Marantz 170 DC repair/recap?

Fredrik 1977

Super Member
a friend of mine had recently bought this amp and the second he hit the power switch the left channel blew, so the seller took it back and "repaired" it and sent it back, only to blow up again in a matter of seconds, so i told him to send it to me instead.

obviously its been smashed in shipping:( bent upper right corner of the faceplate, and right lamp holder is broken. I will take the faceplate with me to work (mechanical workshop) and see if i can fix that up a bit.

upon inspecting the left amp i found that the outputs are fine, there has been a fire ( 3 resistors) on the pcb and components actually fell out of their holes due to previous techs poor soldering, there are broken tracks, soldering that looks like short circuits, all transistors looks to have been pulled probably for checking and then put back, i have to go through them all to see if they are in the right place:)

really dont know where to start out here:) im just looking at the thing and shaking my head.
part of the fun in this hobby is you never know what you will find inside this stuff when opening them up:)

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Q731 (Diode) fell out when i turned the pcb upside down. 3 resistors are burned and a short in one of the drivers.

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underside of Q711. pretty much looks like this everywhere that service has been made:)

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ok q711,712,713,714 and R750,R737,R749 are toast. and the solder pads at Q711,Q713 are gone, geez!
so i guess first thing would be to repair the circuit board itself before i can do anything else.

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think i´ll replace the dead Q713,714 with mje15031/32 and the Q711/712 with mje350/340
 
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thinking id use a small piece of experimental/lab board and wire that,
luckily the q711 transistor is screwed to a heatsink holding it in place.

other pads on the board is lifted but not totally destroyed as the ones in pic.
 
also Q715/716 will need to be replaced left amp has A552/C735, right amp has A552/C1775 luckily thats the only thing previously done to the right amp it seems.
according to schematic they should be A733/C945 so i assume BC547/557 will be sufficient replacements?
 
well however the protection board has got some "repair" treatment too:). broken pads under the relay, i´ll replace the relay since someone has tried to pry it open with a screw driver.
I managed to save 2 pads under the relay by scraping away the green coat and applying solder on parts of the tracks and a bit of copperwire.
and it looks as if the caps in this amp is in pretty tired condition

CN06 on protection PCB. a 47uf 16V cap.
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the Right ampboard is back in with new electro caps and the mentioned Q715/716 trasistors replaced.
it also has new thermal paste since the old paste was dry. this side has what i believe to be original OP´s it says Marantz on them.

the protection board is also back in with a new relay and new caps, also some fixed solder pads, this amp has probably lived a rough life. really sad to see such gear in the shape it was in:(

the broken amp board is waiting for a bunch of stuff i didnt have at hand so thats gotta wait.

so im gonna fire it up with only one amp mounted and check some stuff.
 
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right ch is good. it fired right up and then clicked. haha i actually had my eyes closed during this process, better safe than sorry:)
offset adjusted. so thats good, now i can concentrate on the messed up amp board.

one thing makes me weary though, and thats the left OP´s, obviously they have been replaced. but is there a way to tell if these are some chinese fakes?
i have had some bad experiences with NOS (read chinese) TO-3´s

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amp is up and running now with both channels installed.

left board with a bunch of new stuff.
Mje15031/32 drivers.
Q715/716 replaced with BC547B/557B
Q711/712 replaced with mje350/340
new caps and 4 resistors replaced.

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Foil side Q711

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actually they are pretty good. had to resolder like 10% of the board and repair all pads under those transistors mounted to the heatsinks. for q711 i had to make a new piece of pcb.

i have been listening to it for a couple of days now and it sound good. but i lugged up my scope on the bench to have a look at the signal.
something strange going on at 10khz.
right ch looks fine

10khz Left ch
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and 1khz looks good. power out is about 4-5W on the meters. 8ohm dummy on the outputs.

1khz left ch
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thats crossover distortion right?
there is no mention in the manual how to adjust idle current.
dc-offset is adjusted for 0V out, and seems to be stable at around +- a couple of mV
 
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yeah i seem to find them. or is it them that finds me? i dunno:)
then i have my own stack of stuff to go through.but i cant seem to find the time:( gonna be some good projects when the time comes.

dont know how much of a problem this would be in reality, the amp sounds perfectly fine to me.
a better view.
10khz
 

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I'm not familiar with Marantz, but is it worth comparing bias currents L to R, just to see if there might be a problem in the L bias circuit ?
With no obvious provision for adjustment, is transistor matching needed somewhere ?
 
That was quite the repair, well done.

I can't say as that 10K signal is a problem- actually looks OK to me - ya, not perfect but I'm sure that's not audible.

Solder joints looked OK from here - pic not great but looks like just extra flux around the joints, NBD.
 
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