MC2155 - no right channel

Hi, this is my first post and I researched quite some time before posting.
I have a MC2155 power amp and a C28 preamp which I purchased in 1985.
The MC2155 came from AudioClassics and the C28 came from Audio Resource (I think it was located in New Orleans).
I have done most of the basic things to diagnose my MC2155 power amp's missing right channel, and have found nothing visibly wrong with it.
Today I cleaned the controls and front/back switches with DeOxit.
Before today, I tested the headphone jack and I have both channels through headphones.
Both meters were responding to my input source before I DeOxed the controls.
My input sources were a turntable through my C28, and another source was a walkman plugged into the MC2155 directly, and a CD player plugged into the MC2155 directly.
I have reversed my speakers as well.
When I mentioned above that both meters were working, this was before I cleaned all the controls with DeOxit. Now the right meter does nothing at all. I wondered why the right meter worked previously while the right channel wasn't working.
I carefully removed the top dust cover and then removed the front glass and frame as an assembly before I carefully turned it over onto a double layer terrycloth on my desk. Then I removed the bottom and sprayed a small amount of DeOxit into the front control pots and exercised the controls a bit. I also did both switches on the back.
Also while I had my MC2155 open, I used a strong light and looked into the cages at the back. I could see or smell no burned components. I also looked at the exposed circuit boards towards the front (on the bottom of the amp) and saw no burned or bad looking components. The boards in the front which are under the top dust cover also looked fine.

Before I "cleaned" my MC2155 with DeOxit, I practiced on my old MAC1500
The DeOxing really helped this tired old receiver (MAC1500) a lot.
I pulled the tubes one by one and DeOxed the pins and sockets.
I also did this to all the top mounted transistors and the 2 nuvistors.

I would love to send my MC2155 and my C28 and MAC1500 to Terry after reading all the good things I have read about his top notch McIntosh service he does for people with older gear, but the shipping horror stories scare me to death. I have the original McIntosh shipping box for my MC2155, but I heard united parcel smashers and fed excuse me it's busted, both can break your beloved Mac in transit....
If there is anything else I need to add, I'll make a reply to this post.

opps, about the meter not working...

opps, about the meter not working...
Last night I had my preamp C28 mode selector set to L+R to L
That is why the meter appeared to not be working.
The right meter is working now, but the right channel to the speakers is still not working.

MC2155 no right channel

Hello Jon_j

Deoxit or any other "magic spray" is not the first thing to do

open your amplifier and look inside to see if something is burned or damaged is a much better idea .

Being successfull with deoxit in one MAC 1500 is normal because it is a very very old unit .

MC2155 is not and this is a very reliable unit .

I think (following your description of phenomenon ) that your trouble is coming from the LDR

What is the number on your driver board ?

If you want to be sure about the cause open your amplifier (take off the bottom cover) connect signal to both input , connect speakers , swith he power on and listen if the right channel is back when you short the two legs of the LDR located in my drawing . Do not short anything else and do not touch anything else .

According to my serial number, the driver board is:
045863

I used DeOxit because the left gain had static and cut out sometimes.

Where is your drawing? I'm new to the forum.

MC2155 no right channel

Here is my drawing

The faulty LDR is LDR501 .

Short the legs closer to my "red" point with a flat screw driver (nothing else again) be carefull because the amplifier is under voltages .

Once done you will knox exactly what component need to be replaced . If it is early MC 2155 the original LDR (CLM6500 part number 144-159 ) is no more available and the factory will supply you the new model (see my previous post

http://www.audiokarma.org/forums/showthread.php?t=619254

MC2155 no right channel

Welcome on AK !

Here is the right drawing

The LDR is at the same place (on the left)

See my another previous post

http://www.audiokarma.org/forums/showthread.php?t=532999

clinic-audio said:

Here is my drawing

The faulty LDR is LDR501 .

Short the legs closer to my "red" point with a flat screw driver (nothing else again) be carefull because the amplifier is under voltages .

Once done you will knox exactly what component need to be replaced . If it is early MC 2155 the original LDR (CLM6500 part number 144-159 ) is no more available and the factory will supply you the new model (see my previous post

http://www.audiokarma.org/forums/showthread.php?t=619254

Click to expand...

That is it!
When I touched the screwdriver across the legs, the volume was much louder than the working Left channel.
Thank you, I'll have to order the part if it is available. BTW my serial number is CW179x (I don't know if forum allows serials to be displayed)
Jon
Edit:
I read that post 2 days ago and copied all 3 of the drawings, marked the one that corresponded to my serial number and forgot about it.
I wasn't looking for a solution then, I thought I would need to send it to Terry.
Thanks again.

MC2155 no right channel

Don't take the risk to ship heavy unit Jon

I prefer to help this way . Now your amplifier is safe and you can ship the boards (I recommend to change all the LDR) for years of troubles free , to Terry if you don't want to do it your self .

make a mark ( like L & R ) on boards before shipping to avoid to mix channels when they come back ) they are adjusted in factory for each channel

clinic-audio said:

Don't take the risk to ship heavy unit Jon

I prefer to help this way . Now your amplifier is safe and you can ship the boards (I recommend to change all the LDR) for years of troubles free , to Terry if you don't want to do it your self .

make a mark ( like L & R ) on boards before shipping to avoid to mix channels when they come back ) they are adjusted in factory for each channel

Click to expand...

Thank you very much, I'll do just that and mark and send both boards to Terry.
Are there any special instructions as to removing and replacing these boards?
I'm not an electronics tech, and my hands aren't steady enough to solder in confined spaces.
Thanks again,
Jon

MC2155 no right channel

Jon

Just one tip take a picture of each channel driver board before to disconnect every connector .

Disconnect every connectors on driver boards and then , take off the 3 nuts of each board

Very easy to do

Don't send your boards in ordinary mail . Choose the best way to avoid any parcel lost .

Double check when you put them back in place that you did not made any mistake (bad position of connector) that's all :thmbsp:

I took pictures of the 2 driver boards in my MC2155 and I marked these pics in paintshop with numbers for the 5 connectors. I also used a sharpie and marked the connecting lead-connectors (the plastic rectangle plugs) 1-5 so I wouldn't make any mistakes when re-installing these boards.
I also marked with a sharpie, 'R' and 'L' for Right and Left.
Below is a pic of the LDR501 that clinic-audio suggested I test by crossing 2 of the legs with a screwdriver.
This component with 3 legs looks almost brand new. Look at the pic I posted. It is marked in Redwith a North-South arrow in the South-East corner of the photo.
Photo may not be very good, as I had to resize and shrink it for the forum.

Jon

MC2155 no right channel

Hello Jon

1) I talk about mistake regarding connectors because I have refixed some MC2155 dammaged because the owners (not electronician people) try to make a " recap " and misplaced the connector . Dammages arrived after switch the power on without double check if everything was at the right place .....:nono:

Your picture is clear and the LDR don't have 3 pin but 4 (2 for resistor , 2 for LED) .

Just wait for return now :thmbsp:

clinic-audio said:

Hello Jon

1) I talk about mistake regarding connectors because I have refixed some MC2155 dammaged because the owners (not electronician people) try to make a " recap " and misplaced the connector . Dammages arrived after switch the power on without double check if everything was at the right place .....:nono:

Your picture is clear and the LDR don't have 3 pin but 4 (2 for resistor , 2 for LED) .

Just wait for return now :thmbsp:

Click to expand...

Sorry for late reply. I sent my MC2155 in its original McIntosh double box,
(packed according to the instructions with knobs and faceplate removed) to Terry, 5 days after my last reply here.

Thank you,
Jon