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MC-2255 power on pops

H

handyguy

Member
I thought I would repost this old thread: AKthread as my last question was never answered.
My MC-2255 has minor power on pops, both channels, speakers and headphones. The point of the LDR's (and relays in older models) was for protection and to mute during power on pops.
Is this indicative of the LDR's failing, or the related circuitry failing (transistors, etc.) ? I am not an engineer, so looking for some advice!

I believe there are 4 LDR's in total in the amp. The schematic says ldr201/301 are of type clm6000 and ldr401/501 of type clm6500. However, looking at the circuit board, they are all clm6500's. Although, in my case, one of them has been replaced with a Q28PH 21L682 for which I could find no datasheet/info on.
I do have a couple of the NSL-28 replacement LDR's (which are direct replacement for the clm6500's spec wise) which I have not yet installed.
Thanks for any help,
Barry
 
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Does it pop immediately when you power it up or after a second or so when the self-check completes? One channel or both?

A pop would to me indicate a DC offset somewhere in the audio chain. Probe along the audio chain looking for DCV, possibly a leaking coupling cap. There should be no feed to the OPTs when first powered on / until the LDRs are turned on, unless something is coming through the NFB which on some Mc amps is post-transformer, but I’m guessing at that.

IIRC the LDRs in the MC2255 are two different part numbers, one pair is available through McIntosh parts the other I believe that a suitable substitute is discussed somewhere on these forums (this memory is years old though).

I’m also assuming no input is attached to the MC2255 and there is a shorting resistor on the inputs.
 
Thanks Jeff,
There is almost no DC offset at the speaker terminals, I had it down to <4mv the last time I adjusted it. The pops occur immediately at power on, during the self test count down, when all should be muted. Otherwise the amplifier is quite quiet, notwithstanding the new leaking p/s cap (in another thread).
Regarding the LDR's, the schematics identify two different part numbers, but mine has all CLM6500's (except for the replacement mystery LDR.) I do have a couple replacement NSL-28's ($20 each,) but considering getting two more and replacing all the LDR's...
 
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Once I replace the power caps, i will try to wire up my DVM to the LDR input, and see what the voltage is during power up muted sequence.
I just tested the resistance in-circuit (powered off) of the LDR's, 3/4 read about 10k ohms (value slowly climbs as I read circuit) and 1 that reads infinity.
Now I am thinking this is a clear indication of their failure, but the driving circuitry could also be another problem...
Thoughts?
 
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