Great, my VT52 is shot.

Chris Zach cz at
Wed May 6 20:53:54 CDT 2020

Well, I pulled the E2 op-amp and replaced it with a NOS one of the same 
model. Put the supply together and now I am getting -17 volts on pins E2 
to ground (E10).

I'm thinking of just replacing the power transistor Q12 with a 7912 -12v 
regulator that I have here and bypassing the whole op amp/transistor 
mess. That should give me a solid -12v on the E2 line and provide power 
for the -5 volt divider circuit.


On 4/22/2020 11:52 AM, Jon Elson via cctalk wrote:
> On 04/21/2020 10:09 PM, Brent Hilpert via cctalk wrote:
>> On 2020-Apr-21, at 5:27 PM, Chris Zach via cctalk wrote:
>>> Meantime reading the manual I found an interesting test: If you short 
>>> emitter to base on Q4 (easiest way is to jumper diode D10) the 
>>> voltage on the -12v supply goes to .4 volts. They're saying it's E2, 
>>> R15,R17,R14.
>>> Is there a way I can test the op-amp in circuit? Maybe it's dead.
> Well, if the circuit **IS** regulating, then the voltage on the two 
> inputs will be identical.
> But, since it might not be regulating, then these voltages would not be 
> equal.
> But, if you can see that the + input is more positive than the - input, 
> yet the output
> is pegged negative, for instance, then you know either the op-amp is 
> bad, or another circuit is overloading
> the output and forcing it that way.
> Jon

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