2:90 is it worth saving?

[Brunex1]

Greetings everyone,

I have recently bought a cheap 2:90 from a friend, who had it in a shelf for some years as non working.. He said the amp had suffered a fall and since then he lost interest in it. Once im a lost case lover, i had to bring it with me. I soon realised that the two output fuses are blown. After i took the top lid off, at a first glance it all seams normal with no signs of impact. A closer look reveals a burnt component which i don't know what it is and a bit of black dust which seams to be from the burnt part. I was afraid it had damaged the board tracks, but it comes out easily with cleaning. There's an attached picture with the part i mention, in hope that someone of the mesa lovers could tell me what it is and what it can be replaced with. And if there are any tests to be done after this first repair in order to trace any other anomaly before i put it to work. I have some soldering experience and also im able to use a multimeter. Thank you all for your kind attention. Regards from Portugal.
Ps- the insert image option it doesn't seam to be working, so this is the part reference : vtl5 9339 assm.in
Its a black squared part with four legs. Two on each side. The side closer to a small capacitor has blown. There's another identical part on the other side of the amp.

 

[Henz]

Greetings. Those two black components are called LDR's. ( light dependent resistors).Or optocoupler . One for each channel. They act as a switch. Inside those little black components, One side has a led light and the other a panel. They are polarized. They are for the Half Drive switching matrix. Each one should have a little 100uf 6.4 volt capacitor with them ( the little round cylinder component sticking up). As far as any other damage, you would have to spend some time tracing things out for continuity with your volt meter and take some measurements of resistors to see if all are in specs. The fall didn't break any power tubes? That thing is built pretty tough. Tho the fall could have damaged that LDR, something else could have taken it out. Did you ask previous owner if he/ she was having issues with the half drive mode? Or why they didn't like it anymore or any other issues that was going on with it. Have you tried powering it up? To test the voice modes, use a shorted out 1/4 inch jack and insert it in each of the modes inputs on rear. Or use a small guitar cable and touch it to ground to see if the red led's light up on front. This test can be done in stand by mode. Doesn't have to be fully powered. Be a little careful. There still may be a little residual voltage inside when probing around .I recently rebuilt my 2:90. Im no qualified tech but I know a little about this amp. http://guiguijones.free.fr/Electronique%20&%20Bouine/electronique/Schema%20Amplis%20Effets%20etc/Amplis/Mesa%20Boogie/290pwr.gif http://guiguijones.free.fr/Electronique%20&%20Bouine/electronique/Schema%20Amplis%20Effets%20etc/Amplis/Mesa%20Boogie/2901.gif here are some schematics that may help. Hope you can open them. Let me know if I can help. Jim

 

[Brunex1]

Hi Jim, thank you very much for your reply. Do you know where i could buy a replacement part of the burnt one. Once there are two of them, should i buy both? I think i will replace it first before an attempt of powering it on. What do you think?

 

[Henz]

Yw. I got mine from mesa . call Chris or Rich. He will fix you up. Get the cap as well with it. 100uf 6.3volt. Radial cap. He'll know. No. Don't replace the other at this time. One thing I forgot to ask, is the pilot light blue or red? Blue is older first version, red is second version . little differences inside. Mine is a blue light. If it were me, I'd put it on stand by and just test the voicing jacks as suggested. No further damage can be done , most likely, just in stand by. How are your soldering skills? Its kinda tight in there. Was you gonna do the repairs? The PCB is a hole through, double sided board. But soldering from top side should be ok. Dont. over heat. Also, if it were me, I'd hook a speaker cab to it and fully power up and listen for noises coming from speaker. One channel at a time. The un used channel, keep volume down, turn presence all the way up. Then go to other channel. But the down side is that you are recharging the filter caps with fresh voltages. So that concerns me if your going to be inside the amp. I just checked schematic, even in stand by the four 220uf's will also get recharged. But, take your time and choose your option. Keep me posted. I'm here brother. Blessings, Jim

 

[user.39921]

You know not everybody lives in the states, right?

To OP: For EU You can find them from tubeampdoctor.com, tubetown.de, banzaimusic.com.

I second that some good soldering skills are required and that stands for all Mesa products. The PCBs are very fragile and traces are easily destroyed if not careful.

 

[Henz]

Ty. I completely overlooked that.

 

[Brunex1]

Thanks Dan and Jim,

Jim or Dan, i will leave you my WhatsApp number if you would like to ad me, because i will need some live help if that's ok with you. +351937766332
I'm also a bit worried about those charged capacitors and that i also get burnt like the ldr, if you know what i mean with a little assistance im sure we can bring this old jem back to life. Best regards. Bruno.
If you add me in WhatsApp please feel free to message me and let me know.

 

[Henz]

I do not know anything about that app. That's where i'll need your help. First, get components that you wish to replace. I just looked at my LDR's that I replaced hoping for a crossover part number that you can get there but they basically said the same thing on them . The LDR part number on my mesa invoice, if that will help, is516509/LDR 21L 1048. Did you power it up yet? If not, I doubt if there are any serious voltages still in them. But if they are fully charged, some care has to be taken. There are ways to discharge them. Dont get to alarmed but still one has to respect what's inside. I got zapped a few times fixing mine. Nothing to bad but was enough to say"hey!". Get parts first and lets take it from there. And tell me what i need to do to get that app up and running. Jim

 

[user.39921]

If you don't know what you are doing PLEASE go to a tech. There are many things that can go wrong and Boogie amps are not easy to work on.
Go to a tech - this is the only way I can help you at this moment.

 

[Brunex1]

Hi Dann, thank you for you opinion. At this point i will try to replace the ldr which seams simple, then after i will test it and see what comes up. If something else is broken maybe i will take it to a tech. I have successfully repaired in the past a fender super twin, a studio bass, and a 72 delux reverb. I did get some help from people bit i did it. Let's see how this goes. Thank you again.

 

[Brunex1]

Hi Jim, you just have to download it to your phone and add my number to your contact list. Then we can share live video or pictures. I'm sure it will be a lot easier and faster. WhatsApp, in playstore for android and appstore for ios.
I'm still on vacation, but when i get home i will order the part and replace it. To bad i can't share images here, maybe because I'm in a mobile. I really would like for you to see the overall condition of the amp.. It came with 7 6l6, one is missing to test both channels at the same time. But i guess we can test one channel at a time. What do you think?

 

[Brunex1]

Mine is blue pilot light.

 

[Henz]

Ok. I have the app. I'll add number. Do you need my number? I kinda echo Dan. If you are not 100% certain you can do this, take it to a tech. But since you have some experience working on amps, its your call. Yes. One channel at a time but when powered up, both channels are on. As mentioned, its tight in there when soldering with the PCB not lifted. I removed and lifted my PCB to get to the backside. As you can see, lots of un soldering. I had to take my time and carefully think things out. Using twisty ties to secure and relieve certain places. As mentioned before, you should be able to remove and replace from top but ya gotta be careful with that hot solder stick. Jim

 

[user.39921]

Good idea is to take detailed pictures before you disassemble everything and lift the PCB.
You can always work with a schematic, but glancing over a good pic can save you a few minutes. (Also Mesa schematics are notorious for mistakes/missing components.)

 

[Henz]

That's exactly what I did. And boy, you are so right about their schematics. Tho the one for the 2:90 is really close. But for my C+, wow! Rediculous! From factory. I did find a decent one here in this forum where someone took some time and traced everything out.

 

[user.39921]

The C+ is quite well documented from a couple of forums. But a big problem w/ Mesa is they are changing stuff on amps without designating them as a revision or stripe lets say. Some of the changes are very small, probably just to make them a bit different or what they had as parts that day. But there are also very big changes that you need to dig out information yourself.

For instance my Studio Preamp: they have two versions, let's call them "A" and "B". The original schematic that is floating around (presumably A), shows 15uF cathode bypass cap on V1A - mine has a 3.3uF (lets call it "B"). This trend continues down the schematic. Otherwise - the schematic is correct.

Same for Mark IV "B" - there are very early ones that still have the PT from "A" and some things on the PCB are still corresponding to the "A" schematic.
Why Mesa doesn't designate those "crossover" models and doesn't document them (at least to our knowledge) is beyond me!