Crate BFX100 - Turns on, but no output

[Hokie]

Hello there. I'm new to the forums here, but I studied electrical engineering in school (if that means anything) and I'm not scared to get my hands dirty.

I have a Crate BFX100 that turns on but doesn't output sound. I've sent this thing in twice before, years ago, to have it repaired for similar problems. This time I'm trying to take it on myself. I believe that before the audio techs said the problem was a bad capacitor.

I found another thread on this forum that linked to a schematic: http://music-electronics-forum.com/a...ratebfx100.zip

Any suggestions on where to start? What I should be measuring/testing?

I'll try attaching a picture, thanks for the help people.

[52 Bill]

I have a Crate BFX100 that turns on but doesn't output sound. I've sent this thing in twice before, years ago, to have it repaired for similar problems. This time I'm trying to take it on myself. I believe that before the audio techs said the problem was a bad capacitor.

Welcome to the place. Whenever you start to work on an amp, it is best to try and isolate the problem area. The best way to do this is to start checking things to see what is and isn't working.

You say that the amp powers up, but will not pass a signal. This at least shows you that the basic power circuit is probably working.

If there is no sound, the first thing to check are the speakers. If there is background hiss coming from the speakers then they are probably ok. Listen to the hiss and background noise, does turning any of the controls change the noise character?

If there is no sound at all then test them for open voice coils or bad wiring. A quick test is to unhook or unplug them from the amp and test them with a 9 volt battery across the plug or the speaker terminals. If the voice coil is ok, there will be a loud pop through the speaker.

If the speakers test ok then try plugging in a spare cord into the FX loop and/or pre out-power amp in jacks. If the sound is restored, these jacks need to be serviced. If still no sound try and plug you guitar into the return jack or the power amp in jack. If you hear the guitar then, you can assume that the circuit from that point onward is working and that the problem is in the circuitry before the loop jacks.

Once you have isolated the problem to a general area of the amp, it will make finding the fault much easier.

[Hokie]

Welcome to the place. Whenever you start to work on an amp, it is best to try and isolate the problem area. The best way to do this is to start checking things to see what is and isn't working.

You say that the amp powers up, but will not pass a signal. This at least shows you that the basic power circuit is probably working.

If there is no sound, the first thing to check are the speakers. If there is background hiss coming from the speakers then they are probably ok. Listen to the hiss and background noise, does turning any of the controls change the noise character?

If there is no sound at all then test them for open voice coils or bad wiring. A quick test is to unhook or unplug them from the amp and test them with a 9 volt battery across the plug or the speaker terminals. If the voice coil is ok, there will be a loud pop through the speaker.

If the speakers test ok then try plugging in a spare cord into the FX loop and/or pre out-power amp in jacks. If the sound is restored, these jacks need to be serviced. If still no sound try and plug you guitar into the return jack or the power amp in jack. If you hear the guitar then, you can assume that the circuit from that point onward is working and that the problem is in the circuitry before the loop jacks.

Once you have isolated the problem to a general area of the amp, it will make finding the fault much easier.

Thanks 52Bill. I did everything you suggested to and here is what I found out:
- No speaker hiss at all when the amp is turned on, no change when turning controls.
- I checked the speaker by hooking it up directly to a different amp, and it sounded fine.
- No sound when plugging the guitar directly into the send/receive or plugging an extra cable into the send/receive.
- I also checked the headphone jack and nothing was coming out.

Strangely, at one point before doing the tests it started working fine for about 10 seconds then went silent again.

Any suggestions on where to go from here?

Thanks again.

[52 Bill]

- I also checked the headphone jack and nothing was coming out.

Strangely, at one point before doing the tests it started working fine for about 10 seconds then went silent again.

If there is no signal at the heaphone jack, then there is a problem inside the amp. My first suspect would be the 10 pin connector that connects the power amp to the speaker and to the power supply. Check to see that the two sides of the connector are making good contact and that the connector itself is soldered securely to the pc board.

Before you pull it all apart, you can try giving the amp a good solid Enzo whack with your fist. If the sound comes back on then you know that something is loose inside the chassis.

[Hokie]

My first suspect would be the 10 pin connector that connects the power amp to the speaker and to the power supply. Check to see that the two sides of the connector are making good contact and that the connector itself is soldered securely to the pc board.

Before you pull it all apart, you can try giving the amp a good solid Enzo whack with your fist. If the sound comes back on then you know that something is loose inside the chassis.

I'm not sure what a Enzo whack is, but I've given this thing a few smacks with no success.

I removed and inspected the red 10-pin connector, its getting great contact, and the pins are securely soldered to the board. One of these wires is spliced to feed the speaker (the first pin all the way left).

The one time that the amp did work was when I had the -15dB button pressed and was playing softly, as soon as I played loud it cut out.

I've got my multimeter ready to go to check other stuff! Thanks again for the help.

[52 Bill]

Try this one test. With your meter set to read low ohms connect one lead to ground. Now take the other lead and read from each side of the two speaker terminals. One should read zero and the other should read 3-4 ohms.

The next thing would be to start testing the supply voltages.

[Hokie]

Try this one test. With your meter set to read low ohms connect one lead to ground. Now take the other lead and read from each side of the two speaker terminals. One should read zero and the other should read 3-4 ohms.

The next thing would be to start testing the supply voltages.

Let me make sure I did this right. I unplugged the speaker and tested from both terminals on the board to ground. The red line (coming from the 10-pin) read around 1kohm, the other terminal read infinite.

[52 Bill]

Well I thought that you'd read it with the speaker hooked up, that's why I mentioned the 3-4 ohm reading. In any case, one of the readings should be zero ohms to ground (not the red one from the 10 pin).

If you look at the schematic, the speaker negative goes to the extension speaker jack, which has a normally closed contact that connects the speaker negative to the headphone jack, which has a normally closed contact that connects to ground. If either the ext speaker jack or the headphone jack switch contacts don't close, there will be no return circuit for the speaker ground. Check both of the jacks for oxidation and use your meter to be sure that the closed circuit contacts are making good contact.