MTX BTA 2100

[jkrokenberger]

So... I have an MTX BTA 2100 here. The problem it's now having is it is blowing the MPSA06 in the remote turn-on circuit. Collector is B+, emitter is to Vcc of the 494 thru a 10R resistor and the thermal switch, base is controlled by opto that is switched via remote wire input. If you short C to E the amp will power up and idle fine. I can't find any problems other than this transistor failing. I see this transistor also powers the 494 output collectors thru 75R resistors. Can I install a TO202 device like an MPSU06 to handle the start-up current draw or is there an issue that I need to take care of? I have another one of these amplifiers and it did not need any change to this circuit.

 

[Perry Babin]

This appears to be an Orion circuit. The Orion amps ran with an MPSA06 with no problems. If the transistor has failed more than once, there is likely a fault further along in the circuit.

 

[jkrokenberger]

It is an Orion circuit. I will see if I can track it down. It only supplies power to the 494 and it's outputs. I have a working example to compare to fortunately. I got this amp non-working. The transistor has failed twice in 2 ways. First it shorted CE, second open CE. I've got the output section fixed and the amp will function without drawing excessive current. I imagine I should be looking over the circuits from pins 8-9 of the 494, correct?

 

[Perry Babin]

Measure the DC voltage across the two series resistors to see if either drive circuit is drawing more current than the other.

 

[jkrokenberger]

Thank you for that! I was first thinking of measuring resistance to ground from pins 9-10 but measuring the voltage across them in operation is a better way since I can power it up.

In post 3 I meant from pins 9-10 outwards from 494, not 8-9. My mistake. Thanks again.

 

[jkrokenberger]

I have 110mV across each 75R feeding the collectors of the 494 and 1.6mV across the 10R feeding Vcc of the 494. This is with the amp idling. All regulated voltages are present and there are no issues in the audio section.

 

[Perry Babin]

That's a tiny fraction of the current that the A06 can supply. About 3mA total. Confirm that the same current is flowing through the single series resistor as well. If so, I don't see a problem unless it's possibly intermittent.

 

[jkrokenberger]

That's what I gathered as well, around 3mA total. I think I've found the problem/s. The input power ground planes on either side of the board were no longer connected. They are normally connected thru the board mounting holes and the board has been in and out of the case so many times that the plating was pulled from the holes. I took care of that problem.

Also, I had left out the input smoothing caps for more room to work and noticed that the power LED was slow to illuminate fully. I installed the input smoothing caps(one is connected to the drive supply and one is connected to the transformer input center tap) and noticed the power LED now immediately lights up. I think I may have made my own issue by leaving out the input smoothing caps.