It got too hot. Doing other chores instead. 
Yet another simplifying test is to unhook one end of R48. Then TP4 to U3 output is simply the Twin-T followed by a unity gain buffer. All the other complexity is set aside. Gain should be unity at frequency extremes, 16dB deep null at test frequency.
Good luck. Enjoy what's left of the weekend!
Yet another simplifying test is to unhook one end of R48. Then TP4 to U3 output is simply the Twin-T followed by a unity gain buffer. All the other complexity is set aside. Gain should be unity at frequency extremes, 16dB deep null at test frequency.
Good luck. Enjoy what's left of the weekend!
If you're seeing flat frequency response, it seems like the range-switched capacitor path C11-C15 path must be open--- i.e. the "stem" of the T in the Twin-T filter.
Since the bottom terminal of the stem (TP3) is 90% of U3 output, the voltage across these capacitors is subtle. To make signal tracing a bit easier, I suggesting also disconnecting one end of R50. TP3 should now be at 0V. Now try probing directly on both leads of the pertinent capacitor near the body of the part (e.g. C12 if in the X100 range). One cap lead should be at TP3 potential, the other should should have test signal present.
I hope this will converge on the culprit. Good luck.
Since the bottom terminal of the stem (TP3) is 90% of U3 output, the voltage across these capacitors is subtle. To make signal tracing a bit easier, I suggesting also disconnecting one end of R50. TP3 should now be at 0V. Now try probing directly on both leads of the pertinent capacitor near the body of the part (e.g. C12 if in the X100 range). One cap lead should be at TP3 potential, the other should should have test signal present.
I hope this will converge on the culprit. Good luck.
disturbance in the force
All checks in the S1 area OK all caps OK.
I set up for test 4-25 noise test and there is a disturbance like random crud, not oscillation, present on the meter and scoped at the monitor output. On the meter it is full scale and only there when input selector is at distortion. It is not present at the auto set test point.
All checks in the S1 area OK all caps OK.
I set up for test 4-25 noise test and there is a disturbance like random crud, not oscillation, present on the meter and scoped at the monitor output. On the meter it is full scale and only there when input selector is at distortion. It is not present at the auto set test point.
Assuming I understand your test, it's encouraging that the noise floor is low when in modes than distortion.
What is the present state of A3R48 and R50 connections? Were your able to confirm a Twin-T frequency response TP4 to U3 out? If not, may I suggest the following tests:
Set analyzer frequency to 1KHz.
With external oscillator, test at 100Hz, 1KHz, and 10kHz.
At each frequency, please advise observed amplitudes at TP4, U3 Out, U3 pin 3, and TP3.
I'm not sure what to make of your observations in post #46. It appears the instrument will will allow you to exercise the filters options independently of Distortion mode. If they are well behaved and exhibit expected filter characteristics, then probably as is well in this portion of the instrument.
What is the present state of A3R48 and R50 connections? Were your able to confirm a Twin-T frequency response TP4 to U3 out? If not, may I suggest the following tests:
Set analyzer frequency to 1KHz.
With external oscillator, test at 100Hz, 1KHz, and 10kHz.
At each frequency, please advise observed amplitudes at TP4, U3 Out, U3 pin 3, and TP3.
I'm not sure what to make of your observations in post #46. It appears the instrument will will allow you to exercise the filters options independently of Distortion mode. If they are well behaved and exhibit expected filter characteristics, then probably as is well in this portion of the instrument.
On A2, pulling J3 will remove bias on U13 and it will peg at supply rail. U1C and U1D will peg in response if they they have been selected.
Pulling J1 likely to just confuse matters. But what does TP1 do in both cases?
Pulling J2 might be an interesting test; TP8 and TP9 should both go to 0V.
I recommend reconnecting J1,2,3 after quick checks. Once back in place, does meter respond properly in any configuration?
Pulling J1 likely to just confuse matters. But what does TP1 do in both cases?
Pulling J2 might be an interesting test; TP8 and TP9 should both go to 0V.
I recommend reconnecting J1,2,3 after quick checks. Once back in place, does meter respond properly in any configuration?
progress
I have put a signal of 1kHz from an external oscillator and set it to give the same level as the internal oscillator, then I varied the frequency up and down to find a lovely deep notch at 870Hz so the thing is working but the notch is not in sync with the internal oscillator. I will now try other ranges.
Yup 100Hz notch is at 86Hz, 10kHz is at 8.6 kHz etc
when I get a null on board A3 tp4 is at 5.1vac and pin 3 of U3 is 0.45vac
I have put a signal of 1kHz from an external oscillator and set it to give the same level as the internal oscillator, then I varied the frequency up and down to find a lovely deep notch at 870Hz so the thing is working but the notch is not in sync with the internal oscillator. I will now try other ranges.
Yup 100Hz notch is at 86Hz, 10kHz is at 8.6 kHz etc
when I get a null on board A3 tp4 is at 5.1vac and pin 3 of U3 is 0.45vac
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Ok, this is very encouraging!
The notch frequency bat be at odds with the front-panel due to the "phase control" optocoupler E1. A ~14% discrepancy is a bit more than I would have guessed but not unreasonable. A test would be to short across the E1 photocell and repeat test for best null frequency. It should now be well above the front panel setting.
The notch frequency bat be at odds with the front-panel due to the "phase control" optocoupler E1. A ~14% discrepancy is a bit more than I would have guessed but not unreasonable. A test would be to short across the E1 photocell and repeat test for best null frequency. It should now be well above the front panel setting.
Hi microx,
This should have been posted earlier in regards to cleaning the switches. If overspray gets into any of the trimmer capacitors, that's it. You would have to back the capacitance off all the way. Clean the trimmer and later tune it as you normally would. But the trimmer capacitors must remain dry 100% of the time. Cleaning fluid changes the dielectric constant of the air and can take the adjustment completely out of range. I have a couple 339A that I rebuilt (one is a spare in case the first one dies).
I used to work in a calibration laboratory repairing and calibrating instruments. When we see this taking place, the result is usually "BER". That means Beyond Economical Repair. We have seen some nice equipment trashed due to this. The best fix is to replace the trimmer capacitor and align from scratch.
This advice is applicable to any equipment with trimmer capacitors. So older tuners, CD players, new tuners and especially to test equipment.
If you can tune it into range, it still isn't a good ending. The adjustment will drift like crazy as the solution evaporates.
-Chris
This should have been posted earlier in regards to cleaning the switches. If overspray gets into any of the trimmer capacitors, that's it. You would have to back the capacitance off all the way. Clean the trimmer and later tune it as you normally would. But the trimmer capacitors must remain dry 100% of the time. Cleaning fluid changes the dielectric constant of the air and can take the adjustment completely out of range. I have a couple 339A that I rebuilt (one is a spare in case the first one dies).
I used to work in a calibration laboratory repairing and calibrating instruments. When we see this taking place, the result is usually "BER". That means Beyond Economical Repair. We have seen some nice equipment trashed due to this. The best fix is to replace the trimmer capacitor and align from scratch.
This advice is applicable to any equipment with trimmer capacitors. So older tuners, CD players, new tuners and especially to test equipment.
If you can tune it into range, it still isn't a good ending. The adjustment will drift like crazy as the solution evaporates.
-Chris
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