Marantz 250 restoration query

[quadmatrix82]

I

Perhaps 1N270 would work, you may have to replace all four in that bridge, CR404, CR405, CR407 and CR408

Tom

looked through my parts I don’t have that diode but do have 1n60 do you think I could sub with this instead or just order some 1n270?

 

[quadmatrix82]

Your board looks to be an early revision, the gain of Q501/502 is critical for determining DC offset and then due to a design quirk, the bias. If the gain is too low you'll get no bias, I had to go with extremely high gain parts in my unit to get it working.

Modern MPSA20s work fine to replace the original one.

It is highly recommend (honestly required if you plan to use the 250) that you pull the driver, predriver and output Transistors and test each for gain, if they are not within the proper gain range as defined by the Marantz service bulliten both the bad Transistor and it's complement needs to be swapped and matched. I have seen units where they "worked" but several of those higher power Transistors were nowhere near spec and it was only a matter of time until the 250 nuked itself.

I’ve started checking the transistors and you’re right many of them are out of spec - my question is do both channels need to matched within the same beta range ? Or is it ok to match the compliments per channel?

 

[ocdecio]

Each channel can have its own beta matching independently As long as the transistors in each board respect the gain tables it should be OK.

 

[quadmatrix82]

Each channel can have its own beta matching independently As long as the transistors in each board respect the gain tables it should be OK.

Thankyou! I’ve spent the last hour going through all the outputs I have I got hfe of 67-71 as a range for the 8 outputs all mj21193g/94g they are on the border of the beta ranges but all close together do you think is ok? Or do I need to strictly stick to the beta ranges?

 

[ocdecio]

No worries about the outputs. You should be ok.

 

[quadmatrix82]

No worries about the outputs. You should be ok.

Awesome! Making progress will start to match pre drivers and drivers tomorrow

 

[goldswimmerb]

Awesome! Making progress will start to match pre drivers and drivers tomorrow

What I have found is that, generally, at or above the specified gain range is fine IF all are matched (Just for example, say your drivers are both 300HFE), while it's not in the chart it should be OK as they're matched up. I would definitely swap original parts that are testing low as it usually is a sign of abuse/degredation. My 250 I did my thread on had an HFE of 11 on one driver and 15 on the other, and while it was stable, it was not optimal as those parts were on their way out.

 

[quadmatrix82]

What I have found is that, generally, at or above the specified gain range is fine IF all are matched (Just for example, say your drivers are both 300HFE), while it's not in the chart it should be OK as they're matched up. I would definitely swap original parts that are testing low as it usually is a sign of abuse/degredation. My 250 I did my thread on had an HFE of 11 on one driver and 15 on the other, and while it was stable, it was not optimal as those parts were on their way out.

Good to know thanks I matched the pre drivers tonight 96-98hfe pretty close the best I could get with the batches I purchased do you think this is close enough? Waiting on drivers to arrive and will be almost ready to do a test!

 

[ocdecio]

This is good. No need to be any closer.

 

[quadmatrix82]

This is good. No need to be any closer.

great!

 

[goldswimmerb]

Good to know thanks I matched the pre drivers tonight 96-98hfe pretty close the best I could get with the batches I purchased do you think this is close enough? Waiting on drivers to arrive and will be almost ready to do a test!

I usually consider a match to be within 5%, I shoot for 1-2%. You're spot on.

 

[quadmatrix82]

Quick update predrivers all in with original heat sinks, differential transistors matched and installed too just waiting on drivers now to finish this channel!

 

[quadmatrix82]

Second channel pre drivers changed everyone’s advice on this forum has been spot on - checking the pre drivers hfe varied from 25 to 75 well below the nominated ranges and no way close in matching and q516 was completely shorted not sure how the amp worked previously! I matched both q516 and q517 for both channels using mpsa43 and mpsa93 as recommended here

 

[goldswimmerb]

Second channel pre drivers changed everyone’s advice on this forum has been spot on - checking the pre drivers hfe varied from 25 to 75 well below the nominated ranges and no way close in matching and q516 was completely shorted not sure how the amp worked previously! I matched both q516 and q517 for both channels using mpsa43 and mpsa93 as recommended here

If I recall those are for the current limiting section, probably would've cause some issues shorted but not cripple the thing entirely.

Also, where's your bias diode?

 

[ocdecio]

If I recall those are for the current limiting section, probably would've cause some issues shorted but not cripple the thing entirely.

Also, where's your bias diode?

Agree with the MPSAs they are part of the current limiting.

Is this the model where the diode wraps over one of the predrivers?

 

[quadmatrix82]

Agree with the MPSAs they are part of the current limiting.

Is this the model where the diode wraps over one of the predrivers?

The diode sits under one of the pre drivers

 

[quadmatrix82]

If I recall those are for the current limiting section, probably would've cause some issues shorted but not cripple the thing entirely.

Also, where's your bias diode?

It’s there I’ll take a photo tomorrow was hidden for the shot I have the early black block version

 

[ocdecio]

The diode sits under one of the pre drivers

That's interesting. I really thought the diode would go on top of the pre driver, not under - the idea is to give them as close thermal contact as possible.

 

[quadmatrix82]

Ok I’ll mount them so they touch tomorrow I didn’t know that I thought might be good to give some air to breathe the other pre driver had a plastic sleeve on the base of the transistor the one where the diode is does not

 

[ocdecio]

Ok I’ll mount them so they touch tomorrow I didn’t know that I thought might be good to give some air to breathe the other pre driver had a plastic sleeve on the base of the transistor the one where the diode is does not

I think you did it correctly and I am mistaken:



Sorry for the confusion.