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Tech Chat You can't fix them all!

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WayneD
(@wayned)
Posts: 866
Honorable Member Registered
Topic starter
 

A friend gave me this rather cool looking Tektronix VM700A and I started to do a video on it.

20230930 192509

However, during shooting this happened:

WARNING: CONTAINS A LOUD BANG AND ME SWEARING!

Chris @crustytv very kindly sent me the link to the 600+ page service manual. However according to the manual the power supply is designed to be replaced rather than repaired. I somewhat naively think "Pah, what's so unrepairable about a few capacitors, diodes and regulators eh?" Well, without a schematic quite a bit it seems!

20231004 195240

Still, power goes in, power comes out. It should just be a matter of tracing where the power stops and goes out. Before long I seem to have narrowed it down to a regulator surrounded by diodes and capacitors. Cool, I think. Disconnect the power and start testing components out of circuit. 

I notice there's a neon bulb that's continuing to glow despite the mains being completely unplugged:

20231004 203236

 "That's kinda neat, must be some sort of uninterruptable power supply" As I'm thinking this one of these huge black capacitors lets out one of the biggest bangs I've ever heard on my bench, sprays something that's probably carcinogenic everywhere and at this point I'm like, nah! I'm not fixing this. 

I might see if I can get just the CRT section to work and maybe use it as a green screen monitor but sod doing the power supply.

 
Posted : 04/10/2023 8:47 pm
Lloyd and ntscuser reacted
crustytv
(@crustytv)
Posts: 11903
Vrat Founder Admin
 

As I mentioned to you on RatChat when you first acquired it, Tek kit is for the most part super reliable, but when they develop a fault they're not for the feint hearted, and sometimes you're scuppered for Tek specific chips.

Many years ago I repaired a 2225 oscilloscope which arrived with a smashed CRT. I managed to install a donor tube from a HAMEG, not entirely straightforward as it sounds, it needed some fettling but was well worth it in the end, I still have that scope to this very day.

It's Your choice entirely, and I know your time for projects is limited, but perhaps don't give up so soon? Treat it as a long term project, after all there's no rush and do you really need a green monitor? You can return to it as and when you feel refreshed.

I was reading the manual yesterday and this has many funky features, just one of which is you can FTP/Telnet into the device via RS232. I've uploaded to the library four manuals, Service, Programming, User and operating.

This is such a wonderful piece of sophisticated broadcast test equipment and as you've already seen oozes (Oops, excuse the pun) TeKtronix quality.  These would have cost tens of thousands back in the day, and working ones can still fetch good money, one surplus outfit, I often check for nice bits of redundant kit, has one currently for sale at £600!!

Just some thoughts.

p.s.

I can imagine the little snapper that surprised you in the video, pales into insignificance compared to the black beasty letting go. 😲 

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Posted : 05/10/2023 6:19 am
Lloyd
(@lloyd)
Posts: 1933
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That was a bit of a pop wasn’t it!! I can imagine the big cap letting go wasn’t very nice!

I had a laptop power supply doing funny things once, the fuse blew, so I swapped it, and it came back on, but a few minutes in there were some popping noises followed by a bigger pop and it blew the fuse again, opened it up and saw the main 450V electrolytic had some holes in the end of it, looked like it was caused by arcing, so I did what any crazy person would do and stuck another fuse in it and switched it on! You could clearly see after a minute that it was arcing inside the capacitor! I measured the voltage on it and it was rising to over 500V, and kept going! Turned out that a couple of 0.1uf film caps in some sort of filter circuit before the main cap had changed value a lot and somehow were making a voltage doubling circuit instead, I changed those film caps, and the main 450V one and it behaved itself again.

I wonder if your Tektronix PSU could have done something similar? Either that or it just hasn’t been powered for a long time and the caps were not happy about being woken up!

Regards,

Lloyd

 
Posted : 05/10/2023 7:03 am
WayneD reacted
crustytv
(@crustytv)
Posts: 11903
Vrat Founder Admin
 

Posted by: @wayned

I notice there's a neon bulb that's continuing to glow despite the mains being completely unplugged:

Lifted from manual

A neon indicator, DS1, lights when the primary power is up. Common-mode EMI filtering of the rectified voltage is provided by L2, C6, C7, C8, C9, and C10. Resistors R8 and R9 provide damping for the inductance of L2. The output voltage for either 115 V or 230 V operation is approximately 300 VDC.

psu2
powercct

CrustyTV Television Shop: Take a virtual tour
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Posted : 05/10/2023 8:08 am
WayneD
(@wayned)
Posts: 866
Honorable Member Registered
Topic starter
 

Posted by: @crustytv

p.s.

I can imagine the little snapper that surprised you in the video, pales into insignificance compared to the black beasty letting go. 😲 

I think that's the problem I have with it! I had a ringing in my ear not experienced since I saw Motorhead at the City Hall in Newcastle a few years back. When a component lets out the magic smoke at least I can knock the power off and prevent any further nasty surprises. The problem with this is it gave me a nasty surprise without even the mains plug being connected! I've noticed they're actually blue capacitors with a thick rubber sleeve - I think I've learned why they have that rubber sleeve!

I hadn't noticed that schematic at the end of the service manual, I just assumed that it wouldn't have been in there as it mentions replace rather than repair (I can kinda see why now!) That said, the component layout is for the later Tektronix in-house made version rather than the earlier version that mine has although the schematic seems to be more or less the same and it was a drop-in replacement.

It is probably just going into the Shed of Purgatory for now. 

 

 
Posted : 05/10/2023 11:14 am
Cathovisor
(@cathovisor)
Posts: 6436
Famed Member Registered
 

I have the DOS version of TekVMT (serial control program for the VM700A) running on my W2K laptop - screen cap below:

VMT desktop

I believe there is also a Windows version.

Some time ago I had a right run-in with the PSU in a Tek TSG-271 and it is documented here.

 

 
Posted : 05/10/2023 1:04 pm
WayneD reacted
WayneD
(@wayned)
Posts: 866
Honorable Member Registered
Topic starter
 

@cathovisor Some interesting issues you had there with the wrong type of capacitors. Was that factory or had it been "got at"?

 
Posted : 05/10/2023 2:28 pm
Cathovisor
(@cathovisor)
Posts: 6436
Famed Member Registered
 

@wayned The latter. When I got it, the correct mains fuse (1.25", because American) had been field-replaced with a BS1363 fuse... the semiconductor carnage was epic.

 
Posted : 05/10/2023 3:00 pm
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