I have looked at the power supply schematic for the HP 141T with the intention of improving it, and found something that is very odd (also found others - maybe another time):
Old one - "neon tube" stabilized (pdf-page 35):
141T Display.pdf
New one - zenerdiode stabilized (pdf-page 10..12):
141T_changes.pdf
(both downloaded from here:
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)
The three lower driver and power series regulator transistor base resistors - to high voltages - bothers me. I would prefer e.g. 100 ohm to the emitter.
If the output transistors turn off, just for millisecond, the base is pulled "downward" to the high voltages. The (-)Vbe should never be greater than 5 volt for most transistors - even for the high power. Almost every datasheet tells you so.
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What is your opinion about this? What was the PS designers thinking? I know that the instrument was the state of the art. Please help me to understand this PS design.
Glenn
(the question is also asked in news://sci.electronics.design )
It looks to me like the Q7-8 diff amp could put -20V Vbe across Darlington pair Q6-3 under fault conditions (R37 CW leg open). Pretty unlikely failure mode.
Whether or not they could take it depends on which'un trannysauruses dems was (is). CR13,14, and 18 suggest the answer was "no."
I have three of those mainframes. Two HP141T storage scope and one HP140T non-storage. I also know of four more in use in various homes and labs. They are highly reliable and not really in need of any improvements.
I'm curious. What problem are you trying to solve by making changes to the power supply?
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Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
The old version supply uses a neon bulb as the reference for the 100 volt supply. This regulator drifts over time as the bulb ages. It gets to a point that the supply cannot be adjusted to spec and the special bulbs are hard to find. The problem gets worse as the 100 volt supply is used as a reference for the other supplies.
The new design changed to a 9 volt low TC zener that makes things much better.
You really should post your question in some HP scope forum or newsgroup as it's really a repair problem, not a design problem.
It's probably easier to just replace the neon lamp. Neon lamp failure is a common problem in the really old HP scopes, which usually appears as display jitter. What HP used were Signalite NE-2 or NE-2H type lamps, but stabilized with Krypton 85, which has a 10 year half life and is probably long dead after 50 years. All of the modern NE-2H lamps are made for illumination, not regulation. I don't think anyone makes them for regulators these days and certainly not with radioactive gas inside.
It should be possible to find an approximately 70v zener or build a zener simulator that would suffice as a drop in replacement for the neon lamp regulator. Looking at the schematic, the +100v adjustment has enough range to work with any regulator diode between 75v and 85v. The range can be increased by decreasing R26 (22.1K) or increasing R24 (4.32K). The current through the neon lamp is currently(sic) 3.2 ma, which may need to be increased slightly by decreasing R21 (5.6K).
Note that the nearby VR4 is a 69.4v zener. If you can find another one, it might be usable.
Well, 70v zeners seem to be an out of stock oddity, so perhaps two 35V zeners in series might be easier to find.
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Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
Was done at the same time, but I have followed this group some time and read many posts from many electronics expierienced people. So I thought this group was as good as the HP scope forum.
I also just found a page that made me suspect the problem might be "not enough radioactivity" in the tube stabilizers, but I was not sure. The radioactivity ionizes the gas, which is then much easier to start.
Now - I think that the problem of the sometimes blown fuses are because of the tube stabilizer can not start, and therefore the voltage regulator send max. voltage out which trips the overvoltage SCRs in the plug-ins at -12.6V. SCR trip at approx. 14.4V and the unregulated "-12.6V" is approx. -18V.
I would like to make a new PCB with the zenerdiode circuit, but it should not look like a rat nest...
The "zener" would be chosen to be a TL431-like IC.
As others has written (might be in the HP forum) these zeners is not temperature stabilized.
That 69.4v zener is already occupied, it is used to save the transistors from voltages higher than 70V at start - and at regulator output short circuit. Then the fuses blows som hectomilliseconds later.
The reason was to use component from one or two to repair the others.
But I have repair and calibrated all power supplies.
Two of the 8555A "wheels" has been disassemblied, drilled, glued so they work again. The plastic that holds the contact-metalblade arm had broken.
I have these problem now:
one scope can not get high enough intensity and can not get in focus, but it is useable. Tripler blown? I presently do not own a multimeter or a probe that can measure 6.6 KiloVolts.
two blows fuses occasionally
two 8555A has blown diode first mixers (one blown by me - the antenna cable was not earthed before measurement) - but I have two other 8555A that now works fine. One has a missing "pointer" belt though. Then it should be easy to take a module from the other busted ones - NOT! they use a different socket plug - how can this happen for the same product ;-)
one power supply uses zener - and the other three use tube stabilizers. The difference came as a surprise :-) But later the newer schematic was found.
one chassis is missing many knobs.
I presume that the storage do not work - or I have not read enough about it.
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The reason I bought them is that they for the most part use common components, and I would like to know how they work. The schematics are fascinating to study.
Two years ago I would never expect to own a single multi GHz spectrum analyzer. Now I have approx. 3.5.
I also use the preselector in front of 8555A, so signals do not get aliased into the band of interest.
Yep. That's the likely problem. 3ma through the neon lamp is going to just barely light it. If the radioactive gas decays away, raising the start voltage, it's not going to light up. You might be able to compensate by increasing the current by reducing R21 (5.6K). However, I suspect it will still be unstable.
The bigger the mess, the better it works. Also, if it starts out neat, it rarely stays that way. You might want to look through some of the photos posted in this newsgroup. Very few of the prototypes are neat.
No problem. Shove the zener(s) into a small temp stabilized oven originally intended for stabilizing crystals. You can also build one with a TO-220 case heater and temp controller. This is overkill, but can probably be simplified. (3 parts) Note the mess. Mess is good.
If that's too much, find some nearby component that likes to get hot, and use that to stabilize the zener.
Note that a 5.6v zener is roughly zero temp coeff. Put 13 of those in series, and you have a 72.8v zener with zero TC.
I was thinking you might be able to identify the vendor and part number, and use it for a NE-2 replacement.
--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
Great exercise. I had one more 140T but I sold it.
You could probably fix all of them instead of cannibalizing one or two for parts. I don't have much experience fixing those because they rarely fail.
Common HP problem. The nylon gears shrink and eventually crack. Here's my HP8640B version: Note that I had to file the nylon gear before applying glue or it wouldn't fit on the brass hub.
Possibly the tripler. More likely a shorted turn in the flyback. $25 with a missing probe tip. Use a common screw instead. Check the voltages in the manual as storage scopes are more complicated.
Which fuses? In general, that points to old electrolytic capacitors in the power supply that need reforming or replacment.
Looking at the schematic Pg 8-17, the 5086-7177 mixer assembly goes directly to the RF input connector. That's why I usually have a fused
3dB attenuator connected to the input N connector. It's more than just a mixer having two circulators inside. If you can get it open, the diodes might be replaceable.
I can see why you didn't just replace it: However, you may have just blown the relay switched attenuator, which is located between the RF input connector and the mixer. Disconnect both input and output from the attenuator, and run some signal through it to see if it's still functional. One of my three HP8555A plug ins has a blown 40dB section, which I just ignore.
Very good. Troubleshooting by comparing with a known good unit makes things easy.
I vaguely recall that the 140T mainframe that allowed connecting the HP "storage normalizer" was different. I don't know if this also applies to the 141T mainframe.
Note that you can buy temperature compensated zeners (Microsemi and others). The only reason that HP modified the design so radically so that they could use a 9.1V zener, instead of a 70v zener.
I have a small colllection of HP knobs, cannibalized from various pieces of unrepairable equipment. Send me which one's (via email) and I'll see what I can scrounge.
The storage display is a bit tricky and easy to burn or blow up if you mistreat it. Keep the intensity as low as practical, don't let it bloom, and don't use it with the sweep off.
They're still quite useful. However, they're really dinosaurs which can be replaced by much more modern equipment.
Yep. I have an HP8445B preselector on top of mine. Good for microwave, but not very useful at lower frequencies.
Good luck.
--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
Ya know, I wonder if you could take the bulbs down to the local physics department's "neutron howitzer" and get some neutron activation going. It's not going to replace Kr85, but there's probably a few elements present which will absorb with a reasonable cross-section (i.e., it won't take weeks of exposure to have a useful effect) and half-life (most neutron activation experiments generate half-lives in the minutes range, which is great for detecting them, but not so useful for ionization and stability). It would need to be aged to allow those short-lived isotopes to decay, of course.
Tim
--
Deep Friar: a very philosophical monk.
Website: http://seventransistorlabs.com
That range looks almost right for a SG202B (??202?) type gas stabilizer, its range is nominally 81-86V. These things were made in large enough quantities in the Soviet Union, back when that existed, so that they're still easy to get and cheap. Fleabay has some for a buck plus shipping. Also Pollin (an electronics discount shop from Germany) sells it for 2?. They sell mostly to Germany and Austria, but can ship internationally. link:
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There is no guarantee that it starts (this lamp has a maximum specified strike voltage of 135V and most are about 30 years old now), but that spec is pretty loose to allow for a lot of age-related wear and tear. Maybe try 2 or 3 in parallel and use the one that turns on as it will be the lowest-voltage one. If that works, it may replace the original more or less "as is" without significant modification.
If you decide to put one to use, note the polarity (center pin = anode), otherwise it will drift out of spec before its rated lifetime.
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