Great, my VT52 is shot.

[Chris Zach]

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.

C


On 4/21/2020 5:36 PM, Chris Zach via cctalk wrote:
> Ok, some test results below.
> 
> 1) Are we sure J2-2 should be -12? It really seems to like being -15.
> 2) The voltage across the zener diode there is a nice solid -3.8-3.9v. 
> It appears to be a germanium diode (.1v drop across, both directions) I 
> could disconnect it and see if that makes things better.
> 3)
> 
>> E2.2 and E2.3 should be around 0V.E2.2 is -.8 volts referenced to ground.
> E2.3 is -22mv referenced to ground
> E2.6 is around .6v. Ramps up from about .58 at startup to .61 then down 
> to .60something. Note that is a positive voltage wrt ground.
> 
> Q12 was reading
> -24.4v emitter
> -15v collector
> -23.8v base
> Q12 does not have a CE short
> 
> What is this using as a reference to get the -12v from the transistor?
> 
> Maybe the zener is just out of spec. Can someone with a VT52 cross check 
> my voltages to see if they are accurate?
> 
> Thanks all!
> 
> 
>> E2.6 I expect would normally be within +/- 2V of GND.
>> If E2.6 is way-positive (>Q4.E, > ~ +2.6V), then E2 is attempting to 
>> throttle down the output but failing due to some failure around Q4/Q12.
>>
>> Measure the voltages on Q4 and Q12.
>> They could be leaky or low-gain.
>> It's conceivable Q12 has a CE short, but I'd expect the input to the 
>> reg to be more than -15.
>>
>> The low -5 is odd, zener D14 looks to be a 5.1V type.
>> R45 is probably burning up.
>> I haven't spotted anything in the schematic that actually uses the -5,
>> it looks to be provided for some alternative character generator 
>> (schematic pdf.42),
>> so that may depend on what character generator chip / daughter board 
>> is used in your unit.
>>