Mesa Boogie DC-5 Boost Switch Toggling Automatically

[strato56]

So Uf on the LED from the 4N33 is only 0.7V : should be more, I think.
Can you compare with the other 4N33 ? And give the voltages across this 150R resistor.

[bobloblaws]

So Uf on the LED from the 4N33 is only 0.7V : should be more, I think.
Can you compare with the other 4N33 ? And give the voltages across this 150R resistor.

The voltages for the adjacent 4N33 (B) are pin 1 -> 13.98V and pin 2 -> 14.65V.

Which resistor are you referring to? I don't see a 150R in the circuit.

How does this opto work? The voltages on pins 1 and 2 do not change when the knob is pushed or pulled. Does the LED stay on all the time and voltage difference at pin 6 triggers the connection of pins 4 and 5?

[strato56]

I'm looking at page 7 of the schematic you posted.
The light emitted by the LED (pin 1&2 ) in the opto makes the transistor ( pin 4&5 ) conduct. Enough light will put the transistor into saturation.
On page 7 , there's a 150 Ohm resistor connected at pin 1 for each 4N33.(current limiter )
Also, on that page 7 , I see that the pin 2's of the 4N33's are connected, so that voltage should be the same.
Check that Channel FTSW for bad contact also.
I sure hope we're looking at the right schematic

[bobloblaws]

I'm looking at page 7 of the schematic you posted.
The light emitted by the LED (pin 1&2 ) in the opto makes the transistor ( pin 4&5 ) conduct. Enough light will put the transistor into saturation.
On page 7 , there's a 150 Ohm resistor connected at pin 1 for each 4N33.(current limiter )
Also, on that page 7 , I see that the pin 2's of the 4N33's are connected, so that voltage should be the same.
Check that Channel FTSW for bad contact also.
I sure hope we're looking at the right schematic

No, sorry, you want to be looking at page 13 as opposed to page 7. As I mentioned, mine is the DC-5B and not the DC-5. So the equivalent resistor would be one of the 820R, I'm not sure which one you mean so I'll check both.

I just realized I gave you bad information though. For an unknown length of time I'd inadvertently had the channel selector switch in Lead mode, the middle position. I put it back to Rhythm mode and turned the amp on from cold. As I observed a day or two ago the boost function worked for about 30 seconds, after that the switch does nothing. Then I went and tested the voltage on pins 1 and 2 of the 4N33 (A) again and this time pin 1 is measuring 14.79V and pin 2 is measuring 0V. So when I reported that the difference between pins 1 and 2 was roughly 1V, that was only because I had the channel selector in the wrong position. So please disregard those numbers. Again, what I'm seeing is pin 1 -> 14.79V and pin 2 -> 0V. Pushing or pulling the boost knob does not change those voltages. Pin 6 is roughly the same as I said before (-2.8V with knob pushed in and -2.1 with knob pulled out).

When you say check the footswitch do you mean the footswitch jack as opposed to the footswitch itself? I haven't been using the actual footswitch for this troubleshooting. Are you suggesting a bad contact in the footswitch jack could be overriding the push/pull knob? Note that the knob function works briefly until the amp warms up fully. (Update: I should clarify that I don't observe this every single time I turn the amp on, but it has occurred more than once after the amp has been off for some length of time).

Update #2 : So I went ahead and measured voltages on the resistors. For whatever reason the resistor connected to 4N33-A is in actual fact a 2.21K value. In circuit it measures 2.2K on the nose. The voltage drop across this resistor is 0V. The voltage drop across the 820R that is connected to the 4N33-B opto is 13.75V (the junction of the opto and the resistor to ground is at 1.16V and the other end 14.98V).

I hope we are getting closer to identifying the problem. I still haven't figured it out. I appreciate the help.

[bobloblaws]

Hey, just wondering if anybody can chime in on this. I'm attaching the schematic with only the DC-5B pages to avoid any more confusion. I'm suspecting the 4N33 (A) opto. According to the note at the bottom of page 7 its role is to connect the Rhythm Boost relay to the -3V supply. I've confirmed that I have the -3V on pin 4 (it is -3.5V to be precise). With the push/pull knob pushed in pin 5 reads 0V and it reads +3V with knob pulled out. On a couple of occasions I've seen it function correctly for 30 seconds or so when turning on the amp after it has been turned off completely for a number of hours. In those instances pin 5 would toggle between -3V and some other more positive voltage (most recently I observed -1V w/ knob pulled out before it settled in to the 0V/3V state.). But the thing that confuses me is that intuitively I'm thinking that since pulling the knob out is supposed to engage the boost, and since the note at the bottom of page 7 seems to imply that pin 5 should be connected to -3V when the boost is engaged, in fact it seems to be the the reverse, pin 5 is measuring -3V when the knob is pushed in and is going to a more positive voltage when the knob is pulled out (again, that is when it is actually functioning, more typically pin 5 is 0V w/ knob pushed in and 3V w/ knob pulled out, but still a more positive voltage w/ knob pulled out).

I'm also curious about what is going on at pins 1, 2, and 6 on the 4N33 opto in question. I'm seeing approximately 15V at pin 1 and 0V at pin 2 and the voltages do not change when pushing in or pulling out the boost knob. Would these be expected voltages to sufficiently activate the LED inside the optocoupler? Then on pin 6 I'm seeing -2.8V with the boost knob pushed in and -2.1 with it pulled out. Would those voltages (or that difference in voltage) cause pin 4 to couple to pin 5?

What can I say, semi-conductors are obviously not my forte!

I guess the bottom line is, at this point I'm wondering if the evidence points to needing to replace that opto, and/or whether there is something else wrong, say with the supply voltages maybe?

DC-5B schematic:

MesaBoogieDC5-B.pdf

Here's the datasheet for the 4N33 opto-coupler:

https://www.vishay.com/docs/81865/4n324n33.pdf


Update: Here's another tidbit that may (or may not) mean something. I just observed -3V on pin 5 when I most recently turned on the amp. But this time I didn't toggle the push/pull knob, I just left it pushed in for a spell . I left it like that for a few minutes and the -3V reading did not change until I pulled the knob out. That action caused it go back to the by now default 0V/3V state. Weird!

[strato56]

As the circuit works at cold start, have you checked for bad solder joints around that 4N33 ?

[bobloblaws]

As the circuit works at cold start, have you checked for bad solder joints around that 4N33 ?

I haven't. I guess it's time to pull the PCB out.

[bobloblaws]

Well. for anyone keeping score at home, and for future reference, changing out the 4N33 opto did the trick. Thanks all!