Products > Test Equipment
Vintage HP 3721A Correlator : Repair, Restoration and Enhancement
dazz1:
Just a couple of photos showing the process of doing the fan mod.
I used a wood router to carve away some of the aluminum chassis to allow the fan to fit behind the side panel. This was the only way I could make the fan fit behind the panel.
I made a green fitting template to size and locate the holes in the panel.
I have added a photo of the modified fan mod. I flame polished the 3D print and then sanded it smooth. I then applied some silicone spray. This hid most of the things that identify this as a 3D print. The result is a fan shroud that almost looks like a factory one.
Kean:
Very nice work. That 3D print is very effective at hiding multiple sins.
dazz1:
I ran the HP 3721A for a couple of hours today with my new fan installed.
It ran a lot cooler so obviously a lot more air is being pushed through. It does make a bit of noise, so I might experiment with adding a in-line resistor to reduce the speed a bit.
I am still planning to add TO-3 heat sinks as soon as I get a long enough list of parts to order without paying shipping costs.
Unfortunately I suspect the HP 3721A has developed some new faults. This instrument has been sitting in storage for 8 years, and prior to that, probably something like 20 to 30 years sitting on a shelf. Latent defects are likely to be hiding in there. If I didn't have the manuals, I wouldn't even consider a repair effort.
dazz1:
Hi
This is a call for help. My HP 3721A has a fault that I can't fix.
Symptoms are:
CRT vertical deflection shows exponential response to step input signals.
The displayed amplitude is about 2/3rds of what it should be.
In addition, the displayed signal is sometimes jumpy.
Things I have checked:
Horizontal deflection is unaffected.
The LV vertical deflection signals at the pins of the CRT are OK.
The FOCUS voltage applied to the CRT tube is steady and OK.
The LV power supply voltages applied to the PCB are OK.
The INTENSITY adjustment is working OK.
The Z-MOD input (connected to the same summing point as the INTENSITY) is OK.
The ASTIG voltage is steady and OK.
The feedback signals TP1, TP2 and TP3 are all OK with no sign of any errant signals.
I suspected the connector on the rear of the CRT had corrosion. I removed the end cap and found traces of corrosion that I cleaned off.
I re-melted all of the HV pcb solder joints to get rid of any bad joints. HP soldering is very good.
I cannot measure the 3kV HV supply. None of my instrumentation is rated to that voltage.
I can connect the LV Z-mod and x,y deflection signals to my scope, and they are displayed correctly. These are the LV signals that drive the CRT. These signals are not on the HV pcb.
So it appears I have a component that fails on HV but the only part of the circuit I can't probe has the most likely candidates.
All the components I can probe appear to be working.
If it is a problem on the 3kV HV, then I am not seeing any error signal in the feedback.
There are two separate issues being displayed.
1. The exponential decaying response where there should be a straight line.
2. The unsteady signal displayed on the CRT, consistent with something arcing over, or failing.
Any suggestions would be much appreciated.
dazz1:
Hi
I think I have two options.
1. Replace the HV parts until it works,
2. Buy a HV probe or,
3. DIY a HV probe.
Buying replacement parts in the hope that I end up replacing a faulty part is probably the cheapest path to a repair. I hate repair-by-replacement on principle.
HV probes are an expensive item I would seldom use. The shipping cost only adds to the expense.
The DIY option has some appeal. I already have the materials required to make the plastic items. I would only need the electronic passive parts, caps and resistors.
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