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Marantz 3250B Pre-Amplifier Problems

pppoffice

pppoffice

AudioBear
After having my Marantz 3250B Preamp in storage for a few years I finally drug it out and dusted it off. When I first tried to use it, everything was working just fine except that both of my Phono inputs were totally dead. I took it apart to see if I could see anything obviously wrong but everything looked just fine. I put it back together after replacing the pair of 2SK147's. Now, when I tried it again the two phono inputs are still dead and now I have also lost the left channels of my Tuner and Aux inputs. Both of the tape inputs/outputs are working fine and all of the controls such as volume, bass, treble, etc are also working fine. I am at a loss as to what is going on.
 
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Most issues are caused by dirty switches/controls and/or pots.
Did you try cleaning all of them first?
What led you to change those transistors?
Maybe just put them back in and go from there .

Bob
 
I am already coming to the conclusion that I am going to have to take it back apart and clean the pcb with the switches on it real good. I always clean the pots about every year and cleaned them this time when I brought the 3250B out of storage. The pots are very quiet but I sure didn't think to clean the switches on the PCB. I will do that today and follow up with how it turns out. Thanks

Mike
 
Well, I took the 3250B totally apart and cleaned every inh of it, especially all the switch contacts. After drying it completely and re-assembling it, it now works on all inputs both left and right but the phono section is STILL totally dead. I was getting absolutely no sound out of the phono section prior to cleaning all switch contacts but now I do at least get a loud HUM when I switch to the phono section. There is no signal at all that can be heard in the hum but at least now I get the hum. HELP!!
 
I had a 3250B awhile back that had a problem with a dead phono channel.
It turned out to be a bad solder joint/cracked trace where the phono jacks connect directly to the back of the circuit board.
I think I took some pics to show the owner.
I'll look.
The other thing with the 3250B is that they use alot of pin inter-connected boards which can cause trouble.

Bob
 
Thanks for your reply Bob but I just finished checking all of the traces for a cracked board and I also just finished completely cleaning all of the circuit board, switch contacts, and cleaned all pins and connectors. I'm kinda getting stumped on this one. I'm getting ready to start doing some more work on it this morning if anyone else has some ideas. Thanks again Bob for your reply.
 
Well, in about 50 years of playing with vintage audio and ham radio equipment, mostly tube equipment, I have never run into anything like this before. I was triple checking to see if there were any broken or cracked traces on the phono input board on the back of the preamp. I just couldn't believe there would be a crack or break with such a heavy duty PCB even though some people push very hard when installing the RCA connectors to the back. Just for the heck of it I decided to check conductivity on the RCA connectors on this rear board when I found center post to center post AND center post to ground DEAD SHORTS on this Phono section PCB. I couldn't believe it so I took the RCA section for the phono section out of the PCB (unsoldered). Yep, dead shorts from center to center and center to ground on these RCA connector. Who would have thunk. Now I have to find a good RCA section like this to replace the bad one. Thanks for all your different ideas. The helped me to think.
 
Is it possible to see if maybe you can bend the insides back apart?
Probably someone was hamfisting plugs in there and messed them up!

Those pcb jacks are rare and may be hard to source.

Bob
 
Bob,

That is EXACTLY what happened to the RCA's. When I purchased the 3250B I knew that the phono section wasn't working but figured it would be an easy fix. I had no idea that I was going to spend so much time finding the problem. Boy, was I wrong. I found and purchased 3 of these 4 RCA sections on ebay yesterday for $10.00. Like you, I figured they would be hard to find as no one even thinks about them. I know I don't. Then I went over to my shop and was looking for something else and sitting there in my RCA connector spare parts box was a brand new one. I don't even remember ever getting it. It must have been in one of the estate sales or garage sales I use to go to years ago. I plan on installing it this next week and while I have the 3250B apart for the 50th time I am going to go ahead and recap it. Thanks for your replies Bob.
 
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