Trace Elliot BLX80...issues :(

[7tenths]

Hi All

Been reading quite a while, bit the bullet and signed up yesterday, as I think the valuable knowledge and resources contained here are great and could help me with my very recent problem.

I have a Trace Elliot BLX80 bass-combo just to noodle about on and learn bass (long time drummer, but very new to bass), quite an odd design, manufactured on 04/11/97. Anyway, the problem is that it was working and now it isn't. Specifically, powers-up fine but is now very, very quiet The 10" Celestion is fine as I have connected that to a friends power amp, so no problem with the speaker, well, none that I can ascertain, it sounds fine, no damage to cone etc etc.

It has the GP7 SM pre and an 80w power amp, although the circuit board for the power-amp is marked 'PB130SM/250SMX, so I imagine the same board was used in the 130w combo and 250w head with different caps?

Not sure where to start, I have the schematics for the GP7 150w-300w SM, which is as close as I could get.

Any help/advice gratefully received....I'm not particularly au-fait with electronics. I DO have a multimeter and would like to have a go at sourcing the problem and rectifying it...obviously with the kind help of people here who are infinitely more knowledgeable in electronics than myself.

The main reason for wanting to have a go is lack of funds to purchase a new amp

Drew

PS - Forgot to add, that I have done some forum searching and did come across this, which may or may not be a similar fault (very weak/quiet sound):
http://music-electronics-forum.com/t35402/

In the above post, the culprit was C21 under the EQ board, so somewhere to start I suppose, unless someone here advises me differently. Would this be a good place to start?

[52 Bill]

In the above post, the culprit was C21 under the EQ board, so somewhere to start I suppose, unless someone here advises me differently. Would this be a good place to start?

Welcome to the place. I would not expect a bad cap in another amp to be a very good guide to solving your amps' problem.

Does this amp have an FX loop or power/pre amp break jacks? If it does try plugging a spare cable from out to in and see if the signal returns.

Try and isolate your problem. Listen carefully to the speaker as you turn the controls of the amp. Is there any noise or hiss coming from the speaker? If there is any sound, do the tone or volume controls change the sound?

[J M Fahey]

In the above post, the culprit was C21 under the EQ board, so somewhere to start I suppose, unless someone here advises me differently. Would this be a good place to start?

As good as giving your Doctor some unknown guy X Rays and somebody else's blood and urine tests and expect he gives you proper medication.

[7tenths]

Thanks for the welcome '52 Bill' and the very helpful pointer, I shall try the FX loop with a spare cable I have, as soon as I am able to later today.

Thanks JMF, I actually understand the physiological reference very well, I read Physiotherapy at Uni (called Physical Therapy in the rest of the world I think), so quite like your analogy

I suppose I have a fear of poking around inside this thing (BLX80) with my little multi-meter especially as I don't recognise many of the internal organs...no liver, kidneys, spleen, lungs, heart etc etc ;-) In medicine we DO quite often look at similar problems (symptoms) seen previously in other patients and then work on a differential diagnosis based on our similar or dissimilar findings...but then medicine is not always an exact science......unlike electronics

[7tenths]

Update:

Thanks '52 Bill'; the FX send signal is fine (going to a little Kustom KB16 practice bass amp). It appears then that the pre-amp stage of the Trace Elliot is OK, which is a bit of a relief. I am a little less intimidated by the power-amp stage (altho' still fairly in the dark). As the PCB on the power side is a quadruple purpose board (BLX-80, BLX-130, SMC-130 & SMX-250), there are 2 empty slots for doubling up the mosfets. The configuration that I have has a single BUZ900P and BUZ905P and 2 spare slots for adding extra (in the higher-powered amps), no fan, just the heatsink mounted mosfets.

Now I just need to figure out how to test the mosfets 'in-situ' so to speak.

[Fragger]

Did you plug a cable from the send to the return and see if it 'woke up' the amp. Sometimes make/break switch jacks malfunction.

[J M Fahey]

I like Medicine very much, my Father was an old style Country Doctor who often (always ?) had to rely on his senses,experience and analysis for lack of advanced Medical equipment on the spot, and somehow I approach Amp servicing the same way.

The amp is a System.

Rather than starting part by part checks, let's see how it behaves in full.

You may unsolder and measure every individual part and still not find the problem

That said, start by jumpering send-receive jacks as suggested, because that's a common problem with that symptom.

Here's a potppurri of Trace Elliott schematics, as you see they use the same PCB and basic schematic in a lot of amps, so yours might very well be, say, the "150" with just 2 output transistors.

Mind you, individual part numbers on schematic and printed on PCB might not match yours (R101 - C8 - Q202 - etc.) but what's important is "what's connected to what".