WTB: Keithley TF-245 Resistor Network - Page 1
Forum section: electronics / general

author: petemate
author_rank: Regular Contributor
timestamp: March 20, 2019, 08:10:04 pm
content: Hi guys, I have a Keithley 2000 DMM with a broken TF-245 resistor network. I would like to buy oneat a reasonable price.The Keithley/Tektronix partner in my country wants the equivalent of 75 USD for the IC, which I think is a bit too steep, considering the age of the multimeter and the fact that a "new"(used) 2000 will cost 200-300 USD. Plus, I have verified that I can fix the issue by just adding a few external resistors. Buying some 0.01% resistors will cost me only a few dollars, and will in theory be just as good as the resistor network. But obviously I'd like the real thing.I am in the EU, but due to the size of the item, I expect that shipping from anywhere in the world won't be a problem.Thanks !

author: coromonadalix
author_rank: Super Contributor
timestamp: March 20, 2019, 08:56:09 pm
content: Personally i would do some substitution with thru hole precision resistors

author: petemate
author_rank: Regular Contributor
timestamp: March 21, 2019, 10:13:22 am
content: Quote from: coromonadalix on March 20, 2019, 08:56:09 pmI agree and I will do just that if I can't get any IC.

author: coromonadalix
author_rank: Super Contributor
timestamp: March 21, 2019, 02:26:08 pm
content: I may say easier said than donecheck the attached photo, i do hope you have 1 or 2 dead resistors ....

author: Kleinstein
author_rank: Super Contributor
timestamp: March 21, 2019, 05:38:57 pm
content: Depending on which resistors (or connection) is bad, one may have to replace more than just one. It't not the absolute value that is important but the resistor ratio and TC matching. The accuracy requirements are also different for the resistors.

author: 2N3055
author_rank: Super Contributor
timestamp: March 21, 2019, 06:50:37 pm
content: Actually 75USD for original new one is good price...It is isothermal precision resistor array that it is going to be hard to make without degrading Keithley 2000 to much worse specs...It can be done but you wouldn't save any money not to mention time.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 21, 2019, 07:09:51 pm
content: The TF245 Array is used for several parts in the circuit:The upper 4 resistors are used to scale the reference voltage for the ADC and the Ohms part.The next 2 resistors are for the fine slope of the ADC - these are the least critical ones.The next 3 resistors (around 30 K) are for the ADC integrator input and set the ADC gain and impedance symmetry.The last 3 resistors set the amplifier gain.So if something is broken one would likely have to replace a full group of 2/3 or 4 resistors, not just a part of it.So the main case where a discrete replacement can work it with the fine slope part - this also used an extra external resistor and is thus not that critical.

author: petemate
author_rank: Regular Contributor
timestamp: March 22, 2019, 01:03:53 am
content: Quote from: Kleinstein on March 21, 2019, 07:09:51 pmYou actually helped me out with this back in September/October.https://www.eevblog.com/forum/repair/keysight-2000-repair-adc-behavior-is-weird-looking-for-ideas-or-suggestions/msg1843094/As was found out, the main issue is that there is no connection from pin 14 to the internal parts of the IC. But the resistors work just fine, since I can measure 45.1768k between pins 3 and 4. So luckily the issue is indeed the fine slope of the ADC, which is why I am somewhat confident in getting reasonable accuracy with external 0.01% resistors. But again, i'd obviously like to get my hands on the real thing.

author: z01z
author_rank: Regular Contributor
timestamp: March 22, 2019, 09:01:00 am
content: 3 years ago it costed about 50 EUR with S&H. I can ask about it now, but seems like you won't be saving much.

author: alex-sh
author_rank: Regular Contributor
timestamp: March 22, 2019, 02:04:24 pm
content: Quote from: petemate on March 20, 2019, 08:10:04 pmI have no idea where you found a used Keithley 2000 for USD 200-300. On the ebay, the cheapest is priced EUR 675.In any case, USD 75 does not sound unreasonable if you want to have in spec DMM like this. Do not go for "in theory" because if it does not work and in most cases it does not, you (1) waisted time (2) waisted money (3) back to squire one.

author: TiN
author_rank: Super Contributor
timestamp: March 22, 2019, 08:12:54 pm
content: Onesold for less than 200$last week.

author: Jens01
author_rank: Contributor
timestamp: March 24, 2019, 07:27:39 pm
content: I am also in need of one TF-245 resistor network. I haven't contacted tektronix for a quote, but maybe we can buy it together to reduce the price a bit?

author: petemate
author_rank: Regular Contributor
timestamp: March 24, 2019, 09:58:01 pm
content: Quote from: Jens01 on March 24, 2019, 07:27:39 pmIf you'd look into that, that would be awesome. Here in Denmark, the price quoted from a local company was around 75 USD for one pcs, and I suspect they jacked up the price a bit. If you will contact Tektronix or their distributor in the Netherlands, that would be great.

author: Jens01
author_rank: Contributor
timestamp: April 03, 2019, 07:10:57 pm
content: Quote from: petemate on March 24, 2019, 09:58:01 pmI've contacted our local distributor but they have not replied yet. I will send you a message as soon as they replied.

author: guenthert
author_rank: Frequent Contributor
timestamp: April 04, 2019, 03:06:01 am
content: Quote from: TiN on March 22, 2019, 08:12:54 pmFor parts or not working“bad,shows overflow adc”I know, that wouldn't stop you, but most of the rest of us.

author: marcumr
author_rank: Contributor
timestamp: April 04, 2019, 01:12:17 pm
content: hello everyone guys.I also need a network of TF-245 resistors, I can participate with you. THANKS.

author: petemate
author_rank: Regular Contributor
timestamp: April 04, 2019, 08:43:25 pm
content: Quote from: marcumr on April 04, 2019, 01:12:17 pmSure! Will you contact a distributor in your country and request a quote?

author: marcumr
author_rank: Contributor
timestamp: April 04, 2019, 10:45:42 pm
content: I contacted a distributor in Italy but the cost is very high, $ 100 + tax + transport.

author: coromonadalix
author_rank: Super Contributor
timestamp: April 04, 2019, 11:14:34 pm
content: they know its getting unobtanium errrrrrrrrr

author: Jens01
author_rank: Contributor
timestamp: April 10, 2019, 05:13:37 pm
content: I've emailed the local distributor a week ago but they haven't replied. As the successor of the K2000 is available for a while, i suspect the price increase is planned to move customers to a upgrade instead of repair