MC2205 relay replacement - substitute relay issues and resolution

[Carl in FL]

Folks I wanted to share this in case others experience this. I recently replaced all the electrolytics in my McIntosh MC2205, and decided to replace the speaker relay. I found through research the Tyco Electronics (Potter Brumfield) replacement for the original KUP-12D56 Mac used is the heavily discussed KUP-14D15-48. BTW, if your relay is the 36V version, the replacement works fine.

After I replaced the relay, and started testing everything, I noticed I could activate the relay by turning on the SPEAKER switch, but the relay would not drop out turning the switch off! Perhaps this has happened to others, and they did not notice it if using headphones, but my wife let me know when I was under the headphones the speakers were on (2:00 in the morning; good thing she loves me ;-) ) anyhow, I found the cause; the 10K ohm resistor across the speaker switch (see the circuit sniplet below) was causing just enough trickle current to keep the relay on! So, only 3ma of current keeps it alive. I replaced the resistor with a 39K ohm, 1/2 W resistor and voila! that resolved it.

Now, some of you may ask, why not just take the resistor out. You have to remember the Mac philosophy -make things that last. It reduces the electrical wear and tear of the switch and limits shock to the controlling transistor (located on the Power Supply Board)

The resistor in question


The replacement relay



The original relay

I hope this is helpful to all. Cheers

Carl M

 

[SaturationPt]

Nice.

It would be interesting to compare the DCR of the two relays to see if the new one is significantly higher.

 

[MRL_Audio]

Thanks for posting. Just picked up a 2205 two weeks ago. Already been sorted and updated so I won’t need this for awhile but this will help down the road.

 

[Carl in FL]

Nice.

It would be interesting to compare the DCR of the two relays to see if the new one is significantly higher.

That is what is interesting; they are very close; the old relay DC resistance is 1890 ohms. The new one is rated for 1.8K ohms. However, the construction of the coil and metal core differ; I believe finer wire and more turns to achieve the same DCR. Definitely more efficient :-D. BTW, the 10K resistor measured 9.5 K ohms. Before I replaced the resistor, the hold effect was solid and happened each time reliably!

 

[twiiii]

Don't replace the relay, but first clean the contacts with a clean dollar bill if the relay engages. If the contacts are pitted you will have to use a burnishing tool or very very fine emory cloth. Then clean with a dollar bill. If you don't have a clean dollar bill a clean hundred dollar bill will work, too.

 

[clinic-audio]

Does the one thousand dollar bill exist ?

 

[ramjet]

Does the one thousand dollar bill exist ?

Yes.

 

[jlovda]

If the relay is only $25 why would you even bother trying to clean the contacts? You are doing 100% of the work anyway by removing it and opening the cover.

BTW, I don't think any bill bigger than $100 is legal tender now. Huge notes like $1000 and up were just used to transfer cash between banks way back in the early 1900's.

 

[Carl in FL]

If the relay is only $25 why would you even bother trying to clean the contacts? You are doing 100% of the work anyway by removing it and opening the cover.

BTW, I don't think any bill bigger than $100 is legal tender now. Huge notes like $1000 and up were just used to transfer cash between banks way back in the early 1900's.

Why not replace a 40 year old relay? Sometimes burnishing will not resolve it.

 

[jlovda]

Why not replace a 40 year old relay? Sometimes burnishing will not resolve it.

????

I just said why remove and clean an old relay when you are doing all the work required to replace it.

 

[Carl in FL]

????

I just said why remove and clean an old relay when you are doing all the work required to replace it.

jlovda, I'm sorry, I was trying to respond to twiiii. I totally agree with you! I've worked with relays for years; I've seen them fail after an attempt to revive the contacts. Plus the MC2205 is SO HEAVY, I don't want to have to open it again soon if I don't have to.