732B Crowbar circuit

[alligatorblues]

I have a 732B that instantly blows fuses when power is attached . I drilled apart the transformer assembly to examine the inside, replaced the power-entry module, affixed metal behind the rivet holes, tapped them, and screwed it back together. I measured the output, which is 22VAC, which is correct.

If I have the transformer disconnected from the power supply board, it doesn't blow the line fuse. If it is connected, it blows the fuse. I looked in some older 732B manuals, which had troubleshooting steps and schematics, but I don't reaally understand how the crowbar works.

Fluke doesn't call it that, but thst's what it is. In an overvoltage condition, some component conducts, and shorts the transformer secondary, which blows the fuse. I could use some explanation of how to troubleshoot that part of the board. Newer manuals do not have most of the maintenance and repair info.

In the 732C manual it just says a few test points to check, and if one is off, replace the board. Fluke no longer stocks the part I need. I have to repair the board. It only stands to reasom that Fluke wants to sell its new product, the 732C, and obsoleti g the 732B is a good way to do that.

I don't fault Fluke for it's business model, even though it sucks. No one has offered Fluke  any reason to change. It's the old, "We don't have to care. You don't have any other choice." And Fluke makes some nice stuff. I just wish someone would give them competition! 

[floobydust]

Fluke was acquired by Fortive, so the spirit of Fluke is gone, it's just a profit center for the mega-conglomerate and its share holders.

It's an odd crowbar circuit, it appears to trigger if the transformer's peak voltage is >46V. I think it could only happen if the mains selector was in the wrong position?
You could use a series light bulb on the mains side to save on fuses.

Check SCR CR500, zener VR501 for a short. Otherwise it's not the crowbar causing the fuse to blow, the battery charger could have a fault there. It does run 24/7.

[Kleinstein]

The parts in the crowbar circuit are not that special. So a more more generic zener and SCR will do.  It would normally take serious overvotage to trigger the crowbar. Only if the filter caps had gone bad, just the inductive spike at turn off may be another reason for the extra protection.

Another possible way to have a short is a bad rectifier diode - the 1N400x tend to fail short, though not very often.

[alm]

And Fluke makes some nice stuff. I just wish someone would give them competition!

Regarding voltage standards, it seems like most competitors have stopped playing (e.g. Valhalla, Koep, Guildline/Statronics, ADCMT) or have been acquired by Fluke (Datron/Wavetek). I guess Transmille might be the only current production unit? I have no idea about real-world performance. I'm guessing worse than the Fluke standards.

[mag_therm]

Check the scr has not failed (to short).
If VR501 is indeed a zener, that is a horrible gate circuit, needs a diac to give a healthy pulse to turn the scr on ready for the anode di/dt until the fuse blows.

[alligatorblues]

Thank you all. This is great info! I haven't had time to check out the advice, but I'll let you know when I do. Just for troubleshooting, how long would an inductive surge off the transformef last, 250ms or longer? I can't catch it if there is one. Would a scope be better to detect that?

Thanks everyone! 

[floobydust]

I think this crowbar is relatively useless. It would trip if the wrong (too low) line voltage was selected- otherwise any mains surge has to kick up the main filter capacitor voltage (2,200uF 50V) from around 30VDC to >46VDC which takes a lot of energy maybe >1.3J and C500 adds delay as well. R501 560R 1W is also out to lunch it takes 24VDC to get to 1W but an SCR gate voltage trips around 1V.
The crowbar might have been "good intentions" for a lightning wallop on mains? I think it's just protection for the wrong line voltage.

[bdunham7]

I'm not sure I understand the pissing on Fluke for not supporting (with replacement parts and repair service) a device which is probably 30+ years old at this point.  They still supply the manual and schematics on their website for free, which is more than you get from most manufacturers.  And they'll still provide calibration if you would like.  Eventually things do go unobtanium and they are usually very clear about how long they are going to provide parts and service after production is discontinued.  AFAIK, it is almost always at least 5 years unless something happens in their supply chain.

The crowbar circuit seen here is used in a number of instruments from that era, including the 8840A/8842A meters.  It keeps the PSU regulators from toasting themselves if you select the wrong line voltage, which is a very handy feature that works well.  I don't think it is intended to protect against lightning strikes.  Troubleshooting it should be a straightforward deal and AFAIK it doesn't use any weird parts.  You should start by simply disconnecting one side of CR513 and CR514 to verify that the problem (presumable a short) is in the crowbar circuit.  Make triple sure you actually have the correct line voltage selected, of course.  If the problem is in the crowbar circuit, check all the diodes and VR501 next.  There's not much left after that.

[alm]

I'm not sure I understand the pissing on Fluke for not supporting (with replacement parts and repair service) a device which is probably 30+ years old at this point.

The Fluke 732C was released in 2018, so Fluke must have been selling the 732B until 2018. So any 732B device is at least 4 years old, and at most 32 years old (introduced in 1990).

[bdunham7]

I'm not sure I understand the pissing on Fluke for not supporting (with replacement parts and repair service) a device which is probably 30+ years old at this point.

The Fluke 732C was released in 2018, so Fluke must have been selling the 732B until 2018. So any 732B device is at least 4 years old, and at most 32 years old (introduced in 1990).

Yes, but I'm betting the OP doesn't have a newish one!  But that brings up an interesting point about replacement parts and service.  The OPs complaint was something like he used the 732C manual (?) and decided he wanted a replacement board which he claims Fluke didn't have or wouldn't sell him.  But they list all of the individual TH components in their manual so perhaps those are still available and service (for a price) as well?  In any case, 28-year product runs, published schematics and component-level parts availability are all things we should all be happy about. 

[floobydust]

I think it's unrealistic to think they will stock spare 869185 boards for an older product.
That was (732B/734A) 1992 Fluke customer service, now it's evil Danaher 1998/Fortive 2016- which are not as customer service oriented.
Swapping in a new board requires piles of new soldering? Or is it all pluggable? I think Fluke would repair it for a costly fee and include calibration as well. Or... DIY.

OP what markings are on the SCR? It's a protection component, nothing to do with calibration or accuracy, and there are many parts that would work.
SCR CR500 is "600V 20A" Fluke 886978.
Zener VR501 "43V 3mA 0.5W" Fluke 851584, I'd look at 1N5260B.

These mega-conglomerates expect their Service Dept. to be a profitable cost center yet require no specialist staff. They also frown on having any spare parts inventory.
The end result is an impotent repair dept. and lack of spare parts, railroading users into buying new products. They would make their easy money off calibration services.

[TheSteve]

The crowbar was the failure that required fixing on my 732B when I received it. One of the bridge diodes had shorted.