Re: 8662A - a warning about dangerous service directions
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Dmitry,
It always puzzled me that the manual advised you to bypass the over-voltage protection shutdown in the situation of a potentially damaging over-voltage condition. Seems to me that is only asking for trouble. I went down the route of disconnecting P2 and then capturing the unregulated turn-on transients as you?suggested. That helped me to identify a problem with the -40V line (or rather a problem with the way I was loading it during testing.?
regards
Tony
On Fri, 27 May 2022 at 08:48, Dmitry Teytelman < dimtey@...> wrote: Hello,
During a recent repair my 8662A went into shutdown due to the high
input voltage at the linear regulator (A7A1) input. Service sheet 56
directs one to ground the collector of A7A1Q16 and then measure
voltages on test points TP4, TP5, TP6. Well, if a true overvoltage
condition exists on the -40 V regulator input, that ends up blowing a
bunch of parts, since the absolute maximum voltage rating on A7A1U1 is
50 V. I lost R37, U1, Q1, and Q5 on A7A1.
I think two different approaches would work better:
1. Capture the turn-on transients on the unregulated supplies (-45 V,
23 V, -13 V) and the regulated -5.2 V supply with a scope.
2. Disconnect P2 from the power supply motherboard, remove A7A1
completely, then load the four above mentioned supplies with power
resistors (1 A for -5.2/-13/+23 V, 0.5 A for -40 V). Signals ON and
HI-V on the power supply motherboard must be grounded. Now supply
voltages can be safely examined without worrying about damaging A7A1
or the rest of the synthesizer.
Regards,
--
Dmitry Teytelman <dimtey@...>
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