HP141 Repair and comments by Sam, WB8ZDF
Q1 - HP 1853-0252 = Si PNP SJ 1798
Q2 - HP 1854-0294 = Si NPN SJ 1318
Q3 - HP 1854-0294 = Si NPN SJ 1318
Q4 - HP 1854-0294 = Si NPN SJ 1318
Don't go nuts on these numbers, though. You can probably slap anything in there with a reasonable breakdown and hfe, as these are just current squeezers. Something like 2N3055's for the NPN's is probably just fine for any TO-3 pass substitute. The regulator loop gain will sop up the low hfe, and there you go. Unfortunately, I don't think this will fix things, though.
I had "problems with the pass transistors" also, but found that the real problem was not the transistors, but the CRT tube itself. The 141T uses a big, honking, storage CRT with a front storage grid that is epoxied onto the rest of the CRT.
As near as I can figure, as those CRT tubes get older, the HV section loads down, manifesting as low brightness. Apparently, the epoxy seems to break down in some tubes, and outgasses, making the tube gassy with epoxy resin fumes. The gassy tube then loads down the power supplies, usually toasting the pass transistors, or crushing the HV supply rail by taking out the tripler diodes.
Stick an HV probe on the plate and see what you have. You should have around 7 kV (6600 is spec.) with a healthy vacuum. I have seen as little as 2 kV on ones that were still dimly limping along, but the blanking and storage wasn't working. If you swap out the pass transistors and see 7 kV, you are on the home stretch. I would then attack around the blanking or storage mesh pulser circuits and see what's up there.
If the HV stays low, then you probably got gas. Try disconnecting various electrodes and see what that does to your HV.
In my playing, I have also seen arcing inside the CRT, somewhere around the storage grid. I could not fix this CRT. It was probably gassy, but I could never get the arcing to stop long enough to glow the phospor. This one's my footstool, now.
I have also seen arcing on the three-conductor flexible circuit strip that has the HV, storage mesh, and collector mesh in it. After getting a shock, I noticed that the flexible circuit was coming apart. I glued the flexible circuit strip together with crazy glue, and now it seems to be OK. (->this is my only working 141T!)
If you have trouble getting the voltage up, it's probably time for a new tube. HP probably don't sell them. I saw a new one at a fest, but it was $200. I have seen mainframes for around $150, but they might have gasy CRT's also.
In true amateur (A.K.A. cheapskate) fashion, I am also playing with reducing the filament emission on a tube that I have that is not real gassy, but bad enough to keep the blanking grid from ionizing. I want to see if I can limp a tube that's kinda gassy and get some more hours out of it by cutting down on the number of electrons availible inside.
If anyone else knows anything to help us poor 141T owners, let us in on it. We faithfully lug our hulking units around with pride, only to be greeted by dim, fuzzy CRTs and the distinct odor of ozone eminating from the vent holes topside.
Here is some added advice from a reader:
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This page was last upadated on 8/31/99
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