8502a worth fixing? - Page 1
Forum section: electronics / testgear

author: whitevamp
author_rank: Regular Contributor
timestamp: May 31, 2016, 05:32:03 am
content: ok i have a 8502a and i am trying to figure out if it's worth trying to fix it, the dc auto rang quit working and when in manual mode the readings are way off.except when i apply 5v to it reads correct, but nothing else.and then the ohms card setting are all over the place, no i never did replace that bad resistor yet. ( i did pull that resistor out and put in a temp resistor.) but i suspect more then just that resistor is bad. its now when i have that board plugged in the reading will go verry wild and then hit HHHHHH across the read out and buy this time you will start hearing a whistling sound, and the auto ranging silinoid you can hear clicking away. ( could be what took out the auto range function, and this is before i noticed that the auto rang quit, and dc volts way off.)and also one of the ic's get really hot.so is it worth fixing or should i part it out on ebay and sell the manual and use the $$ to get a different meter like the Fluke 8840a , or a HP 3478A or something else.and if i was to fix it im not too sure where to start.

author: Vgkid
author_rank: Super Contributor
timestamp: May 31, 2016, 05:45:04 am
content: Yes, since these meters are modular pull every card that is not needed. Also check the contacts on the card connectors.

author: dacman
author_rank: Frequent Contributor
timestamp: June 01, 2016, 12:46:24 am
content: If DCV doesn't work, then usually, nothing else will. I'd pull the other functions out and then fix DCV. The 8502A is not an easy meter to work on. Whether or not it is worth fixing depends on your circumstances. If time is money, then I'd look for another meter.

author: whitevamp
author_rank: Regular Contributor
timestamp: June 01, 2016, 03:15:47 am
content: Quote from: dacman on June 01, 2016, 12:46:24 amNop time is not money. It gives me something to do.From looking at trouble shooting list i think for auto range i need to replace the relay but parts thats a different story.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 01, 2016, 04:31:34 pm
content: Looks like a service manual is available - so a repair might be possible. One should start with the manual DC volts ranges, starting with the 10 V range.I won't expect different relays used in auto-ranging mode. So if manual ranges work, but autoscaling not, this could be a problem with the controlling part, maybe an old EPROM.

author: whitevamp
author_rank: Regular Contributor
timestamp: June 03, 2016, 02:01:24 am
content: Quote from: Kleinstein on June 01, 2016, 04:31:34 pmGot the schmatics. Part of the manual i have.Ok ill ltear into it more and see what i can figure out.Ohh and only 5vdc works i increase or decrease voltage and it dont change. Manual range mode.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 03, 2016, 12:49:57 pm
content: If it's only working at 5 V, it's a little like a watch that is not moving - it's still correct twice a day. So it's a kind of coincidence that it show's 5 V.There are mainly 3 parts for the DC measurement: range switching, input amplifier and the ADC. The first two are usually relatively easy to check if you have the schematics.

author: whitevamp
author_rank: Regular Contributor
timestamp: June 03, 2016, 04:42:22 pm
content: Quote from: Kleinstein on June 03, 2016, 12:49:57 pmOk will look more at those 3 items.

author: Vgkid
author_rank: Super Contributor
timestamp: June 03, 2016, 09:37:19 pm
content: I would work backwards from the adc forwards. Seperate the adc input from the input / ranges. Apply a small signal(1v, and vary it) see if you can get an output. If that works then proceed to the range/ input circuits. If not then, that is where you start.

author: whitevamp
author_rank: Regular Contributor
timestamp: June 06, 2016, 02:52:05 am
content: After some testing i narrowed it down to the dc conditioning card. I can get my in put voltage to appear on a dmm on pin 15 of the K1 relay ( R40-E0186-2 ) but not on display of the 8502.i did a search for that relay and its a fluke # i did find a pic of ohms card that has DS4E-ML2-DC5V on it my ohms card and dc card have same relays.So am i thinking correctly that the relay is bad? Or did i chase a rabbit down the hole?Now if it is that relay thats bad the DS4E one is also discontinued.

author: Vgkid
author_rank: Super Contributor
timestamp: June 06, 2016, 03:13:06 am
content: Is the relay socketed i the board, or is it soldered. If socketed, the socket could have gone bad(it happens). Have you tried checking the relsy out of circuit. Its driver could have gone bad, or the componentry that controls it.

author: whitevamp
author_rank: Regular Contributor
timestamp: June 06, 2016, 04:52:52 am
content: Quote from: Vgkid on June 06, 2016, 03:13:06 amSoldered. And nothing on the out put side.Nop havent tryed to desolder the 15 pin monster yet.

author: whitevamp
author_rank: Regular Contributor
timestamp: June 07, 2016, 04:36:29 am
content: So i gingerly desolder that 18pin monster and tested it tested out fine, applyed 5vdc to it and it switch just like it should also tested continutue (sp)all good.Had an isue resoldering that thing back on, its not copper traces its like exposed solder traces, and i messed a couple pin traces upso that leaves the controle cercit at fault i think. But im not sure what/ where that is (ic U2 ?) Yes i know i have the schmatic but im learning.