marantz 2325 idle current

[jspernc]

I have a 2325 which suffered a breakdown that took out the finals,1 driver and a couple of resistors. Now I have sound again but idle current is only 2mv. I can adjust it down to 0 but not any higher. the bias transistor 715 is conducting as it is getting warm and I do have .6 volts at the drivers and voltage to the finals. I have double checked the final transistors and probed around the sockets to see if I could find a bad connection. No luck. If someone has experience with this type of issue please let me know.

Joe

 

[fgsutton]

I have a similar problem with my 2325 even after a complete recap. The left amp is fine but the problem is the right amp. Transistors H701, H702, H703, H704, H708 and H709 have been replaced, as have all of the diodes on the board. Voltages across the board are at or close to spec EXCEPT the bias. It stays close to 0 mv no matter how R740 is adjusted.

 

[orbanp]

Any chance that R743 and R744, those 1.5 Ohm resistors in the emitter of the driver transistors got damaged? I would check those.
Measure the Vbe voltage directly on the driver transistors, and not to the ground. If the above resistors are fine you should read close to 0.6V

Good luck, Peter

 

[fgsutton]

Thanks orbanp. I checked R743 and R744. Although both you and the Marantz parts list calls for 1.5 Ohm values, both of mine are 3.3 Ohm. I unsoldered one end for an accurate measurement and they are both 3.3 Ohms. Following your suggestion I measured H708 and got .600 volts, while H709 read .580 volts. Note: I have a salvaged amp board and it has 3,3 Ohm resistors instead of 1.5...curious. At this point I am stumped.

 

[Ian Finch]

.... I have a salvaged amp board and it has 3,3 Ohm resistors instead of 1.5...curious. At this point I am stumped.

Having higher value driver emitter resistors there is not a big issue. It was quite normal for early designs, including Marantz's Japanese production versions, to have teething problems and require revisions during and after manufacture.
So you may be assuming your issue of the schematic/manual is concurrent with your 2325 amplifier boards when it is'nt.

 

[fgsutton]

Bias?

Thanks Ian. What I am stumped about is the inability to achieve anything other then 0 to .4 mv at J702 - J708 for bias adjustment. With all the work and on-board components that have been changed I don't know where to go next. In spite of no bias adjustment I heve my Marantz 2325 on all day long. It sounds wonderful, but will I hurt it playing with no bias adjustment?

 

[Ian Finch]

...It sounds wonderful, but will I hurt it playing with no bias adjustment?

Not in principle, as no bias means less average current and less power for the output stage to dissipate. Usually it's just your ears that hurt due to crossover distortion becoming obvious.

This is a CFP output stage where bias voltage is only half that needed for the usual EF types so there will be difference to the usual figures you see quoted here.

Still, I'm puzzled that you say it sounds wonderful with almost no bias current. Just how are you measuring bias; across either pair of output transistor collectors (that's the same as J202-J208) with no speakers connected, no sources connected and volume down to zero?

Otherwise, plenty of people have trouble like solder bridges, dry joints, lifted and broken tracks, broken end caps on old resistors etc. but in this case, maybe failure of the preset pot. It is probably an open type, so you can get right down and compare voltages on the slider arm and resistance element for an accurate assessment.

 

[anatech]

Hi Joe,
Most amplifiers want to see 15 ~ 20 mA per output. High bias ones sat around 50 mA, a bit silly high. Bias can be as low as 2 mA with matched transistors and sound great, so how it's sounding isn't a great indicator of trouble.

I'll have to have a look, but I have rebuilt tons of 2325 receivers. The amp sections are pretty well behaved. I would want to check the bias circuit for one. If you are looking at voltages in the output section, put your meter common on the output terminal. That way the readings will be more stable, and they will make more sense. Also, an emitter resistor on the output can be open, but measure a normal drop. It is the driver transistor that delivers energy to the speaker at low volumes in that case. If you crank it, the driver could very well fail. Don't do that as a test.

-Chris

 

[Wurlyman]

Thanks orbanp. I checked R743 and R744. Although both you and the Marantz parts list calls for 1.5 Ohm values, both of mine are 3.3 Ohm. I unsoldered one end for an accurate measurement and they are both 3.3 Ohms. Following your suggestion I measured H708 and got .600 volts, while H709 read .580 volts. Note: I have a salvaged amp board and it has 3,3 Ohm resistors instead of 1.5...curious. At this point I am stumped.

The service manual, according to other forums, is in error. 743 and 744 are 3.3.