Electronics > Repair

FLUKE 332A pre-regulator cycle dropping

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Vollumscope511:
Howdy All-
I'm new here-- this is my first post.

I have a very clean Fluke 332A DC standard. I've owned it for at least twenty years. It looks like it's been updated (way before I acquired it) because many of the boards have 1984 copyright dates-- and device date codes, but the transformers are stamped with a 1966 date code.

I want to put my ol' side kick to use but....

The unit's pre-regulator is on the fritz.  It drops every other mains cycle-- that is the standard's voltage transformer should be energized every 8.333ms (60Hz US power main) for a period of the mains cycle proportional to the voltage drop across the pass transistors.  But the pre-regulator is "ticking" off at 16.67ms...

The UJT oscillator works. It responds to the voltage drop across the pass transistors, there's drive out of the pulse transformer. The driver circuitry seems to work right but when it comes to driving the pass transistor every 8.33ms-- there's base drive every 16.67ms.

I'm stumped.

The diode bridge around the switching transistor is OK...

I performed a pile of other inconclusive tests but I'm not going to drone on...

Anybody have experience with the pre-regulator in this beast?
BTW-- it's a relatively common pre-regulator set up...


Any thoughts, help, musings hints greatly appreciated.





Roman oh:
I can’t immediately find my notes on the pre-regulator, but over the years I have made a number of mods to my 332 mainly with a view to reducing its power consumption and hence warm- up and post-turn-on stabilization time. A replacement pre-reg was part of that, so I have fiddled in this area. 

IIRC, the UJT circuit associated with the Series Pass element fires a stream of pulses through T1 to the pre regulator, which is synced to the mains by +V applied through R18 to the quasi-SCR circuit made up of Q6 and Q7. +V is (in your case) 120Hz positive half sine wave coming from CR1/CR3 driven in turn from a nominal 10V AC transformer winding. If one of those diodes is U/S, that would explain your symptoms.
Have you checked these, and the +V waveform? BUT BE CAREFUL, because the pre-reg circuit is connected to the mains, so unless you have some isolated form of working, don’t try to probe +V directly with any un-isolated instruments. But test the diodes in the normal way.
Roman

Vollumscope511:
Howdy Roman--

Thanks for replying to my query about the Fluke 332A pre-reg SNAFU I'm having.
I've got more experience in electronic design & repair than the average bear but this 332A pre-reg issue has me stumped.

I'd like to avoid a massive redesign of the pre-reg circuitry-- altho it's tempting.  Over the production cycle of the 332x and like voltage standards, Fluke made a few minor changes to the pre-reg circuit(s)-- very likely improvements having to do with dynamic range and regulation stability.

The basic pre-reg circuitry seems to be fully functional.  I can energize the 332A's DC regulator with an external supply providing the pass transistor energy and "steer" the pre-reg UJT pulser/drive all the way to the switching transistor. Of course the AC mains to the voltage power trans is disconnected (that's the transformer that provides the output energy of the 332A and not the trans/supply that runs the 332A).

It looks like the UJT and associated circuitry is firing at the 60Hz mains zero X-ings and a "turn-off" time delay varies in proportion of the voltage drop across the pass transistors. 

The circuitry on the "other side" of the pulse transformer seems to be functioning as well.  I can't say correctly.

In my 332A the original pre-reg switching transistor was replaced. I can not find any doc on that original transistor. My guess is it was custom made for Fluke NPN silicon device with somewhat higher hFE and maybe 250-300V Vceo / Vceo(sus) than what was commonly found back then (even today).   I know it was made by Westinghouse but the device bears a non-JEDEC number. Search-engine look-up of the device number provides nothing useful.

I am very suspicions that transistor static hFE is at the heart of my problem. It could be that a lack of base drive keeps the transistor from saturating or the transistor pops out of saturation as the AC mains voltage rises and hence the wattage demand increases. The replacement transistor is rated at an hFE of 7 at 4A Ic. The way Fluke has the switching transistor driver rigged, it looks like they depended on over an ampere of base current to the switching transistor.

I replaced the present switching transistor with a device I found in a junk box-- and found some data on-- it had safe Vceo and Ic capabilities. Also rated hFE of 35 and 5A Ic-- and low and behold it worked but failed (shorted) at the 332A's 1000V range-- I suspect second breakdown.

I replaced that transistor with a Darlington pair device. A big assed brute TO-3 switching transistor BUT-35.  I used my transistor curve tracer to establish base drive requirements vis a vis likely transistor IC.  With some changes to the 332A's pre-reg driver load resistor(s)-- for a mere 50ma base drive to the Darlington-- low and behold it works! But... but..  pre-regulator dynamic range is severely compromised.  The drive to the new Darlington transistor will stop if I raise the mains voltage (to the switching transistor) to the nominal 120V.  It will also grind to a halt if I draw too little wattage from the 332A's output terminals..   transistor looks like it's saturating as it doesn't heat at all.

One last thing before I close-- that pre-regulator switching trans is straddled in a diode bridge so that the alternating current flows through the switching trans in one direction. But as the mains current flow alternates the transformer primary looks like it appears in the collector circuit and then on the alternate 60Hz cycle in the emitter circuit.  The base drive return to the pre-regulator common is through the diode bridge.  I'm wondering if this affects the switching transistor biasing?  I haven't come up with a safe probing approach. I'm using battery powered and thus isolated 'scope 'n' meters. Also running the pre-regulator thru an isolated variac.

All for now.. and thanks for replying to me..

Regards,

Robert (aka Vollumscope)




Dr. Frank:
I also had an old 332B/AF with a defective pre regulator board.
The power transistors Q1, Q2 were dead, unobtainium.
What I also found out, that nearly all of the medium power transistors Q3-Q9 showed very low hFE, below 10, or so, as if they all deteriorated over time.
If I remember correctly, these sit in sockets, so it's easy to check and replace if necessary.
Frank

Vollumscope511:
Thank you Dr Frank for replying to my plea for help with my Fluke 332A pre-regulator..

I suspect that somehow the mains switching transistor's (Q107) emitter voltage is rising above it's base voltage during transformer primary current draw in that cycle dropping issue. While the base drive and DC from Q107 return directly back to the pre-regulator power supply, somehow voltage drop across the regulated power transformer primary is also lifting the emitter voltage above ground (although there is no common circuit between Q107 base and mains/regulated transformer primary)-- and therefore above the base voltage. This back-biases the switching transistor B-E junction and cuts off the transistor (Q107).  The driver transistor for Q107 is also a saturated switch and its base drive current is part of Q107's base drive current. So when Q107 becomes reverse biased, the driver transistor no longer conducts because its base driver current stops flowing.

This is a guess. I have to probe the circuit to verify if my guess is plausible.

See my other reply post to Roman Oh.

Again,
Thank you for your help.

Regards

Robert, aka Vollumscope





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