Replacing PS board in B-52 AT-100

[blckmynnse8]

Someone brought one of these into the shop, and the power supply board was scorched. So far nothing else appears to be damaged upon examining it as much as I could without the amp being in a fully functional condidtion. We ordered a new one from the company, and the somewhat shoddily refurbed board has arrived from the factory.

The owner plays this exclusively in Tube A mode and typically seems to play guitar for periods of time as long as two or more hours a stretch... usually taking a break for 30minutes or longer then goes back to playing (very loudly as well). From what I understand not the best conditions for playing this amp trouble free.

It had been in another shop, and they told him that the rotary switch for the rectifier selection was bad. It was replaced. They also told him to disconnect the reverb tank, only telling him it was "bad" but giving no further explanation. So he got it back and played the amp until the crackling and popping sounds got louder while the guitar sounded weaker and weaker until no sound at all came out.

I see that a considerable amount of trouble is experienced with these, and after reading a few of the other threads about them I figured it may be wise to ask if anyone has any further commentary on what I may expect or encounter when replacing the PS.


Thanks.

[Amp Kat]

This is the most failed amp on the market and make sure the owner is aware that replacing the power supply may only shortly fix it as something else will happen. After fixing one of these that was blown to shreds and I mean literally black crud everywhere the only sense I can make is the connectors are the main problem. The guy that replaced all the ribbon connectors resolved most issues with it. One of the best sounding little amps out there they just are horrid road amps.

[blckmynnse8]

Thanks for the reply. Yes, this repair makes me a tad nervous due to the numerous stories I've read about this amp failing again shortly after a repair has been done. I've worked very hard to maintain a good reputation so I'm trying to do what I can to prevent any little fires developing with this one. This amp was brought in to our store because it burned up after being repaired at another shop, the owner blamed the previous place but now it is very apparent that it most likely wasn't their fault... after more probing I found out that he's not even sure what the impedance of his cab is!!!

I'm also wondering about this refurbished board and the shoddy work. Instead of completely removing a couple of the resistors and one of the diodes for replacement, all they did was cut out the previous component leaving the original leads in, then soldering the new component to those old leads instead of removing the solder and mounting the new leads through the holes!

[blckmynnse8]

Welp, this amp is going to require more troubleshooting, to make a long story short. The owner has already sunk about $250 into it from previous repairs at other shops during the short time he's owned the amp so we'll have to wait and see what he wants to do next.

If the repair proceeds I'll report back with what I've found.

[Prairie Dawg]

Doesn't it seem like there are a lot of shoddily built tube amps around with doodley squat for product support lately? If the guy's already sunk $250 in it he's fast approaching the 'not economic to repair' point.

I've just learned a lesson from you about burned up B52s and protecting your reputation. My friend Kevin says one of the great revelations of his life was that you don't have to fix every amp that shows up.

[Gtr_tech]

I've just learned a lesson from you about burned up B52s and protecting your reputation. My friend Kevin says one of the great revelations of his life was that you don't have to fix every amp that shows up.

Ya sure don't. I don't turn too much away, but occasionally I just don't feel good about repairing stuff that has not been designed properly.

Example: I recently had a Studiomaster powered mixer come in with blown output amps. Std issue stuff I thought....until I popped the hood. Its no wonder the amps blew up....this thing has the shittiest heatsink area I've ever seen. No way it can dissapate what several to-3 devices can produce heat wise. Just basically sheet aluminum with minimal fins....not even cast. I politely declined the repair and explained to the owner why. He didn't argue.....

[blckmynnse8]

This is a tough one, because now that the owner has invested in the current repair I want to do the right thing and do as much as I can for his money, but it's at that pivotal point where the labor intensity is going to require a slight bump in the labor charge (and he's already getting a super low bench rate in the first place). I definitely don't believe in turning down a repair, and this is the first time where there's a real possibility that there are going to be tons of little things that need to be chased down (and still no guarantee that all problems will be solved). I feel bad for the guy because of the investment made so I'm giving it the honest effort but at the same time he needs to be aware that this will not be the last time this thing will need serviced (as was suggested above). So I still need to give him a call and have this discussion before proceeding any further to make sure that this is what he wants to do.

I just found some very elusive burn marks on the reverb transformer primary leads, and r137 is blown just like another thread regarding this amp. There is also alot of carbon on the connectors that lead to the power supply board as well. Basically in order to be able to offer any sort of guarantee (though most likely relatively short term) this thing is going to require alot of examining with a fine tooth comb.

Ha ha... I can't wait till this situation has come to a conclusion, I'd much rather jump to the Fender Champs and Line 6 that are waiting for me. But that's just how it is, every blue moon or two you get the nightmare that comes in.

[blckmynnse8]

Doesn't it seem like there are a lot of shoddily built tube amps around with doodley squat for product support lately? If the guy's already sunk $250 in it he's fast approaching the 'not economic to repair' point.

I've just learned a lesson from you about burned up B52s and protecting your reputation. My friend Kevin says one of the great revelations of his life was that you don't have to fix every amp that shows up.

I'm highly skeptical about the possibility of mass producing a good tube amp and maintaining a high degree of quality while being able to have a high profit margin combined with a price that will make it attractive to a large crowd. It's obvious that when the parts were sourced for this thing it was all about going with the lowest price possible. Certain industry trends also make it difficult, such as things discussed elsewhere like lead-free manufacturing which has a lot of bugs that need to be worked out. I also agree with another comment made in another thread about this probably being designed by a computer guy. Sure the amp has this crushing tone but it just seems like they took it too far and you have all of these potential things that can go wrong... then Murphy seems to have a really good time imposing his famous law on these amps.

[blckmynnse8]

Chalk this one up as one of those "textbook" failures common to this model. The owner just wanted me to "get it running", meaning he wants to be able to play it but isn't too interested in having it restored to factory condition, which is fine with me. After giving the caveats about repeat issues with this amp, I proceeded.

Bad reverb transformer took out R137 on the power supply board, so going by what I was instructed to do I just disconnected the reverb circuit and tank and replaced the resistor. The amp now works, and I left it powered up for over three hours after re-biasing with no issues. Now I see what people mean regarding the differences between the different rectifier settings... mainly when running in Tube A. I recommended that he not use the Tube A setting anymore and in the interest of trying to preserve the life of the amp I set it so the power tubes have about 33mA in Tube AB, seemed safe enough to me and it sounded good. Since the PS board was toasted when it came in I have no idea how it was biased previously. Replacing the PS board and cleaning up all of the burned connections took a heck of alot of time... it would be a real nightmare to be expected to comb through this entire amp in order to get it into "perfect working order". A whole sea of cheap plastic connectors... I'm not a fan of relays in amplifiers, this one has about seven if my memory serves correct. Lots of loose connectors.

So last night this guy was at the store talking to someone else about his son's B-52 amp and how amazing it was... how it has this really great feature where you have three different rectifier settings... etc... I just stood there and didn't say anything but I wish I could have seen the look on my face (the reaction was instantaneous so I had no control for those first few seconds).

[blckmynnse8]

Doesn't it seem like there are a lot of shoddily built tube amps around with doodley squat for product support lately?

I could easily be wrong, but I got the general vibe that on the other end of the phone line was just a small room with a few people in it. Every time I called I got the immediate response that the person whom I needed to talk to was not at his desk, and my calls were never returned. It seemed like their main interest was getting off of the phone with me... I was always polite, friendly, and laid back but still the main objective seemed to be to get off the phone. Now I remember that other thread about why techs are so grumpy...

[Billy R aka DynaFreak]

This is interesting, because I am working on a AT-100 with a burned PS board right now. The root cause appears to be the connections to the reverb transformer. The connection arced btwn the Faston tab and the ground plane, which is VERY close to the tab.

[micri5446]

made in china.....lol....did you know egnator is made in the same facility as b52 in china. eti is the company that imports them. i know, i am an authorized service center for both (only one on li as the others have closed).

this company leaves alot to be desired in the customer service game. even with an authorized center, we deal with the same vibes you got from calling in...not at desk, no call back..ect...

if you need help with b52...pm me and ill help if i can...
peace.............mike

[Enzo]

I have had arcing at the push-on connectors for the reverb transformer on a couple B-52 amps. Grind away the arc residue and a little reroute with jumper wire and all was restored.

[Billy R aka DynaFreak]

Yep, I did the grind thing on the main pcb at the reverb tabs, and on the PS board. The previous repairman had replaced the pwr resistor on the PS board and then put electrical tape over the charred part of the board (Git r done!], he failed to find the arcing problem on the main pcb so it eventually failed again {for the new owner}. The figure 8 etch pattern on the pcb for the faston tab puts the ground plane nearly touching the little pointy wings on the tab that is spose to stablilize it. sharp point on high voltage connector near a ground = arcing!

[armaddy63]

Saw your note about contacting you on the B52.

I have had mine for 6 years now and about once a year the 5watt 90ohm resistor gets
so hot it falls out...lol...I love the amp so I put it back in the first time (soldered) and a
year later it fell out again...So I bought a new one with TABS I put through the holes in
the PCB and bent/soldered them..

Here's the thing:
Now that it CAN'T be the weak link and CANT fall out Im wondering WHAT will happen next..
I found a swollen JUN FU capacitor and googled them...They are bad quality and appear to involved
in some kinda deal where companies dont actually want to put their 'REAL' name on them...

Anyways...I replaced them ALL....do you know WHY (R124 5W 100ohm) may be getting so HOT?
I have the schematic if you want to take a gander at it...

Thanks in advance...
ARay
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