Crate BX-15 Dead

[Waterppk]

Hello,

I've got a Crate BX-15 amplifier that's died.

Symptoms:
When plugged in the red light comes on when the switch is on. I checked and I am getting good power to the main amplifier (TDA2030 is getting +/-15VDC clean) but when the the amp is flipped on I get no speaker pop/no hiss/nothing.

The interesting thing is when I placed the multimeter across the speaker out (wires directly to the speaker) I suddenly get a speaker pop on turn off and a light pop and hiss on turn on. I disconnected my probe leads from the multimeter and am still able to get a popping speaker with just two bare wires running in parallel with the speaker. [capacitive effect??]

The board is manufactured by SLM Electronics
PN 06A248-01 REV1
Really simple board (cheap practice amp) and nothing looks scorched/dead as far as visual inspection on the board components goes.

Anyone have any ideas? If you need more information I can check anything on the board (have o-scope, capacitor checker, desoldering/resoldering tools, etc) and would be happy to test anything. I would like to save the amp from the garbage

Thanks!

[Mark Black]

Perhaps one of the speaker wires has a poor connection to its terminal or is broken inside the insulation?

Another possible candidate is the headphone jack, which has normalling contacts (like a patch bay) that carry the signal to the speaker when no phones are plugged in. If those are dirty or the contact tension relaxed you will get no speaker output.

Also all the SLM-made amps are notorious for solder joint cracking at jacks, controls, switches, and large or heatsink-mounted components.

I'm curious - to what function did you have your meter set when you tried it across the speaker leads? Were you looking for DC voltage, and did you find any? Or did you have it set to measure resistance which would have injected a small DC voltage and perhaps stirred up some protection/mute circuitry?

[Robert M. Martinelli]

How to sort out the problem

Hi,
to sort out if the failure comes from the preamp or the TDA2030 find the electrolytic cap which has its negative terminal connected to the pin 1 of the 2030, unsolder the positive terminal from the board and try to inject a 1 KHz sine wave 100mV p-p between the positive of this capacitor and the ground, this way you should be able to hear something through the speaker ( in case you don' t have a signal generator, a simple square wave oscillator will suffice, you can easily build one using an NE555, there are tons of schematics on the internet ).

If this works you have a preamp failure, otherwise replace the 2030. To cross check, connect the core of a shielded wire to the preamp' s output ( the place to which the positive of the cap was connected ) obviously connecting the shield to gnd, and try to send this signal to another amp to see if it gets there...

If neither the 2030 or the preamp look bad after these tests, don' t overlook the speaker... with the amp turned off disconnect the lugs, connect a multimeter with alligator clips to the speaker and check the dc resistance of the coil, for a 8 Ohm spkr it should read about 6 Ohms, about half of this with a 4 Ohm spkr. With the meter still connected, lightly press the cone ( membrane ) to see if the DC resistance changes with the movement ( this would indicate a bad coil ).

I have had experience of TDA2030-2040-2050 failing due to poor heatsink design/dimensions, usually these small and cheap amps fail if they' re driven too loud for a long time - hope this is your case as this is very easy to fix.

Hope this helps...

Good luck!

Bob

[Enzo]

Hi Bob.

Can I suggest your idea can be done simpler. Just touch pin 1 of the TDA2030 with a small screwdriver or even a stright piece of wire held in your fingers. SInce the test here is for sound or no sound, the fidelity is not the issue. If the touching of pin 1 makes a hum out the speaker, then the amp chip is working.

Water, I am with Mark here. Try connecting another speaker to the external speaker jack. Does it work? The headphone cutout doesn't affect the external speaker jack. And try it with headphones. if they work, the power amp is working.

In my experience, in this type amp with no sound, I run down a list like this:

Verify speaker works.
CHeck amp with headphones or external speaker.

Then inside:

solder under the jacks
cutout contacts on phones jack
TDA2030
SOlder under controls along the panel
Solder under input jack
Cracked controls along panel
And then everything else.

[Robert M. Martinelli]

Enzo' s got a point

Hi Enzo,
you' re absolutely right, if things can be done in a more simple way achieving the same results, that is the best way! Maybe I let myself go a little when trying to explain things.... And Mark' s got a good point too, cheaply-made amps are built using wave soldering techniques ( which I absolutely hate ) so bad solder joints are always an issue with these amps....When I find myself facing such issues, I simply go through resoldering by hand the whole thing. It' s boring and time-consuming, but at least eliminates this issue for good.

Men, I LOVE this forum! Only hope my English is good enough....

Ciao

Bob

P.S. - Enzo, I' m the one who bothered you via e-mail with a Valvetronix issue some time ago....but I wasn' t a member yet....thanks again for your help!It' s nice to hear from you again.

[Waterppk]

I took a look at the amplifier this morning. I hooked up a microphone to the input and a pair of headphones to the speaker out and to the headphone out.

The headphones received an amplified microphone on both the headphone and the speaker outs. When I put the multimeter wires in parallel with the speaker (not hooked to anything) I can get an amplified microphone out of the main speaker (in the cabinet). So everything works perfectly except there must be an issue with the headphone/external speaker cutout. I'll try removing (desoldering) the headphone out and see if the problem is fixed and work backwards from there.

Thanks for the great information, I'll post back when I get it figured out.

[Enzo]

The headphone jack is PART of the speaker circuit. If you remove it there will definitely be no sound from the speaker.

Look at the phones jack. It is the type with the large plastic nut, right? Are there not three cointacts on th jack? They are the three metal blades lying across the plug shaft. Note when you plug something into that jack, those three contacts lift up a little. What is important here is that when something is NOT plugged into it, those contacts come back down and TOUCH a lower contact under them.

So try this: with the amp running and a signal applied - so there SHOUD be sound coming out the speaker though it is not now working - take a small screwdriver or your meter probe or something and gently press down on the metal blade closest to the front panel. Does that make sound happen? If so, then either that contact is dirty underneat, or the jack contact blade is sprung and no longer touching.

[Waterppk]

Alright - I ended up with the tip contactor on the external speaker out open (like you said, the jack has to have connectivity across it for audio to be carried to the speaker, the jacks act like switches when there is something plugged into them). I found a new plug on Mouser for ~$.60 (PN:550-10201 )

With regard to the multimeter lead magic:
I think connecting the multimeter leads in parallel with the speaker was just causing enough of a shift in capacitance on the speaker to cause the amplifier to bias audio out to the speaker and the external out.