OK, will measure those TestPoints, cannt seem to find them now.
meanwhile, I was studying the HP339a Manual and read section 8-31 . it explains a notch of about 16dB is expected without the Fed-Forward and summing by A3U3.
This is suspiciously close to the -14.5dB notch I am measuring.
meanwhile, I was studying the HP339a Manual and read section 8-31 . it explains a notch of about 16dB is expected without the Fed-Forward and summing by A3U3.
This is suspiciously close to the -14.5dB notch I am measuring.
A4 TP1 is +9V, allways, no matter which way I mis-tune.
A4 TP2 varies between 0 and +9V, slowly varying as I mis tune .
Yep, the oscillation is gone when I probe with a series resistor to my scope.
A4 TP1 is +9V, allways, no matter which way I mis-tune.
A4 TP2 varies between 0 and +9V, slowly varying as I mis tune .
A4 TP3 is a clean sine, 4Vpp, 90 degrees phase shifted with input
A4 TP4 is a clean sine, 2Vpp, in phase with input
A4 TP5 is a clean sine, 4Vpp , in phase with input
Sure hope it is not the U2 demodulator that is defect 🙁
A4 TP1 is +9V, allways, no matter which way I mis-tune.
A4 TP2 varies between 0 and +9V, slowly varying as I mis tune .
A4 TP3 is a clean sine, 4Vpp, 90 degrees phase shifted with input
A4 TP4 is a clean sine, 2Vpp, in phase with input
A4 TP5 is a clean sine, 4Vpp , in phase with input
Sure hope it is not the U2 demodulator that is defect 🙁
Good progress!
It's quite possible that Demian's optocoupler call was spot on. I believe we're getting close.
One or two additional checks: the voltage across R23 should be identical to TP1 (within a few mV). This will confirm the amplitude opto LED driver is working.
The fact that TP9 is at at 9V indicates the analyzer wants to LOWER the A3E2 cell resistance. What appears to be at odds is that you indicate A4 TP4 is IN phase with TP5; with too high cell resistance, I would expect TP4 to be 180 degrees out of phase re TP5. An additional experiment it to short voltage across C8 then release. TP1 will start from 0V, TP4 should start large and 180 out of phase with decreasing amplitude as TP1 voltage rises. With failed optocoupler, TP4 amplitude will never decrease to 0. But if amplitude passes through 0 and then grows amplitude and becomes IN phase, then opto probably ok and problem may be associated with U2.
Please advise and good luck!
It's quite possible that Demian's optocoupler call was spot on. I believe we're getting close.
One or two additional checks: the voltage across R23 should be identical to TP1 (within a few mV). This will confirm the amplitude opto LED driver is working.
The fact that TP9 is at at 9V indicates the analyzer wants to LOWER the A3E2 cell resistance. What appears to be at odds is that you indicate A4 TP4 is IN phase with TP5; with too high cell resistance, I would expect TP4 to be 180 degrees out of phase re TP5. An additional experiment it to short voltage across C8 then release. TP1 will start from 0V, TP4 should start large and 180 out of phase with decreasing amplitude as TP1 voltage rises. With failed optocoupler, TP4 amplitude will never decrease to 0. But if amplitude passes through 0 and then grows amplitude and becomes IN phase, then opto probably ok and problem may be associated with U2.
Please advise and good luck!
-voltage across R23 is identical to A4 TP1.
-Shorting C8 makes the signal on TP4 large, upon releasing the short, it goes through a dip in amplitude and grows again within 2 seconds. TP4 starts very large, out of phase, upon releasing the short across C8, it fades to allmost zero, and grows to a large signal IN phase
-Shorting C8 makes the signal on TP1 zero, upon releasing the short across C8, it risis to about 9V within 2 secs.
-Shorting C8 makes the signal on TP4 large, upon releasing the short, it goes through a dip in amplitude and grows again within 2 seconds. TP4 starts very large, out of phase, upon releasing the short across C8, it fades to allmost zero, and grows to a large signal IN phase
-Shorting C8 makes the signal on TP1 zero, upon releasing the short across C8, it risis to about 9V within 2 secs.
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So issue is probably is associated with U2.
The waveforms and bias voltages should be similar to those seen on U4. I would poke around and look for any dramatic differences.
While you investigate, I'll try to think of other experiments.
Good luck!
The waveforms and bias voltages should be similar to those seen on U4. I would poke around and look for any dramatic differences.
While you investigate, I'll try to think of other experiments.
Good luck!
Thanks.. I had the same idea, to watch the pins of U2, see if they make sense.
If U2 is shot, the Analyzer is beyond repair, not? No way to figure out a replacement part number, and prolly not available anyway.
If U2 is shot, the Analyzer is beyond repair, not? No way to figure out a replacement part number, and prolly not available anyway.
I wouldn't jump to that dire conclusion.
Other members may to able to help here. If memory serves, the IC is in a round package. I don't know if they are still available. The IC appears to be very similar to MC1496 (originally Motorola and still available from various manufactures), though you might have to adapt to different pinout/packaging.
Other members may to able to help here. If memory serves, the IC is in a round package. I don't know if they are still available. The IC appears to be very similar to MC1496 (originally Motorola and still available from various manufactures), though you might have to adapt to different pinout/packaging.
Yeh, that demodulator is a 10pin metal can. Looks very vintage 😎
I'll also try to figure out a way to determine if U2 is shot or somewhere around it another component is defective...
I'll also try to figure out a way to determine if U2 is shot or somewhere around it another component is defective...
The scan I have of the manual is really poor on the parts lists. We need the HP part number for the demodulator. The blurry parts list I have references opamps (Harris) for those locations which is not right. However certain opamps can do the task. There are several HP cross references to standard parts on the web. Since these landed in the US military there are mil support info as well.
The pinout on the 1496-G matches the schematic https://www.rf-microwave.com/resources/products_attachments/5a4387051df08.pdf and a quick search found this: MC1496G MODULATORS/DEMODULATORS IC - Leads Leg | eBay So its not hopeless. (I would get one and store it inside the box just in case.)
Hi Demian,
I just looked at my scan of manual--- also very poor. But much to my surprise the description field(s) read "IC 1496 MODULATOR" and "LM1496G". Still not sure about package availability though.
Pinouts share many of the same numbers, so perhaps an adapter can be made to work.
I just looked at my scan of manual--- also very poor. But much to my surprise the description field(s) read "IC 1496 MODULATOR" and "LM1496G". Still not sure about package availability though.
Pinouts share many of the same numbers, so perhaps an adapter can be made to work.
The ebay link was to the can which seems to match HP's pinout. Adapters would be a pain in that space but sometimes you have no choice. (I have been there.)
Since I haven't heard any updates, I'll offer another wild guess/hail Mary thought. 🙂
I can't help being suspicious of pot R116, especially since I've seen trimmers fail in other HP equipment (not 339A).
If problem still unknown, you could experimentally short all three terminals of R116 to +15V. You'll lose ability to do deep trim on notch depth but, if bad pot is the only problem, the instrument will probably null!
Good luck!
I can't help being suspicious of pot R116, especially since I've seen trimmers fail in other HP equipment (not 339A).
If problem still unknown, you could experimentally short all three terminals of R116 to +15V. You'll lose ability to do deep trim on notch depth but, if bad pot is the only problem, the instrument will probably null!
Good luck!
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HI,
yeh busy with the kids this weekend. Being divorced etc... and one of them needs special attention. Didn't have opprtunity to wotk on the HP339a
But this morning:
-Shorting A4 R116 out makes no difference. I shorted A4 R116 by finding A4 R14 and A4 R15, and used their legs to short out the tiny pot.
-The other legs of R14 and R15, both have a DC offset of about +8V, and a AC 150mVpp, containing a 2nd harmonic! The signal of R14 and R15 are exactly out of phase.
yeh busy with the kids this weekend. Being divorced etc... and one of them needs special attention. Didn't have opprtunity to wotk on the HP339a
But this morning:
-Shorting A4 R116 out makes no difference. I shorted A4 R116 by finding A4 R14 and A4 R15, and used their legs to short out the tiny pot.
-The other legs of R14 and R15, both have a DC offset of about +8V, and a AC 150mVpp, containing a 2nd harmonic! The signal of R14 and R15 are exactly out of phase.
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I think that 2nd harmonic on both R14 and R15 is good news. It might indicate that the demodulator is functing OK.
Also on the other demodulator (R42 and R41), I see similar signals.
But the output of U3A, pin1, is fixed at -13Volt DC ...?
Also on the other demodulator (R42 and R41), I see similar signals.
But the output of U3A, pin1, is fixed at -13Volt DC ...?
It's getting complicated, so I need some summary:
Shorting the integrating cap and releasing the short again used tell me:
-Is A4 U1 working correctly? watching TP4
-Is U2 (demodulator) U3A and U3B working correctly? : watch TP1
Keeping in mind:
-The unit suffered a high voltage injection (possibly causing IC defects)
-The unit is old, electrolytics might be dry.
Shorting the integrating cap and releasing the short again used tell me:
-Is A4 U1 working correctly? watching TP4
-Is U2 (demodulator) U3A and U3B working correctly? : watch TP1
Keeping in mind:
-The unit suffered a high voltage injection (possibly causing IC defects)
-The unit is old, electrolytics might be dry.