Mesa Boogie Single Rectifier Solo 50 motorboating

[Mick Bailey]

I've got a frustrating problem with a Mesa Boogie Single Rectifier Solo 50 - essentialy a Rectoverb without the reverb. It's a Series 2 and I've posted a schematic request but no luck as yet. The circuitry has quite a lot of differences to the Series1.

The problem is motorboating when the output level is turned up beyond 75%. It doesn't start by itself, but requires an abrupt signal to get it going - especially delay repeats in the FX loop. Once started, all other controls can be turned to zero and it continues at about 3hz.

My first thought was "decoupling caps, dead easy!" So did a cap job and it marginally changed the frequency, but nothing else.

I next isolated the stages to leave V4 (V4a - FX send/V4b - FX return), V5 (PI) and the output tubes. Touching the grid of V4a or V5a with a meter probe will start the amp motorboating if the output level is higher than 75% - all other controls set to zero.

Scoping the output off the back of the Output pot shows that below 75% there is a classic 'damped oscillation' trace each time the grids are briefly touched with a meter probe. This coincides with some low-frequency distortion evident on the clean channel when low notes are played. Further scoping shows that the power supply is being modulated back to the first filter caps when the control is turned up.

If the output tubes are removed the motorboating (as seen by the scope) stops, same if either V4 or V5 are removed.

Removing the NFB has no effect.

Changing the EL34s to 6L6s improves matters some - the control will go to 90% before motorboating.

Increasing the 30uf filter cap value (the one immediately after the choke) to 100uf almost fixes it.

Unlike the Series 1, there appears to be no decoupling of V4 or it sections - it is fed straight from the 30uf filter cap via a 2.7k resistor. Pin 1 connects to this resistor directly, and pin 6 via a 120k resistor. All associated resistors measure ok.

The amp's voltages all check out fine and the PI is internally matched. Joints and ground connections look ok, and the amp does not buzz or hum. I've swapped out V4 and V5 just to be sure.

I'm wondering if anyone else has seen this problem, or can offer any assistance?

[Steve Conner]

If you temporarily add decoupling to V4 (bang a cap from pin 1 to ground) does that fix it? (Maybe Mesa Boogered up the design? )

Does changing the speaker load impedance (mismatching one tap) have any effect? If you're testing with a dummy load, does the problem also happen when using the speaker?

Any sign of RF oscillations?

[Mick Bailey]

I tried a cap to ground off V4 and it didn't make any difference. Just tried a deliberate mismatch and that didn't either. I gives the same problem with a speaker as with a dummy load.

Can't find any evidence of RF oscillation.

I've searched around quite a bit on the web and there are quite a few reports of motorboating with these amps. Some people lower the gain of the FX recovery to 'fix' this, but the owner says it used to be fine, but has had the problem for over a year with no other changes to gear. I tried a 12AT7 in V4 and it still does it.

[Steve Conner]

OK, so, if you put an unconnected jack plug in the FX return socket (breaking the FX loop) does that make a difference? Will it stop the motorboating once it's started?

If there's a return level control, does that have any effect on the problem?

Me thinks it could be a grounding issue in the FX loop jacks, and they removed the decoupling from V4 because they experimented and found that made it better.

If all else fails, making some coupling capacitors smaller somewhere should fix it!

[Mick Bailey]

Breaking the FX loop still leaves it connected via the parallel 25k mix pot, but I removed this and the problem stopped. Inserting a smaller coupling cap as you suggested also stopped it. Looking at the Series 1 schematic, the coupling cap to the loop was 0.6uf and in the Series 2 this was reduced to 0.22uf - possibly as a result of the problems with motorboating with those earlier amps.

Experimentally I replaced the 0.22uf with a 0.1uf plus 470k to ground off the send tip and this completely cures the problem without having a detrimental effect on the sound.

I've just tested the removed preamp decoupling caps and 2 out of 6 are no good, but still weren't having any noticeable effect on the amp.

Thanks Steve, Much appreciated.

[soundguruman]

well there is nothing new about a mesa amp that oscillates and motorboats.
However I don't have a schematic to it.
Now, is there diodes in the output stage connected to the plates of the output tubes?
Like in this amp? http://www.drtube.com/schematics/marshall/2000pwr2.gif
Now you see how the diodes were fitted to the plates of the output tubes? Believe it or not, sometimes adding these diodes stops the amp from motorboating.
I have not tried it on that particular model, but I have used the same idea in other mesa boogies, and it really worked.
Stopped oscillations and motorboating by installing the diodes, yes. Did not work on all amps but did work on many.

Now there are two things that go wrong on many mesa boogies,
#1 resistors laying on top of circuit tracks- the resistor starts to burn down to the track that it is laying on top of. The resistors must be lifted off the circuit tracks so that there is a space between the board and the resistor.
#2 circuit tracks spaced too close together. sometimes the circuit track will form (burn) a carbon thread through the fiberglass, and start to arc to another track. This usually occurs between the grid and the plate track, or the plate and the heater track.
Inspect the board closely with a magnifier, and look for signs of tiny burn marks, between 2 circuit tracks or under resistor that is laying directly on top of the board's circuit tracks...

[Mick Bailey]

No diodes fitted to this amp, but the diode mod is an easy one to carry out as 'belt and braces'. Interesting that this will sometimes stop motorboating in Boogies - not heard this before and I need to think through how this works.

The tracks are very close with this amp, in some places almost touching adjacent solder pads on the main circuit board. You would expect that the proximity and routing of tracks would be considered in relation to the voltage differential - not always the case, I know. The fairly large orange drops obscure the top layer to a large extent, making it difficult to inspect. Can't see any signs of tracking on the Power tube board, though. Given that this amp has been used in a faulty state for well over a year I would have expected any tracking to have developed into something pretty serious by now.

[soundguruman]

On many of them, I have to remove the parts to inspect the board. I can't assume that there has been no arcing.
The arcing occurs when the amp is living in humidity, near the ocean especially. The coating on top of the tracks is supposed to prevent it, but it breaks down.
On earlier models, there were ceramic beads installed, to lift the resistors away from the board. Not on the new ones.
Sometimes you gotta lift the resistors one at a time and inspect it. I make a habit of that, although it's a very time consuming amp to repair...
Sometimes it helps to shine a flashlight through the fiberglass to spot a burn mark. The burn can be the size of the head of a pin.

Yes, the diodes can help on some of these amps, use a very low leakage diode(s), some diodes just have too much leakage. As you can see in the Marshall, they use two 4007 in series.
And then also, reducing the value of a coupling cap(s) can also stop a motorboating problem...but unfortunately, do not have the schematic to make suggestions.
Basically you have too many parts, crammed on to a tiny circuit board, with almost no space between the circuit tracks. I have complained about that for decades.
I remember the first model, that came with exploding preamp tubes, I called them up and chewed them out for about two hours, no kidding.

I know Randall loves me...

[nickvivid]

Breaking the FX loop still leaves it connected via the parallel 25k mix pot, but I removed this and the problem stopped. Inserting a smaller coupling cap as you suggested also stopped it. Looking at the Series 1 schematic, the coupling cap to the loop was 0.6uf and in the Series 2 this was reduced to 0.22uf - possibly as a result of the problems with motorboating with those earlier amps.

Experimentally I replaced the 0.22uf with a 0.1uf plus 470k to ground off the send tip and this completely cures the problem without having a detrimental effect on the sound.

I've just tested the removed preamp decoupling caps and 2 out of 6 are no good, but still weren't having any noticeable effect on the amp.

Thanks Steve, Much appreciated.

Hey Mick and Steve thanks - sorry im resurrecting an old thread but i had a motorboating solo series 2 50 watt head (like an earlier post it would start motorboating when i made a loud ding sound on the guitar - flicking the pickup switch could usually trigger it). I put a 1/2 watt 470k between the send tip and ground and a .1uf 50v coupling cap between return tip and ground (dont know if that was the right spot for it, but it seemed to work - i thought maybe if some dc was escaping through the return this would be the fix). The amp's been running fine so far with no issues, so thanks!

[Mick Bailey]

Nice to hear. Also, if it helps anyone here's the series 2 schematic - Mesa Boogie kindly sent it me as the amp is no longer in production.

sRec_sers2.pdf