Tektronix DM44 + 465B time measurement repair

[Warsp]

Hi,
The DM44 connected to my 465b does not work in the time or 1/time modes, and does not show the ∆Time cursor. I have found that the voltage from the buffer for the ∆Time potentiometer (U3275) only rises to around 1.9 volts and stops rising after that point. I have fixed the issue of the voltage not rising and the missing cursor by removing p3071, a 100Ω resistor to ground, but this does not fix the measurements being completely off and the time convertor circuit maxing out the A/D converter. I have also tried removing connections to the delay pick off selector and the function selector, but the issue still persists. I have also swapped op amp U3275 and measured the voltage from U3256, and I believe both are working properly. The entire scope has recently been recapped so the power supplies are fine. If anyone knows what may be causing this issue, please share. Thanks!

https://w140.com/tekwiki/images/a/a8/070-2036-00.pdf p124

[Chris56000]

First of all, have you got a COMPLETE 465B manual?

You can download one that has been corrected with all the missing diagrams (4, 6 and 10) plus associated tables and waveforms from my Dropbox here :–

https://www.dropbox.com/scl/fi/vunh6xwlmaoz3navx8sz1/465B-Svc.-Complete-OCR.pdf?rlkey=68dcvpz6y854tez3lq6535mhu&dl=0

. . .The next step to take is to follow through the Troubleshooting Chart in page 137 of the DM44 service manual, 070–2036–00 – this has specific tests and setup checks for a defective "Time" Function!

. . .I presume you're referring to the 100 Ω resistor R3073 that is switched between the input common and chassis 0V when the "Temp" button is pressed? – this should not be in circuit when any of the other D.M.M. functions are selected!

. . .Also, is your DM44 PCB correctly setup.to match your 465B Oscilloscope?

. . .There are various component, diode and link changes in the power supply/voltage divider circuits on diagram 3 to suit different Oscilloscope Models – this is because the oscilloscopes's main H.T. supply voltage differs between them!

. . . Altho' only instructions for the "465" are quoted, you can use the same instructions for the "465B" as the 465 and 465B both use the same +55 V H.T. supply!

. . .I would pay special attention to :–

U3211, U3213, U3223, U3256 and U3282, these are all common linear, CMOS and TTL "vanilla" ICs that one of those Chinese Digital IC Testers can test for you (they have unfortunately been rising in price alarmingly over the past two or three years!), and if you don't have one, it might well be cheaper to bung the lot in the bin rather than buy a Digital IC Tester new!

. . .If you're still stuck, check the "ALT SYNC" switching pulse from the main 465B on P3201/J3201 pin 1, the "ALT" pulse itself on P3215/J3215 and the "CH 1" signal on P3201/J3201 pin 2 is being switched OK by the main Oscilloscope!

. . .I would also make a careful test of all the function switch contacts on switch board A4 in case you've got a yakky one – Diagram 1 in particular has a great many of them all associated with the common input return on P71 pin7/P3071 pin 5, any of which could be causing trouble!

. . .You also need to run through the 465B Performance Checks specified for use with DM44–equipped models as both the "Time" and "1/Time" functions of the DM44 unit are dependent on the 465B operating correctly!

. . .This really is a fault where you have to follow BOTH manuals carefully, firstly to make sure your main Oscilloscope is working correctly, then apply the sections applicable to the 465B + DM44 and work through all the appropriate fault-finding charts as necessary!

. . .If the worst comes to the worst, download the DM44 Installation Instructions from the "TekWiki" and make sure there's no mistakes been made (there shouldn't have been if it was originally factory fitted!).

Chris Williams

[Warsp]

The DM44 came with the 465b so it should have all of the right components. The only thing not working as specified that I could find is that the alt sync pulse from U3211B being inverted, but I believe this is normal for my configuration. U3211, U3223, U3256, and U3275 have all been swap tested and are working properly. All switches have been sprayed with contact cleaner as well.

The correct waveform appears to be present on TP3280 which is on the outputs of U3282. Assuming that U3282 and its control circuitry is working correctly, is it safe to rule out the delay pickoff selector logic (U3211, U3213, and U3223) as being the source of the problem?

The main issue I am finding in terms of the measurement issues is that the voltage coming from the attenuator on the oscilloscope should be less than 2V but is usually much higher. Even when R3073 is kept in circuit and the voltage from U3275 is prevented from rising past ~1.9V, the voltage being sent to the A/D converter by the attenuator can be as high as 4V. When R3073 is removed and the ∆Time buffer is working correctly, this voltage can be as high as 12V. The attenuator on the oscilloscope is composed of three resistors and three switches, and it appears to work fine. I believe the temperature function uses the same path to the A/D converter, so seeing that it also works should mean that anything past the attenuator should not be the problem. With this information, can I assume that the issue is in the time converter circuitry, or should I continue to look elsewhere? Thanks!