Marantz tech help please

blhagstrom

Mad Scientist, fixer.
Subscriber
I have a 4270 on the bench.

Initial report.
Intermittent loud short circuit like sound and act on one amp.

There was some small popping and crackle like a bad cap after it finally started to act up.

Some finger poking the input coupling cap seemed to cause more noise, popping and crack.

Tossed all new caps on the amps.

Amps adjust very well but DC offset at zero when trimmer at max turn on flacky amp.

Runs great, sounds great. Then bzzzap. Tossed into protection. Then pops back and plays fine.

It is the amp. Noise was isolated to the amp using the IN/OUT jacks. Noise would occur at zero volume.

Trimmers seem ok, adjustment shows smooth changes when moved.

Blob diodes replaced.

Intermittent sucks.

Anyone figure out one like this?
 
Bad transistors in the pre will cause offset to go wild. Plug a pair of rca's in the pre and measure dc on the offending channel. Monitor it as you would measuring dc offset on the amp side.
 
Last edited:
Thanks Brad.

I was thinking it was isolated to the amp but the noise may have been just the amp and the hard crash may be a second issue that hammered the amp.

I think there was work done in the pre before, things don't look factory.
The audio wires are different and a ground wire seems different.
Yet the components didn't seem hacked. I'll need to look closer.
This is a multi-voltage P model.

I have a multi-voltage E model but didn't notice the pre in there.
The power supply in the E is totally different.
 
Due to the fact that it's a quad I think it also has a tone amp board- the pre and tone amp are made up of 20 transistors per channel and a FET- if i'm reading it right.
 
I've had issues in the tone amp and pre-amp and about everywhere else.

Jumpers out, tapped the pre-out with the scope, amp open.
Playing fine
Playing fine
Playing fine
.....


BBBZZZZZZZZ!

It's the amp.

I shut it down quick and hooked up the meter to the amp.
Fired it back up and the DC offset was all over the place.
Bias looked fine.
Nothing is hot.



Differential pair, maybe?

Damn, damn, damn I HATE intermittent problems.

Nothing on a 4270 is easy to work on so the less trips in and out is a good thing.
 
Yeah, gotta start with the diff pair. Have you tried gently touching each transistor with a wood stick to see if any are mechanically intermittent? I've also found loose hard hardware on driver transistors but that was on the 2270 type output boards. Power resistors tend to get their solder joints cystallized and/or cracked due to the many heating/cooling cycles too.
 
Yeah, gotta start with the diff pair. Have you tried gently touching each transistor with a wood stick to see if any are mechanically intermittent? I've also found loose hard hardware on driver transistors but that was on the 2270 type output boards. Power resistors tend to get their solder joints cystallized and/or cracked due to the many heating/cooling cycles too.

I eyeballed the joints real close when I had the amp out to recap.

Damn 4270 are so packed in that it’s tough to get at the amp parts when operational.

I have to study the schematic a bit. Then maybe take a stick to it and try the poke test. I’ve found busted stuff above the board a few times as you say.

Splaying it all out with jumpers ties up so much room waiting for it to fault. I need a second bench.

Hell, I need another building! I have 2 lifetimes of projects here.
 
I dropped in new diff pair

the old ones measured hfe 338/162
offset adjusts more to center now.
playing
waiting to see/hear

intermittent is a lot of four letter words
 
You should have this licked in no time considering it's just a 4270. :D I have a 4400 on my bench right now.

I'd figure those diff pairs are gonna fix your problem though.
 
You should have this licked in no time considering it's just a 4270. :D I have a 4400 on my bench right now.

I'd figure those diff pairs are gonna fix your problem though.

Well, working on Quads takes a certain sort of sadomasochism. You and I both like North Dakota and that says something about us.
 
Back
Top Bottom