Products > Test Equipment
Vintage HP 3721A Correlator : Repair, Restoration and Enhancement
dazz1:
Attached is the cross correlation of a swept (FM) frequency verses the triangle wave and sine wave output.
Note I didn't record any settings. I was basically just trying different things.
I think I might have a fault on the HV for the CRT. When I increase brightness, the Y deflection decreases. I suspect a weak negative voltage somewhere.
dazz1:
Hi
The correlator has a fault revealed by the CRT intensity (brightness) adjustment. I can' figure out what the cause is so putting this out to ask for help.
The symptoms are that that as CRT intensity is adjusted from zero, the trace moves vertically (Y) from the bottom of the screen. At about 1/2 intensity the trace settles at the crt mid point, where it is supposed to be.
As the intensity is further increased, the trace stays in the middle, where it is supposed to be. The fault is not linear across the range of intensity adjustment.
In addition, the crt trace can also be randomly unstable.
Also the vertical gain varies in discrete steps.
When I vary the Intensity setting, the Y trace position does not change instantaneously with intensity adjustment. There is a long RC delay. The intensity setting does adjust the intensity.
The X of the trace is unaffected and behaves normally regardless of the intensity adjustment. Only the Y is affected.
When I connect up the X,Y,Z Correlator outputs to an oscilloscope, they are normal regardless of intensity. The output of the Y deflection amplifier is rock solid on both legs of the output amplifiers. This eliminates the X,Y deflection amplifiers as the source of the fault. I have my scope connected to the diff outputs of the vert deflection amp. It is clean and working perfectly.
The only thing separating the Y deflection amp outputs and the crt is wire.
I don't have a HV probe to look directly at the CRT 3kV HV.
The heater circuit is floating at 3kV above ground, so not touching that with a meter.
Referring to the attached drawing:
The HT oscillator is the lower left part.
The CRT HT feedback for voltage regulation is the lower part.
Upper part is the Intensity and Z modulation. Both controls are working correctly.
R26 provides feedback to Q11, a constant current source. Then there is a chain of transistors that end at Q8, which modifies the switching point of the HT oscillator Q6 & Q7.
All the test points are normal and are not affected by adjustments to the intensity.
I have just turned on the HP3721A and the crt trace was unstable in Y. X is always OK. I will make additional posts with photos.
I suspect leakage or breakdown of an HV part. Unless I have missed something.
dazz1:
The two attached images show the normal display and the unstable Y trace.
This Y displacement is the full height of the screen. This is full scale, so I will call it a gain of 1.
dazz1:
The attached photo shows a normal display (no noise or instability).
This shows the gain is about 40% of full scale. It should be a gain of 100%.
tautech:
Old drifted carbon comp high value resistors. :horse:
Might as well replace the 4700pF caps too....
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version