MC2505/MC2105 how safe to use with expensive speakers?

[PEQuestrian]

I decided to pick up a MC2505-2 to experiment with ruining my (overly) perfect system. Realized - after pulling the trigger - that these first gen SS models don't have much in terms of protection circuity and am seeking advice from folks who know these amps about what I could / should do to minimize risk for speakers and the amp itself? (Aside from: "don't do it!" and "take it to an authorized tech first" (it will happen, but I'm not sure how well equipped he is to test for DC voltage leaks and these guys don't grow on local trees. Want to avoid shipping)

Basically: are there any specific parts in the circuitry that you recommend focusing on getting thoroughly inspected or simply replaced to minimize risk of damage from degradation? (for example thermistors that help "slow" start the amp, power caps leaking DC voltage, rectifiers, output transistors etc.)

Or: do I just wire in speaker fuses?

Thanks!

 

[Panelhead]

I do not know how old the unit is. If age is a concern, put on a power strip with circuit breaker.
Check for dc on outputs on power up and after being powered up overnight. If this is normal, less than 20mV in both cases, hook a five volt LED to check for oscillation. If LED glows at all there is an issue.
If the dc stays low, no sign of oscillation, and you do not smell anything burning you are off to the races.

 

[Panelhead]

Guess I should have looked. Old enough to have transformers. Should have almost zero dc on output.
It is fifty years old. Or more. Hook up cheap speakers and let it play. After a day if still working you are fine. Tough stuff.
Look for smoke, checks for smells.

 

[Bruce Morgen]

I decided to pick up a MC2505-2 to experiment with ruining my (overly) perfect system. Realized - after pulling the trigger - that these first gen SS models don't have much in terms of protection circuity and am seeking advice from folks who know these amps about what I could / should do to minimize risk for speakers and the amp itself? (Aside from: "don't do it!" and "take it to an authorized tech first" (it will happen, but I'm not sure how well equipped he is to test for DC voltage leaks and these guys don't grow on local trees. Want to avoid shipping)

Basically: are there any specific parts in the circuitry that you recommend focusing on getting thoroughly inspected or simply replaced to minimize risk of damage from degradation? (for example thermistors that help "slow" start the amp, power caps leaking DC voltage, rectifiers, output transistors etc.)

Or: do I just wire in speaker fuses?

Thanks!

You might want to review this repair/restoration series on U-Toob to learn about your beast's innards. The MC-2100 is the same amp as the big brother of yours, the MC-2105, without the cool glass front panel and meters:

 

[PEQuestrian]

Good call on hooking up cheap speakers to test all controls, check for noise, etc. I'll look for a pair on Marketplace and watch those vids while the amp is en route.

Any other tips on where to focus to fortify the circuitry or eliminate signal degradation hotspots like rca jacks?

 

[PEQuestrian]

Got the amp a couple of weeks ago and ran it on my workbench with some test speakers, as suggested.

Unit worked beautifully, minus the meter switch that needed cleaning. Opened her up and could tell she was serviced previously. Cleaned the switch, RCA jacks and replaced the cheapo, bright white LED kit with better, OEM color matched ones, put her back together and moved her into the main system.

Shockingly (?), this 50+ year old amp sounds identical to my Onkyo M588F and my 2nd gen Purifi Boxem. Rarely have I been more OK with a mission failed though: an full grain aftermarket L12 wooden case is ordered and even though there's no audible difference, this beauty will be staying in the family, maybe for use in a second system or the kids later on if they show an interest.

Next up in "let's add distortion": a 90s Conrad Johnson Preamp...

 

[GXAlan]

The easiest thing to say is that you have to be careful, so that you don’t blame random internet stranger for saying it’s OK.

The reality is that the autoformers are a key element of safety since it shunts DC to ground (according to McIntosh literature). You may lose noise as the power supply caps get older though…

Still, there is a reason why the old McIntosh amps hold their value AND a reason why the newer McIntosh amps have a premium even though you wouldn’t be able to easily hear the difference in real world listening…

 

[PEQuestrian]

That's helpful to know about the Autoformers! - I would like to get the caps tested for DC leakage but the answers amd attitude I got from the local authorized service tech over the phone turned me off to dropping it off there so that might be a while.

Ultimately, since there is no audible benefit, the amp won't be staying in the main system for long and I do want something with better protection circuitry.

On the note of Mcintosh reliability: until recently, I didn't realize that Mc amps apparently run mostly or completely (?) in class B, hence the pretty low heat and long component life.