sparkos on Marantz 2325

[ducati2]

My bias pot was not cooperating on the left channel, just impossible to set bias as it was too sensitive. So, after reading on this forum for possible fixes, I decided to spring for a pair of sparkos BA312-M upgrade kit for my tone board. After a careful install and triple checking everything, I fire it back up to find the bias was still the crazy same (replacing pots tomorrow), but NOW the right channel dc offset pot has no effect at my meter. Left dc offset is working fine. ugggh.
question - I suppose a new problem in the tone board would cause this? As that is all I touched since the last time I set dc offset a few days ago. the sparkos product sure looks great, and surely they test them prior to sending out...

 

[sanjib]

I had the same problem with the bias pot in one channel after a full restoration. Replacing the bias pot with 25 turn bourns solved the problem. Post #28
https://audiokarma.org/forums/index.php?threads/marantz-2325-project.910187/page-2

 

[sanjib]

I don't think bias problems related to BA312 but DC offset can be.

 

[Oerets]

Were the Differential Pair transistors replaced in the amplifier board?

 

[ducati2]

I had the same problem with the bias pot in one channel after a full restoration. Replacing the bias pot with 25 turn bourns solved the problem. Post #28
https://audiokarma.org/forums/index.php?threads/marantz-2325-project.910187/page-2

Having trouble finding multiturn trimmers that are not the size of a microchip... have any idea which one you used?

 

[ducati2]

Were the Differential Pair transistors replaced in the amplifier board?

Not sure which transistors are referred to as a ‘differential pair’, but I replaced a good number of transistors per lists I found on this forum. I would need to know the reference number printed on the board.

 

[sanjib]

I used normal bourns. I soldered three extra legs to fit in.

 

[sanjib]

Differential pair is H701 and 702 and usually replaced by gain matched KSA992

 

[ducati2]

This gets even more weird.. with the meter hooked up to measure the dc offset for the right channel, just for kicks I adjusted the left channel r713 and it appears to change dc offset for both left and right channels! Not the same readings, but it certainly adjusts both sides. I must have a short or something somewhere... this really sucks.

 

[ducati2]

Differential pair is H701 and 702 and usually replaced by gain matched KSA992

Ah.. yes, h701 and h702 were replaced with new (and checked) ksa992’s on both boards.

 

[sanjib]

disconnect the pre-amp from the main amp and try setting the DC offset, then you will know for sure who is the culprit.

 

[sanjib]

Did you match the gain of the differential pair?

 

[ducati2]

Did you match the gain of the differential pair?

Yes , they were pretty close. Is that pretty important?

 

[ducati2]

disconnect the pre-amp from the main amp and try setting the DC offset, then you will know for sure who is the culprit.

So just unplug the tone peo1 board?

 

[sanjib]

Yes , they were pretty close. Is that pretty important?

Yes, very important regarding DC offset or balancing the amp. I think the main amp connected to the preamp through a small Molex connector on the amp board at pin J701. You can disconnect that one.

 

[ducati2]

disconnect the pre-amp from the main amp and try setting the DC offset, then you will know for sure who is the culprit.

Ok I disconnected molex from both amp boards an now neither side will allow me to set dc offset.

 

[sanjib]

Instead of disconnecting the Molex (I was wrong, pre input actually an RCA jack connector, not Molex) connector, insert an RCA cable in pre-out.

 

[ducati2]

Ok, so with molex are plugged back in and RCA cables in the pre out ports, I see no change in the weird behavior. Am I correct to assume my issues are on the amp boards? Again, my issues are;
LEFT CHANNEL = bias pot r740 works great, bias set spot on. Dc offset pot r713 works great, but also effects dc offset on right channel at a different value...
RIGHT CHANNEL = bias pot r740 is way too touchy to properly adjust. DC offset pot r713 unresponsive.

if plugging in the rca’s disconnects the front end, then I am leaning toward problems in the right amp board. Maybe I start with replacing the pots and double checking the gain match on h701 & h702. What are your thoughts?

 

[sanjib]

Here what I understood about the present state of the receiver: After powering on, the relay is clicking, and with everything connected you can set bias and DC offset perfectly on the left channel but you can set bias and DC offset in the right channel. If you disconnect the pre-amp RCA connection from both of the amp boards you are having a problem setting both DC offset and bias in both channels.
Can you confirm?

 

[ducati2]

Yes the relay is clicking fine.
I disconnected the red rca connector to both of the amp boards and tried to set dc and bias on both boards in 2 ways; 1, with the cables still in the pre-out in the back and 2, with them removed, as you did not indicate which.

With the cables still in the pre-out
LEFT CHANNEL = r713 dc offset set fine, r740 bias set perfectly.
RIGHT CHANNEL = r713 no response, r740 too sensitive to set.

With the cables removed from the pre-out,
LEFT CHANNEL = r713 set ok but floats +/- 10mv. r740 no response.
RIGHT CHANNEL = r713 no response, r740 too sensitive to adjust

I am scratching my head.....