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Adjust MC2255 channel gain question

H

handyguy

Member
Hi,
I have been fine-tuning my very nice mc2255 amp. I have found that the gain on one channel is slightly higher (6%) than the other channel with both gain knobs set to max. I have cleaned both pots, but no change. I was thinking of adding a small resistor in series to the louder channel, but before I do that, I was wondering if there are any other gain adjustment pots on the amp boards anywhere.

Thanks for any advice.
Barry
 
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How do you know the gain is different......do you have a calibrated microphone to measure speaker output, a bench test set up with the proper load resistors, or are you observing the meters are reading different?

When we used to do bench tests, whether just on our retail sales floor or as part of a Mac amp clinic this is something we would measure.

I suspect the meters have gone out of adjustment over the last 30 years so a bench test and recalibrate is due.
 
If found this on the Marantz forum. Would this work for Mac amps like MC2255? See the note at the end.

restorer-john said:
The easiest and most extremely accurate no-equipment-required way to balance, is to hit the mono button and lift the two negative speaker wires at the amp/receiver. Connect them together, leaving the positives in place.

Play some music at the level you normally have the volume control and adjust the balance to completely null out the sound. The balance is then perfect. What you are essentially doing is nulling out the difference in level between the two channels.

note: only to be used on common ground, non-BTL amps
Click to expand...
 
Was that 6% or 6 dB? 6% is only about 0.5 dB difference. Are you worried about a technical problem or a concern that the lines on the two gain knobs are not in identical positions? This is probably the reason the input level controls on my MC300 were moved to the back. Many years ago I owned a Soundcraftman power amp with no level controls and one side seemed a little louder. I made my own input level control by putting a dual 50K audio taper clutched pot in a Bud box and put it between the pre and the amp. Set it once and it sat in the back for five years. Using a clock analogy, the two pointers were about five minutes apart.
 
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The OP has not provided near enough info so I tried to cover many options.......for example at my house the MQ107 bass gain is turned up for the left channel compared to the R....therefore to be in balance as measured with a mic and 1/3 octave test tones, the LH channel meter will read higher as expected.

Meters go out of adjustment.....parts age.
 
I would use a wye cable from one output of your pre-amp to feed both channels of the 2255. Then find a volt meter and play a tone below 1000 HZ through the system. For get the meters for the moment and set the controls so your amp reads the same on the outputs. Then look at your meters for any meter errors. . 4 volts ought to be plenty loud. 2 watts across 8 ohms. It doesn't matter if the volt meter is accurate or not you just want the levels to be the same. After that remove the wye and reconnect the pre-amp and play the same tone while in the mono mode. If the balance does not stay the same you have further problems. Then use the balance control to balance the system. Then put the mode selector in the stereo position and see if the balance stays the same if not, wye one channel of the source into the input you are using everything should now be balanced. if not you're pre-amp has further issues too. If the balance is OK then return the stereo hook-up of the source and if there is a difference then its the source. Odds are everything is going to be off a little bit, either adding to give a worst case scenario, or averaging everything out to come up with your answer for the 6% difference.

What I hated was when every time you change the setting of your volume control on the pre-amp the balance changes. C-28 and 22 were notorious for that issue. Just monitor the out put with a MI-3 while in the mono mode and then switch to stereo. Having meters helps, but they are not always the final solution. Early Analog recording not only had level issues left to right but spectral issues, too. Thats where the brighter instruments are on one channel where the background and bass instruments are on the other. The channels never seemed to balance.
 
Hello Andyguy

I was thinking of adding a small resistor in series to the louder channel don't do that because
1) in serie is not the way
2) don't touch inside your MC2255
A simple question : is it an old MC2255 or a late ?
I need to know if your amplifier is using old CLM6500 or VACTROL VTL 5C9
To know if your trouble is coming from there , unit on , gain to max position , make a short with a screw driver on the top of each LDR and see if difference still there or not
I am pretty sure this is coming from your LDR
 
Hi,
Thanks for all the replies!
I am sure I have just enough knowledge to be dangerous!
It is a small difference, for sure, but I am am a little anally retentive (and that's part of the fun of vintage audio gear, anyways!)
I measured it with a dummy load, signal generator (1 khz sine), and RMS volt meter at max rated output.
The serial number is CT3385, which I think makes it a later one, according to the notes on the schematic.
There are several LDR's in the amp, two on each preamp input/power output/power guard board: LDR 201/301/401/501.
The schematic labels them all as CLM6000, but it guess I have to open it up to see what's really there.
It will be a few days before I can open up the beast for biopsy/surgery. I will update upon close-up.
If off-spec, what would you suggest I replace them with?
Barry
 
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Most volt meters are not very reliable throughout the audio frequency range unless they were purposely designed for audio use....hence the HP, Sound Technology et al distortion analysers.....

The LDR for muting could possibly be off but unfortunately so could any of a dozen other resistive elements in the path. i do not know if a modern replacement would be even closer. The tolerance of most resistors used was 5% so to find two in alternate channels off by 5 % is certainly possible.

A voltage difference of 6% is what less than 2.8 volts and on a log scale I think that would be less than .25dB...2dB I would be concerned.

I think you are chasing a windmill but go for it and check each resistor in the signal path with a very expensive Ohm meter, probably way beyond my Fluke 77 to find that little difference.
 
I did not say that.......there are 1 % tolerance parts out there but trying to match between each channel for so actual little difference considering the way we hear will be difficult.

I just did a frequency sweep on a SX1050 and the tone control tracking between channels exceeds 2dB but the volume control is with 2dB....sometimes it just is what it is.
 
ahh, change of subject - my dad owned a stereo shop back in the day, and i worked there. We had a Pioneer dealership, and I recall selling SX-1050/1250's. Nice receivers; Stereo is in my blood!
 
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