Pioneer M-73 amplifier from the 1970 era - Looking for schematics and info

[AndersJ]

A friend of mine is trying to repair a Pioneer M73 amplifier from the 1970's, not the 1990's model.
He tells me he is stuck, since it easily oscillates and/or gets instable.

Does anyone have:
* Insights into possible difficulties with the older M-73?
* Schematics or other service information?
* Any clues to why the 2SK389 dual FET easily and often fails

[drussell]

I don't see any links to the 1977-1980 model M-73 manuals, only that it was supposedly a Japanese-market-only model and the schematic is supposedly similar to the SPEC-4, but it is physically smaller (and only rated 85 W/channel instead of 150?) and the board layout is different... 

It might help with tracing the basic circuitry, though... 

Pioneer SPEC-4 Stereo Amplifier Service Manual
(File is slightly too large to post directly to the forum.)

https://audio-database.com/PIONEER-EXCLUSIVE/amp/m-73-e.html
(This does list the input voltage spec as 100V.)

[floobydust]

2SK389 you can get from Linear Systems. They've re-released 2020-01-21 their LSK389 to even lower noise en = 1.3nV/√Hz.
These parts have no overload or ESD protection in most circuits and get damaged from connecting RCA jacks.

Given the amplifier's age, it will need to be re-capped first. You can sketch a schematic too, I do that using pics of the top and bottom side of the pcb.

[coromonadalix]

from 1992 to 1996 ??  not sure of the similarities

https://www.hifiengine.com/manual_library/pioneer/m-73.shtml

Register its free ...

[drussell]

from 1992 to 1996 ??  not sure of the similarities

https://www.hifiengine.com/manual_library/pioneer/m-73.shtml

The service manual for the 1990 model is readily available, but that is not the unit the OP is looking for, which was made from 1977-1980.

I also have that version mirrored here:
Pioneer M-73 (1990 version) Stereo Amplifier Service Manual

(Yes, Pioneer likes to re-use identical model numbers for completely different products.)

[drussell]

Additional data points from the compiled Pioneer model number database, for reference:

[drussell]

Oh, and here is a copy of the SPEC-4 (M-75) service manual with more readable schematics:

Pioneer SPEC-4 Stereo Amplifier Service Manual w/readable schematics

[floobydust]

Pioneer did the same with the A5, the newer one is just a cost improvement, circuit is totally different, no dual JFETs-  just to cash in on the old model number fans I guess.

[AndersJ]

The saturn-tech links seem to be broken.
Perhaps it is a temporary issue.

[drussell]

The saturn-tech links seem to be broken.
Perhaps it is a temporary issue.

Try them now... 

That server is multi-homed and the main address on the fiber wasn't routing properly, apparently...