Marantz 2325 bias and DC offset trim pot replacements

dbxdx5

Super Member
The bias trim pots in my 2325 are super sensitive, to the point that achieving 15mV is impossible, even after repeated cleaning. The traces and joints look fine, so I've decided to swap in new pots, including for DC offset. What have other folks used? I'm considering the Bourns 3368 series.
 
Not familiar with te Marantz 2325 however the replacement "must" be a multi turn type like the 3296 series. Just
need correct orientation.
 
Some folks like the multi turn, some go for the Bourns single turns depending on the application. The 2325 uses 1Kohm pots. Here is the Bourns part number for a multi turn 1K ohm pot with the three legs offset:

3296Y-1-102LF

There are four suffix types: W, X, Y, and Z.

W and X have their three legs in line, Y and Z are in a triangular pattern. The 'Y' pattern is usually the one to use on vintage Marantz amplifiers.

W and Y have the adjuster on the top, and X and Z put it on the side. The X and Z won't work on a 2325, so you end up again with the 'Y' designation.
 
Some folks like the multi turn, some go for the Bourns single turns depending on the application. The 2325 uses 1Kohm pots. Here is the Bourns part number for a multi turn 1K ohm pot with the three legs offset:

3296Y-1-102LF

There are four suffix types: W, X, Y, and Z.

W and X have their three legs in line, Y and Z are in a triangular pattern. The 'Y' pattern is usually the one to use on vintage Marantz amplifiers.

W and Y have the adjuster on the top, and X and Z put it on the side. The X and Z won't work on a 2325, so you end up again with the 'Y' designation.

Thanks, especially for the explanation about the leg patterns.
 
The 2325 uses bigger, wire-wound pots. I wonder if the power rating of the bourns (500mW) is appropriate ?
 
Here's my take on this. If anyone has a better understanding of Vbe multipliers I hope they chime in, I wish I understood them better.
I don't know what wattage the original controls are.
Let's assume the circuit is operating normally.
The Vbe multiplier circuit consists of H714 (connected as a diode), H715 and the bias control R740 (connected as a rheostat).
The voltage on the collector of H715 should be about 0.6VDC, the voltage on the emitter should be about -0.6VDC with respect to ground.
Let's assume that the Vf drop across H714 is 0.6VDC, it is probably more, but we are assuming worst case.
This puts zero (0) VDC on the junction of H715 and the bias control R740. There is -0.6VDC on the other end of the bias rheostat.
Let's assume that the control (1000Ω) is in the middle at 500Ω. Ohms law says Power = E(squared)/R, therefore, (0.6 times 0.6) is 0.1296/500 equals 0.259W.

Tom
 
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Power = E(squared)/R, therefore, (0.6 times 0.6) is 0.1296/500 equals 0.259W.

I get .36/500 = 0.00072 in this scenario. That's on both sides of the pot though so the pot wattage = 2 * .00072 = 0.00144. I don't have the schematic here at work but isn't there 1.2V across it? That would make for 1.44/500 = 0.00288. Anyway you cut it then, the Bourns pot is fine.
 
Power = E(squared)/R, therefore, (0.6 times 0.6) is 0.1296/500 equals 0.259W.

I get .36/500 = 0.00072 in this scenario. That's on both sides of the pot though so the pot wattage = 2 * .00072 = 0.00144. I don't have the schematic here at work but isn't there 1.2V across it? That would make for 1.44/500 = 0.00288. Anyway you cut it then, the Bourns pot is fine.
All this math is hurting my brain. I'll put my finger on the pot and let you know if it's hot. :biggrin:
 
The new 3296Y trimmers have been installed, but aren't responding/changing DC offset or bias when I attempt to adjust. They can only go in one way, so I know the orientation isn't wrong (unless the 3296Ys aren't the right trimmer in the first place). The only other thing I can think of is that the original trimmers have two outer legs that were soldered into the board, but the 3296Ys don't have them and so I left those spots empty. Color me puzzled.
 
With original trimmers you got down to 15mV. If there is no response to the new trimmers there is a likelyhood that it's an installation
issue somehow. Also no response to both DC offset and bias trimmers using same technique. Will chase up the artwork unless others
more familiar...
 
Service manual contains no artwork. Check trackside that there are traces going to the legs of the trimmers.
 
I think those are 25 turn trimmers so they won't seem to do a lot. Keep cranking!

I was thinking it could be that, but I turned them a fair amount and don't want to crank them too far. Should there be a light "click" felt with each full turn?
 
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