Boogie Mark IV B with bad optocoupler on lead channel help!

[Mick Bailey]

Those voltages look good to me. I wasn't expecting that! The 12v will be OK, otherwise the 'high' voltages would be much lower.

What's bugging me is that the Lead channel indicator LED on the front panel flashes and goes out. It could be faulty - too much of a coincidence - or losing its ground connection, or losing its DC feed. The grounding is done via the selector switch and this is working OK. The two resistors that would affect the DC feed to it are R122 and R9, but if these were faulty they'd affect the other channels.

Are you certain that in switch position LD, the connection LD is staying at 1.78v?

[Harrysong]

Yep. Checked it twice..

[Mick Bailey]

I'm thinking that something could be shorting out the panel indicator LED voltage when you're switched to lead. The other rails measure correctly as 'high', but possibly because their voltages are coming via the relay coils from the 12v feed side of R122.

Can you check the voltage across R122 when the switch is in each position - R1, R2 & LD? Amp can be in standby.

If there's a much greater voltage drop when switched to LD it would point to a shorting optocoupler LED which is pulling down the voltage after R122. This would bring down all of the LEDs on the LD rail, preventing the lead channel from switching correctly.

The ones which could do this are LDR1, 4, 8, 9,11,15,27.

You would check by measuring the voltage across the LED side of each of those optocouplers - where the wires come out of the device. A shorted one will measure 0v or close and a good one around 1.8v. I use a little pair of grabbing hook probes wired to banana jacks to check their operation, as sometimes they can be difficult to get to with standard meter probes. Just take care not to short anything out when checking.

[Harrysong]

Thanks Bill! Any a clue to where R122 is located inthe PC board? I can´t find it

[Mick Bailey]

If it isn't marked take a look at the schematic - R107/R122/R292/C431 are all connected - find one and try to trace and locate the rest. Mesa sticks stuff all over the place so it isn't always easy. In addition circuit boards get revised and part numbers can change. The picture's not much help - a bit like trying to see a pair of shoes from space!

Bill who?

[Harrysong]

Sorry Mick

[Harrysong]

Well, this is going beyond my technical abilities. I decided to take it to a tech and see what´s up with it. Thank you very much for your time and your help.

[Mick Bailey]

When it's fixed let us know what was wrong. It could help the next person who travels this way.

[Harrysong]

Sure! Shall do. Thanks again.

[Harrysong]

Well..more than to months after I delivered the amp to a amp tech here, he finally phone me and told me that he can´t fix it without a diagram of the amp. He has the schematic but he says that is not enough and that without a Diagram he won´t be able to find the components because he don´t have that much time as he has many gear to fix. In other words this jerk won´t fix it without a diagram. Since we have no mesa Dealer here in Uruguay, anyone knows were I can get a diagram of the amp? I know no other amp tech who would work in a Boogie here.
Thanks

[Jazz P Bass]

As far as I am aware, there are not any board layout diagrams.

It sure sucks having a lazy ass tech.

We are not concerned with the complete layout, just the LDR matrix.

[Zouto]

Could you get it fixed in the meantime? If yes, what was wrong, which parts had to be swapped? I have a similar problem with a mark4 a, loud hum in lead mode, probably caused by some switching matrix defect, see https://music-electronics-forum.com/...ad.php?t=51016.

[Mick Bailey]

The OP hasn't logged in here for 6 years, so perhaps unlikely he'll read your reply.

[Harrysong]

Hi!!! Im still here. Can't Help. My boogie is gathering dust on a corner at home. Probably tomorrow I will sent it to another tech and hope he can fix it once and for all.

[nevetslab]

Hi!!! Im still here. Can't Help. My boogie is gathering dust on a corner at home. Probably tomorrow I will sent it to another tech and hope he can fix it once and for all.

I looked thru my Mesa Folder to see if I have more schematics on the Mark IV. Not all of the schematics are present. It's missing the Switching Matrix, where all of the LDR's are shown being switched from the transistors being turned on to turn the LDR's LED's on, doing the switching. And, it's also missing the power supply schematic. I had to look in a Lone Star Schematic to see what was missing. The Lone Star's are not as densely packed as the Mark IV. When any of these in our rental inventory go seriously wrong, and I'm not able to find a cure quickly, they get sent to Mesa Hollywood (from Burbank, CA), and I can breathe a sigh of relief. Most folks here on this forum are not overly fond of servicing these amps, as they are extremely tedious and NOT service-friendly. I know that doesn't help you at all in Uruguay, but, that's why there hasn't been an overwhelming onslaught of assistance on your amp from when you first posted. I've never seen component layout maps issued from Mesa Engineering. Those who work for Mesa no doubt have.....they DO EXIST in the Engineering Dept's files, and something as densely populated as most of their products are, they SHOULD provide them. No doubt the reason they don't is to discourage non-Mesa Service Techs from working on them.