Products > Test Equipment
Vintage HP 3721A Correlator : Repair, Restoration and Enhancement
factory:
If the problem is vertical deflection I would be looking at the vertical circuitry. HV fault would affect the horizontal deflection, trace usually gets wider/longer if HV drops.
As for measuring the HV, are there no AVO lamp foots over there? some of the model 8 have a 3kV range, probably enough for this as the HV isn't that high compared with higher frequency scopes. Only problem in finding good one, took me a few attempts, they go for about £10 here.
David
dazz1:
Hi
Yes, Avo's are available here but they are now rare enough to sell for prices 10x to 20x your £10.
My highest rated meter is a Fluke 77 @ 1kV so well short of being useful. Even if it was rated higher, the 10Mohm meter input impedance would load down the 3kV supply.
The HP 3721A has a 3kV supply with a tolerance of +/- 2v. That tolerance will provide accurate measurements off the CRT image, but it actually makes more sense to read the data from the pre-GPIB data bus that uses a 50pin Centronic connector. Rationally I should ignore the CRT fault and spend my time and money emulating a HP 3720A Spectrum Display.
I think I have been dealing with at least 3 separate faults. There was the carbon compound resistor identified by TauTech. Cleaning the corrosion of the CRT pins seems to have fixed the Y deflection modulation by the brightness control, and apparent breakdown of a mystery HV component. None of the low voltage things I can measure correlate with what I see on the display.
dazz1:
Hi
So while a rational person might just leave the CRT fault as is, I never claimed to be rational.
I took a harder look at the circuit diagram to figure out where a fault could exist that did not generate an error signal. I narrowed it down to R17, a 1Mohm LV resistor. When measured unpowered, this resistor had and does check out OK.
So I changed that resistor out and changed the fault.
The time constant significantly reduced and the Y-deflection went full scale. The unstable trace is now stable. In addition, adjusting the intensity no longer has a significant effect on the Y-deflection. Almost BINGO.
The photos show the before and after changing R17.
Based on these observations, I now believe that R17 was failing to a higher resistance state that created a significant time constant re-charging C7 and maintaining the grid voltage.
I now suspect that C7 has or is failing to an open circuit. Without the full capacitance of C7, it takes time for the current to flow through R17 to build up the voltage on the control grid of the CRT.
My next problem is obtaining a new HV cap. I just can't wander down to the local corner shop and pick one off the shelf. I have to wait until I have a big enough parts order to take advantage of the free shipping. Typically I order parts about twice a year. I end up with multiple projects on hold until I get parts to progress them. So expect radio silence on this topic for quite a while.
tautech:
--- Quote from: dazz1 on November 15, 2024, 12:26:06 am ---My next problem is obtaining a new HV cap.
--- End quote ---
2m search:
https://www.aliexpress.com/item/1005006067184429.htm
dazz1:
--- Quote from: tautech on November 15, 2024, 02:54:01 am ---
--- Quote from: dazz1 on November 15, 2024, 12:26:06 am ---My next problem is obtaining a new HV cap.
--- End quote ---
2m search:
https://www.aliexpress.com/item/1005006067184429.htm
--- End quote ---
I buy a lot of stuff of Aliexpress but I hadn't considered buying HV caps. I will look into that.
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