Mesa Boogie Mark 3 Gain Loss

[pdf64]

All those heaters need 6V across them, ac or dc. You need to measure the voltage between pins 4/5 and 9
Can you locate those 4 diodes on the board (from the schematic kindly posted by NorCalTuna)? If so then test them.
Have you contacted Mesa to request a full schematic?
The board is probably double sided / through plated. Peter.

[sixstring63]

Could this lack of voltage on the heaters cause the gain loss I'm experiencing with this amp ?
Mike

[sixstring63]

Think I got it fixed

I think found the problem on my Mesa tonight. I went down to my garage workshop and started playing around with the oscilloscope on The circuit I thought might be the culprit but could not find the bad component in the DC power supply to the first preamp tube heater. Started checking things again and was still getting an AC signal in the rectifier circuit and all the sudden it would flatten out intermittent to a DC signal and then saw my finger was resting on a radial cap in that circuit so I pushed it to one side and it held the DC signal and the first tube lit up like normal, I must have a bad solder joint under that cap. Wedged a piece of cardboard between that cap and another then plugged in my guitar and it sounded great, all the warmth and crunch was back though not as good as it was new but the rest of the components are 23 years old . Now to get that pain of a PC board out of the chassis to fix the problem and replace some of the other tone bearing caps. Thanks for all your help guys,I have learned a lot and hope to continue.

Mike

[pdf64]

Well spotted. Mesa pcbs I've seen are through plated, so you could get away with soldering from either side, though good practice is to solder from the pad side.
I've just been through an early 80s Fender Rhodes amp which was crackling and cutting out. All 3 pcbs had numerous dry / fractured joints, so I decided to resolder all pcb pads. The obviously fractured joints were on the big electros and connector headers, but more reliable just to do them all to avoid missing one.
If your pcb is single sided, suggest you go through the whole board, it's unlikely that there's only 1 bad joint. Peter.

[sixstring63]

Bit off more that I could chew

I guess I should have left this job to a pro ! Changed the 3300uf cap in the rectifier circuit for the DC heater supply that was bad and also changed out the 3 30uf 500V caps plus the 2 50uf 75V bias caps.Put the thing back together and what a nightmare these things are to work on ! Now I know why some repair guys don't like to mess with the Mark amps. Fired it up and it works but don't have half the volume it used to have and is not getting enough plate voltage to the pre-amp tubes only around 90V or less on some , I don't know what went wrong , with the guitar plugged in when you roll back the volume on the guitar you get an awful scratchy sound like a dirty pot but I know it is not that because I used it at practice last night in my other amp and it was fine. I dread the thought of taking this thing back apart. Any ideas guys ?
Mike

[stokes]

If you are only getting 90vdc to your preamp tubes,you fouled something up when you changed those 3 30uf 500v caps.Those feed the dc voltage to your preamp.The scratchy guitar pot sounds like you have dc voltage on the grid of V1

[sixstring63]

New caps bad ?

Could the new caps be bad ? I checked all my connections before I put it back in the chassis and everything looked good, can't think of any reason it should not have worked.
Mike

[stokes]

Something aint right with your power supply,since all you did to the power supply was to re-cap it,the problem is there.Are the volts to the power tubes okay?Its hard to tell from here if the caps are bad,but I cant imagine a bad cap would load down the B+ that much without blowing a fuse.

[sixstring63]

The voltage coming in is good ,about 420v and after the 5.6k resistor it drops to like 130v when it should be up in the 300 and something volt range as best I can read from the unreadable schematic I have and drops to around 90v after the 1k resistor where it should still be up in the 300v range. The caps I put in are F+T caps from Germany and the only things I could come across at a reasonable price,I used these same brand caps on the main power rectifier circuit and seem to work fine.Only thing I see that is different is when you check the voltage on the caps after power down it is up around 55v and on the original caps it was around 12v and they are also quite warm after the power is on for awhile and I don't recall this on the originals.When I check the resistance on the 5.6k resistor it reads around 8.5k on the meter and I don't know if it was like that before or not.From what I see the caps should not drop the voltage like that. I'm just about ready to put the old ones back in and see what happens. I'll attach the schematic so maybe someone can take a look if you can read it. Mike

[sixstring63]

Bad 5.6k resistor

I checked it out some more and found when I checked the voltage after the 5.6k resistor after turning the the standby off the voltage would drop 30-40 volts over a period of time so I started thinking that this resistor was breaking down with heat and sure enough I hit it with air and the voltage started to come up. I pulled the resistor out and the board was scorched a little underneath and I noticed that the original caps had a little scorch on them too that was next to the resistor, I did not notice when I removed them.I'll have to wait and see how it works since I have new resistors on order but I think it will do the trick.
Mike

[pdf64]

'after turning the the standby off the voltage would drop 30-40 volts over a period of time' What would you expect?
Maybe the resistor is bad, but perhaps a circuit it is feeding is drawing too much current, causing it to drop a lot of volts during operation, and so get too hot.
When you've replaced it, the new one might give the same result. If so, remove the pre amp tubes it is feeding. Sould get same voltage either side of the 5k6 dropper. Then replace pre amp tubes 1 by 1, monitor voltage until you find the tube which causes the voltage to drop a lot. The problem will likely be related to that tube or the circuit around it. Peter.

[sixstring63]

Resistor

The 5.6k resistor in question had a value of around 8.7k when checked with a meter out of the circuit and from the looks has been heating up for awhile and when I talk that it dropped I'm saying it started out 200v below normal and dropped 30 to 40 more when it heated up.I've read that the carbon comp resistors sometimes go bad over the years and the values tend to drift. I'm figuring things out and reading a lot of books on tube amps as I go , so far I fixed the tube heater rectifier circuit and I'll figure the rest out too,as I've stated before I'm new to this and do not have a lot of experience like some of you guys and I would hope to get a little slack on some of my questions.When I spoke of the voltage drop after putting power to the pre-amp tubes only the voltage pulled down after the 5.6k resistor not before and would come back up some but not to where it should be when cooling the component.

[oc disorder]

To reiterate what pdf64 said, if you remove all the pre-amp valves so there is no load on the power supply after the 5.6k (2 watt) resistor you should get
the same reading either side of the 5k6 resistor.
If not something must be dragging down the supply eg the .003 cap between the anode and cathode of valve 4a the reverb send half of tube 4.
Well thats an example ..so by putting the pre tubes back in one by one
while monitoring the voltage drop across the 5.6k resistor (probe either side) should indicate if any tube is drawing more than it should which may give a clue to where the problem is.My hunch would be around V4 as it has 334volts on the plate and only a 33k dropping resistor before the reverb trans primary.
This comes from point C in the power supply.
Have attached pre schematic may be a tiny bit clearer

[sixstring63]

Out of Ideas

Replaced 5.6k resistor that had a funky value and fired it up without the tubes in and blew the fuse after turning off standby , put tubes in and blew fuse. The more stuff I replace the worse things get , I can't find any thing shorted or what is pulling down the voltage,I'm thinking of putting the old caps back in to see what happens and if that fails I'm done trying and will take it to a friend who is a wiz on this stuff , just hoping to learn myself how to repair it.

[pdf64]

Are you sure you've got the right (slow blow / time lag) fuse in?