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MC2255 click at startup through speakers

H

handyguy

Member
Hi,
I have a nicely operating mc2255 which has a very small click (through the speakers) right at the start of power up, before the check sequence is completed. I am wondering if this is normal, or what might component might be failing if this is not normal? Can any other mc2255 owners confirm that power on is completely silent?

Thanks for the help!
 
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Was trying to remember when we received our first MC2255s into stock, I thought it was '82 or '83, the compendium says 1981 til '86 so all these amp's are at least 35 years old......these will have parts with 20 year expected service lives which are now 35-40 years old.

Mac engineering stopped using relays in the output circuit starting with the MC2255s so any parts in the output circuit that are exceeding tolerance now after decades of faithfull service could cause noises.

Do you know the service history of the unit? How long have you owned it?

Many new owners of these vintage pieces do find that a modern restoration by a restoration tech will solve these small hiccups.......
 
13th june 1983 is the date of arrival of the first MC2255 in France

Normally at the first switch ON , nothing can be heard in the speakers, it is totaly quiet
 
There is a circuit that basically nulls the input into the last gain stage of the amp until the countdown is finished.

This circuit should null all preamp noise if it is working correctly.

It is reasonable that after near 40 years of service that one or more channels will not come up to stability uniformly. In fact I would expect it.
 
Certain power conditioner creates a crackle (tick) through speaker when it is turned on.
Have you try to turn the Speaker Switch to OFF before turn the amp on?
 
My MC2250 is dead quite through start up and after until music is supplied.

I think it’s the same amp, sans meters.

I would think the power guard should keep any sound from the speakers until it’s happy.

Do you have a speaker wire wrapped around the power cord?

Are you using powered speakers on the same AC line?
 
It would be interesting to see on a scope if the noise is a -, + or full AC pulse (pulses).

Being in both channels it would seem to be a imbalance between the positive and negitive power supply sections.

I would suggest it will continue to degenerate as should be expected in a near 40 year old unit but should not at this time impose any possible speaker damage.

The autoformer will block any purely DC thru it but spikes of imposed AC will come thru.
 
no powered speakers, no speaker wire wrapped around ac cable.
I don't think the autoformer blocks dc, otherwise how could you measure and adjust the dc offset at the speaker terminal?
 
Autoformers block DC, you cannot check and adjust any DC at the speaker terminals.

Mine is completely silent (unless the "beep" is turned on) until the countdown is finished and music starts.
 
Gonna check the schematic for you, I thought that the 8Ohm output stages was a 1:1 winding to the 8Ohm taps, might be straight through.

<<edit>>
The output (from transistor stage) is 2.1Ohm, all outputs go through the transformer.

Looking at the auto-transformer schematic, it does appear that there are convoluted paths that the power can take through the windings to provide DC to the output taps.

IMG_4687.jpg

Have you checked the offset? It sounds like that would be suspect The protection circuit (turn-on) is between the Preamplifier and power amplifier stages in this amp, not between the output and speaker taps. Therefore the amplifier is connected to your speakers at all times (including during power-on).

Also, if you have an electronic / scan of the 2255 service manual / schematics, will you please emal them to ak.pdfs@gmail.com so that they can be added to the database? I have not found electronic ones anywhere.
 
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handyguy said:
Here are the dc/bias adjustment instructions from the mc2255 schematics:View attachment 1870867 View attachment 1870867
Am I missing something?
Click to expand...

Yes you are. You are fixated on a quick and easy shotgun fix without diagnosing if the "problem" is a problem or just a benign issue that you need to chalk up to aging parts.

Have you looked at the pulse on a scope or even to see which speaker driver is pulsing or whether it pulses in or out?

How many volts is the pulse?

There are so many diagnostic issues a tech needs to examine before attempting a "real" repair.

A tech might have "the usual suspects" from years of experience but any good tech knows that the next unit across the bench could be a new learning experience.

You have a multi thousand dollar amp that has been appreciating at a startling pace........why risk a large drop in value by attempting amateur repairs?
 
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