Test Equipment Anonymous

Problem child Tek 475A. It's had multiple issues and the last time I attempted to do compensation of the attenuators I pretty much gave up getting it calibrated properly. The attenuators are typically aligned with a 1kHz signal. Turns out the real issue is with the high frequency response. It has considerable peaking. So it's going to take a complete alignment of the Vertical Pre-amp and possibly the Final Vertical Amplifier. The manual is quite extensive and it's going to take hours. I'll post up the results.

Typical 10MHz square wave from the Heath IG-4244 Scope Calibrator. Channel 2 is worse than Channel 1.



What it should look like.



The 475A has a B/W of 250MHz vs the 2465 B/W of 300MHz so I would not expect it to fully resolve the ringing on the trailing edge of the square wave which is a normal artifact of the IG-4244 at high frequencies.

Looks like it needs a CAT scan.

Finally got some time and inclination to start the 10 page calibration of the vertical pre-amp/amp. Everything was going good until I started channel 1 gain adjustments. It was way low and couldn't get it anywhere near spec. If I pressed with the screwdriver or my finger on R165 (10mV gain) the trace would jump significantly to almost correct level. So either the pot is open or possibly a cold solder joint. If it's the trimmer pot I probably have one in my parts stash. But either way the vertical pre-amp board has to come out. Oh joy. But at least on this 475A it's mostly modular with few connections to be unsoldered. But not today. I just lost interest.

Best of luck with it.

I don't like those delicate push on connectors. I always heave a sigh of relief when I push one back without wrecking it.

If you mean the Petolta cheap/small coax connectors, then they do feel fragile. But I've never broken one.

Definitely have to use care when inserting them. I did bend one once but luckily it did not snap off when straightening it.

tggzzz wrote: ↑Thu Mar 21, 2024 9:27 pm If you mean the Petolta cheap/small coax connectors, then they do feel fragile. But I've never broken one.

Yes. I'd forgotten the name. I've always been aware than it wouldn't take much to damage one, although I haven't had a problem yet. I recall I did bend the wire once and had to straighten it.

I have a 465 parts mule where some of them are well and truly mangled.

The other slight annoyance I've found with Tek 475s are the TO-8?? heatsinks on hybrids which can be missing and I couldn't find a source for, apart from parts mules.

These are the joys of messing with 40+ year old scopes which were leading edge in their day, and even today are pretty good, if they are working.

I've had to completely strip a couple of these - never damaged the coax connectors at all. They are surprisingly strong. The delay line is where it gets horrible.

As for the heatsinks, yes they appear to be difficult to find. I did have a few around a few years back.

The Vertical Pre-amp board pulled out and on the bench. The suspected trimmer pot is indeed intermittent.



The grey trimmer is the replacement. Bad trimmer on the bench. Now the PITA task of re-installing the board.

Well done. I have a 465 which I am going to have to remove the vertical preamp board from and deal with, plus another from a parts mule, which may be substituted. Inspired by your fine example, I'll stop finding excuses not to tackle it.

Zenith wrote: ↑Mon Mar 25, 2024 9:58 pm Well done. I have a 465 which I am going to have to remove the vertical preamp board from and deal with, plus another from a parts mule, which may be substituted. Inspired by your fine example, I'll stop finding excuses not to tackle it.

As I recall the 465 vertical pre-amp board is a much more difficult removal process. The 465B is more modular like the 475A.

You think after sitting disassembled on the shelf since March that perhaps I should get my arse in gear and install the vertical board and finish the calibration? Yea, I guess so. The vertical board went back in without much difficulty but it is time consuming. And all was well when I powered up. Unfortunately I'm going to have to start the vertical calibration all over again. But first I have to verify that the channel 1 vertical gain is at least close to spec. There are 2 gain adjustments per channel. 5mV and 10mV. I'll do that section of the manual first and then go back to the beginning.



The Heath IG-4244 Scope Calibrator is the perfect tool to check the vertical gain as it provides a calibrated square wave from 1mV all the way up to 100V. Once the gain is adjusted I'll slug thru the rest of the calibration and post the results.

Now you're making me wanna drag out the GriefKit scope I picked up at the spring swapmeet and still haven't gotten past triaging and "Yup, dead as a doorknob".

mnem
I hate you a little right now.

Progress today getting the overall vertical gain, DC balance, etc set on both channels but also ran into a major issue. Channel 2 fell into place with no issues but channel 1 would intermittently lose proper gain. Traced it to the attenuator switch deck assembly. There's major slop in the main shaft causing contact issues. I guess it's not surprising after many years of Gorillas abusing the switch. The best fix is total replacement....if I could find one in decent condition. But replacing it would take major, MAJOR surgery which I don't think is worth the effort. So I came up with a band-aid bodge. IPA'ed and cleaned the assembly then with careful counter torquing with the attenuator cover screws I was able to minimize the slop and get a consistent conection of the finger switches. Sometimes you just have to get creative.

Pix of channel 1 100mV input set to 50mV. Tomorrow I'll start at step 28 of the Vertical calibration which is the high frequency compensation adjustments.

Continuing with the vertical calibration. The high frequency adjustments took a lot of fiddle farting but finally fell into place. Compare this 10MHz square wave with the posted one at the beginning of this thread. I call it a win and this repair a success.

MED6753 wrote: ↑Wed Sep 25, 2024 6:40 pm Progress today getting the overall vertical gain, DC balance, etc set on both channels but also ran into a major issue. Channel 2 fell into place with no issues but channel 1 would intermittently lose proper gain. Traced it to the attenuator switch deck assembly. There's major slop in the main shaft causing contact issues. I guess it's not surprising after many years of Gorillas abusing the switch. The best fix is total replacement....if I could find one in decent condition. But replacing it would take major, MAJOR surgery which I don't think is worth the effort. So I came up with a band-aid bodge. IPA'ed and cleaned the assembly then with careful counter torquing with the attenuator cover screws I was able to minimize the slop and get a consistent conection of the finger switches. Sometimes you just have to get creative.

A neat solution. It would nice to do a proper job on everything, but sometimes you can't find spares or the amount of disassembly and reassembly is enormous, and hard to justify to change a bad cap or two. So cut the old cap off leaving the leads and solder the new one to the leads.

Is the attenuator assembly different from that on the Tek 475? 475s are very much more common than 475As and can sometimes be picked up as parts mules for surprisingly little.

Zenith wrote: ↑Wed Oct 02, 2024 11:01 am
Is the attenuator assembly different from that on the Tek 475? 475s are very much more common than 475As and can sometimes be picked up as parts mules for surprisingly little.

Don't know. I'd have to compare manuals and part numbers.

I'm clearly getting old as I have had both but I don't remember. The 475A was however the one that had a Friday afternoon job in one of the attenuators.