Odyssey oscillator repair

[Joachim Verghese]

On Wed, 17 Apr 1996 gstopp at fibermux.com wrote:

>      I remember fixing a VCO - it was the 4011 CMOS NAND gate. Can't 
>      remember if it was "A" series or "B" series CMOS, or if it mattered 
>      (Joachim, are you reading this?).

Yes, this is what makes Odysseys with the B-I oscillator board
potentially harder to fix. This early (1972) design uses a CD4011AE
gate to determine the sawtooth retrace point of the VCOs. This mechanism
relies on the 4xxxA-series transfer characteristics, according to
which any input voltage lower than VDD/2 is treated as logical
zero, and any input higher than VDD/2 is a logical one (1). In other
words, they utilized the analog property of a digital IC.

The newer 4xxxB-series devices have totally different transfer
characteristics and cannot therefore be used as replacement in
this particular application. The A-series devices have long been
obsolete and are very hard to find. Some manufacturers make an
unbuffered version of the 4011 (eg Philips HEF4011UBP) which has
characteristics similar to the 4011A and should work as replacement.

Sometime in the late '70s, ARP issued a service note in which they
recommended that all faulty 4xxxA chips be replaced with the
corresponding B-series chip. This was because they were running
the circuits on +15V which is the absolute maximum for the A-series,
and thus made the chips prone to failure. The absolute maximum supply
voltage for the B-series is +20V.

In conclusion, if you must replace a 4xxx-series CMOS chip in any ARP
synth, use a 4xxxB chip, unless you're dealing with a 4011A (Z1 and Z2)
on the B-I Odyssey board, which should be replaced with an unbuffered
type such as the one mentioned above.

BTW, the B-I oscillator board was used in the white-face model and
the black/gold units without interface jacks on the rear panel.

>      The Odyssey is a very cool machine, 
>      very ergonomic IMHO. The patching method of parallel input attenuators 
>      on each module, with a source select switch, is really ingenious...

I agree, and I'm glad they carried that idea on to the Chroma as
well (albeit in software).

-joachim