Once again need help fixing Orion HCCA 225 Digital Reference

[Chopperbuilder]

As you know I am no stranger to the effects of leaked electrolyte. I just cleaned around 494 with acetone and the voltages are now reading:
Pin 1: +2.3
Pin 3: +2.5
Pin 4: +10.5

 

[Perry Babin]

Pin 7 is directly tied to ground. It can't read more than a tiny fraction of a volt unless there is a broken trace (not likely).

 

[Chopperbuilder]

I connected the jumper. The amp powered up(red light on). The rail voltage across the rectifiers is +42.8 volts. I also monitored for components getting hot, nothing was heating up.

 

[Chopperbuilder]

Oh and correction on that about pin 7....It was pin 4 not pin 7. (I corrected the above post).

 

[Perry Babin]

With the 4-7 jumper. Are pins 1 and 2 back to the earlier readings?

 

[Chopperbuilder]

The voltage on both pins is oscillating...at the lowest +1.3 to the highest +2.5

 

[Perry Babin]

Sweeping/rubbing your meter probe on the board around those pins, do you see any voltage?

Confirm that there is nothing connected to each of those pins but a 10k ohm resistor (which goes to ground)

 

[Chopperbuilder]

Sweep with the probe showed no voltage. Attached are pictures of the trace on the other side of the board. There is a resistor on the top side connected to the trace for pin 1 (the yellow "2" is on top of one of the pins of that resistor in the one photo).

 

[Perry Babin]

I see that they go elsewhere. I'll dig out the board I have when I get back in the shop.

 

[Chopperbuilder]

Here are a couple photos of my old board if they help...?

 

[Perry Babin]

Sorry for the delay.

On the amp I have here, removing the 6488 did nothing to the voltage on pin 1. Pins 2 and 3 follow pin 1. That's why they all changed. You can reinstall the 6488.

Pin 1 is driven by the protection circuit transistors. Pull them to see if that allows the amp to power up.

 

[Chopperbuilder]

I reinstalled the 6488. Will you show me where the protection circuit transistors are located?

 

[Perry Babin]

I hit the wrong button when I posted. I edited the post (#51).

 

[Chopperbuilder]

Sorry that took a minute. I had to clean out my desoldering station...it was clogged.


It did not power up. However the power light did flicker on and off a few times. It did not immediately start flickering after turning on the power supply, it took about four seconds.

 

[Perry Babin]

Is the jumper still on pin 4-7? It should be, for now.

What is the voltage on pin 1 of the 494?

 

[Chopperbuilder]

I pulled the jumper since it was powering up with it on. Do you want me to put it back and then check the voltage on pin 1 of the 494?

 

[Perry Babin]

Yes

 

[Chopperbuilder]

Jumper is back on. Amp did not power up. Voltage on pin 1 voltage is oscillating between +0.3 and +0.15 would intermittently briefly spike higher up to +1.6 volts.

 

[Perry Babin]

Are the two protection transistors out and the pads clean, so no solder bridges?

 

[Chopperbuilder]

Yes.