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Model 18 fire!

You will get it going!

Since the hum is on both sides immediately at power up it's probably in the power supply to the amp.

If C22 was in backwards for 15 MIN then a bunch of current would flow through it to ground getting it very hot, I suspect that's why R24 lit up like a candle. The current to R24 would be supplied by the rectifier diodes, CR7 and CR9. They probably got a bit toasty as well. Not sure how much if any additional current would flow through C8. My bet is that the cap that was in backwards had it's electrolyte cooked like an egg...

I did the same thing on a 2500. I re stuffed the main filter caps and had them wired wrong internally. I was lucky and caught it before anything was damaged, I could hear the cap protesting as I ramped up the voltage.

Good luck!
 
You should not hear anything through your speakers until the relay kicks in....... is it welded together now????
Ask me how i know this:D
 
Before you spring for an expensive axial cap, chances are you can install a cheaper , better (or at least as good) radial cap in its place. As long as the leads reach. You can sleave with spaghetti, wire insulation or heat shrink in it runs by anything . I would change that cap before i did anything else. like someone else said if you still have the old cap and it wasn't damaged during removal you could put it back as a test. It could be as simple as that (read likely, no promises).
 
Okay, so I replaced both questionable caps and still have a hum when the speakers come on that does not increase with the volume, but does when you press the 'low filter' I was told to check cr6,7,8,9, .... can I test these in circuit and what would be the appropriate replacement (SM just lists 'rectifier diode' and the Marantz part #). Thanks Rich
txturbo said:
am almost certain that the relay on the 18 is normally closed. It only opens if you have DC on the outputs.
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Yes, there is not relay click, the speakers just kind of 'warm up' like an old tube amp. Rich
 
tested the rectifier diodes, started checking 36v, first at driver boards and it is 77v across the 36+ and 36- pins, then checked the output of r22 -38v and r24 +38v. Any insight? The hum comes in when the amps power up. With the volume all the way down, you can hear a little white noise and the hum. Ii doesn't increase with the volume control, but if you press the 'low filter' it about doubles in volume and is very noticable. Thanks for any help!! Rich
 
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txturbo said:
CR7 and CR9. They probably got a bit toasty as well. Not sure how much if any additional current would flow through C8.
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checked the rectifier diodes and they seem good, C7 and C8 were replaced after the short with nichion 8200uf 63v hi-sound caps
 
When you did the recap what did you replace?

The assumption is that the problem started after the fire but maybe it was due to the recap.
 
txturbo said:
When you did the recap what did you replace?
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Using your list C-6A 20/250, C-6B 20/250, C7/8 8200/63, C9/10 470/63, C-11 10/600, C-12/13 330/100, C-14 470/63, C-16/16 470/16, C-15/16 470/16 and C-17/2/21/22 470/63, all the part #'s you supplied except C7/8 are nichion 8200/63 hi-sound. It did not have the noise before the re-cap/fire. I tested after each section and the failure was 15 minutes after the last section (36v supply) and damaged the last section. The initial hum is livable, but when you press the low filter, it is very noticeable, that's what confuses me, is something on the lo/hi filter board served from the 36v supply I blew up? The cap the blew was C-22, and I replaced C-22 and C-21 along with the 68ohm 1/2 watt at R-24. Thanks for your help Txturbo! Rich
 
txturbo said:
I need to stare at the schematic...
Click to expand...
I am not good at schematics, but it looks like the 36v feeds the tone amp which in turn feeds the hi/low filter and then the drivers. Also, both bass and treble controls operate as the should on both channels. I just would not have a clue what I would have damaged, I'm hoping you can help, I hate to ship it off for repair again, it is so heavy and the shippers are so bad. Rich
 
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