Tek 2445B Bandwidth mod - Page 1
Forum section: electronics / testgear

author: andy2000
author_rank: Frequent Contributor
timestamp: October 09, 2016, 04:02:07 pm
content: I've heard about the person who was modifying 2445s and passing them off as 2465s. Apparently there are some bandwidth limiting circuit board traces on the delay line output which can be bypassed to increase the bandwidth the be the same as a 2465.I'm not planning to scam anyone, but I did recently acquired a cheap fixer upper 2445B that I wouldn't mind experimenting on. Has anyone here either modified a 2445B, or used one that had been modified? How does it compare to a real 2465B?Assuming it's worth trying, I'd like to see some more details before I start cutting traces. Even some close up pictures of the front and back of a real 2465B A1 board in the area of the delay line output would help me decide what to try. I do have a genuine 2465 (original version) A1 board as a reference, but I don't know how closely it matches the 2465B.

author: BFX
author_rank: Frequent Contributor
timestamp: October 09, 2016, 09:32:09 pm
content: Hi here is some poor info about ithttps://forum.tek.com/viewtopic.php?t=136149

author: David Hess
author_rank: Super Contributor
timestamp: October 10, 2016, 01:30:27 am
content: The filter built into the circuit board does more than limit the bandwidth; in the 300/350/400 MHz 2465/2465A/2465B, it compensates the amplitude and phase response of the delay line so if it is removed from the 150 MHz 2445/2445A/2445B, you get high bandwidth and messed up amplitude and phase delay. The 300 to 400 MHz oscilloscopes are fast enough however that this is difficult to test or verify without a fast reference level pulse generator like a tunnel diode pulser.

author: andy2000
author_rank: Frequent Contributor
timestamp: October 10, 2016, 03:57:21 pm
content: Quote from: David Hess on October 10, 2016, 01:30:27 amThat post from the Tek forum is all I have been able to find.I have read some comments about the filter, but it was a few years ago, and I haven't been able to find it again. Obviously, looking at the board for a 2465, the delay line isn't just connected straight through. The traces which form capacitor plates on the 2465 are smaller, but still there. The 2445B has a pair of large spiral traces that are virtually absent from the 2465.I guess I could try to duplicate the 2465B's filter with discrete components.I guess my real question is how bad are the problems? Considering how many modified 2445's there must be out there, I'm surprised more people haven't noticed.I may just have to try it for myself and see, but without a real 2465B as a reference, it won't be conclusive.

author: woodchips
author_rank: Frequent Contributor
timestamp: October 10, 2016, 05:45:56 pm
content: Go on Tekscopes forum, this was discussed quite a few years ago, 5 or more, but when I looked I couldn't find it. More regular visitors might know more.There is also a link to change on the timebase chip to get the faster sweep speeds.My memory is that all the scopes are the same, apart from the lp filter components. This might have been true with the original, non-suffix letter, scopes but not with the newer ones. The 2455 seemed to disappear after a few years. They all use the same CRT, even the 2430 and 2440.

author: David Hess
author_rank: Super Contributor
timestamp: October 10, 2016, 11:32:23 pm
content: Quote from: andy2000 on October 10, 2016, 03:57:21 pmThat would be a major undertaking but certainly possible. Take a look at the delay line compensation circuit in oscilloscopes like the 7904 which is fully documented.QuoteMost users do not notice it because they either lack a fast enough signal source to reveal the issue or they are using high impedance passive probes which will load the circuit under test enough to cause more of a problem anyway.QuoteThe only signal source you need to verify the performance is a 100 picosecond reference level pulse generator.Quote from: woodchips on October 10, 2016, 05:45:56 pmI participated in that discussion or at least the follow up ones.QuoteI suspect Tektronix may have graded the hybrids in the vertical amplifier chain for the faster models.The 150 MHz 2445/A/B was discontinued after 1994 while they were all discontinued after 1996.The 250 MHz 2455A/B was discontinued after 1990 and there was no 2455 at the beginning.There was no 2467A. The 2445A, 2455A, 2465A, and 2467 were contemporaries. The 2445B, 2455B, 2465B, and 2467B were contemporaries.There are more changes than just the delay line compensation network and sweep configuration between the 2445/A/B and 2465/A/B.

author: andy2000
author_rank: Frequent Contributor
timestamp: October 11, 2016, 03:11:00 pm
content: I found the big discussion about this on the Tekscopes Yahoo group. One post has a list of other differences (I haven't verified it)."Actually, there are a handful of differences between a 2445B anda real 2465B:1) a jumper that tells the A5 board to enable the fastest sweep speed.2) PCB T-coil that limits frequency vs T-coil that enhances risetime.3) L403 is missing, and circuit around L403 is slightly different.4) C403,C404 are 2-10pf trimmers should be 1-5pf trimmers.5) R410 is a 1K trimmer, should be a 250 ohm trimmer.6) 5K pot, R417, HF ADJ, is missing from Channel Switch U400.7) 5K pot, R802. TRANS RESP, is missing from U800.R659 is 332 ohm, should be 1.1K, with 7.68K to -15V on VERT CTR pot.9) R4335, 5K trimmer on Z-axis transient response (board A9)."

author: femtohertz15
author_rank: Contributor
timestamp: October 13, 2016, 01:05:24 am
content: I'd say true T-coils are not implemented to limit frequency response. They are a peaking network to and extract a little more bandwidth when pushing transistors to the edge of their frequency response yet keep as flat a response as possible. The 465 I have is lousy with them.Notice the schematics for both the 2445 and 2465 are still easily found on w140 and BAMA and other places. Decide for yourself! I also see numerous tiny differences in the emitter peaking network in the final vertical amp. The 2465 delay line comp is not just different values, it is more elaborate, and differential instead of two single ended circuits.I bet besides changing out the filters and trying to recalibrate, to turn a 2445 into a 2465 and keep a good step response would involve swapping out higher graded ICs in some or all of the vertical chain. Where to get them? A 2465.

author: andy2000
author_rank: Frequent Contributor
timestamp: October 13, 2016, 03:32:06 am
content: Quote from: femtohertz15 on October 13, 2016, 01:05:24 amI haven't been able to find a schematic of the 2445B. Anyone know where I can find it?

author: David Hess
author_rank: Super Contributor
timestamp: October 13, 2016, 04:05:28 am
content: Quote from: andy2000 on October 13, 2016, 03:32:06 amWell, this is embarrassing. I recently acquired a 2445B and assumed that I had the service manual but the copy I have lacks schematics. A full scan of the 2445B service manual can be had from the usually sources for a low price.Even if they are not identical, comparing the 2445A and 2465A will give you some idea of the difference.

author: edavid
author_rank: Super Contributor
timestamp: October 13, 2016, 04:14:55 am
content: Quote from: femtohertz15 on October 13, 2016, 01:05:24 amI bet you're wrong.

author: tautech
author_rank: Super Contributor
timestamp: October 13, 2016, 05:12:38 am
content: Quote from: David Hess on October 13, 2016, 04:05:28 amFrom this thread:https://www.eevblog.com/forum/testgear/looking-for-tek-2445b-schematics/This evidently is it: "070-6873-00_2465bSvc_Sep89.pdf"Google for it.

author: David Hess
author_rank: Super Contributor
timestamp: October 13, 2016, 05:21:03 am
content: Quote from: tautech on October 13, 2016, 05:12:38 amI have that one and it is only for the 2465B and 2467B.