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Vintage HP 3721A Correlator : Repair, Restoration and Enhancement

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ch_scr:
Great job, looks factory  :-+

dazz1:
Hi
Now working on improving the thermal management.  Cooling might have been sufficient in the chill of Queensferry in the north of Scotland, but not so good in a temperate climate.  It is summer here.

I have fitted heat sinks to the TO-3 power transistors on the rear.  The aim being to dump heat outside the enclosure before it gets into the chassis.  The heat sinks will fit under the factory cover.
I have also fitted a thermostatic controller to the fan so it only runs as fast as necessary.

I have also replaced the HV cap but now have an issue with power supply fuses blowing.   I can't yet test the fix.   Before you ask, the heat sinks are electrically isolated as they should be.
The problem is on the 5V supply which draws a LOT of power on this thing.   One of the stud diodes on the 5V rectifier has previously been replaced so I am not the first to have a problem.

There is a before and after fitting the heat sinks.
Also a photo showing the fan controller fitted to a 3D printed bracket, wiring not yet fully loomed-in.

dazz1:
Hi
I went through and did a lot of static checks on the power supplies to make sure there were no obvious broken parts. 
The power supplies have a cascading protection scheme starting with the 50V supply.  The +24VDC supply requires a 50V input to the reference.  If the 50V supply fails and goes to zero, the 24VDC reference is zero and the 24VDC supply goes to 0VDC. 

The +12VDC supply reference also depends on the 50VDC supply.  The +5VDC supply reference is driven by the +12VDC supply.  Other supplies also have dependencies.

As a result of all this interconnection, if one fuse blows, multiple power supplies drop out.  It looked really bad, until I dragged out the schematics to see how everything works.  Once I had completed static checks of the key components in the power supplies, I refitted/replaced the fuses as required, and flicked the power switch.  It all burst back into life again as if it was just another day. 

The +5VDC power supply has some interesting features, if you are a power supply geek.   It runs 2x 2N3055 power transistors in parallel because one was obviously not enough.  That is a lot of power.   The transistor that supplies the base currents is itself a power transistor.  Being all linear power supplies, there is a lot of waste heat.

The newly fitted heat sinks are definitely doing their job.  The heat sinks combined with the fan controller are working well together.  I can't quantify how much heat is being dumped by the heat sinks, but it is significant.    I have tuned the fan controller to start accelerating from a low base speed at 25 deg C, and go to full speed at 50 deg C. 

With the power supplies now fully operational, I checked the repair to the CRT power supply (new HV cap).  The good news is that the symptoms consistent with a failed HV capacitor have gone.  The bad news is that the power supply output is unstable, consistent with arc-over somewhere.  That brings the tally to 4 defective/replaced components and at least one more to discover.  I now have a used HV 1000:1 probe so I can now poke around to see new things I could not see before.

Three of the rotary switches were loosely mounted, so I tightened those up.


To the HP employee that left the message "FIXIT" 55 years ago please note, where ever you are, I am trying. 

So the next step is to figure out where the remaining HV fault(s) is.  So far no magic smoke has been observed during test flights.



dazz1:
Hi
Photo below IMG_3185 shows the crt display before the HV cap and resistor were replaced.  The cap value had degraded.  The "dot" are near vertical as the HV supply dropped with the increased current required to draw a dot.


Photo IMG_3316 is the after the fix.  The dots are closer to being dots.  Vertical deflection is still about half what it should be so still a fault there to fix.  There is no droop in the line of dots.  The line is straight.

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