Fluke 8800A power supply ±7V ref problem [SOLVED] - Page 1
Forum section: electronics / repair

author: GregDunn
author_rank: Frequent Contributor
timestamp: December 27, 2018, 10:55:07 pm
content: I have 2 problematic 8800A units left, since I got the A/AF version running last week. One seems to have a bad U11 controller, the other may only have a bad power supply. Knowing what I need to fix one of them, I decided to move on to the next one to see if any more parts were required before I placed an order.I've fixed a couple of these, but it seems that the PSU has different problems in each one.Usually, replacing the caps will bring out of spec voltages up and all is well. This one, however, has a problem just with the ±7V supply. I used the Fluke recommended method of shorting pins 3-4 on the ±18V reference (lifting R120), and the ±18 and -13V supplies look good but the ±7V are actually at ±15.3V. I measured across the reference U9 pins 3-4 and got 6.2V (should be 7V on a working unit - but that's with the feedback loop closed). The +7 and -7, although way off, track exactly so my suspicion is that U7 is bad since it supplies the reference to U8? Or is it possible that the ref chip U9 is still defective? I don't understand the circuit well enough to see how it could be causing that large of an offset.Nothing looks burned or poorly soldered on the board; the only issues I saw were a starting-to-leak C22, which was replaced along with the other electrolytics. I don't see any tantalums in the ±7V circuit which might cause problems.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 27, 2018, 11:14:18 pm
content: U7 provides the -7 V Reference. The + 7 Reference it than made from this be inversion. Checking U7 would be a first step - if broken there is still a chance that the actual reference chip is still not fried, though it may have got some stress beyond suggested ratings. It is the 3.4 K resistor at the base to save the reference.6.2 V would be about right for the Zener only. The total reference is the zener plus base emitter voltage, and that should be a around 7 V.IC sockets can be a big help, but also a source of error.

author: GregDunn
author_rank: Frequent Contributor
timestamp: December 28, 2018, 01:12:35 am
content: The zener voltage (pins 1-4) on a working unit is about 6V vs. 7V for the zener + transistor (pins 3-4). On the non-working unit, the zener is about 6.1V vs. 6.2V for the zener + transistor.That is collector-to-emitter + zener which doesn't seem right - possibly the transistor portion is damaged?

author: Kleinstein
author_rank: Super Contributor
timestamp: December 28, 2018, 09:09:33 am
content: It is possible that the transistor section of the reference is damaged. At least I expect it to be the more sensitive part.The important voltage is base to emitter. The collector voltage is used by the feedback loop and usually (could be different in other circuits) is close to the base voltage if the loop is working. A broken OP (U7) could lead to high base current and thus the low emitter to collector voltage.

author: GregDunn
author_rank: Frequent Contributor
timestamp: December 28, 2018, 05:54:42 pm
content: I have two broken 8800A units now, one of which (I think) has a broken U11 and the other possibly a bad reference. I can afford to replace U7 on the second one, and if that doesn't fix the supply, the next step may be salvaging the U11 controller for use in the first one - because trying to obtain spares for either part is just another name for buying yet another parts unit.Speaking of which, the Fluke part # for U11 seems to be the same as the controller for the 8810A U11 and 8600A U8 (PN C2506/407734) so conceivably I could use one from any of the units as long as it's the right IC type. The manual talks about both CMOS (PN 354895) and PMOS variants but other than the circuit board jumper it gives no info about how to distinguish the parts. Of the 4 units I have in the repair queue, here's how the ICs are marked:8600A #1: unmarked ceramic package - has sticker which reads "86"8600A #2: 78051X C2506 8600 KOREA (plastic)8800A #1: 7621XB C2506 8600 KOREA (plastic)8800A #2: 79260F C2506 8600 KOREA (plastic)8600A #1 might be the old CMOS IC, especially since it's a low SN unit. But the others "seem" to be the later PMOS component? If they're a match, I can at least swap one into my dead 8800A and confirm whether it's the U11 or not. There's a hint in the schematic about jumper W1 and resistor R121 (not present on any of my units, which to me indicates they're all intended for the same IC) being used for the PMOS IC on certain boards, which I think means there was an interim board design which would handle both ICs. Maybe they removed that later, when production shifted to all version 407734?Anyway, any info which might shed some light on this would be appreciated. If one of the 8800A is beyond repair other than by stealing parts from the other, I can live with that.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 28, 2018, 07:40:45 pm
content: It should be possible to check the reference by measuring at the base terminal. Even if the external loop is out the base - emitter + Zener voltage should be somewhere around 7 V if there is a reasonable current flowing. So no real need to swap around the references. That reference is likely a predecessor of the LTFLU. So maybe an Motorola SZA263 or similar.For the CMOS and PMOS versions, I would expect the CMOS version to be lower power (especially useful for battery operated versions) and more modern. PMOS was more like a simple to handle process that was relatively slow and high power. If there is no analog amplification going on inside, I would expect the CMOS version to be really low power, like < 1 mA and the PMOS version considerably higher power (e.g. > 10 mA).Chances are the first numbers are date codes, so chips from 1976 to 1979.If the reference is dead one could test the board with a substitute circuit, e.g. made of an NPN and 5.6-6.5 V zener, just like the drawing.

author: GregDunn
author_rank: Frequent Contributor
timestamp: December 28, 2018, 10:08:25 pm
content: Pretty sure the original 8800A references were SZA263 chips; one of mine is marked thus and the other has had the markings removed but seems identical.The annotations were confusing in the manuals - pretty clearly the 8810A, a much later model, uses only the PMOS version (407734) but at least one of my battery-powered 8600A also has a PMOS chip and the other one is completely without markings except for the sticker - so unknown. The later 8600A manual from 1977 does not talk about the different chips (only the PMOS), whereas the earlier 8600A (1976) and 8800A (1975) manuals refer to both, and the later 8810A manual (1978) mentions only the PMOS. It seems that the CMOS chip was phased out sometime between 1976 and 1977 and replacement parts as well as new manufacture all used the PMOS chip.Anyway, it's easy to replace U7; whereas replacing U9 with the correct part is unlikely without a donor unit. Additionally, removing U9 will subject the chip to more stress and probably upset its stability and characteristics. So my plan is to replace U7 and possibly a couple of the passive components and if that doesn't resolve the reference issue I will probably take U11 and move it to the other 8800A to salvage one good unit.I do see that the LT1021-7 is a 7V reference, so maybe a circuit could be cobbled together to replace the Fluke chip permanently?

author: Kleinstein
author_rank: Super Contributor
timestamp: December 28, 2018, 10:50:33 pm
content: If a different reference is used, one would still somehow have provisions for adjusting the -7 V / -1 V level. So one would need a trim option and a -7 V reference. So the choice may not be that easy.With a not so high resolution a different reference could be an option, especially for the 8600.This might also be interesting for other, as a direct replacement is hard to come by. At least the 8600 does not really need a reference of that grade, especially low noise.Starting with CMOS and later switch to PMOS is odd. Normally it was the other way around. There could be one reason: Fluke at a time had a rather crude own fab to make there own chips. AFAIK PMOS is one of the easy to manage processes they may have been able to to make there own PMOS version instead of an external source CMOS version.

author: GregDunn
author_rank: Frequent Contributor
timestamp: December 28, 2018, 11:29:18 pm
content: Yes, I was kind of reaching a bit there, because I know the LT1021 is a fixed reference and I don't fully understand how the Fluke circuit is using the SZA263 with feedback to trim the voltage.Fingers crossed that it isn't the reference or I may be unable to solve this...The earlier ceramic chip is unmarked, but the later plastic cased chips are all molded with "AMI" in big letters; I just assumed those are the PMOS chips because the parts list refers to the PMOS as "C2506/407734" and the chips are marked C2506. I really don't know what Fluke did for chip sources over the years.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 29, 2018, 12:08:31 am
content: The 2 pot R97 and R99 seem to be for calibration, so that the +-1 V readings are right. They set the negative reference voltage and the gain of the invert to get the positive references. AFAIK the SZA263 reference is a little below 7 V and the pot is used to add a little bit of gain.

author: GregDunn
author_rank: Frequent Contributor
timestamp: December 29, 2018, 05:24:56 am
content: Yes, and I tried tweaking the pot with no effect. That does make me think U7 might be the culprit though. Anyway, ordering a replacement part and we'll soon see!

author: Kleinstein
author_rank: Super Contributor
timestamp: December 29, 2018, 09:25:20 am
content: One could check the function of U7: if the 2 inputs are at about the same voltage, U7 is likely OK. If not it is very likely broken.I would not expect anything special for U7 - could be an 741 or OP07 , though a different speed could be tricky with the compensation.When keeping U7 it should be possible to use a different reference and still do an adjustment. The slight problem here would be that the circuit would be different for a reference slightly high or lower than the nominal "7 V".

author: GregDunn
author_rank: Frequent Contributor
timestamp: December 29, 2018, 10:48:03 pm
content: Agreed. On the working unit, U7 is maintaining the input at a ~0V "virtual ground" but on the bad unit, it's considerably different - just about as far off as the output is from the 7V required, which is not a coincidence. U7 is an LM301 or equivalent, which is an easy find.

author: GregDunn
author_rank: Frequent Contributor
timestamp: January 04, 2019, 12:32:20 am
content: Success! I bought an LM101 to replace the LM301 (U7) in the reference power supply, and installed it. [I chose the '101 because it was the only one I could find readily in DIP-8 format. As a bonus its specs are better in almost every instance than the '301.]Now the ±7, +13 and ±18VDC are all correct. A quick run through my resistance standards showed the unit to be well within spec, and the HP calibrator convinced me AC and DC were pretty linear all the way through the ranges. Throwing it on my +10VDC reference shows it's very close (I haven't let the unit cook long enough to zero it, so it may be even closer than I think).I'm so glad it wasn't the unobtainium Fluke reference chip. Counting parts (caps and op amp) it's probably still the cheapest 8800A on my bench. Now, time to clean it up a little and put the case back on.

author: elecdonia
author_rank: Frequent Contributor
timestamp: January 04, 2019, 01:09:28 am
content: I think a better grade op amp would be desirable for U7.The OP-07 is a good candidate. IIRC Fluke used the OP-07 in later production 8800 units. The LM-101 LM-301 is a very crude early general purpose op amp and was obsolete by the late 1970’s.That said, it is clear that a functional LM-301 works a whole lot better than a broken one.As stated by another post, in most cases the + and - inputs of a functional op amp will always remain within +/- 50 mV DC of each other when tested “in-circuit.”

author: GregDunn
author_rank: Frequent Contributor
timestamp: January 04, 2019, 01:30:56 am
content: True, it's a better part (the specs are as much better than the LM101 as the '101 is to the '301) but for what I'm doing, it wouldn't make much difference. I'll probably order a few on my next parts binge though, to upgrade if I lose another one; they're certainly affordable.

author: David Hess
author_rank: Super Contributor
timestamp: January 04, 2019, 02:20:07 am
content: The 301 (actually 301A) is not completely obsolete because its input common mode range includes the positive supply and its compensation terminals allow some extra flexibility. In a circuit like this where the closed loop gain is low and the common mode voltage is fixed, its shortcomings compared to a precision part are not a problem. A 741 and any number of other parts would work just as well here.The OP-07 became the jelly bean part of precision operational amplifiers because so many companies second sourced it. It would be my choice for a improved replacement because of its low cost.

author: coromonadalix
author_rank: Super Contributor
timestamp: January 04, 2019, 02:34:29 am
content: Niice, another classic saved congrats