what happened

I'm figuring the capn might be able to give me some insight, but I welcome any help.

Last night at my gig my Fender Princeton 112 solid state amp (USA made), started giving me trouble. I was getting distortion, it sounded like the speaker coil was damaged. Today I was going to hook up an auxiliary speaker but before I had a chance the amp started making a loud humming sound. It wasn't coming through the speaker but through the amp itself.

The amp is about 12 years old.

Any thoughts?

jake

jmac said:

... getting distortion ... the amp started making a loud humming sound ...

Click to expand...

Hi Jake: other than something cute like 'it's time to get a tube amp' ... I don't think I have anything really useful to offer. I barely get by with tubes but ... If it were here and I had a schematic for it with design voltages, I'd probably test the nodes in the power supply; filter caps in transistor amps are just as susceptible to fatigue / failure as caps in tube amps ... if something 'normal' were going to let go, it'd be there. Past that, I wish I had something more helpful to say. Good luck with it. John

John,

Thanks. As it turns out I've been looking into getting a tube amp anyway, but still if this amp is repairable I'd like to save it.

I don't have a schematic diagram, but I'll search online for one. If I can find one, I'll test the nodes as you suggested.

Thanks again

Jake

I think you can find the schematics online through Fender.

jmac said:

Thanks. As it turns out I've been looking into getting a tube amp anyway, but still if this amp is repairable I'd like to save it ...

Click to expand...

I don't know what that amp is worth but if I didn't need to sell it to finance a replacement, I'd get it fixed and keep it too. J

Paddlefoot,

Thanks, I got a schematic online.

Capn,

I think I understand what you're saying. Use a multimeter to check the voltages at the transistors (it look like there are three of them). If I see proper voltage then its probably a transistor problem, but if I'm not seeing the proper voltage, I may have a power supply problem. This is assuming I can navigate the circuit board, which I'm not sure I can. If I feel like I'm in over my head I'll take it to someone qualified.

jmac

Deezez one?

http://www.freeinfosociety.com/electron ... on_112.pdf

jmac said:

... Use a multimeter to check the voltages at the transistors (it look like there are three of them).

Click to expand...

Hi Jake; I think you'd want to start at the power supply ... and not the transistors. Before turning it on, wiggle and push the edge connectors ... make sure they are snug.

If that's your amp in the linked schematic, then the image below has excerpts indicating where to find the power supply test points (lower left) and the design values (upper right). Note that TPs 1 and 2 are AC ... not DC; be sure to set your meter accordingly. If all the values are in range, then go on and check all the TP voltages shown on the schematic. If everything still checks out, then try taking a chopstick or other non-conductive stick-like object and gently thump pretty near everything in sight ... see if you get any change in the noise.




But, don't forget to ask yourself - as every good LTGer does - 'suppose I find a bad part, can I fix it?'. If not, and you both shock yourself and ruin the amp, then you will have won the LTG Harry Callahan Manly Man of The Week Award ... "a man has to know his limitations".

Maybe you'll get lucky when you open it up ... there'll be some evidence of the problem; burned resistor, discoloration of the circuit board, a split open capacitor ... if not and if everything checks out ... then you're done. Got pics of the chassis?

Disclaimer: when turned on, the amp chassis has all kinds of potentially lethal AC and DC voltages in it. Even when it's off, the power supply caps retain enough DC to stop a person's heart. Caution ......... but good luck. John

John,

Thanks for enhanced schematics and good advice. I have opened up the amp, and nothing was obviously wrong. I spent the free time I had today making a work space for myself to start doing some testing. My work bench for cluttered with the remnants of outdoor/summer projects. But I will go VERY slowly as you suggest.

Oh yeah, "Just my imagination"...got it! I'm telling you though, last week when I played through a 15 Watt Blues junior, I was having a hard time controlling the feedback. Maybe I was playing louder than I usually play, but I didn't think so.

I asked if tube amps tend to feedback more than solid state, because I'm beginning to realize there's so much I don't know about amplifiers, so I figured it was just one more of those things other people knew but I didn't.

Anyway, thanks for the amp help and I'll keep you informed.

Jake

Ok; I hope you can get the amp straightened out. J

John,

I fixed my amp...largely due to your advice of starting at the power supply. There were a couple solder connections that were bad. It wasn't obvious and I would have missed it if you hadn't suggested wiggling around the power supply.

So after spending days preparing for a big job, the actual repair took me less than an hour.

I'm back in business. Thanks so much for your advice.

Jake

Hi Jake; 8) ... glad you got it straightened out. Would that everybody's amp headaches - including mine - were that straightforward. Does this mean no new amp in the near future? J