Mesa Boogie DC-5 Boost Switch Toggling Automatically

[bobloblaws]

A guy brought me a Mesa Boogie DC-5 to fix. There is a push pull switch on the gain pot on the rhythm channel which engages a boost feature when pulled out. What's happening with this one is that with the switch in the normal pushed in position the amp works correctly. But if the boost switch is pulled out the channel toggles between boost mode and non-boost mode periodically, with a frequency of once per second or so. I don't understand the schematic in terms of this boost feature. I know that the "no" designation on the switch symbol at the top of the tone stack stands for "normally open" and "nc" on the other switch symbol at the bottom of the tone stack stands for "normally closed". But it this diagram showing two different switches or two different states of the same switch?

In any case, is this even a viable fix? I understand that some (most?) of you guys won't service Mesa Boogie amps and refer repairs to the company itself. Should I just hand it back to my client and say "sorry, no can do"?

B

Here's the schematic...

[g1]

RY1A and RY1B are 2 sides of the same relay, acting like a double pole switch. You need the rest of the switching circuit diagram as there is no actuator shown on that gain pot. It must be shown elsewhere in the schematics.

[bobloblaws]

Oh, I see, here it is...



I'm saving screenshots, the entire pdf won't upload, too big I'm guessing.

[Pedro Vecino]

Optocoupler 4n33 is connected to relay RY1, which is the one that controls these two functions simultaneously. You could locate the relay, activate the boost through the pushpull and measure the power supply voltage in it. You should find 6V between the ends of the 4007 diode that is connected in parallel.

[bobloblaws]

Optocoupler 4n33 is connected to relay RY1, which is the one that controls these two functions simultaneously. You could locate the relay, activate the boost through the pushpull and measure the power supply voltage in it. You should find 6V between the ends of the 4007 diode that is connected in parallel.

Sorry, I can probably do the voltage test, but I'm not really following. Which two functions do you mean?

Do the two 4n33 optocoupler symbols represent a single optocoupler?

Also I don't see a 4007 diode connected in parallel.

As you can tell, I'm pretty confused by these two diagrams and how they relate to each other in terms of the boost function.

[Pedro Vecino]

The scheme you have included is incomplete. The relays do not appear, but one of the two optocouplers (similar to an integrated circuit and with six pins) must be connected to the relay that activates these functions. And the relay must have a diode in parallel to mitigate the voltage peaks. It´s the ideal point to measure the activation voltage.
The two functions that the relay controls are the ones that you see in the diagram: add a capacitor in parallel on the treble potentiometer and establish a 100K resistance between the end of the potentiometer of mids and ground.

[bobloblaws]

The scheme you have included is incomplete. The relays do not appear, but one of the two optocouplers (similar to an integrated circuit and with six pins) must be connected to the relay that activates these functions. And the relay must have a diode in parallel to mitigate the voltage peaks. It´s the ideal point to measure the activation voltage.
The two functions that the relay controls are the ones that you see in the diagram: add a capacitor in parallel on the treble potentiometer and establish a 100K resistance between the end of the potentiometer of mids and ground.

OK, thanks for the info, I'll try to figure this out.

[bobloblaws]

OK, thanks for the info, I'll try to figure this out.

So here's where I'm at. First of all, I zipped up the pdf with the schematic that I believe is the correct one for my amp, different than the screen shots I uploaded previously (DC-5B vs DC5).

In any case, I really can't make heads or tails of the 3 different diagrams that work together to illustrate the circuitry in question. I mean, If I was given these diagrams and asked to stuff the board I wouldn't know what to do.

Anyway, I'll tell you what I do know. I located the diode which appears to correspond to the rhythm channel boost. Initially the boost was toggling on and off (with the pot switched to the ON position/pulled out) as I described above. I put my meter leads across the diode and observed that it would read about -4.5VDC when the boost kicked in and 0V when it turned off. I'm assuming if I had switched the lead polarity it would have read +4.5V instead. Anyway, by the time it occurred to me to try measuring the ends of the diode to ground the toggling behaviour stopped. Now when pulling out the switch it simply does not engage the boost function at all. The voltage on the diode (cathode to ground) does change when activating the manual switch (push/pull pot). With the switch pushed in the meter reads 0V, with the switch pulled out it reads 3.3V. Does this mean that the LDR is working correctly and the corresponding relay is bad?

Update: I just went to double check I had it straight in terms of 0V when pushed in and 3.3V when pulled out, or whether it was the other way around. The amp had been off for about 10 or 15 minutes and I turned it on and out of standby immediately. I initially observed that with the switch off the meter read -3.3V and when I pulled it out the meter read +3.3V AND the boost was switching on as it is supposed to. However this state only lasted for about 30 seconds and the -3.3VDC quickly reverted to 0V as the amp warmed up to full power. I'm hoping this is useful information.


Schematic:

MesaBoogieDC5_Schematic.zip

[strato56]

Please check the voltage across the electrolytic capacitor from the diagram in post #6

[g1]

You were not clear as to whether it is a DC5 or DC5B. Schematics for both are in that package you zipped in post #8.

[bobloblaws]

You were not clear as to whether it is a DC5 or DC5B. Schematics for both are in that package you zipped in post #8.

Sorry. I think mine is DC-5B as opposed to DC-5.

[bobloblaws]

Please check the voltage across the electrolytic capacitor from the diagram in post #6

For what it's worth, voltage across the cap is about 7 volts DC. To ground I'm measuring 3.5V on the positive end and -3.5V on the other side.

[bobloblaws]

For the relay supply I'm measuring 3.5V on the positive end of the rectifier diodes and -3.5V on the other side. I'm also getting -3.5V at pin 4 on the 4N33(A) opto. The problem seems to be that based on my previous observation the expected behaviour is that pin 5 on that opto should toggle between -3.5V and +3.5V (when pushing or pulling the switch), but in fact it is toggling between 0V and +3.5V. Can I safely assume the 4N33 is the culprit here?

[strato56]

What are the voltages at pin 1 & 2 from that 4N33 ?

[bobloblaws]

What are the voltages at pin 1 & 2 from that 4N33 ?

Pin 1 is 15.2V and pin 2 is 14.5V.

In case it helps, for pin 6 the voltage is -3V when the pot is pushed in and -2.2V when it is pulled out.