Tektronix dm-44 multimeter option with gigaohm input impedance? - Page 1
Forum section: electronics / general

author: thomastheo
author_rank: Regular Contributor
timestamp: June 24, 2016, 09:33:13 pm
content: Hey all,I just repaired my tek475 with dm44 option, with an eye towards selling it in exchange for a more modern scope, and in the documentation for the multimeter I came across an option to remove a bridge wire in order to raise the input impedance of the volt meter from a sane 10 meg to 1 gig! It only works for the low ranges, but I am confused, is there a use case for this that warranted the inclusion of this option?On a side note, holy hell these machines are built well. Every transistor in a socket, every single part is the highest quality available for the time. They're an absolute dream to work on. It's getting tougher to sell it by the minute...

author: Gyro
author_rank: Super Contributor
timestamp: June 24, 2016, 09:40:35 pm
content: It will have been included as an option to minimise loading when measuring high impedance circuits. I suspect that the reason it is an option is that in 1G mode, the meter reading will float when the probe is removed rather than settling back to zero. This characteristic is shared with high-end meters that can have input resistances of 10G on lower voltage ranges.Maybe try it and see how you get on.

author: tggzzz
author_rank: Super Contributor
timestamp: June 24, 2016, 10:04:26 pm
content: Quote from: thomastheo on June 24, 2016, 09:33:13 pmUnless, of course, you have to remove a complete board in order to, say, replace large electrolytics.QuoteI know what you mean. I have a 465, a 475A+DM44, a 485, and an HP1740A. Two have to go, but which two?Not the 485 since it has a great screen, 400MHz-ish, a 1ns calibrator, and is easier to repair than the 465/475. The HP1740A is nice, and is easy to repair. I suspect I'll offload the 465 and 475 because they should fetch good prices. Now all I have to do is understand exactly how to pack them for shipping.Does anyone have a guideline selling price for a working 475A+DM44; ebay doesn't.

author: thomastheo
author_rank: Regular Contributor
timestamp: June 24, 2016, 11:50:11 pm
content: Quote from: tggzzz on June 24, 2016, 10:04:26 pmI'm wondering about that myself. Over here in the netherlands it seems a euro per MHz is somewhat of a guideline for working dual trace analog scopes, but honestly I'm not comfortable asking 200e just for this scope, even if everything works a song. I do have a front cover, one TEK probe and even an original P6430 temperature probe to go with that dm44 though, together with original paper manual/service manual. I'm aiming for 200-225e with everything included. Maybe i'm delusional :p

author: tggzzz
author_rank: Super Contributor
timestamp: June 25, 2016, 12:01:28 am
content: Quote from: thomastheo on June 24, 2016, 11:50:11 pmI've used £1/MHz for low-end uninteresting scopes, but I don't know whether I should now revise that up or downProbably down for "collection only", and up for "exports".Googling has now lead me to two 475+DM44 that sold for 280-290e in February.QuoteNobody has to buy it unless they want it. The corollory is that if they buy it for 200e, then they are happy. Choose your time, and let other people decide what it is worth to them.QuoteMaybe you are unnecessarily thoughtful and caring

author: thomastheo
author_rank: Regular Contributor
timestamp: June 25, 2016, 12:29:16 am
content: I've done a lot of trading over the years, but among electronics hobbyists i've always been treated more fairly than in most every other area, so just looking to pay it forward I suppose.

author: tggzzz
author_rank: Super Contributor
timestamp: June 25, 2016, 05:53:26 am
content: Quote from: thomastheo on June 25, 2016, 12:29:16 amAnd I hope and expect that would continue to be true in both directions. Keeping your self-respect is averyimportant consideration.But if you are ruthlessly honest about what you know about the scope, then I wouldn't see any problem.When I sell mine I will show pictures of it operating, e.g. risetime, delayed traces, A+B operation. I will also note what I have done to the scope e.g. replaced capacitors/CRT/controls, added a heatsink to a bridge rectifier, remarked legends etc. I won't make any guarantees about it being calibrated, even though looking at the traces should make it obvious that I've twiddled most of the internal adjustments.I would regard that as fair.