Keithley TF-245 Replacement Options? (K2000 models) - Page 1
Forum section: electronics / repair

author: nidlaX
author_rank: Frequent Contributor
timestamp: October 12, 2015, 03:01:53 am
content: I'm in the process of repairing a Keithley 2000, 94/95 vintage, and I suspect that the TF-245 resistor network might be out of spec. This is preemptive, but I am wondering what solutions people have come up with to replace said resistor network? I heard fromthis threadthat newer 2000 revisions use a different circuit instead of TF-245, but page 2 of that thread is still broken.

author: TiN
author_rank: Super Contributor
timestamp: October 12, 2015, 04:34:23 am
content: I measured my TF245 before,here are results, also with other owner's measurement data.You could replace broken resistors with stable standalone resistors, or perhaps order new TF245 from Keithley or resellers..

author: nidlaX
author_rank: Frequent Contributor
timestamp: October 12, 2015, 05:08:41 am
content: Quote from: TiN on October 12, 2015, 04:34:23 amI saw that TiN, thanks so much for all of your documentation! It's been one of my best resources for this repair, especially with the references to other people's repair logs.Do you know what parts the newer K2000s use for the resistor network? I see some blurry pictures on 38hot, but nothing with part numbers or a circuit.

author: TiN
author_rank: Super Contributor
timestamp: October 12, 2015, 10:00:40 am
content: Could not be possible without people contributing, I cannot have all the meters in the worldNewer 2000's usingBI Model 668 5K array, Tracking TCR 5ppm/K and few other foil networks:Photo from 38hot BBSYou can try bodge fixed resistors on top of TF instead of broken ones to see if that fixes issues, and if it is, then you can think of investing more money into more stable resistors.

author: nidlaX
author_rank: Frequent Contributor
timestamp: October 14, 2015, 11:32:42 am
content: Quote from: TiN on October 12, 2015, 10:00:40 amThanks. I actually see a lot of random suppliers who list a 16 pin TF-245 part for sale in large quantities.

author: TiN
author_rank: Super Contributor
timestamp: October 14, 2015, 11:35:12 am
content: Most I think are fake nonstock , taken from google search requests :-)

author: nidlaX
author_rank: Frequent Contributor
timestamp: October 17, 2015, 03:07:48 am
content: I put 6.4K in parallel with 9-6 1K (which measures ~1.19K in circuit) and my 304.1 and 200.2 passed, so I suspect TF-245 will need replacement.Next step is debugging amps circuit and 100.1 / 100.2. I'm not sure if those are related, but I'll start with the dead amps signal and hope for the best.

author: nidlaX
author_rank: Frequent Contributor
timestamp: October 19, 2015, 10:29:51 am
content: Does anyone have any good leads on sourcing these TF-245s? costas' original supplier no longer has them in stock, and the prices I'm getting from other sources are a bit exorbitant. It just leaves a sour taste in my mouth knowing that I'm installing an out of production part with a high failure rate.The other option I'm considering is making a small adapter board for extant resistor networks to solder in place of TF-245, but I'm worried that performance will suffer.Update: Pricing fromKeithleyTektronix USA is reasonable, $17 for the TF-245 with a $30 minimum order. Not sure if these are new (probably not) or old stock, but as of today they had around 20 units left.

author: pigrew
author_rank: Frequent Contributor
timestamp: January 07, 2019, 03:46:11 pm
content: Prices have gone up to $20 each, with a 13 week lead time if not in stock. I'm about to order two (to meet the minimum order).

author: Robert
author_rank: Newbie
timestamp: June 22, 2021, 06:24:40 am
content: Hello,I'm looking for TF-245 replacement too.Did any of you have one left over or a contact to Tektronix/Keithley? I didn't get a response to my request.Has someone already built a replacement circuit using single resistors? What do you think? Are 0.05% 10PPM Resistors good enough to match the unknown spec of the TF-245? How ciritical is the ratio between them? Is that something a new calibration would compensate?Best regradsRobert

author: Kleinstein
author_rank: Super Contributor
timestamp: June 22, 2021, 08:19:07 am
content: 0.05% ration accuracy should be well good enough. A new calibration should be able to compensate for most ratios. There may be a few ratios that may not be fully compensated with a calibration. This are those effecting the slow slope of the MS ADC and to a small part the symmetry of the pos and neg reference. The slow slope is at 1/128 the normal reference and thus a special ratio, so there may be no correction. Anyway it uses an external resistor too, so this part should not be that critical and even 0.5 % may be OK. The symmetry also enters a little and not sure if there is a correction - if not used for the slow slope, I would not expect if the the symmetry either. The importance / required accuracy should be about comparable to the slow slope part.The amplifier gain part is very likely fully corrected - so no extreme needs there.The absolute gain for the reference is definitely corrected / part of the calibration, but the resistors are together with the symmetry anyway.From what I have heard the failure may effect only some resistors (e.g. failure of bonds). So one may get away with only a partial replacement.For the TC, it is not so much about the absolute TC, but the TC matching. The TC matching in the array can be very good (e.g. 1 ppm/K range).For separate resistors it can help to use with resistors from the same series/construction, as there is a good chance that the square part of the TC is relatively fixed and mainly the linear part varies. So the matching between 2 resistors at 5 ppm/K has a chance to be better than 5 ppm/K.It also help to get the best TC grade, as the lower grades may have the best ones selected to go to the better grades.The resistor for the integrator input can effect the linearity. So this resistor should be low TC (preferably better than 10 ppm/K) for good performance. With not so good thermal couling here the abloute TC can matter, not just the relative. Many Keithley DMMs use rather frequent measurement of the ADC gain in the background and thus may not need very good TC matching for the other resistors. However I don't know for the K2000/K2015.

author: Robert
author_rank: Newbie
timestamp: June 22, 2021, 09:43:54 am
content: Thank you Kleinstein for your detailed response!As far as I figured out, only the 27.42K resistor between pin 10 and 12 failed, see also my more detailed description here:https://www.eevblog.com/forum/testgear/keithley-2000/msg3590281/#msg3590281As easy patch for the moment I might try to remove R300 (in series with the failed one) and use a repacement with same overall resistance from pin 8 to 12 of R271.Does anybody know anything about the TC of R300? Looks as an quiet usual 1206 1% for me. The repair manual only states:RES, 2.15K, 1%, 125MW THIN FILM (1206)Such parts usually have a TC from 25 to 100 ppm or even more, so how could the TC and absolute value of the 27.42K be critical then?As long as I don't have a reasonable replacement or fix I don't like to spent the money for a real calibration, but I can check the meter against two other 6.5 digit DMMs I have access to.Since U163 is able to connect the reference to the ADC input (this is also done in test 101.2 which failes in my case) I would say it is likely that the Keithley 2000 do ADC gain measurements at least when powering up or maybe in intervalls.Best regardsRobert

author: Kleinstein
author_rank: Super Contributor
timestamp: June 22, 2021, 09:58:59 am
content: R300 and the 27.4 K set the ADC gain - not more. The exact value should not be really critical (some -2% to +10% should be OK).My personaly fovorite would be using something like 10+20 K from the PTF65 series with 5 ppm/K. The large form factor would help a little reducing the self heating effect.The thin film resistors are usually in the 25 ppm/K class, rarely worse. As the values for R300 is relatively small, It would not matter that much. It is still a little odd to have the extra resistor. It may be something to trim the full scale range - possibly after designing / ordering the TF245 arrays.