Testing transistors accurate? - Page 1
Forum section: electronics / repair

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: October 01, 2015, 06:41:39 am
content: I am still waiting on a replacement to find out for sure, but I think a particular transistor is bad.However, the transistor tests OK with a multimeter HFE test, and one of those AVR based "transistor tester" units.Is it possible for those tests to pass, and yet the transistor to not be functioning properly? Anyone experience this?Thanks.

author: ez24
author_rank: Super Contributor
timestamp: October 01, 2015, 06:52:20 am
content: How about more info on your tester.

author: mzzj
author_rank: Super Contributor
timestamp: October 01, 2015, 07:36:17 am
content: And how about more information on those transistors and where you use them?Yes, its possible. For example high voltage transistor leaking above 100 volts but works fine on most testers/tests

author: dave_k
author_rank: Frequent Contributor
timestamp: October 01, 2015, 01:05:08 pm
content: That's when something like a curve tracer becomes useful, especially ones like the Tek 576 or 577 which can test with a collector supply voltage up to 400V

author: Stray Electron
author_rank: Super Contributor
timestamp: October 01, 2015, 01:56:35 pm
content: OP, HP and other companies frequently use "matched" or "selected" transistors and other parts in their equipment. It's one reason why they often put their own part numbers on them instead of using the manufacturer's part number. If that is a selected part in your item then you're going to need something a lot more sophisticated than an AVR tester or the Hfe test function on a DVM. Go to E-bay and look at the pictures on some of the auctions for the Tektronix 57x Curve tracers. You'll see that they display multiple curves based on different bias currents vs collector voltages (and that's just ONE of the test that they will do). Other cheaper testers with video displays such as the kit that was sold by Groote, only show a single, generalized characteristic curve of the device under test. Digital meter type testers don't even show you that much, they only display a calculated Hfe based on a single fixed collector voltage and a single bias current.Long story short, digital testers are ok for a quick Go - No Go test for non critical applications. Testers that show a two-dimensional Characteristic Curve are better but still limited. However they are VERY good for simply identifying general types of devices; i.e. resistors vs diodes vs capacitors vs chokes vs zener diodes, etc. BUT to MATCH a device or to get exact values of the various parameters, you need to use a curve tracer such as the Tektronix 57x series, that lets you control ALL of the test conditions and is calibrated (more or less) and can display accurate results.There actually was a pretty good discussion about curve traces on this forum several years ago.https://www.eevblog.com/forum/beginners/dirt-cheap-and-simple-scope-based-component-tester-curve-tracer/45/

author: krivx
author_rank: Frequent Contributor
timestamp: October 01, 2015, 02:28:57 pm
content: Quote from: Stray Electron on October 01, 2015, 01:56:35 pmI don't know if this is true. To match any parameter just requires the ability to measure those parameters under similar conditions. If you want to match Vbe of some transistors you just need a test circuit similar to the final application and a voltmeter. If you need matched temperature coefficients you also need a thermometer and a source of heat. The value of a curve tracer is not having to design or build a test fixture every time, you have a configurable system. Considering that a curve tracer is a system of programmable voltage/current sources and voltage/current meters I don't know of any test that absolutely requires one.Tbh, I think the high prices curve tracers command is due to them becoming niche instruments that are no longer widely manufactured. You can get a new SMU with better specs for the some of the used prices on Tek 57x series.

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: October 01, 2015, 03:35:30 pm
content: The transistor is an RCA 40250 from a Fluke 8100B Nixie tube DMM. It is the primary transistor used in the -18v power supply regulation. I have tested and swapped the other transistors, as well as measured the resistors and capacitors in the -18v supply. Things seem to be pointing at this transistor.The "Transistor Tester" is one of those AVR based testers like this :http://www.ebay.com/itm/GM328-transistor-tester-ESR-table-LCR-frequency-meter-square-wave-genera-/271821270337?hash=item3f49cf6d41The HFE tester is an old Micronta DMM.I have some replacement transistors coming-- 2N4232 since I cannot find the original.Thanks.

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: October 01, 2015, 05:31:53 pm
content: I have seen Alan's video on Curve Tracers. Good stuff. But don't you need a good component to compare it to (or a datasheet that has the trace)?

author: Paul Moir
author_rank: Frequent Contributor
timestamp: October 01, 2015, 05:59:52 pm
content: I have a carcass of one of these multimeters if you need any more parts. I have that transistor in it.Just paid too much for an 8120A out of nostalgia for the 8100B, which broke 25 years ago and I didn't have the ability to fix then. Not much left of it though: circuit board, some of the frame.EDIT: Just remembered, a couple years before it had the fault I could not fix, it lost it's power supply due to a failure of Q6 - a little TO-92 near that RCA part which I replaced with a 2n3904 or 6. How come I can remember that and not names?EDIT2: I have a Micronta 22-174B I can plug it into if you want a comparison hFE.

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: October 01, 2015, 06:38:57 pm
content: Quote from: Paul Moir on October 01, 2015, 05:59:52 pmThanks. I did just find some of the proper transistor on eBay.Quote from: Paul Moir on October 01, 2015, 05:59:52 pmDoes any of the power supply work? If this transistor doesn't fix it I will be looking for somebody else with a working 8100 who can do some measurements.Quote from: Paul Moir on October 01, 2015, 05:59:52 pmYes, Q6 is a 2N3906! I think that is part of the 15v rail. I isolated the -18v with the procedure in the service manual, and am still having problems. I tried replacing Q2 and Q3.Quote from: Paul Moir on October 01, 2015, 05:59:52 pmThat would be quite helpful actually. If you have a chance please let me know. That is actually the same meter I am using. I have a Fluke 87V and a bunch of bench meters, and some analog meters. But only the Micronta has the hFE. It's actually a decent meter, save for the slow autoranging and likely "CAT Negative One" ratingIf this turns into a repair thread I will start a new thread. Would like to post some pictures anyway.Thanks.

author: Paul Moir
author_rank: Frequent Contributor
timestamp: October 01, 2015, 07:10:32 pm
content: Yeah, can you imagine 1000v through those wires, talk about irresponsible. Relegate it to the bench: I rushed out to help my brother in law with his water heater and only had this meter handy. Well I blew out the current source checking the elements. But volts, current and hFE work fine still which is why it's around still. Love the thing still, even though you gotta take out a mortgage to change the batteries... OK desoldering iron should be warmed up.nPn hFE=61.8EDIT: We'll have to see about it's power supply. It hasn't been started in 25 years and is missing a *lot* from the forward end. I raided the nixies years ago, and the ASICs are gone (which is what I thought at the time had failed). Nixie transformer is awol and the switches have been removed. Still have the switch caps though!

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: October 01, 2015, 08:09:42 pm
content: Quote from: Paul Moir on October 01, 2015, 07:10:32 pmIt does take a lot of batteries, but man they last a while. I don't think it has an auto-shutoff either. I have left it on overnight a number of times. Mine still works, though I was always blowing the fuse measuring voltage with the meter set to current! I think I even did that with 110AC a couple of times, and the thing still survived. It was my first digital meter ever, which I bought new 25 years ago or so.Quote from: Paul Moir on October 01, 2015, 07:10:32 pmInteresting... Mine reads hFE = 33 on the same brand meter. I suppose one or the other meter could be out of cal by now. But I think I "misunderestimated" what is "good" when I read that. Hopefully a replacement transistor will fix this guy. Now if I could only get the ePray seller to respond.QuoteI may hit you up for parts if I ever get further than the power supply. Thanks for your help.