Ampex 601 Reel to Reel plays back fine but has only AC hum in record mode

[mark]

The input section has severe AC hum when recording via the Microphone
& direct line but playback through the output & headphone jacks is
fine. If anybody is familiar with working on these any help with
listing the usual high failure suspects & what parts should be
upgraded would be greatly appreciated. I just moved & misplaced my ESR
tester so I'll definitely need to find it to test all the old caps in
this one some of which obviously need to be replaced. I tried printing
out the schematic off the net but for some reason my computer
(Powerbook G4) wouldn't print it. I repaired TVs/VCRs/stereos for 35
years but not many vintage tube audio units.

Thanks, Mark

[Phil Allison]

"Meat Plow"
>
> What's the URL of the schematic?

**See:

http://recordist.com/ampex/schematics/601man/601schem.gif

** Wanna bet the mic input tranny is missing and /or the hum balance trim
pot is burnt out ?

... Phil

[Dave M]

Meat Plow wrote:

> How is the OP is recording, from Line or MIC? Don't remember and you
> snipped the quote.
>
> Hum balance is R-147?

OP says that the hum is present when recording from both mic and line
inputs. Sounds like a ground connection is broken somewhere around V101,
V102 or V103. Also check those tubes for heater-cathode short. Check line
record level pot and mic record level pot for broken grround connection or
open wiper.
Make sure that the bias oscillator (V103B) is running in record mode.
--
David
dgminala at mediacombb dot net

[Robert Macy]

I once had a bias oscillator kick up to a very high frequency [part
failure] intermittently. You could not believe the high quality of
the recording when the oscillator went berzerk. Down side, got super
hot, even the plate on one tube just starting to glow.