AT-100 B-52 Bad Mojo

[ChazAjem]

This amp first acted up when the tube setting quit and the tube/solid state setting worked and then nothing all together. The tubes all glow but no signal out. I thought maybe it was an input or a fuse inside and I pulled the chassis and found some shuff that looked to be not right and an obvious de-solder, all the fuses were fine. I snapped a few pix of it and wonder what people may think to do about it. If I had to take a stab at it I would fix the bad solder and what looks to be a blown up transistor that I pointed out in the pix, but is that just fixing the symptom and not the cause therefore makeing things worse. The blown trasistor is located at
R137( can some one tell what value it is so I can buy one) on the board that goes straight to the power / standby switches.

Anyway, let me know what ya think, if the resistor solder messing up caused the transistor to blow up and I can re-solder the one (resister) and replace the other (trasistor) I can do that. Just don't want to make thinks worse!
Thanks man.











First post so I'm not sure if the pics are going to work, I sure hope so!

Peace

[Jazz P Bass]

Here is the schematic.
Can't help much more than that .
Now, if I had the amp in front of me......
I think what the amp needs is professional help.
It very well may have been bad soldering on the power resistor that caused the damage.
But to properly repair & test the amp, you are going to need test equipment & a healthy respect & understanding of how a tube amp functions.

[g1]

The burnt part that you are calling a transistor (R137) is a resistor. It's value is 1.2K 1 watt (suggest replacing it with 2 watt).
This is your main area of concern, and you must find out why it burnt or you will do further damage to the amp. Something is causing excessive current flow through it which is why it is burning. It supplies power to everything downstream of the B+3 node of the power supply.
If you are not able to determine the cause, then you should take it in for service.
The larger resistor that broke loose (R95) is the cathode resistor for the power tubes. Being broke loose would result in no sound from the amp, so it must have come loose after the other fault and would not be related.
The other part you showed with the goo is the surge suppressor (RT-1). If it was bad it would be the same as if the main fuse was blown, you wouldn't get any power light or tube glow or anything. So it should be fine.

[ChazAjem]

Thanks for the posts. I know very well to take care working on the amp. I have a bit of history with old marshell gear but first time I have delt with one of these, this is for a friend as a favor.

And old friend of mine took a look at this post and sent me an email and told me the following:


1st. solder in that loose 15 watt resistor.
Remove the burnt resistor and be careful not to pull up the solder pad, or board trace in the process.
I'd clip the resistor out first leaving a short portion of the lead in the board.
Then after heating up the solder joint grab the lead w/ some needle nose plyers and pull it out.
If you have some solder wick, that can be use to clean the pads in prep for the new resistor.


Once everything is back together, you can power it up, first w/ the stand by OFF. And speaker connected.
Check the low voltage supply w/ a voltmeter. Connect negative to the chassis ground and then use the positive to probe for proper voltages.
Note: if you have a clip on lead, you can use a small screwdriver as the probe.
Once you are ready, flip on the standby and wait for a few seconds and make sure the same part is not starting to smoke.
After 10 sec or so, the high voltages should be up and stable.
You can then check the B+ voltages on the board at the components shown, or you can also check at the tubes if you follow the diagram to see what pin.
The voltages will start in the 400 - 475 range, and step down from there. the last B+ will probably be under 200 V.
If all that looks good, plug in and jam.


Best of luck young grasshopper.


Dan

Sorry for having my componants wrong I have been looking and can not find any place to get the resister. I did find them but 40 bucks for 1000 of them. Any thoughts? He has texed me a couple things also but thats the main thing. So I guess I need to track down a resistor and give it a shot.

Any other thoughts are welcome, thanks again I will post a follow when I the part and see what happens.

Chuck

[Mark Black]

Resistor choices for the 1.2k 2W:

Mouser Electronics - Electronic Component Distributor resistor 1.2k

Look at the descriptions and/or download the data sheets to get physical sizing & decide which one(s) might fit the best.

Mouser, Digi-Key, MCM, Newark, and Allied Electronics are all great online sources/resources.

[drewl]

Same thing happened to one I worked on a few years ago.
You need to remove that power supply board, and resolder that power resistor making sure it has a good physical connection, as it gets very hot in cathode bias mode.
Or if possible replace it with a higher power resistor.

[soundguruman]

You need to go with a chassis mount resistor. It's made of metal and screws to the chassis. Then jump the leads to the board with teflon covered wire.
http://www.vishay.com/docs/30201/30201.pdf
It's pretty common these days for hot resistors on circuit boards to come loose from the solder, due to constant cycle of heating and cooling. Fender has the same problem.

[Amp Kat]

I would also recommend doing a search on this Amp Topic that have been discussed by us from the past. Not to be a bearer of bad news but this amp is a nightmare of on and on problems. The white connectors through-out the main PCB are very shady and should really be hard soldered in as they can and will cause some serious problems in the future.

[soundguruman]

Want to borrow my framing hammer?

[Prairie Dawg]

I think the polite term is "dog waste".

[drewl]

Oh, they're not that bad.....
Install the factory fan mod to keep them from overheating, rework some pc board connections and the board connectors and it should be okay.
I've repaired about a half dozen or so, and none have come back yet.
Unless of course the owners used the "framing hammer"

[ChazAjem]

Thanks for all the help. For some resaon I have had trouble getting on to the forum as the page kept timing out. Going to order parts today to get this thing rolling. Updates to come.

[ChazAjem]

Hey guys, I got the parts and did the resolder on the one resistor and replced the other that was blown up. powered up the amp with the standby off and all was fine and good. Had the amp hooked up to my cab. When I flipped the standby switch it started doin the snap, crackle, pop manuver with little sparkes snapping around the tube sockets, and I flipped the standby switch back as fast as I could. Looks like I neeed that framing hammer! I do have a 26 oz estwing if that might do the trick, but was thnking a sledge hammer may be a better option. Any thoughts at this point are welcome but I feel like I have done all I can do but if here is anything more that a talented layperson can be directed to do I'm all ears. Other than that I think its time to fine a pro in the Flint, MI area that I can hand this one off to. Even with the seback on this amp I have desided to go forward and train in the electronics field. I have been a tech for years and a life long auto mechanic but fighting cars has taken its toll on my body and IU think working at a bench on music gear is something I enjoy and would be less taxing on me. Any thoughs as to where to start as far as training goes would be great. I can self study but at least to get started I think I would be better served to take a basic class first so I have some sort of foundation before doing home study. Again thoughs and opinions are welcome. Ans thanks again to all who have helped out on this. I'm new here but have found everyone on theses boards to be friendly and helpful where many time people don't want to share what they know too much. I know you guys have put it your time to learn and know you do and to share freely as you do is a great help. Now a days many people are not sharing anything with out the "whats in it for me" factror. I will be running around here reading alot and posting little while I begin my journey into a new career field. OPne ears and closed mouth type deal. I open to all thoughts and opnions on what to do with the AT-100 and how to go about persuing electronics as career. All the best and trhanks for listening and all the help so far!

[soundguruman]

If the sockets are arcing, they have to be replaced.
Once carbon forms on the socket or circuit board, it will continue arcing / burning until all the carbon is removed.
This means cutting, drilling, it cannot be cleaned off by normal means, such as spray cleaners.