Magnasync

Does anyone have any info (schematics/service manual) for the Magnasync G-924 remote microphone mixer? (http://www.preservationsound.com/wp-content/uploads/2011/05/Magnasync_G924_mixer_1954.jpg)

I've got one that's not working properly, and though it'd be easy enough to trace through, I'd love a bit of reading material on this thing.

Anyone used one before?

Had one many years ago but never had a schematic.  Sounded good but not a ton of headroom IIRC.  Here's another piece of ad copy for you:

So I was just looking through the Audio Cyclopedia (1st Ed) for something else and came across the 924 schematic!

Wow! Who would have thought? Thanks a ton.

Am I missing something?
It seems that the anode of V810 is connected to C824.
At C 824 the voltage is 290 V, but at the anode of V810 it is suddenly 115 V!
Also C823 wouldn't get any AC voltage, because any AC would be shorted by C824.
I suppose there should be a resistor and/or a choke in the anode of V810.

And wow, a gain of 100 dB for the microphone input!!!

RuudNL said:

Am I missing something?
It seems that the anode of V810 is connected to C824.
At C 824 the voltage is 290 V, but at the anode of V810 it is suddenly 115 V!
Also C823 wouldn't get any AC voltage, because any AC would be shorted by C824.
I suppose there should be a resistor and/or a choke in the anode of V810.

And wow, a gain of 100 dB for the microphone input!!!

Click to expand...

and our voltage climbs back up to 120 at V809 after another 330k  resistor...

you are right, something smells like billowing smoke with this schemo

(we have a magnasync G 803 passive EQ here that sounds great,  most likely because of the lack of voltage errors )

[quote author=QUEEF BAG]
and our voltage climbs back up to 120 at V809 after another 330k  resistor...
[/quote]

Could be, because of R836 and R845

> Am I missing something?

No.

It is a SAMS re-drawing. SAMS became notorious for adding errors to schematics. Following House Style was more important than being correct.

By inspection you would think there is a 175V/4mA= 44K resistor in V810 plate path. As the 4mA is probably rounded, and there are no other odd-number resistors in the thing, nor any reason this one would be oddball, I would assume 47K.

You would also like to find a capacitor between V808 plate at 180V, and V809 grid at zero V. Otherwise it's not TalkBack but BIG POP (no talk).

There's nothing exotic here. Use more common-sense than SAMS bought from their re-drawers.

Yes, 100dB gain is high but a lot of old stuff was rated in that area. You would NEVER run it wide-open. Taking 20dB in-hand at both input and master pots gives 60dB. We see lots of folks moaning that 60dB is not enough.

Not to dig up a corpse thread, but I just worked on one of these and I can confirm PRR is right as usual - 47k plate resistor.  Also this one had R846 fitted as 2k2 instead of 1k.  R816, which is not given a value on the schematic, was missing because someone had hacked the preamp side of things all to hell.  I ended up using a 47R, 5W because I had it and it worked.

I made a cleaner scan of the schematic and added some corrections and notations.  It is uploaded to the Tech Docs section now.