Orion HCCA 2100 no rail voltage.

[Chopperbuilder]

Yes, the collector of that standing 6488 is connected to the collectors of the drivers for the power supply.


Using the photo in post #260 as a reference for the A06 drivers:
Top left bank DC voltage:
1. +1.712v
2. +2.027v
3. +3.407v

Bottom left bank DC voltage:
1. +1.708v
2. +2.040v
3. +3.380v

Top right bank DC voltage:
1. +1.670v
2. +2.015v
3. +3.407v

Bottom right bank DC voltage: (driver for the 6488's)
1. +1.689v
2. +2.011v
3. +3.380v

 

[Perry Babin]

Post #260?

 

[Chopperbuilder]

Correction, post #259

 

[Perry Babin]

I don't see a problem with any of those.

How do the waveforms compare?

 

[Chopperbuilder]

Waveforms all look the same on the scope.

 

[Perry Babin]

With everything identical, I can't explain why all of the FETs in one bank are heating up.

 

[Chopperbuilder]

Is it possible one of the PS FET's in that bank is causing it?

 

[Perry Babin]

I don't think so. If that channel was drawing excessive current, the regulators would have significantly different readings between the two supplies.

How hot do they get when clamped?

 

[Chopperbuilder]

Hadn't noticed them heating up at all when clamped. Searching for the cause of the amp intermittently shutting down and so I was thinking that might lead us somewhere.

 

[Perry Babin]

This amp uses PAM, not PWM for regulations. It's incredibly inefficient at the threshold of the FETs (where it would be at idle or low power). That could explain why they're heating.

 

[Chopperbuilder]

The amp hasn't been shutting down for very long so when it does again, do you have any suggestions on what I should check first?

I do know when it shuts down rail voltage is at +/-0.650v.

 

[Perry Babin]

Does it shut down with the 4-7 resistor?

 

[Chopperbuilder]

I can't remember. I haven't had the 4-7 resistor installed in it for a while.

 

[Perry Babin]

I would reinstall the resistor and just let it run for... ever or until it does something odd. Don't leave it powered up unattended but if you need background music, use this amp. With any luck, the problem will become more obvious, in time.

 

[Chopperbuilder]

That's basically what I was thinking of doing. Back to my question of in the event it should shut down, do you have any suggestions on what to check first?

 

[Perry Babin]

It depends on what changes to make it shut down. There are just so many things that feed into 494-4. If all else fails, you start pulling everything that feeds that pin. Until it starts to shut down more consistently, that may not work to find the fault.

 

[Chopperbuilder]

Do you happen to have a schematic of everything that feeds into 494-4?

 

[Perry Babin]

No. If you do this, just follow all traces away from that terminal and desolder (only 1 terminal of resistors, capacitors and diodes is good enough) all components that connect to that terminal's trace.

Leave the 10k resistor that is between 4 and ground. Remove your added 4-7 resistor.

You can pull them one by one and wait to see if the problem returns. You can pull all of them and see if the voltage is fluctuating on any of them.

 

[Chopperbuilder]

Question about the muting circuit in this amp. All of the other Orion amps of this era I've worked on have three 2N5639 transistors in the muting circuit. Does this amp only use two or should there be three?

 

[Perry Babin]

I think this is driven by one of the optocouplers, directly.

In the older(?) amps, the optocoupler connected to the 3rd jfet that was connected to the negative voltage.

Here, a high-value resistor (1% resistor with a yellow multiplier) is connected to a negative voltage. The optocoupler holds the muting gates high (ground?) and when the opto switches off, the high-value resistor pulls the gates down to defeat the muting.

I could be mistaken.