What’s wrong with these McIntosh Monoblocks?

Most tube amplifiers require a speaker load present, therefore, if the amp is ran unloaded, the fuses will fail to protect the output transformer.  

Test pilot is correct. Hooking them up without a load is a no no.

I am an electrical engineer who restores vintage audio equipment as a hobby.  The previous posts are not really correct - the MC40 does not need a load connected to verify the DC circuit voltages, as the circuit is perfectly stable at idle.  Just do not connect a signal at the input.

But the real issue here is that you plugged in ancient amps into the wall outlet!  YIKES!  You are lucky they are well fused or you might have gotten a fire on your hands.  Thought I suspect previous people made the same mistake.  You needed a variac at a minimum.

I can tell you all the caps are shot.  The electrolytics are probably shorted.  Hopefully the output transformers are ok - that's the most worrisome part.  Replacing all the caps is a huge job (I've done it 5 times and don't want to do it again lol).  But it would be worth it.

Thanks for the response, but I have a hard time understanding how to apply a variac to this equation. Says the amps are rated up to 125v, so I just plugged them in. But evidently there’s much more to this than I thought.

If fuses blow right-away, chances that you have bad output stage are dominant and you will need to inspect output stage components such as DC supply literally one by one if you don't have schematics certainly starting from output tubes. After testing one or few bad output tubes, don't be in the rush placing new tubes!!! Inspect components around tubes such as caps, resistors, large filter caps and resistors around large filter caps.  

Take the amps to a tech who can work on them any of a number of things could be wrong. And as said above always use a variac and ramp up the voltage slowly. A sudden inrush of current can do damage to long unused electronics.

As Clint said..."Are you feeling lucky?"

Good luck. 🤞👍

@aball - correctamundo - I am in New Jersey if you need repair assistance.

Happy Listening.

problem now is the same as what blew the fuses in the 1st place. unusual that both amps fuses were blown, and both blew again.

fuse specs: perhaps someone put the wrong fuse in them. find the fuse specs, get a couple pairs of them, see what happens

how rusty, i.e. if they need work, fix spend money, could you re-sell them for a profit?

Not that rusty, just a really light layer. I would definitely have to refinish the pieces though. Checked the fuse and the correct one has been used, 1.5A sloblo. 

Hi OP:

Variacs are used to gently turn amplifiers on, letting you turn the power off before really major damage/fire has occurred. Technicians use them both for recently repaired gear as well as for amps like the one’s you are describing, which may have been stored for a long time and or improperly.

At the very least, letting a piece of gear like that dry out for a couple of days inside before turning them on is a good idea too.

I am agreeing with the consensus, the caps are "dried out". Capacitors need exercise (especially vintage, older and idle caps). A variac allows you to start the voltage going to the amp through the power cord very low and slowly bring it up, over a period of days if necessary. This will give the capacitors the slow start up that they need when coming out of storage or recent repair, kind of a break in-warm up period for old caps. I am not familiar with Macintosh equipment, I'm thinking that if two fuses are blowing they are on the output side. I would not hook up speakers until you find the shorted component common to L and R. You do not want to put line voltage AC (or DC rail voltage for that matter) to your speakers. They are probably output (speaker) fuses (1.5 slo seems low for even speaker protection fuses), and if both blow immediately I would be worried that something on the amplifier side is shorted. With no speakers attached there should be no current flow because there's no complete circuit without the speaker coil. You might have inherited a problem requiring a Mac technician. Over the years I have learned it's much easier to destroy an amplifier than repair it. It's too late, but Amazon has Chinese variacs for around fifty bucks. You can, of course spend much more. AC in fuses for line voltage would be much higher, maybe 5 amp, or higher, and usually only the hot leg of the line voltage is fused (single fuse). There probably are in circuit fuses coming off of the PS. If you don't have a call out on a schematic or service manual, I would not assume the fuses you removed were correct to begin with (I have doubts). Good luck, and send these amps to a tech, after capacitor or transformer replacement the amp will need bias adjustment anyway...