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2245 Pre Amp P700 board

Mike Sweeney

Mike Sweeney

Well-Known Member
Every board in this unit has been a challenge at some level :) I think the Karma gods are making up for the last one which was a cake walk.

This has blown caps, lift traces, odd transistors with no marks swapped in and then I just found a resistor buried behind on the big ugly caps broken in half.

The heavy red is the busted resistor, the thin red is the unknown transistor which was considerably lower HFE than the rest and the green on the replacement films I just put in.

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I've replaced the caps and the troublesome drop diodes and the two resistors with new carbon films. I replaced the broken one and it's mate on the other channel.

So now it sorta works. One channel is completely dead. And the sound is thin and harsh on the channel that works. I've got signal ( via scope) at R724 but nothing past it's mate on the other side.So thats H702 which has been replaced ( and all the rest) a ceramic cap ( C706) and a few resistors. I've cleaned the board and looked for solder issues but didn't see anything obvious. I checked the transistors before they went in because they are all matched. The caps were checked also to make sure they were in spec.


I'll pull the board out and jump it back in so I can more easily work on it. Meanwhile, any ideas? I'm guessing maybe some issues with the carbons given the abuse I've seen so far. Its a "learning opportunity" :D And since I'm pulling it out, I'll put in new shielded cables like I did for the power amp boards. Always a bright side..

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If you changed out the 2SC458 transistors, their pinout is 'BCE'. If you replaced them with KSC1845's, their pinout is 'ECB'. The flat on the transistor would be facing the other direction.
 
For some reason the Marantz phono boards, which seem simple enough, have caused myself problems as well. Once on a 1060 phono preamp, there was a glass capacitor that was open, but when I would unsolder the cap, it would heat up and test good. Then, after I put it back in, it would work o.k., and I'd think it was fixed, until it cooled and then would go open again. The symtom was it was about 1/3 more quiet and no bass when compared to the other channel.

Frustrating.
 
Test the BCE voltages of each transistor against the SM.
When I got the power amp working on another model one of the phono channels was dead, a voltage check lead me to the culprit.
 
Got a scope, easy to troubleshoot- signal goes into the pre and comes out, you'll find where it drops using one. I had one of those glass caps fail in a unit and I lost all signal, scope lead me to it.
 
At least I did figure it out. I had the output on a oscilloscope and could see as the cap cooled down, the sine wave actually shrink quickly. It didn't slowly get smaller. It "jumped" to about 1/2 to 2/3's of the good channel.

That's how you learn.
 
PS- OP this is the phono board P700, 400 is the pre is it not? Are you just missing a channel when in phono or the entire unit?
 
Got!!! It took some time and a magnifying glass to find the issue. I had to scrap off the resist coat to verify it. At some point, someone pressed hard on this board or or something and managed to put a hairline crack in it. It cracked and took out the trace connecting R704 to the base of H702. I was looking because I had some odd readings in that corner of the world when I was troubleshooting. It wasn't a hard failure, just low voltage but all the parts were testing fine. I even pulled C706 thinking it was bad.

I put a solder bridge over it which fixed it but I will be going back and putting a left piece of a lead there keep the solder from cracking. The crack doesn't show on the edge or on the top of the board :/ PIA to track down.

cracked_board.jpg where-crack.jpg
 
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