Crest CA-18; one channel died peacefully

[Peter S]

I have just purchased a Crest CA-18 used. I checked DC off set, and listened with a signal generator at time of purchase and seemed fine.
Installed in my system, heard some quiet clicking sounds, mostly from L chan. Now L chan produces no signal, and Left clip light is on. DC off set still only 4 mV,
Has anyone had a Crest (CA-18) fail like this?
Thanks for any advice, Peter, in Canada

 

[deafbykhorns]

Op amp on input
OP275 is more than likely bad, the output is relay protected so you wouldn't read much on a fault.
A scope would be very helpful if you can borrow one

 

[wg_ski]

The fault protection circuits themselves can go haywire on these amps, too, giving a false positive for a fault. Leaky electrolytics, the usual stuff, causes this.

If it really is a fault, the op amp front end is a good place to start - those OP and OPA series *really* don't like to be driven beyond their rails, even momentarily during startup.

 

[Peter S]

Thanks for your help so far. I have a scope but have not done any real trouble shooting so far.
I have a small confession; Shortly after bringing the amp home, I opened it to investigate installing resistors in series with the fans. I had to lift the AC line up to get access to some slack in the fan wires. The AC wires were a bit tight..... Am I correct in assuming the pre-amp board is piggy-backed on the amp (driver ) board?
I noticed the clip light illuminate slightly with signal at first. Now it is on more or less constantly with no signal. I have not yet tested with scope because there is no signal at the output.
I have repaired and rebuilt my Phase Linears, but this amp may be beyond me.
Is there a service manual available for this amp? I have the schematics
Thanks again, Peter

 

[sajti]

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[sajti]

Hi,

first time I would check why the CLIP LED is ligthting. This means, that the limiter circuit attenuate the input signal. The maximum attenuation can be the reason of the missing output.

Sajti

 

[Peter S]

Here's were I was fiddling. Can he amp be run on the bench with the pre-amp board removed? Maybe I should find the service manual before I exhaust your patience with basic questions.

 

[sajti]

Here's were I was fiddling. Can he amp be run on the bench with the pre-amp board removed? Maybe I should find the service manual before I exhaust your patience with basic questions.

I would not turn on without the preamp board, as the half of the power amplifier circuit will be removed too.

Sajti

 

[Peter S]

Hi Sajti; Thanks, yes I see multiple pin connections between the 2 boards and would not turn on the amp with components removed. But what board would you remove first to get access to test points for my scope?
I was wondering if wire harnesses allow the modules to be removed but remain connected for testing.
The (first stage) OP 275, is that located on the rear pcb behind input connections?

 

[sajti]

Hi Sajti; Thanks, yes I see multiple pin connections between the 2 boards and would not turn on the amp with components removed. But what board would you remove first to get access to test points for my scope?
I was wondering if wire harnesses allow the modules to be removed but remain connected for testing.
The (first stage) OP 275, is that located on the rear pcb behind input connections?

I would start with checking why the clip LED is lighting. It's driven by the IGM detector, and the CLIP detector circuit. I would check them first.
They are on the preamplifier panel, I guess.

I think, that You can remove the whole amplifier module, together with the heatsink. But it should be done very carefully.

Sajti

Sajti

 

[deafbykhorns]

Best way is test it live with a signal, start at opamps to see if signal is passing.
I suspect they have gone DC and tripping the protection ckt

 

[sajti]

Best way is test it live with a signal, start at opamps to see if signal is passing.
I suspect they have gone DC and tripping the protection ckt

There is coupling capacitor between the power amplifier and the input stage. So DC will not trigger the protection. Anyway checking step by step is good idea

Sajti

 

[Peter S]

Hi every one;
Thanks for the guidance so far. I have a scope and signal gen, and I know I can compare Left and Right channels but; it seems like the amp modules, then capacitor bank (or maybe whole power supply) has to be removed to gain access to the input board.

Can someone advise me or direct me to a dis-assemble procedure for the CA-18. Is there a service manual in addition to the schematics found on the web?

Thanks for your help, deafbykhorns and sajti, Peter in Canada

 

[Peter S]

PS; It looks like the capacitor bank will prevent removal of the input board. I am assuming the first op-amp is on that pcb (on back panel, between fans) Is there a trick to getting at this pcb?