McIntosh MC-2255 #2 - Driver board update considerations

I've added a 2nd MC-2255 to the collection. My first 2255 is a later production unit that has all of the circuit modifications / updates that are noted in the service manual. The service manual lists 3 different versions to the driver board. My latest acquisition is a early production unit - the 357th made. The driver boards in this unit match the part number in the service manual for the first series board however my driver boards lack the current source sw circuit. Updating the boards to match the first version listed in the service manual should be no problem but doing all of the updates to match my later unit could be difficult as the board changed.

How necessary are the updates for these somewhat newer amps? I plan to use the amps as mono blocks, and I know the right answer is to do the updates so both amps are the same, but I wonder how many of "newer" units actually receive the updates when they go in for service / restoration.

I was able to locate pictures of another early 2255's being restored. There are very detailed pictures of boards before and after restoration and no service updates were performed that I could see.

The picture below shows how McIntosh added the current source sw transistor and two resistors to the early boards. The transistor is a "Selected" 2N6428A, McIntosh part number 132-195. I was going to sub in a KSC1845F as I had them on hand but McIntosh still has the 2N6428A's so I had Gail add them to my last order. Installation is pretty straight forward. You'll see a resistor that is pulled up and soldered to what is the collector of the 2N6428A. The emitter gets soldered into the hole that the resistor came out of. The base gets soldered to two resistors - a 22k (R453) & a 47k (R454). Both are 1/4 watt carbon film resistors. The R453 gets soldered to one side of R432 and R454 goes to R425.

After doing this update to my boards I noticed that all of the LDR's are CLM6000. The service manual notes that each board should have one CLM6000 (LDR201) for the power guard and one CLM6500 (LDR401) which is for the speaker muting. In the last board revision, the circuit around LDR201 changes a bit as two resistors and a cap are added. I find it interesting that they were originally using CLM6000's and then switched to the CLM6500's without any circuit changes. I now wonder if the VTL5C9 could be used to replace both the CLM6000 and the CLM6500?

I'm following this thread with interest.......

I wonder if the sound is different? Can't imagine it would be, they never changed the specs with the updates did they?

Although if I was running them as mono blocks, my slight OCD nature would want both amps to be identical.....

Looks like you're all over it, so all good......:thmbsp:

kevzep said:

I'm following this thread with interest.......

I wonder if the sound is different? Can't imagine it would be, they never changed the specs with the updates did they?

Although if I was running them as mono blocks, my slight OCD nature would want both amps to be identical.....

Looks like you're all over it, so all good......:thmbsp:

Click to expand...

Hi Kev! Today's the big day for you, right? Congrats!

They should sound identical and the published specs didn't change that I'm aware of. I've noticed that these early boards have many dark area's from exposure to heat. The boards in my newer amp do not have this problem.

Audioclassics lists the last revision board in stock unpopulated. I may just grab a pair of them and build them. We'll see.

jeffs79 said:

Hi Kev! Today's the big day for you, right? Congrats!

They should sound identical and the published specs didn't change that I'm aware of. I've noticed that these early boards have many dark area's from exposure to heat. The boards in my newer amp do not have this problem.

Audioclassics lists the last revision board in stock unpopulated. I may just grab a pair of them and build them. We'll see.

Click to expand...

Not quite, its sitting in customs at the moment according to the tracking information, I would say its likely tomorrow at this stage, but its very close.....nervous times indeed!! I'm going to have to take the day off work!! Or at least come home when I see its on the local delivery courier....

I would imagine the mods for the MC2255 may well have been about reliability and running things a little cooler, I'm sure one of the gurus like c_dk or clinic-audio would know....
Sounds like a good option to grab those boards from audio classics, that's what I'd be doing......

kevzep said:

I would imagine the mods for the MC2255 may well have been about reliability and running things a little cooler, I'm sure one of the gurus like c_dk or clinic-audio would know....
Sounds like a good option to grab those boards from audio classics, that's what I'd be doing......

Click to expand...

I agree. I ordered the boards from Andy at Audio Classics today. As always, Andy was extremely helpful! I have an "extra" board coming so I'll build one with new parts and see how it performs with new transistors. If this is successful, then I'll do a second and update the first amp. If not, then I'll pull the transistors from the original boards.

what are the serial number ranges of the stages of the amps development?

jeffs79 said:

I agree. I ordered the boards from Andy at Audio Classics today. As always, Andy was extremely helpful! I have an "extra" board coming so I'll build one with new parts and see how it performs with new transistors. If this is successful, then I'll do a second and update the first amp. If not, then I'll pull the transistors from the original boards.

Click to expand...

Nice, well looks like you have a nice little project to focus on there.....
Will be interesting to see how it works out.....:thmbsp:

jdwk said:

what are the serial number ranges of the stages of the amps development?

Click to expand...

Driver board V1: CT1001-CT2449, V2: CT2450-CT3263, V3: CT3264 and above. Some part changes here and there as well.

kevzep said:

Nice, well looks like you have a nice little project to focus on there.....
Will be interesting to see how it works out.....:thmbsp:

Click to expand...

It will indeed. I'm looking forward to it.

Stuffing

The new, bare boards arrived from Audio Classics earlier this week. Here's a shot of the progress so far. New Molex connectors with the same specs as the originals and new Bourns pots. A few resistors are missing but they should be coming soon. I'm stuffing two boards but only soldering one at this time. After the first board is complete I'll test it out. If all is well I'll complete the second board.

Hello jeffs79

MC2255 is one of the best and reliable amplifier that MCINTOSH labs produced

In your post you ask some questions :

The service manual lists 3 different versions to the driver board.

yes ! but what "service manual" do you have to say that ? MCINTOSH labs never print any service manual for MC2255 . They just print 3 differents types of "service papers" (with schematics but no parts list) for the first type (generation) and 3 big papers print with schematic and separate parts list for second generation and then 2 big papers recto verso with schematics and parts list all together on the same sheet for the last issue.
Try to "upgrade" as you said a first generation of MC2255 is possible if you have the right service papers information and I don't think that you have them .
The "changes" are not only on driver boards ....

You said : After doing this update to my boards I noticed that all of the LDR's are CLM6000. The service manual notes that each board should have one CLM6000 (LDR201) for the power guard and one CLM6500 (LDR401) which is for the speaker muting. In the last board revision, the circuit around LDR201 changes a bit as two resistors and a cap are added. I find it interesting that they were originally using CLM6000's and then switched to the CLM6500's without any circuit changes. I now wonder if the VTL5C9 could be used to replace both the CLM6000 and the CLM6500?

My answer is : YES ! Sound technology use them in their unit and they got same troubles in time with these CLM6000 , same like in the MCINTOSH amp !.

If I was you , I will not touch one working unit and I am pretty sure that you will not hear any sound difference between the first MC2255 you bought and one of the last produced !

Again , be very very carrefull with your MC2255 , because if you are not, you will cause drastic dammage to your working boards if you think that substitution is possible from one to the other one !
Never forget this : if your boards look identical but don't have the same number , think that they are really different and don't mix them ! :nono:

I will send you picture for example soon :

MC2255 Driver boards "update"

Here is the example :

See my pictures and watch carefully the small differences ....C703 and C709 are in reversed polarity position !!

third picture is my " MC2255 service manual "

4th is the complete parts list :thmbsp:

clinic-audio said:

Here is the example :

See my pictures and watch carefully the small differences ....C703 and C709 are in reversed polarity position !!

third picture is my " MC2255 service manual "

4th is the complete parts list :thmbsp:

Click to expand...

Good to hear from you, clinic-audio!

Yes, I have the schematics that you have shown. They include the parts list. The first updates that I did were to auto test board, including adding R757, R758, & C715.

The schematic clearly shows the correct polarity for C703 & C709 as pictured below. The polarity of C703 & C709 in fig 7-1 are drawn incorrectly.

clinic-audio said:

Hello jeffs79


If I was you , I will not touch one working unit and I am pretty sure that you will not hear any sound difference between the first MC2255 you bought and one of the last produced !

Again , be very very carrefull with your MC2255 , because if you are not, you will cause drastic dammage to your working boards if you think that substitution is possible from one to the other one !
Never forget this : if your boards look identical but don't have the same number , think that they are really different and don't mix them ! :nono:

:

Click to expand...

I thank you for your concern.

My main concern is that the early driver boards show signs of excessive heat. I'm certain that some of the revisions listed in the schematic were to address this issue. Building new boards will allow me to keep the original boards without altering them too much as there are many parts that need to be added.

jeffs79 said:

Driver board V1: CT1001-CT2449, V2: CT2450-CT3263, V3: CT3264 and above. Some part changes here and there as well.



It will indeed. I'm looking forward to it.

Click to expand...

My 2255 (just got today) has "CT4450" on a label on the back.
New to all of this

So, am I in luck and got the "good / latest" design?
Thanks
Cad55Ken

cad55ken said:

My 2255 (just got today) has "CT4450" on a label on the back.
New to all of this

So, am I in luck and got the "good / latest" design?
Thanks
Cad55Ken

Click to expand...

Congrats on your new amp!

Your 2255 is a later unit so it would have all of the updates to the test board and the last version driver board.

MC2255 Driver boards "update"

Hello cad55ken

MC2255 serial number CT4450 has been manufactured in January 1984 :thmbsp:

Success

Finally finished up the first board. Testing was done in my first amp which already has V3 driver boards. Initial testing showed that some voltages were off. This was due to a couple of 10 ohm MF resistors being installed where 10k MF's were needed - R411 & R412. I installed the proper values and voila!

I should note that KSC1845F was used to replace all of the 132-185 & 132-195 transistors. The KCS1845F's have a different lead arrangement. The predrivers were supplied from McIntosh parts and are 2SA1360 (different lead arrangement) and 2N6553 (drop in). The LDR's for the power guard and speaker muting were sourced from McIntosh and are different - they are not using one LDR for mass replacement.


Ran a quick test with my HP8903b this morning. Brought it up to 50 volts, 1Khz @ 8 ohms = 312.5 watts, .039% distortion. I'll test it further tonight to see what my max power is before the power guard kicks in. The original board in that channel would max out at 52.5v, 1Khz @ 8 ohms = 344.53 watts, .0321% distortion before the power guard would kick in.

All in all, a fun little project.

Old thread, I know, but I have a new to me MC2255 and it's the first version. It sounds very nice to me and after replacing the bulbs, I'm happy just enjoying it, but I'm wondering if is worth updating or doing any revisions, driver board or otherwise. I don't listen very loudly, so I'm not worried about pushing it hard.