If you have drive back on both sides, you can remove the jumper on the 6488 and reinstall the resistor from 4-7 on the 494. I think that's where we were before the FETs blew.
Not applying any heat, the amp only powers up for about 30 seconds before shutting down. Ounce the voltage on pin 4 of the 494 rises above +2.5 volts it shuts down. Should I change the resistor to a lower ohm one? If so, what do you suggest? It has a 10k on it now.
Yes. I don't know what you have. 4,7k?
You want something that will allow the amp to stay on but won't be such a low value that it won't allow the fault to show.
You want something that will allow the amp to stay on but won't be such a low value that it won't allow the fault to show.
I put a 5.2k in it and it seemed to have stabilized it just enough to test with the heat gun.
When I apply heat to the area of the 494 the voltage of pin 4 goes high.
When I apply heat to the area of the 494 the voltage of pin 4 goes high.
I thought that the heat sensitive area was near the FETs on the other side of the amp. Is it sensitive in those areas as well?
As I mentioned, I wasn't 100% sure that area was effected by the heat.
I blocked off the lower portion of the amp with a piece of card board and applied heat to the rest of the amp, including the area near the FET's. It had no effect.
I then hit the 494 area and the voltage once again went high on pin 4.
I blocked off the lower portion of the amp with a piece of card board and applied heat to the rest of the amp, including the area near the FET's. It had no effect.
I then hit the 494 area and the voltage once again went high on pin 4.
I had a similar problem with high pin 4 voltage shutting down drive on a BTA 2100, which is very similar. On the BTA 2100 there is a voltage divider that comes from 5V ref to pin 4. One of the components in the divider had been knocked from the board before I got it. The pads had one square and one round for it so I thought it would be either a diode or capacitor. I installed a 2.2uf tantalum cap in place and had the same symptom - drive for a few seconds then shutdown as voltage came up. I pulled the cap and looked at the pull-down resistor value, it is 10kohm, pin 4 should be low so I installed 1Mohm resistor - Pin 4 volts were under 0v1 and the drive was restored and amp works good.
See if there is a voltage divider from Vref. If there is, look at value of pull-down resistor. The divider should be made so pin 4 receives 50-100mV from 5V, a 1Mohm feed resistor worked well with the 10kohm pull-down for my amp.
This may help, it may not. This is how I was able to fix my high pin 4 voltage on an Orion made amp of similar design and size.
See if there is a voltage divider from Vref. If there is, look at value of pull-down resistor. The divider should be made so pin 4 receives 50-100mV from 5V, a 1Mohm feed resistor worked well with the 10kohm pull-down for my amp.
This may help, it may not. This is how I was able to fix my high pin 4 voltage on an Orion made amp of similar design and size.
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Touch your soldering iron to one end of the 494 (not in the center) for a second or two to see if it's the sensitive component.
I found the heat sensitive component, the cap that is in circuit with pin 4 but it was not the entire problem. While the cap was out of circuit I checked power on the pad and it was still high. Following the trace I found two solder pads that where ever so slightly touching, a diode and resistor next to each other(see photos for reference). De-soldered and re-soldered both. Pin 4 voltage is now down where it should be and amp is powering up as it should with resistor removed.
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I was starting to see this repair job as a career and was going to ask if you were going to offer a 401K plan.
I was starting think the same thing! You are going to love this Perry... I've got a different problem child, a 2250sx, that I've started working on, may have to enlist your help on it. If I can't figure it out I'll start a new thread.
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