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Old 1st Jun 2012, 5:55 pm   #21
Radio Wrangler
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Default Re: Fluke 8600A Fault

There was a problem with reliability of ICs from this era. The palstic packages weren't gas tight and humidity and air pressure variation got moisture in. The water leached phosphorous out of the glass passivation ('Glassivation') on top of the silicon, and formed dilute phosphoric acid, which slowly ate the aluminium connection layer.

CMOS suffered badly, but TTL made enough heat to drive the moisture out... except for stuff which wasn't used for a while.

Rather nastily, this mechanism goes more for parts in storage than for equipment in use, which makes finding good spares a bit of a problem.

David
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Old 1st Jun 2012, 7:13 pm   #22
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Default Re: Fluke 8600A Fault

I suspect that the meter, which was second hand, had not been used for a while so that might explain it.

Here http://www.youtube.com/watch?v=bk4tJ...ature=youtu.be is a youtube clip of the signal trace. It is actually reading around 4v.


Another bit of information now I can see a decent trace is that the "lows" on pin 6 are 1.5v. The data sheet for the 7440 shows tha maximum low as 0.8v. The other input pins are all 0-5v so pin 6 seems to be stuck high. Dos this confirm a fault with the main chip? (I sound as if I understand what I am talking about but my son has given me a quick lesson on it all!)

The output of the 7440 shows a pretty random trace.

Paul
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Old 2nd Jun 2012, 1:17 am   #23
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Default Re: Fluke 8600A Fault

Paul

If you want to be adventurous, you could read the output from U8 (the fluke chip) using your osciloscope (a logic analyser would be better though!).

The display is "multiplexed" and data effectively sent serialy to the display board.

What that means is each digit is sent individualy, one after the other, and the display board decides which LED digit to display it on. The digit information (in binary form) is sent on lines w, x, y, and z ("0 to 9 in binary) and the correct digit is determined by the stobe lines.

For example a value of 3.50 would be sent:-

w x y z
0 0 0 0 (0) Strobe 1
then
0 1 0 1 (5) Strobe 2
then
0 0 1 1 (3) Strobe 3

Therefore to "de-multiplex" or read the data:-

Attatch channel one of the scope to U8 pin 17 (strobe 1), and set this to trigger the scope.

You should see a stable square pulse on the left hand side of the scope display, and perhaps again near the righthand side of the scope display.

Attatch the second scope channel to w, and see if there is a square pulse corresponding directly with the other pulse. If there is then this is a 1 for "w"

_n______ Channel 1 (strobe)

____n___ Channel 2 (w=0)

_n______ Channel 2 (w=1)

Repeat moving the 2nd probe to x, y and z in turn and record if there is a pulse directly where the first channel pulse is.

You can then read this binary code and find the value for digit 1. (the first digit may not be stable, due to the sensitivity and errors in the Analogue to Digital stages)

Move the first scope input to Strobe 2 and repeat for the second digit, and third etc

You can then evaluate if U8 is giving the correct data to the 7440 chip.

If the digits are being sent from U8 correctly, then either chip could be causing the high "0V" you see, but only the 7440 is available for replacement, and relatively cheap. Fit a socket when replacing it!

regards

peter
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Old 2nd Jun 2012, 9:53 pm   #24
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Default Re: Fluke 8600A Fault

Thanks Peter - I have passed the suggestion on to my son to try.

I suppose we could remove the control chip and see whether there is still a voltage on the 7440.

Paul
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Old 14th Jun 2012, 7:04 pm   #25
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Default Re: Fluke 8600A Fault

Sorry about the delay in getting back about this.

I removed the 7447 chip and the waveform on pin 6 was far from clean but did at least drop to zero so I ordered a new chip and socket. I have just soldered in the new socket but the fault remains and the minimum is no longer zero.

It seems that the fault must be with the main chip unfortunately and so the unit is scrap. Thanks for the help all, I have learned a lot.

Paul
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Old 15th Jun 2012, 3:09 pm   #26
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Default Re: Fluke 8600A Fault

There is a bodge approach: try pulling pin 6 of the 7447 down to ground with a resistor, say about 470 ohms. That might encourage the signal low enough to actually work!

If you're intending to scrap it, I'm interested in it for parts since I have a pile of these meters which I use regularly.
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Old 15th Jun 2012, 7:50 pm   #27
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Default Re: Fluke 8600A Fault

Although the readout is garbled, the parts you can see no longer accurately reflect the voltage input which I suppoose may mean that there is more wrong with the chip. I wondered about pulling down the voltage as suggested and might give it a try.

The meter actually belongs to my son and I will ask him about parting with it, but I suspect he may want to hold on to it.

Thanks

Paul
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Old 15th Jun 2012, 7:58 pm   #28
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Default Re: Fluke 8600A Fault

There may indeed be more wrong with the chip, but anything's worth a try even out of curiosity I understand about it belonging to your son - I'm not desperate but would like to keep it out of landfill!
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Old 15th Jun 2012, 8:13 pm   #29
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Default Re: Fluke 8600A Fault

Well, many thanks for the suggestion. It is working again with the resistor in place. The reading is actually ok, it was the reading on the power supply that was slightly out.

What would I do without this forum?

Cheers

Paul
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Old 15th Jun 2012, 8:34 pm   #30
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Default Re: Fluke 8600A Fault

Excellent! I'm very happy that it's working again for you. That's a quality bodge!
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Old 4th Apr 2014, 11:57 pm   #31
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Default Re: Fluke 8600A Fault

Quote:
Originally Posted by cmjones01 View Post
There is a bodge approach: try pulling pin 6 of the 7447 down to ground with a resistor, say about 470 ohms. That might encourage the signal low enough to actually work!.
I happened to stumble across this thread (and forum) while trying to repair my 8600a problem where ALL decimal points were lit ALL the time. The meter seemed to work properly otherwise.

I tried the "bodge" technique using a 560 ohm resistor. By pulling pin 9 of U8 low in this manner, the darn thing works like a charm! Just wanted to say thanks!
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Old 5th Apr 2014, 10:27 am   #32
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Default Re: Fluke 8600A Fault

Quote:
Originally Posted by Postal666 View Post
I tried the "bodge" technique using a 560 ohm resistor. By pulling pin 9 of U8 low in this manner, the darn thing works like a charm! Just wanted to say thanks!
Glad it worked for you too, and thanks for the feedback!

Chris
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Old 10th Jun 2014, 7:40 pm   #33
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Default Re: Fluke 8600A Fault

I too just came across this thread, and would appreciate your help. I just received a used Fluke 8600A, which seems to work fine, except in the ohms range, with nothing connected I get the flashing +18888 display. When I short the inputs the display shows 0000 as would be expected. I have read the previous postings in this thread, and notice that the flashing +18888 in the ohms range is mentioned, but I can't seem to see the resolution.

Thanks in advance.

Cheers, Paul.
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Old 11th Jun 2014, 9:57 am   #34
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Default Re: Fluke 8600A Fault

What's the problem? These meters are supposed to show a flashing +18888 on over-range, such as with an open-circuit on the ohms range. I've got seven of them (!) and they all do that, and it's what's described in the manual. It sounds like yours is working properly if it shows 0000 when you short-circuit the input on ohms.

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Old 11th Jun 2014, 11:25 am   #35
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Default Re: Fluke 8600A Fault

Yes, it's their way of saying "infinity" i.e. an open circuit.

N.
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Old 11th Jun 2014, 2:56 pm   #36
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Default Re: Fluke 8600A Fault

Thanks for the replies. I was wondering if that might be the case, and had not come across that info in the manual. Thanks for setting me straight. As it happens the meter is defective. Now, when you turn it on the display comes on for a few seconds but then goes blank. Are these things worth fixing?
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Old 11th Jun 2014, 6:21 pm   #37
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Default Re: Fluke 8600A Fault

Quote:
Originally Posted by PaulR View Post
I was being wooden about getting the 7447 out of its socket because it seems to be soldered directly to the circuit board
It sounds like you're sorted now Paul, but if at some point you did need to remove and replace the IC, the safest way to do it without risking damage to the PCB - especially if it has plated through holes - is to snip through all of the pins of the IC as close to the package as you can, so you leave the pins in the PCB and discard the IC, which - if you tried to de-solder the pins with it in situ, would act as a heat sink. Then one pin at a time, fix Spencer Wells forceps onto each pin, heat up the pin beneath the board with the soldering iron and gently pull the pin out when the solder has melted. It only takes a couple of seconds and is far less risk than trying to use a solder sucker or braid on each of the 16 pins with the IC in place. You could then fit a low profile turned-pin IC socket and plug the new IC into that.

In the late 80s, I replaced scores of ZX Spectrum CPUs that way, mounted on delicate PTH PCBs. (28 pins as I recall).

If you do need a replacement 7447, they are available 'buy it now' at modest prices - like here for example:

http://www.ebay.co.uk/itm/SN7447AN-T...item53f9a0b7da

The 'SN' prefix simply denotes the manufacturer (in this case Texas Instruments), the N suffix denotes dual-in-line plastic package.

The later version of the 7447 was the 74SL47 (low power schottky), referred to in Ray Marston's 1994 book 'Modern TTL Circuits Manual' as 'the modern equivalent of the old 7447', so the 7447 was already old in 1994.

Here we are, 20 years on. How time flies!

Hope the Fluke 8600A continues to behave itself.
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Old 12th Jun 2014, 1:55 pm   #38
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Default Re: Fluke 8600A Fault

I have one myself waiting for inspiration. There's something wrong with it but I can't remember what... Maybe a good start is trying to get a manual.

Tjerk, 9ZZ
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Old 18th Jun 2014, 5:44 pm   #39
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Default Re: Fluke 8600A Fault

Well, pulled the thing apart, and discovered some corrosion underneath the battery
connections. Cleaned that up, and cleaned the switches and now the thing is up and running fine. Has anyone replaced the battery supply with an AC only supply? From searches I see that running this without the batteries does not work.

Thanks in advance.

Cheers, Paul.
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Old 19th Jun 2014, 2:50 pm   #40
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Default Re: Fluke 8600A Fault

Don't try this. The 8600A uses a mains transformer fed via a series capacitor (value chosen to suit differing markets of mains voltage and frequency) in order to charge the NiCd cells at something like constant current.

I've given my 8600A manual away to another Forum member, but if you search "Fluke 8600A" you should find some of my previous comments.

This power supply works well enough as designed, as long as a good set of cells is fitted.

Leon.
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