Tektronix CMC251 Frequency Counter - Page 1
Forum section: electronics / testgear

author: xrunner
author_rank: Super Contributor
timestamp: April 15, 2015, 07:36:34 pm
content: Just got this in - a Tek CMC251 Freq. Counter, 1 HZ - 1.3 GHz. Listed on Ebay for parts or not working, but it seems to work perfectly. It's displaying my 10 MHz frequency ref right now. Another score, this one for $70. Will open it up and post some pics later.

author: dom0
author_rank: Super Contributor
timestamp: April 15, 2015, 08:30:52 pm
content: When did Tek crank out all this rather low-end gear like this counter or the CDM250 and so on?

author: rdl
author_rank: Super Contributor
timestamp: April 15, 2015, 08:32:06 pm
content: If that's from the lot of many (8 was it?), I almost bought one also. I backed out at the last minute because there was no detailed photo of the actual unit I would have been sent. I was was burned that way once, five of an item available but photos only of one, and of course it was the one in the best condition. The one I received was considerably worse looking.Anyway, yours looks good. I'm looking forward to see what's inside, and $70 was a fair price. Most often those C series Tektronix devices are way overpriced, I guess because of the Tek name. I got a CPS250 not long ago for $99, which isn't bad I guess.

author: rdl
author_rank: Super Contributor
timestamp: April 15, 2015, 09:29:07 pm
content: Quote from: dom0 on April 15, 2015, 08:30:52 pmAround early/mid 90s, but I don't know when they first became available or when they were discontinued. BK Precision also sold their own versions of many of these "Basic Bench Test Instruments", as Tektronix called them. In the pdf copy of the 1993 Tektronix catalog that I have, the standard 1.3 GHz CMC250 list price was $495, the CMC251 added a "High Stability Time Base" for an additional $50.

author: xrunner
author_rank: Super Contributor
timestamp: April 15, 2015, 09:31:49 pm
content: Quote from: dom0 on April 15, 2015, 08:30:52 pmSometime in the '90s. I see chip date codes in this unit of '97 - '98.They had a line using the same enclosure style. I've got several of the DMMs (CDM250). Gave one away in the beginner's contest. Then there's this Freq. counter (CMC251). I've also seen a function generator, and there may be more things as well.Quote from: rdl on April 15, 2015, 08:32:06 pmYea that's the lot. They were listed a while back for $100 a unit, but I wrote the guy and said that was a lot to ask for if it's listed "for parts or not working". He took them down and told me he would re-list them later for less, and he did. I took a chance on them because I also got two working CDM250s listed the same way for dirt cheap, and they both worked just fine, so I thought this was worth a chance.QuoteYea I've seen this counter listed for well over $200. Let me get the case off and we'll take a look.As I was typing this -Quote from: rdl on April 15, 2015, 09:29:07 pm^^^ Thanks for the info. That concurs with the chip date codes.

author: xrunner
author_rank: Super Contributor
timestamp: April 15, 2015, 10:36:56 pm
content: Board top view -Most of the chips are 74HC logic and GALs (I think GAL1 is under the low frequency can) -80 MHz - 1.3 GHz input. The A06 part is a DC - 2 GHz amp -http://www.datasheetarchive.com/dl/Datasheet-077/DSAE0043177.pdfTCXO and a ICM7226A, an 8-Digit, Multi-Function, Frequency Counter/Timerhttp://www.intersil.com/content/dam/Intersil/documents/icm7/icm7226a-b.pdfRed LEDs, but encased in yellow plastic

author: Unknown
author_rank: Unknown
timestamp: April 16, 2015, 12:00:45 am
content: Quote from: rdl on April 15, 2015, 09:29:07 pmThey popped up under a few brand names (e.g. Tek, BK Precision, Iso-Tech, Escort), with minor feature differences between brands & comparable models. As far as I know all of them were were OEM'd by Escort - they certainly look like Escort, inside & out.Most of them are actually pretty good basic test gear, although the prices asked on eBay do tend towards the stupid. I've owned & used a few in the past - mostly grabbed from the bin at work after a cleanout - but I've still got & use a CDC250 I picked up for cheap.

author: xrunner
author_rank: Super Contributor
timestamp: April 16, 2015, 12:28:07 am
content: I've got an off-the-shelf 10 MHz TCXO module I zero beat to WWV 10 MHz using my ham rig to listen to the sigs, and I got that module adjusted to better than 1 Hz accuracy, and I calibrated my HP 5315 to that. Right now the Tek unit is reading that reference about 6 Hz high. The CMC251 has been on for a couple of hours so I reckon it could use a touch-up but that ain't bad for something that has no cal stickers on it anyway.The Ebay seller that sold these and the CDM 250s seems to have a line on stuff like that, and they must be getting them as donations because the seller is Goodwill - "All proceeds go to charity". I think they are getting them from a tech school or the like.

author: xrunner
author_rank: Super Contributor
timestamp: April 16, 2015, 12:10:48 pm
content: Here's one for $300 (or best offer). Good luck on the $300 ...http://www.ebay.com/itm/Tektronix-CMC251-1-3-GHz-Multifunction-Counter-/321268371618?pt=LH_DefaultDomain_0&hash=item4acd1650a2Escort counter -

author: Unknown
author_rank: Unknown
timestamp: April 16, 2015, 01:18:15 pm
content: Quote from: xrunner on April 16, 2015, 12:10:48 pmTell 'im he's dreamin'!IIRC the EFC 3303 is the Escort version of the Tek CMC251 - it had a TCXO, while the EFC 3203 in your pic only has a 10ppm crystal. The EFC 3203A, which Google tells me went up to 2.4GHz, certainly only had a standard xtal.

author: el.designer2012
author_rank: Newbie
timestamp: May 14, 2024, 07:04:46 pm
content: Hi, my dear friends! Can anybody help to find the service manual or schematic of this counter?

author: xrunner
author_rank: Super Contributor
timestamp: May 14, 2024, 07:19:21 pm
content: Attached ...

author: el.designer2012
author_rank: Newbie
timestamp: May 14, 2024, 07:37:35 pm
content: Thank you, my friend! But it is user manual without schematic. The service manual contains the schematic diagramm...

author: el.designer2012
author_rank: Newbie
timestamp: May 14, 2024, 07:40:57 pm
content: I have the CMC250 service manual and it is not the same, it has no tcxo but use the same ICM7226.