HP 339A oscillator problem

[BSST]

Lack of null not too alarming I think. I recommend exercising S3 through all positions all confirm all switch contacts and associated resistors.

That done, I think we're back to trouble shooting null control loops. Probably best to use external oscillator so that you can search for null frequency in the event it's at odds with analyzer setting. Would you advise any behavior on A4 TP1, TP2 and frequency arrows as you vary oscillator frequency?

 

[BSST]

BTW, how deep was the null you reported in post #55? I might have overlooked a useful clue.

 

[microx]

fresh problem

Now the 15v both plus and minus rails are being pulled down to about 5vdc and the two regulators are getting hot, could be anywhere. It happened while the unit was running, one minute 15v next 5v so will have to fix this first.

 

[BSST]

Poke around, sniff around for the component that's gotten hot. But don't get burned!

 

[microx]

strange that both rails are down by the same amount

 

[BSST]

I haven't searched out the regulator IC. Often dual supplies are "tracking,' the negative supply using the positive as its reference. So they may appear to overload symmetrically.

If you're having tracking down the failed component, here's a technique I'm often forced to use. Attach one voltmeter lead to the power supply output (say +15). Put VM on a sensitive scale and look for voltage for increasing voltage drops along the +15 supply distribution. Voltages drops will probably be only a few mV. You sniff along, trying to find maximum drop, following the highest current path. Often you can lead yourself right to the culprit.

Good luck.

 

[microx]

unplug board A4 restores 15v and U6, drives the frequency leds, is getting hot. Can I use a TL074 here like I did on the oscillator section

re null depth question, when I get a null on board A3 tp4 is at 5.1vac and pin 3 of U3 is 0.45vac

 

[BSST]

Another measurement that might offer insight: On the A3 power supply, R301 and R310 are the current overload sense resistors. Voltage drop across these resistors will let you determine current draw from each rail. I note the R301 is smaller than R310. I infer that +15 current draw is normally a bit higher than -15. I wouldn't be surprised if current from each supply is nearly equal. Might be a failed opamp conducting between both rail.

 

[BSST]

Excellent!

 

[microx]

replaced U6 with TL074 works great AND the right hand arrow led comes on sometimes. Measured depth of null 14.5dB

 

[microx]

re overspray

Chris, thanks for that. I did tape up the trimmers, I learned that lesson working on capacity tuned UHF tuners

 

[BSST]

So what voltages to do you observe on A4 TP1 and 2?

 

[microx]

very little TP1 has 1.2vac and 0.18vdc, TP2 has .02vac and 0.13vdc.
I am now very suspicious of the quad amps used in this rig. I have now had two faulty, I think I will change them all for TL074

 

[BSST]

Failures very strange. Not sure what to make of that.

 

[BSST]

I should have confirmed this earlier: at some point I recommended that you disable R48 and R50. I assume everything is back to original now? Same for shorted E1?

 

[microx]

Yes everything is returned to normal. What do you think of the voltages on TP1 and 2

 

[BSST]

I'm still going strong here in the USA, but you might be getting tired in Spain. I'll be up and about 8:00 AM tomorrow (2:00 PM Spain?) if you'd rather continue later. Entirely your call.

 

[BSST]

Voltages don't sound at all correct. Wasn't sure if other opamps were under suspicion and contributing to abnormal readings?

 

[BSST]

Just to followup on TP1 and 2: Voltage on R23 = voltage on TP1? Same for R55 and TP2?

 

[microx]

That was my last TL074 so I just ordered some more. BTW the three trim pots on A4 don't have any effect on the meter reading. Yes it is getting late here and I have closed the workshop so we can continue tomorrow with regard to checking voltages etc/