Author Topic: Fluke 8502A - fuse blown  (Read 9294 times)

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Offline RaxTopic starter

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Fluke 8502A - fuse blown
« on: November 22, 2021, 09:03:56 pm »
Hi all,
Going through some of the other units I got - see my other thread(s) - there's also a Fluke 8502A that worked for a minute, then blew its 1/2A fuse.

I've been going through the PS with one of those universal testers, and most electrolytics check OK, except C7 (which gave weird, either non functional, or resistive readings). Pulled it out and measures OK out of circuit. I will likely simply replace all electrolytics soon, but in troubleshooting this there's also a C10 right across C7, just very near the regulator's legs (of course). This is a tantalum 5.6uF (and I'm generally deeply suspicious of tantalums...). Due to where it's placed, checking it and/or taking it out will likely mean replacing it - what have others used for this? I am tempted to go for a 4.7uF film cap (maybe even further bypass it with a .1uF or so). Datasheet says 25uF for an electrolytic if not going tantalum, but I'd rather go film there.

Not at all sure this may cause my fuse meltdown (I'd say not likely), but other things like the safety caps seem to check OK. Any suggestions for what to check or look for are very welcome!
 

Offline RichardM

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Re: Fluke 8502A - fuse blown
« Reply #1 on: November 22, 2021, 09:44:26 pm »
Hi, we are on a similar journey, see me recent post on the 8502a.

I replaced the three tantalums near the back heat sink and regulators with 10uf tantalum caps and I seem to have the correct voltages now. After powering the unit on the first time, 2 of those old 5.6uf caps let out the magic smoke. The negative voltage regulator was also dying and these are socketed and easy to replace.
 

Offline RaxTopic starter

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Re: Fluke 8502A - fuse blown
« Reply #2 on: November 22, 2021, 10:43:25 pm »
Richard,
Yes, I was just reading your notes... :) As soon as I posted my thread, I went back to the listing of all topics, and for a second I wasn't sure which thread was mine... ;)

I am tempted to do an all-recap, but only after I conclude it's worth going through the expense. So far so good, as during the minute or so it was working, I didn't see much obvious signs of trouble. Not a lengthy examination, for sure, but at least I get good heart to keep at it.

On those tantalums - I never stock any, as almost all my work is in audio or FM, and dread their reliability record, but I'd be tempted to replace them with film caps to keep the esr low. I have WIMA caps that are very miniature for their values - essentially, almost the overall size of the electrolytics for given value, and the same typical lead spacing - and may use here. I am not yet sure these are blown, but I'll try to correct things I suspect or measure out of whack, as hopefully they correct whatever blows its fuse, as I don't have the comfort to fire it up and measure for issues at my leisure. I got a variac and a "dim-bulb" fixture, but not sure if I should try it here.

Looking forward to your progress!
Radu.
« Last Edit: November 22, 2021, 11:53:30 pm by Rax »
 

Offline RichardM

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Re: Fluke 8502A - fuse blown
« Reply #3 on: November 22, 2021, 11:32:20 pm »
Hi Radu,

Your repair may be much smoother if it was working at some point. Mine came in a non booting state.

I went with tantalums as there is polarity shown on the circuit board and schematic I think.

Richard
 

Offline RaxTopic starter

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Re: Fluke 8502A - fuse blown
« Reply #4 on: November 24, 2021, 05:31:57 am »
Hi Radu,

Your repair may be much smoother if it was working at some point. Mine came in a non booting state.

I went with tantalums as there is polarity shown on the circuit board and schematic I think.

Richard

Richard,
If you could drop a quick indication of how you gained access to those parts (7912, tantalum caps). Does the heatsink pop out? Been looking at this many different ways, but before I start disassembling too many parts, some of which I may not need to touch, I though I'd ask you as you've just done this.

Thanks!

[meanwhile: loudly slaps forehead, repeatedly, and hides in a corner for a while]

Jeez, I guess reading the SM (which I finally could print today, after getting the darn toner order) was too much to ask of me... It slides right out from the back!
« Last Edit: November 24, 2021, 05:49:19 am by Rax »
 

Offline RaxTopic starter

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Re: Fluke 8502A - fuse blown
« Reply #5 on: November 24, 2021, 08:56:06 am »
Alright, so tonight I got as far as replacing all those three regulators - U6, U7, and U8 - and their caps (used 22uF low esr lythics, per datasheet), and now I have solid +5V, +12V, and -12V coming out of those. I forgot to how much I hate the PCBs of the time... barely got away with lifting just a small number of pads (checked continuity everywhere and I think I'm OK). Brittle as heck.

Also, the SM is not very pretty.

****************************************************************************************************************************

Yeah!... Tonight I got further than what I just said above. PS seems to be sorted out. Turns on, no fuse burn or anything.

But now, when I switch it to measure VDC/AUTO, it goes into "Error 5." If the SM would be legible, I'd figure what the heck that is.
« Last Edit: November 24, 2021, 08:57:43 am by Rax »
 

Offline RaxTopic starter

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Re: Fluke 8502A - fuse blown
« Reply #6 on: November 24, 2021, 04:56:31 pm »
Given Error 5 is "R^2 A/D Error - the recirculating remainder analog to digital converter module is faulty or not installed." I can't find this module in the SM (an index would have been nice...). I did remove the "R^2 A/D Converter" module and cleaned the contacts (deoxit) and reseated a couple of times. Still the same issue ("Error 5"). Not right away, and may need triggering by switching modes, but it occurs ultimately.
 

Offline RaxTopic starter

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Re: Fluke 8502A - fuse blown
« Reply #7 on: November 25, 2021, 01:24:56 am »
Not much positive progress to report... Tried all available modes and there's fault codes to most:
  • ohms gives me "error 3"
  • all others, immediately or after a while (which may include measurements) ultimately give me "error 5"
Worst of all, I don't think I ever got it to credibly measure anything. The startup screens are correct and uneventful, but sooner than later an error code will pop up. And the instrument doesn't seem responsive to the trigger of a stimuli applied to it.

Adding insult to injury, the SM is dead quiet on workflow around "error 5." For that matter, any error codes. Please point the page to me if I'm wrong... Would love to be wrong.

Thanks all!
 

Offline RichardM

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Re: Fluke 8502A - fuse blown
« Reply #8 on: November 25, 2021, 05:40:52 am »
Hi Radu, sorry. Work got in the way for the last few days.

Unfortunately I have not seen error 5. I have only had seen errors 2, 6  & 9.

There is a flow chart in Table 4.6 worth following if you haven't already in addition to the accompanying tables.

My own repair is going around in circles!! I think I have cleared the signal conditioner module and now believe it may be my filter module as the measurement from RT2 to RT5 is not right. Another rabbit hole to follow :)

Richard
 

Offline RaxTopic starter

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Re: Fluke 8502A - fuse blown
« Reply #9 on: November 27, 2021, 05:26:21 am »
A bit of an update/summary.
  • Power supply - repaired and adjusted, including confirming 16VAC on the 60Hz ref.
  • AD Module - replaced the 470uF axial on the analog PCB (which was bad, though not exploded or shorted, just completely out of spec), and confirmed the rails and Vref (7.0000V). I observed some oddity where some ground(s) (pin 30) seemed for a while - which I couldn't replicate later - elevated by about 0.3V from the rest (TP6 Ref Common) - which is visible on the included scope plots (collected at TP5 to observe switching). Redoing the plots referenced to TP6 restores symmetry around 0V/x axis, although currently all grounds on the analog board of AD seem to be at same potential. Would love to hear thoughts on what's going on at TP5, but seems OK to me.
Evaluating further steps, but not sure what it is I should look at next. Also, feedback and interpretations of this data are appreciated. If possible, comps with similar readings from known good units would really be great.
 

Offline RaxTopic starter

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Re: Fluke 8502A - fuse blown
« Reply #10 on: November 27, 2021, 07:41:12 pm »
Progress!!  8)
 

Offline RaxTopic starter

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Re: Fluke 8502A - fuse blown
« Reply #11 on: November 29, 2021, 05:00:25 pm »
Update on my status.

As the picture included a few days ago shows, it seems my work on the PS and AD module restored operation on DCV (and ACV) most of the time. And yet, "Error 5" (when operating on DCV, especially in high res mode) and "Error 3" (when operating in ohms mode) can still occur. I am currently focusing on "Error 5," and leaving "Error 3" for later.

When this happens, TP5 hangs either on a high state of -7V, or 0V (instead of the switching behavior I placed screenshots of above; during "Error 5," it flatlines). After trying this extended weekend to provoke the error so I'd figure what triggers it, it seems to be the "high res" mode tends to by far. I'm actually pretty stable if I simply up the scaling of the measurement as far as it goes. If I go to "high res" though, Error 5 is pretty quick to occur. Not sure if some else has dealt with this and has any recommendation to what to look at first.

Also, I'm looking for a compatible extender/riser card (I'm told the 5100B work), which would allow me to poke further into the functionality of the AD board.
 

Offline bdunham7

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Re: Fluke 8502A - fuse blown
« Reply #12 on: November 29, 2021, 05:26:58 pm »
If you take the isolator board out of its shell, what part number and revision level is on the PCB?
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Offline RaxTopic starter

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Re: Fluke 8502A - fuse blown
« Reply #13 on: November 29, 2021, 05:59:56 pm »
If you take the isolator board out of its shell, what part number and revision level is on the PCB?

486415 Rev A

Thanks!
 

Offline RichardM

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Re: Fluke 8502A - fuse blown
« Reply #14 on: November 30, 2021, 11:36:31 am »
Update on my status.

As the picture included a few days ago shows, it seems my work on the PS and AD module restored operation on DCV (and ACV) most of the time. And yet, "Error 5" (when operating on DCV, especially in high res mode) and "Error 3" (when operating in ohms mode) can still occur. I am currently focusing on "Error 5," and leaving "Error 3" for later.

When this happens, TP5 hangs either on a high state of -7V, or 0V (instead of the switching behavior I placed screenshots of above; during "Error 5," it flatlines). After trying this extended weekend to provoke the error so I'd figure what triggers it, it seems to be the "high res" mode tends to by far. I'm actually pretty stable if I simply up the scaling of the measurement as far as it goes. If I go to "high res" though, Error 5 is pretty quick to occur. Not sure if some else has dealt with this and has any recommendation to what to look at first.

Also, I'm looking for a compatible extender/riser card (I'm told the 5100B work), which would allow me to poke further into the functionality of the AD board.

Well done. Unfortunately I have not been able to get any further with my problem. I have non-zero readings at boot and they do not change with input to the front jacks.

I am back looking at the A/D board now and was wondering if you are able to probe a few pins on J1 or J2 on the back of the analog board using J6 are common ground ? Apparently the pulses need to be 2us max rising and falling but I am seeing much greater times when rising.

This is becoming an obsession :)

Regards

Richard

 

Offline RaxTopic starter

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Re: Fluke 8502A - fuse blown
« Reply #15 on: November 30, 2021, 03:07:15 pm »
Update on my status.

As the picture included a few days ago shows, it seems my work on the PS and AD module restored operation on DCV (and ACV) most of the time. And yet, "Error 5" (when operating on DCV, especially in high res mode) and "Error 3" (when operating in ohms mode) can still occur. I am currently focusing on "Error 5," and leaving "Error 3" for later.

When this happens, TP5 hangs either on a high state of -7V, or 0V (instead of the switching behavior I placed screenshots of above; during "Error 5," it flatlines). After trying this extended weekend to provoke the error so I'd figure what triggers it, it seems to be the "high res" mode tends to by far. I'm actually pretty stable if I simply up the scaling of the measurement as far as it goes. If I go to "high res" though, Error 5 is pretty quick to occur. Not sure if some else has dealt with this and has any recommendation to what to look at first.

Also, I'm looking for a compatible extender/riser card (I'm told the 5100B work), which would allow me to poke further into the functionality of the AD board.

Well done. Unfortunately I have not been able to get any further with my problem. I have non-zero readings at boot and they do not change with input to the front jacks.

I am back looking at the A/D board now and was wondering if you are able to probe a few pins on J1 or J2 on the back of the analog board using J6 are common ground ? Apparently the pulses need to be 2us max rising and falling but I am seeing much greater times when rising.

This is becoming an obsession :)

Regards

Richard

Richard,
Here's a view of TP2. I am using a 10:1 probe, but the reading on the screen of the DMM changes, so I know the probing affects the instrument. So take the risetime value with a grain of salt (this is a quick read before work... not much time for thinking this through). But I think what you can take from this is that the risetime and fall are the same.
 

Offline RaxTopic starter

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Re: Fluke 8502A - fuse blown
« Reply #16 on: November 30, 2021, 03:09:32 pm »
Here's the plot
 

Offline RichardM

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Re: Fluke 8502A - fuse blown
« Reply #17 on: November 30, 2021, 08:27:20 pm »
I meant to say use TP6 as common ground, apologie. Is that what is shown in your image ? The rise and fall times are much greater than 2us ?

Thanks again, Richard
 

Offline RaxTopic starter

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Re: Fluke 8502A - fuse blown
« Reply #18 on: December 01, 2021, 03:31:56 am »
I meant to say use TP6 as common ground, apologie. Is that what is shown in your image ? The rise and fall times are much greater than 2us ?

Thanks again, Richard

Richard,
Sorry, I may have misunderstood your request. What's included above is a read off of TP2 (as referenced to TP6, of course). The probe is loading the circuit, so the voltage reading is truncated while the probe is attached and the waveform looks like you see it (I assume the risetime is much slower than what it actually is without the probe loading the circuit). FWIW, there's no malfunction while doing this, as soon as the probe goes away the reading restores and no errors are being thrown out.

Could you please explain what you need measured again? A picture is worth a thousand words, so that would be gorgeous, if at all possible. I will do my best to help. Thanks.
 

Offline RaxTopic starter

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Re: Fluke 8502A - fuse blown
« Reply #19 on: December 01, 2021, 09:36:41 pm »
At this point, I have a pretty high degree of confidence "high res" triggers "Error 5." I left it run measuring about 13Vdc with all digits possible outside "high res" for just about 24hrs, the error never came up. I switch it to "high res," it locks up on "Error 5" right away (may take up to a minute). I did the same with my AP box - for a calibrated AC level - and it locked up when I requested "high res."

This happens on both ACV and DCV.
 

Offline RichardM

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Re: Fluke 8502A - fuse blown
« Reply #20 on: December 01, 2021, 11:08:47 pm »
I meant to say use TP6 as common ground, apologie. Is that what is shown in your image ? The rise and fall times are much greater than 2us ?

Thanks again, Richard

Richard,
Sorry, I may have misunderstood your request. What's included above is a read off of TP2 (as referenced to TP6, of course). The probe is loading the circuit, so the voltage reading is truncated while the probe is attached and the waveform looks like you see it (I assume the risetime is much slower than what it actually is without the probe loading the circuit). FWIW, there's no malfunction while doing this, as soon as the probe goes away the reading restores and no errors are being thrown out.

Could you please explain what you need measured again? A picture is worth a thousand words, so that would be gorgeous, if at all possible. I will do my best to help. Thanks.

Hi Rax

My explanation was not very clear that's for sure.

I have attached a photo of the Analog side of the A/D twin board. J1 and J2 are circled in red and TP6 is also. The Fluke is turned around so the back of the unit is closest to me and obviously the cover is taken off the A/D module. With the scope ground attached to TP6 I am looking to see what you get when you probe the pins on say J1 (easiest to reach). Most pins will show pulses and the rise/fall should be <2us. My rise times are much greater than 2us.

Just fyi, I have also attached scope readings for TP2,3,4,and 5. I don't see much at all at TP2. With the exception of TP5 the results don't appear correct, although I am unsure what to see at these test points ?

Thanks again, Richard
 

Offline m k

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Re: Fluke 8502A - fuse blown
« Reply #21 on: December 02, 2021, 08:15:35 pm »
What manuals you are using?

Shouldn't be a problem if A/D board it the interesting one.
Advance-Aneng-Appa-AVO-Data Tech-Fluke-General Radio-H. W. Sullivan-Heathkit-HP-Kaise-Kyoritsu-Leeds & Northrup-Mastech-REO-Simpson-Sinclair-Tektronix-Triplett-YFE
(plus lesser brands from the work shop of the world)
 

Offline RichardM

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Re: Fluke 8502A - fuse blown
« Reply #22 on: December 02, 2021, 09:36:39 pm »
What manuals you are using?

Shouldn't be a problem if A/D board it the interesting one.

I have the Instruction Manual. I still there something else available ?

If it is the A/D board I thought it would be a big problem as I doubt you can easily find things like the opamps ?

Regards

Richard
 

Offline m k

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Re: Fluke 8502A - fuse blown
« Reply #23 on: December 03, 2021, 10:21:02 am »
I meant that the schematics is not a problem since it seems to be always the same.

Instruction Manual is good.

I have 4.
Instruction Manual 1978 rev 7/81
Instruction Manual 1981 (AT)
Operating and Service Manual
Operating and Service Manual (Artek)

Original 1978 manual I dont have but not Artek O&S has old Isolator schematics.
Yet that O&S has new general pictures, including Ext Trigger connector and it's not dated.
Artek version is missing all but -01 options but it has single page schematics.
1981 vesion has AT stuff only.
« Last Edit: December 03, 2021, 05:33:50 pm by m k »
Advance-Aneng-Appa-AVO-Data Tech-Fluke-General Radio-H. W. Sullivan-Heathkit-HP-Kaise-Kyoritsu-Leeds & Northrup-Mastech-REO-Simpson-Sinclair-Tektronix-Triplett-YFE
(plus lesser brands from the work shop of the world)
 
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Offline RichardM

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Re: Fluke 8502A - fuse blown
« Reply #24 on: December 03, 2021, 01:47:42 pm »
Success!!!!!

I pulled Q27 and Q9 for the A/D board which are dual n-channel jfets and Q27 tested badly on the curve tracer, basically no curves at all on both sides. It’s late at night but I thought I would just try to replace Q27 with 2 J113 jfets that I had and the meter has come to life. I guess there may be flow on effects but for now I will run the meter until something fails I guess. I will try to run a calibration tomorrow before ai get too excited. I may try to find a data sheet for the old jfet but the markings and manual don’t provide much of an indication of the model and maker.

Thankyou to all who assisted with suggestions etc

Richard
 


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