Re: 466 +15 supply and U1724 (1458)


If the +65 volt supply is actually +60 volts, then that explains why
all of the other supply outputs are slightly low. They all directly
use the +65 volt supply as a reference so there is no mystery there.

Interesting the other half U1724A both inputs are 4.5 should be 8.9. Output 5.1 should be 8.3. And the Vcc is also 4.5. The reference voltage from VR1726 is correct enough at 9.3.
If the output of U1724B was driving Q1752 and Q1756 hard, like 8.8
milliamps hard which is within the output current limit of a 1458,
then Vcc would be pulled down causing problems with the +65 regulator
as well so I think we are looking at a problem with just the +15 volt
regulator. C1757 which you identified is the prime suspect. From the
Tektronix 466 parts list:

10uF 25V 20% 90201 TDC106M025FL Mallory

I was originally suspicious of a socket but now I think it was Colonel
Mustard, in the Conservatory, with the Candlestick. Err, I mean the
solid tantalum capacitor, in the +15 volt regulator, with a short.

There might be a short somewhere else on the +15 volt supply but
removing C1757 is the place to start. If you have a 10uF to 100uF
aluminum electrolytic capacitor laying around it can be used in its

All voltage measure slightly low but the +15 is way down to 1.4V. Unregulated +15 is slightly high 25.9 vs 22.5 presumably because of no current through pass regulator Q1756.
Is it possible that the 10uf cap to ground at the + input [actually +15 volt output] is bad pulling that input down?
Given the above, this could be the whole problem. A shorted C1757
will pull the +15 volt output down, Q1754 will then limit the current
by pulling the output of U1724B down and that will cause Vcc of U1725B
to be pulled down.

On 25 Jan 2016 03:57:16 -0800, you wrote:

Thanks David for the input. Yes, I was thinking of the .02uf at the input but wrote the 10uf on the output. My bad.

The 65V is around 60V and adjustment is non-reponsive; but that is to be expected since U1724 is not working. No sockets on this board. So I'm going to check the supply to U1724 - R1727 and VR1725 and the U1724 itself.

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