Marantz 2275

[GregS]

Wondering if anyone is familiar with this series. I was
testing it out and the bias circuitry is not
functioning. There is shown on the schematic a bias transistor
along with one diode and pot.
There is no bias transistor on the two stages, just looks
like one diode. I intend on adding a diode or two to get it going.

There is a place on the boards for the transistor, but it was never there.

greg

 

[GregS]

Wondering if anyone is familiar with this series. I was
testing it out and the bias circuitry is not
functioning. There is shown on the schematic a bias transistor
along with one diode and pot.
There is no bias transistor on the two stages, just looks
like one diode. I intend on adding a diode or two to get it going.

There is a place on the boards for the transistor, but it was never there.

Just to clarify. You can see the holes in the board for the transistor, but the copper
is solid on the back plane. I don't figure how it could have worked right
from the factory. The diode under the driver heatsink seems normal, about
6 volt drop. I might need at least 4 diodes in addition to the one.
H709 is MISSING.

http://zekfrivolous.com/misc/marantz2275.JPG

greg

 

[David Farber]

Wondering if anyone is familiar with this series. I was
testing it out and the bias circuitry is not
functioning. There is shown on the schematic a bias transistor
along with one diode and pot.
There is no bias transistor on the two stages, just looks
like one diode. I intend on adding a diode or two to get it going.

There is a place on the boards for the transistor, but it was never
there.

greg

I found a service bulletin from 10/29/1975 that may or may not have an
effect on your problem. If your serial number is below US 7701 or Europe
51001, replace diode H720 and diode H721 with a 1S1555. When you say the
bias circuitry is not functioning, do you mean there is zero bias current?
It is supposed to be 10mv from TP701-TP702 and TP703-TP704.

Good luck.

 

[David Farber]

Just to clarify. You can see the holes in the board for the
transistor, but the copper
is solid on the back plane. I don't figure how it could have worked
right
from the factory. The diode under the driver heatsink seems normal,
about 6 volt drop. I might need at least 4 diodes in addition to the
one.
H709 is MISSING.

http://zekfrivolous.com/misc/marantz2275.JPG

greg

If both channels are missing that transistor, and one of them works
properly, then it probably was meant to be like that. You can see in the
schematic that there is still a path from the base of H713 to the base of
H714. If anything, H709 would lessen the voltage between the two points and
decrease the bias even more. What is the reference number of the diode which
you say has a 6 volt drop?

 

[GregS]

If both channels are missing that transistor, and one of them works
properly, then it probably was meant to be like that. You can see in the
schematic that there is still a path from the base of H713 to the base of
H714. If anything, H709 would lessen the voltage between the two points and
decrease the bias even more. What is the reference number of the diode which
you say has a 6 volt drop?

Neither channel has any current. point 6 .6 volt.
I'll check on the diode specs.

greg

 

[GregS]

I found a service bulletin from 10/29/1975 that may or may not have an
effect on your problem. If your serial number is below US 7701 or Europe
51001, replace diode H720 and diode H721 with a 1S1555. When you say the
bias circuitry is not functioning, do you mean there is zero bias current?
It is supposed to be 10mv from TP701-TP702 and TP703-TP704.

Good luck.

H720 and H721 are for the bias of a couple stages back. I'll
check that out too.

greg

 

[GregS]

H720 and H721 are for the bias of a couple stages back. I'll
check that out too.

One mystery solved. There is a bias transistor. Its hidden under one driver
heatsink. There are two positions for it on the board. One
away from the heatsink. That position I first saw is vacant.

So far a bad pot on one channel is the only thing I have found wrong.
I still have to figure why the bias on the other channel is
not working.

greg

 

[William Sommerwerck]

One mystery solved. There is a bias transistor. It's hidden

under one driver heatsink. There are two positions for it on
the board, one away from the heatsink. That position I first
saw is vacant.

So far a bad pot on one channel is the only thing I have
found wrong. I still have to figure why the bias on the other
channel is not working.

Doesn't each output stage have its own bias transistor?

 

[GregS]

Doesn't each output stage have its own bias transistor?

Right. There are seaparate assemblies.

I was fixing some Sansui integrateds some years ago. A particular series
had a resistor that would change value and screw up the bias. It
was an esy fix, except for the conductive glue on the power supply
board.

greg