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Marantz 2500 scope help

quadmatrix82

quadmatrix82

AK Subscriber
Subscriber
Hi Marantz 2500 experts, hoping to get some guidance on repairing a Marantz 2500 scope, there’s no centre dot and whilst works otherwise the scope display is pretty faint, I tried adjusting the the rear adjustments but not much difference.

AM and FM okay great very strong signal on all stations

Any suggestions on how I should tackle this repair? Thanks!
 
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It could be that the tube is just gotten weak. But the power supply should be looked at.
It's a high voltage circuit so be careful.
 
Thanks I rebuilt the power supply it has new caps and diodes and transistors so I’m going to rule it out for now and think it’s elsewhere
 
Here is the power supply board it has been recapped and zener diodes replaced anything else I should look for here?
 

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This is as bright as I can get it and that’s with the house lights off

Replaced q807-809

Reception is great
 

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I’m starting to look for the FETS for the p900 board and found 2sk30a-y locally, will these suit?
 
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Lavane said:
It could just be that the cathode tube itself is getting weak? The scope in the model 120 tuner I gave my son is much brighter.
Click to expand...
True it could be I just wanted to rule out all the other areas first ensuring the tube itself is getting everything it needs! I’ve had this in storage for almost twenty years so recently hasn’t had use but who knows how many hours before that
 
quadmatrix82 said:
True it could be I just wanted to rule out all the other areas first ensuring the tube itself is getting everything it needs! I’ve had this in storage for almost twenty years so recently hasn’t had use but who knows how many hours before that
Click to expand...
Tubes don't normally go bad just sitting around unless they go gassy and start to loose vacume. So if it was fine when you stored it the tube may infact be ok. I would think cathode tubes are similar in that they don't really age in storage.
 
Lavane said:
Tubes don't normally go bad just sitting around unless they go gassy and start to loose vacume. So if it was fine when you stored it the tube may infact be ok. I would think cathode tubes are similar in that they don't really age in storage.
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Good to know! It was a long time ago but I do recall it being brighter as I wasn’t concerned with its brightness when I first got it…and my 2110 is certainly brighter also r13-15 resistors look a bit dark on the p900 board and the board looks all original also
 
What’s the safest way to remove the p900 board out to check all the components? Any advice conscious of the high voltage of the tube itself
 
Let the receiver sit unplugged for a day. You want those high voltage caps to drain. You can Google ways to quickly discharge them too. I usually just let a tube amp or preamp sit for awhile unplugged before working in it.
With tight spaces like that board, discharging them is probably best.
 
Have you looked at the data sheets to compare the two?
Are they known to be bad? These are low noise J-fets and unless they took a big hit, they should be ok. You can use an O, OB, B, GR,Y, etc., but it can effect the square wave. The values are a little different between them all.
doing a search I did find some other J-fet subs as well, but again, you want to get as close to the original value of the 2SK30A-OB as you can.
 
Last edited:
Lavane said:
Have you looked at the data sheets to compare the two?
Are they known to be bad? These are low noise J-fets and unless they took a big hit, they should be ok. You can use an O, OB, B, GR,Y, etc., but it can effect the square wave. The values are a little different between them all.
doing a search I did find some other J-fet subs as well, but again, you want to get as close to the original value of the 2SK30A-OB as you can.
Click to expand...
Thankyou! I did try look up the spec sheet I couldn’t find much on the OB and I’m not too familiar with FETS so not sure what to look for. I notice R13/15 on the P900 board are a little dark, I will try remove this board and check some components the 2sc1756 x 4 on the board also look pretty crusty so I might change them - any recommendations for a substitute here? To remove the board is it just a matter of unscrewing the two screws and plugs and Sliding it out from the front pins?
 
Lavane said:
I didn't find much doing a search for subs, but I did find a surplus supplier with
old stock 2SC1756's.
Click to expand...
Cheers Thankyou I found some 2sc1756 too once I remove the board I’ll test out the components however r13 and r15 are first on the list looking a little darker than the rest on the board may have been stressed at some point
 
quadmatrix82 said:
Cheers Thankyou I found some 2sc1756 too once I remove the board I’ll test out the components however r13 and r15 are first on the list looking a little darker than the rest on the board may have been stressed at some point
Click to expand...
Are those resistors higher watt dropping resistors? If so, maybe go up in wattage some if they will fit. Either way, install them up off the board some for better cooling of the resistor. If close to any caps try to bend the resistors away some from any caps if possible. I've run across hot running resistors that were factory installed against the board and just fried the board. And in some cases cooked electrolytic caps next to them.
 
Lavane said:
Are those resistors higher watt dropping resistors? If so, maybe go up in wattage some if they will fit. Either way, install them up off the board some for better cooling of the resistor. If close to any caps try to bend the resistors away some from any caps if possible. I've run across hot running resistors that were factory installed against the board and just fried the board. And in some cases cooked electrolytic caps next to them.
Click to expand...
I think they are originals are 1/4 watt I have 1/2 watt on hand so will use those and get them off the board cheers
 
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