this is way after the Master volume pot.)
Any ideas?

Thanks much!

[Chuck H]

If it's 60hz it's heaters, right? I'd check the ground for the 330k resistor, the cathode bypass caps for V2b and V3a and also the filter ground for those sections. And though it would more likely result in 120hz hum I would also check the coupling cap between V2b and V3a for leakage. For starters.

[soundguruman]

Hi Guys,

Having a hum issue with this amp... 60 cycle stuff that's pretty annoying. I've just changed the power supply filter caps, but the problem persists. I've replaced V2 and V3 with new tubes today, and no change in the hum issue.

I've decided to trace the 60cycle signal on a scope, back from the Master volume, as the hum goes away completely when the Master is turned down to zero. The hum follows the Master volume knob setting, and is there even with the Gain knob (the first volume knob on this amp) at zero. So, this seems to eliminate V1 and V2a completely from being part of the problem. Scoping the power supplies at the top of each plate R...., this shows that V2B, V3A & V3B plate supplies are totally clean at the most sensitive scope setting.
The most hum signal appears at the plate of V3B, the driver for the reverb section. There is a little 60Hz at the grid of V3B, maybe .004V p-p, if my scope is accurate at that level. At the plate of V3B, I see about 200mV p-p.

So, it seems that the 60HZ first appears somewhere between V2B and V3B. It would seem the tubes are at fault, but changing them didn't fix it. (I also see some 60Hz on the bias supply, but with both grids getting the exact same signal, they should zero out, right? Also, this is way after the Master volume pot.)
Any ideas?

Thanks much!

Replace light dependent resistors...
Check for arcing between resistors that are sitting on top of circuit tracks, and the board...a real common problem in these amps. Lift the resistors off the circuit tracks, so there is a space between the board and the resistors.
check for burning between circuit tracks, especially plate track and heater track, grid track and plate track...high voltage arcing between circuit tracks, forming carbon tracks between traces on the board. The tracks are spaced so close together that burning fiberglass, can form carbon threads through the board. The carbon tracks must be cut out.
check for open reverb windings....
removing parts from the circuit, then check with a megaohm meter for leakage between plate and grid or plate and heater tracks. bypass / cut defective tracks.
In areas with high humidity, near the ocean, rivers, lakes, where it rains a lot, expect arcing, burning between resistors sitting directly on top of circuit tracks, or from track to track, through the fiberglass...
I have fixed several boogies with these defects. The defects are often misdiagnosed as cold solder connections, bad capacitors, etc...even by the factory techs. Look VERY closely, with a magnifier, for tell-tale black spots of carbon between circuit tracks.

[Chevy]

Thanks for the tip!

This is one bit of esoterica that comes from hard-won experience... Would not have thought of it myself... much appreciated!

I dont' quite follow the "replace the LDR" comment though... can you fill me in on how they might cause hum?

[soundguruman]

The LDR is about 10 meg ohms when the light is off and about 200 ohms when the light is on. If you are amplifying a 10 meg resistor or an open resistor, it can hum. The LDR's occasionally fail to engage when the light is (supposed to be) on.
It's a pretty big effort to go through, disconnect (the resistor side) each LDR, and test it with an ohm meter...but you may find that some of your LDRs have opened up, or are out of whack.

The thing to remember in a boogie...stop assuming that the insulation between tracks, or the insulation between resistor and circuit track is working. The insulation breaks down when the amp is exposed to humid conditions. In Hawaii, New Orleans, other humid climates, this spells big problems.
When you have so many parts crammed together in such a small space, and almost no distance between circuit tracks, the insulation can fail.
Experienced techs often mount resistors, capacitors, LDR's to the OTHER side of the circuit board, to eliminate the overcrowded conditions, and to make the amp easier to service when it fails.

Of course we have been complaining about these problems (to mesa) for years. And guess what?

[Chevy]

LDR values.... Soundguruman or anyone?

Thanks for the LDR info... I measured the LDR resistance on and off, and this is what I got:

LDR1 2.6K/OL
LDR2 2.7K/OL
LDR3 1.5K/OL
LDR4 154 ohms/6M
LDR5 251 ohms/OL

"OL" means overlimit on my Fluke 111 meter, which is a pretty high reading... not sure, but likely over 40Megs. The meter reading was not constant on LDR4, as in the resistance seems to be changing as I'm reading it, so I think I should replace that one. But as for the rest, why the great discrepancies in values? Which ones should I replace? Should they all read around 200 ohms/10-20M ? Do all of them need replacing?

AES carries the Vactec LDR's... will their single unit work in all the spots for this Boogie?

Thanks much!

[Jazz P Bass]

Are you checking the LDR's in circuit.
If so, that may explain the variance.
FYI, on a Fluke meter, O.L. means Open Line.
Meaning just that.
No resistance whatsoever.

[Chevy]

Are you checking the LDR's in circuit.
If so, that may explain the variance.
FYI, on a Fluke meter, O.L. means Open Line.
Meaning just that.
No resistance whatsoever.

I'm checking the LDR's with one R lead disconnected, so no, they are not in circuit. So... why the variations?
OL is not "no resistance", it's actually "Overload" .... what you get when the input value is out of limits or over-range for that meter.

[Jazz P Bass]

?
OL is not "no resistance", it's actually "Overload" .... what you get when the input value is out of limits or over-range for that meter.

Not knowing which Fluke meter you have is not helping.
But I think it is irrelevant.

I have one Fluke 73 & (2) Fluke 78's.
All three indicate "O.L" when the leads are not connected to anything.
The largest value resistor that I have on hand is a 10meg.
All three indicate 10M.
Three in series reads 30M.
There is no indication of "Over Range" on the resistance scale.

My point is, I think the LDR that reads "O.L" is actually open.

[soundguruman]

Not knowing which Fluke meter you have is not helping.
But I think it is irrelevant.

I have one Fluke 73 & (2) Fluke 78's.
All three indicate "O.L" when the leads are not connected to anything.
The largest value resistor that I have on hand is a 10meg.
All three indicate 10M.
Three in series reads 30M.
There is no indication of "Over Range" on the resistance scale.

My point is, I think the LDR that reads "O.L" is actually open.

The .50 uses 5C1 and 5C4 LDRs (they are labeled) the difference being the on / off resistance. The data sheets showing resistance are posted on line at the manufacturer websites.
http://www.aikenamps.com/TI_datasheets.htm
The resistance depends on the current flowing through the resistor and the LED voltage. But it is not unheard of to have an open LDR, which could cause hum for sure. I many times just replace em all, and mount them to the TOP of the board, so they can be changed without taking the board out.
You may notice 2 LDRs in parallel, this is an area where one or both can fail.
I am sorry but working on this amp takes a huge commitment. The wires will start breaking off the board, and the transformer connections, as soon as it is removed...and really is a pain to work on for sure.
Check also the bypass caps for the preamp cathodes, which sometimes short out or become leaky. These are tantalum capacitors, and it's best to get high reliability, high temp caps (125C) to replace them (digikey, Vishay).
In the crowded space between V1 and V2, check under the resistors for burned spots under the resistors, which are laying on top of the circuit tracks. Check between these tracks for burned spots in the fiberglass.
you can check with the ohm meter on Meg setting, for leakage between heater tracks and plate voltage track. Also check for leakage between grid and plate voltage tracks.
Typically when leakage occurs, it will read about 2.2 megaohms through the burned spot in the fiberglass, between plate and grid or between heater track and plate....
I have been getting a good price on LDRs from MAMMOTH electronics.
I will try to give you some more info shortly...

This amp is for sure a brain strain, just try to work through it patently, take your time.

Anyhow, testing the LDRs is an important step to boogie maintenance, and so regardless if they are good or bad, you have not wasted your time. That's one of the fundamental tests, if you have removed the circuit board, along with checking the bypass caps..
and as long as the board is out, replacing the power resistors may not be a bad idea ether, if they look toasted.

[soundguruman]

If it's 60hz it's heaters, right? I'd check the ground for the 330k resistor, the cathode bypass caps for V2b and V3a and also the filter ground for those sections. And though it would more likely result in 120hz hum I would also check the coupling cap between V2b and V3a for leakage. For starters.

It is not unheard of for the heater track to short to, or develop leakage to other circuit tracks, which explains how AC gets into the audio path. Ive drilled a hole right through the board to stop it sometimes...

[Chevy]

Soundguruman... Which LDR's to get for my .50 Cal+ ?

Thanks so much for the info!... I've found a high voltage track that was arcing to the adjacent LDR power supply runs. Bypassed it, and now will replace all the LDR's (1 thru 4 anyways).

So... looking at the LDR's, they all seem the exact same unit to me, and don't have any 5C1 or 5C4 markings on them. Only difference seems to be what looks like a batch number or something (21L883 and 21L884 are the only ones I can see).

Can I just go with all 5C1's? It has a 50M "off" resistance, compared to only 400K for the 5C4. Also, the cell voltage rating for the 5C1 is 100V, but only 50V for the 5C4. (This seems low to me... for either of them.)

So... can I just order up 4 5C1's and pop them in there... any worries about that?

Thanks much again!!

Chevy

[soundguruman]

See? I told you!
When bypassing the track, make SURE the burned part of the fiberglass is COMPLETELY removed. Cut, drill, scrape...until its all out of there.
The burned fiberglass is a RESISTOR made of CARBON threads, and cannot touch any of the remaining circuit tracks.
Or else it will start burning across the tracks again.

[Chuck H]

+++
Absolutely. Seen it a few times. "CARBON" makes a nifty resistor.

Incidentally... The black used in sharpie marers (or any marker) and to pigment rubber or plastic...etc. IS ALSO CARBON!

The black pigment used in almost all media is "lamp black". It's the equivallent of the soot that builds up inside glass oil lamp wind shield globes. Thus the name. It's no big wonder that those old Fender boards can become conductive spontaneously.

I've even seen this phenomenon cause fires in commercial lighting fixtures.

I NEVER build on vintage correct black fiber board for this reason.

[soundguruman]

DISCLAIMER
If you have been reading up to this point, I wish to clarify that ALL Mesa Boogie amps DO NOT arch and burn. Only some of them, which generally have been exposed to high humidity. Also, spilling beer inside of the amp should be discouraged, and is not recommended.
I think Mesa Boogies sound good, even though I (personally) may not like the design of the circuit board(s). This is just my own personal preference of construction methods...

But, one more repair tip for amps with fiberglass circuit boards:
The board can be cleaned with denatured alcohol. It can be wiped with a paper towel, or Q tip that has been dipped in alcohol. You can drip alcohol on the board and scrub the board with a clean toothbrush. This does not harm the board or components, and dries quickly.
This will remove flux, loose carbon grains, magic marker, and other residue. It will clean it up properly.
This will not remove carbon threads or burned fiberglass, you have to cut or scrape that to remove that.
Denatured alcohol is flammable and toxic, and should be used outdoors, with precautions.
But, the alcohol is probably the ideal thing to get the board cleaned, after it is repaired.

Denatured alcohol is cheap, much cheaper than de-fluxing spray. And it works just as well or better, try it.
after the old parts are removed or the circuit track has been cut out and scraped, clean the board with denatured alcohol to remove the flux and other junk,...before soldering in the new parts.
This gives you a cleaner repair, removing the crud that might cause further arching.