Marantz 240 possibly blown

[James Cox]

So my friend called me up today and was very distraught. He purchased a Marantz 240 and 7t preamp a few months ago and was experimenting with some separate phone preamps he had. The problem came when he was switching around the phono preamps with the amp and stuff on and accidentally hit the RCA output cable from the pre to power amp. When he did he said there was a loud crazy sound and he jumped up to switch off everything. He said he though it may have been a corroded RCA terminal so he cleaned the RCA jacks with deoxit and hooked everything back up. When he did the sound was extremely weak so he plugged in his headphones to see if the issue was in his speakers. He he did there was another loud popping noise from the headphones and no more sound from speakers at all after that and the relay no longer clicks on the power amp.

He called me because he knows I work on vintage equipment but solid state is not really my forte, I mostly work on tube stuff. Ive worked on solid state stuff before but mostly just simple recaps and stuff like that. My question is, what would have blown on the Poweramp by accidentally shorting the RCA cable from the preamp? I'm leaning towards the outputs because of no sound but if there's another place I should look I'm all ears. Also is it likely, if the outputs are blown, that other stuff down stream may have been damaged as well?

 

[quadmatrix82]

There’s some good threads on restorations for 240/250 models here we’ll wroth a read through - without knowing the history of the unit it’s possible it hasn’t had the factory bulletins done to help improve stability and if original would need a restoration anyway.

The pre driver and driver and output transistors need to be matched - check all of these and make sure the differential transistors are in spec most likely they aren’t and need changing too with all of this done and a recap you are a long way forward in having it back to sounding it’s best good luck

 

[James Cox]

He planned on having it restored soon but since it was working properly he wanted to listen to if for a bit before. And just to note it does still power on and doesn't blow any fuses, the relay just doesn't click on

I should have time to look at it Tuesday or Wednesday. In the meantime Ill do some research on the service bulletins and some of the previous restoration threads. My goal would be to troubleshoot and find the initial problem then once it's working go through with the full restoration. I appreciate the info on where to start looking.

 

[rickl]

I've worked on a few.

If the protection relay doesn't engage, one or both channels are blown.

You can measure the protection wires to see what channel is blown.

Then, disconnect the b+/- from the good channel and get the bad channel working. Use a DBT.

Like quad said, there are many rebuild threads.

 

[James Cox]

Cool thanks for the info! I have a DBT, what size bulb would you use with a beast like this? I think mine is 60W but I should probably double check. And when I measure the wires from the protection relay what readings am I looking for and what point to I take my readings from?

 

[rickl]

I have a DBT, what size bulb would you use with a beast like this?

I'm using a 150 w bulb. you might be able to use a 100w bulb.

And when I measure the wires from the protection relay what readings am I looking for and what point to I take my readings from?

you might not be able to reach pins 5 and 7 on the protection board. pin 19 on the amp boards might be easier. post what you find. I think i had to open up the amp boards (unscrew from chassis) to get access. it is kind of tight space.

here is a 1200 but you get the idea