SUCCESS! The "sick" 475A is now the "fixed" 475A!


 

After going over the other elements of the z-axis amp carefully, and not finding any other blown parts, I replaced the blown transistors with parts from stock (and, for Q1358, a part from one of the parts scopes), powered her up and I have a clean trace that responds to the beam intensity control.

Now I can mop up some issues with a 2215A that got a few weeks ago (just so I could play around with a scope that had a dual time base with an ALT horizontal display mode), return to evaluating my dad's 475 and 2213 (there's something screwy about channel 1 on the 2213 which might be fun to investigate), or start work on another scope (another of the "for parts" 475s is probably in good enough condition to warrant significant effort, and I've got a 2236 coming this week which will, if I am lucky, have some interesting disfunction). I've also got a PS502 that blows the mains fuse in my TM503 every time I have it power it up.

I'm having so much fun learning things and fixing things.

Thanks for all the guidance and support, I really appreciate it.

-- Jeff Dutky


Tom Phillips
 

Hi Jeff,
I'm glad you were able to get the 475A working.
Just to let you know...When you change the subject line the groups.io system starts a new thread that is separate from your previous postings on the subject. If you were to add the success message as a reply to your original discussion then all the information will be kept in the same thread. This is especially useful when someone finds your thread in the archives because they will be able to easily follow your information from start to conclusion.
Congratulations,
Tom


 

Congratulations, Jeff!

Raymond


Roger Evans
 

My congratulations as well. I don't think the PS502 will challenge you too much, if it does (and you may well enjoy it all the more) it means that the problem is more subtle than electrolytic capacitors whose rated maximum voltage is exactly equal to the voltage they see in normal service.

Roger


toby@...
 

On 2020-11-30 11:16 a.m., Raymond Domp Frank wrote:
Congratulations, Jeff!
Indeed, well done Jeff.

I was pretty pleased when I fixed a similar Z amplifier issue in a 603.
At first the intensity and focus controls were non-responsive. After
suspecting and testing almost every component on the board, including
the dual JFET, and swapping out the push-pull transistors, I eventually
found it was just one bad 2N3904, Q542...

Took weeks, because I'm a noob, but a satisfying fix.

--Toby


Raymond





Michael W. Lynch
 

Excellent! Good Work.


--
Michael Lynch
Dardanelle, AR


 

Hi Jeff,
Another working 475A cannot be bad. My recently acquired 475A is number B020399 and appears to be now working in all respects after a little slide switch cleaning. The Y attenuation was noisy but has cleared after use: gold plated contacts are apparently self-cleaning. EHT voltage was spot on when checked. In TEK products high quality resistors do not go high over time!
Regards


John Gord
 

David,

You wrote: " In TEK products high quality resistors do not go high over time!"

Well, that may be true, but if so, they did not always use high quality resistors.
The focus divider string in many of the 2200 series oscilloscopes used a bunch of 510K resistors which seem to frequently drift high and eventually open. It may be the most common failure in those scopes.

--John Gord

On Tue, Dec 1, 2020 at 03:33 PM, David Collier wrote:

Hi Jeff,
Another working 475A cannot be bad. My recently acquired 475A is number
B020399 and appears to be now working in all respects after a little slide
switch cleaning. The Y attenuation was noisy but has cleared after use: gold
plated contacts are apparently self-cleaning. EHT voltage was spot on when
checked. In TEK products high quality resistors do not go high over time!
Regards


Chuck Harris <cfharris@...>
 

Same thing happens with the 2710/11 SA's... It has a long string
of SMD resistors, that fail in odd ways... mostly open circuit.

-Chuck Harris

John Gord via groups.io wrote:

David,

You wrote: " In TEK products high quality resistors do not go high over time!"

Well, that may be true, but if so, they did not always use high quality resistors.
The focus divider string in many of the 2200 series oscilloscopes used a bunch of 510K resistors which seem to frequently drift high and eventually open. It may be the most common failure in those scopes.

--John Gord

On Tue, Dec 1, 2020 at 03:33 PM, David Collier wrote:

Hi Jeff,
Another working 475A cannot be bad. My recently acquired 475A is number
B020399 and appears to be now working in all respects after a little slide
switch cleaning. The Y attenuation was noisy but has cleared after use: gold
plated contacts are apparently self-cleaning. EHT voltage was spot on when
checked. In TEK products high quality resistors do not go high over time!
Regards





 

David Collier wrote:

Another working 475A cannot be bad.
He says as the bench groans under the weight of another 400 series scope.

In TEK products high quality resistors do not go high over time!
That was not my experience with this scope: there are a pair of resistors (R1354 and R1356) that are marked as 7.5 kΩ but currently test as about 9 kΩ, which is about 20% drift. They are 1/2W AB carbon composition resistors. The z-amp circuit, however, must not be very sensitive to their specific values.

-- Jeff Dutky


 

Hi Jeff,
Yes I know 'ordinary' carbon comp resistors go high, but most resistors in TEK products (and old Fairchild Dumont scopes) are high quality IRC or Welwyn.
In my TEK 191 sig. gen. the very few ordinary resistors were a bit high, but were in non-critical parts of the circuit.
Now to resume work on two 545B's, which do indeed require a substantial workbench!
Regards.


Dave Peterson
 

I get to jump on the "me too!" bandwagon!

The parts scope lives!

It was very weird in that the first throw of the power switch yielded ... nothing! No lights, no DC voltages at the test points. Oh well, here we go down the debug path.

What's funny is that I have no idea what I did to fix it! I unhooked the power switch so I could get at the back and verify it was working - it was. I pulled the High/Medium/Low supply jumper in the main fuse holder. Nothing weird going on there. I plugged it back in and switched on and was blindly probing the output of the transformer on the A9 board and was seeing voltage. So I went back to the DC test points and low-and-behold, there was voltage! Even correct voltage! Then I noticed that when I touched the 15v test point the trigger light was flickering. I looked at the face, and sure enough, the CH1 1X light was on! Wo-ho!

I had the F1419 fuse out to keep the HV circuit off for first power up. I plugged it in and turned the scope back on. Turned the intensity up, and there's the trace! The focus is bad - I think there's noise coming from the vert assembly, but the calibrator is putting out a square wave, and CH1 is displaying it.

Time to get the calibration procedure out and go through this thing. But my first oscilloscope repair job! What a feeling!

Thanks everyone for the help so far. I'm sure I'll have more to share and questions to ask. It's great to be able to work with you guys and share the success!
Dave


Tom Lee
 

Congratulations, Dave! Your "Scope Fixer" merit badge is in the mail.

-- Cheers,
Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 12/6/2020 21:33, Dave Peterson via groups.io wrote:
I get to jump on the "me too!" bandwagon!

The parts scope lives!

It was very weird in that the first throw of the power switch yielded ... nothing! No lights, no DC voltages at the test points. Oh well, here we go down the debug path.

What's funny is that I have no idea what I did to fix it! I unhooked the power switch so I could get at the back and verify it was working - it was. I pulled the High/Medium/Low supply jumper in the main fuse holder. Nothing weird going on there. I plugged it back in and switched on and was blindly probing the output of the transformer on the A9 board and was seeing voltage. So I went back to the DC test points and low-and-behold, there was voltage! Even correct voltage! Then I noticed that when I touched the 15v test point the trigger light was flickering. I looked at the face, and sure enough, the CH1 1X light was on! Wo-ho!

I had the F1419 fuse out to keep the HV circuit off for first power up. I plugged it in and turned the scope back on. Turned the intensity up, and there's the trace! The focus is bad - I think there's noise coming from the vert assembly, but the calibrator is putting out a square wave, and CH1 is displaying it.

Time to get the calibration procedure out and go through this thing. But my first oscilloscope repair job! What a feeling!

Thanks everyone for the help so far. I'm sure I'll have more to share and questions to ask. It's great to be able to work with you guys and share the success!
Dave




 

Dave,

That's quite similar to my experience with the first fault on the 475A: I had bad voltage on the +110 V rail, so I started testing components in the circuit for that rail. I found what seemed to be a shorted ceramic disc capacitor, so I unsoldered it to be certain. When I tested it out of circuit it tested fine, so I soldered it back in place. Just for giggles I powered it back on and the +110 V rail was fixed!

Sometimes just taking something apart and putting it (carefully) back together does the trick. It doesn't always work, and it feels like it shouldn't, but it can.

It does feel good to get something like this fixed up. Part of it is, I think, that these are such well crafted objects that it feels good to make them whole, but there's also a fair amount of pride in skill and determination.

Congrats, and wishes for further success!

-- Jeff Dutky


Jim Ford
 

I'd be wary of problems that go away by themselves.  They often come back by themselves as well.  Just an FYI.    Jim Ford Sent from my Verizon, Samsung Galaxy smartphone

-------- Original message --------From: Jeff Dutky <jeff.dutky@gmail.com> Date: 12/6/20 10:36 PM (GMT-08:00) To: TekScopes@groups.io Subject: Re: [TekScopes] SUCCESS! The "sick" 475A is now the "fixed" 475A! Dave,That's quite similar to my experience with the first fault on the 475A: I had bad voltage on the +110 V rail, so I started testing components in the circuit for that rail. I found what seemed to be a shorted ceramic disc capacitor, so I unsoldered it to be certain. When I tested it out of circuit it tested fine, so I soldered it back in place. Just for giggles I powered it back on and the +110 V rail was fixed!Sometimes just taking something apart and putting it (carefully) back together does the trick. It doesn't always work, and it feels like it shouldn't, but it can.It does feel good to get something like this fixed up. Part of it is, I think, that these are such well crafted objects that it feels good to make them whole, but there's also a fair amount of pride in skill and determination.Congrats, and wishes for further success!-- Jeff Dutky


 

Jim Ford wrote:

I'd be wary of problems that go away by themselves.
Well, if the problem had simply stopped without my touching anything, I would agree with you (and then some: a problem that goes away by itself is likely to return with friends). In my case I suspect that one lead of the cap had been bent down and was making contact with a neighboring signal trace, shorting out the cap. I straightened the leads after desoldering it while measuring it on my multimeter, so when I reinstalled the cap I did so with straightened leads that couldn't touch the circuit board in the wrong places. I still find it amusing that I didn't replace any parts and still managed to fix the problem, but some problems on the 475 are like that: cleaning drum switch contacts, and cleaning and reseating socketed components are the two examples that leap to mind. Removing heavy dust build-up could also improve the performance of a really dirty scope, but you might expect that if the dust were causing significant overheating then some components might also be permanently damaged.

-- Jeff Dutky


Dave Peterson
 

Yes, intermittent transient problems are the worst.

I figure any recovered parts scope comes with 0 assumptions. When it at first did nothing I wasn't the slightest bit surprised. I was more surprised that it did work - and is working pretty darn well. My primary guess is that the "Regulating Range Selector" - the jumper in the main fuse holder? - was the culprit. That removing and reinstalling it broke whatever surface corrosion was interfering. The power switch had already been cycled a few times (without power) during the disassembly/reassembly process. Should it be taken back out and and cleaned? Sure, along with every other contact surface in the scope? I'm not that patient and thorough, yet.

It's doing a lot of off-nominal things that can't be of any surprise: pots being "jumpy", etc. I have to wonder how long this thing has been sitting on a shelf in the most non-ideal conditions. There have been a lot of water spots on internal metal parts. I think mostly likely condensation. The boards have appeared rather pristine, though it also appears to have had some boards and shields removed. Obvious solder work on the wires and BNCs necessary to disassemble/reassemble. I suspect several attempts to fix it have been made before.

Things are cleaning up as I use it - position pots are settling down with use, BNCs are being noisy and intermittent, but getting better. Small oxidation and corrosion on contacts has to be expected. I'll get into cleaning methodologies as I get into the calibration and determine what is and isn't working.

Just part of the fun!
Dave


 

Dave,

When you say "calibration" are you actually doing a full calibration on this scope -- do you have the full set of calibration equipment -- or are you doing something more abbreviated?

-- Jeff Dutky


Dave Peterson
 

I'm talking about going through the instructions in section 5 of the 465 manual.

Calibration is definitely a loosely used term here. I'll only have a function generator, a DMM and a "working" existing 465 scope.

I was thinking about opening a calibration conversation with you. I suspect you're familiar with real calibration considering your father was using these scopes for real work. I'm sure he had to send them to a certified calibration lab every six months. Does such a lab even exist for these old scopes anymore? I doubt it. In other words, I doubt a strict certified calibration of these scopes is ever possible anymore. And calls into question the concept of full/"certified" restoration. It's all hobbiest restoration at this point anymore. I suspect.

If I resell a parts scope as "restored", what does that mean? It's a rhetorical question I'm pondering for myself at this point.

Dave

On Monday, December 7, 2020, 12:06:05 PM PST, Jeff Dutky <jeff.dutky@gmail.com> wrote:

Dave,

When you say "calibration" are you actually doing a full calibration on this scope -- do you have the full set of calibration equipment -- or are you doing something more abbreviated?

-- Jeff Dutky


 

Dave,

I am not, in fact, familiar with "real" calibration, except in as much as I know that I can't easily do it with what I have to hand (I suspect that I might be able to rig something up, but the price of doing so would probably rival the prices for the three important calibration devices on eBay), and if I DID rig something up, I'm not sure I could rely on the calibrations that resulted.

My father apparently sent his 2213 in for regular calibration through the late 80s. That calibration appears to have been annual, judging by the dates on the stickers. I'm not sure why he stopped, but his service business started to dry up about that time, so I can imagine several explanations. He never bothered to get the 475 calibrated after he got the 2213. Again, I'm not sure why, but the fact that he was using a 60 MHz scope as his "daily driver" suggests that the 475 was well in excess of his needs, so maybe strict calibration wasn't too important either?

I haven't tried any significant calibration of anything, only small stuff that (hopefully) doesn't throw off the rest of the scope (e.g. adjusting the invert balance on channel #2, and recalibrating the V/div balance). I haven't touched the sweep timing calibration, for example, because I know that would require equipment that I just don't have.

My impression (from this group and elsewhere) is that you can still get these things calibrated by professional shops around the country (the place I'm looking at is www.custom.cal.com which has locations in New Jersey, Florida, and Oregon), and that it's not absurdly expensive (but more expensive than the old scopes themselves). I'm not sure what, if any, the extra cost is for a traceable cert.

I'm pondering the same thing you are pondering, if only because it would be a possible way to defray the cost of acquiring these parts scopes. Also, I only need so many parts scopes, and I'm well beyond that number right now. I also need only so many working scopes, and I am probably beyond that number as well. I'm not sure what obligations you are under if you sell a scope as "working" or "restored". If I were going to charge the kind of prices I still see for "working" 475a on eBay, I would want to include at least a front cover, pouch, probes, and operators manual, and the scope would have to be in pristine cosmetic condition (but that's just me and my ridiculous scruples).

The number of working scopes I need is a complicated question: While the 475 and 475A are on the bench (as subjects) I need a scope that I can use to work on them, which should be a 100 MHz scope (the 2236). However, the 2236 is currently on the bench (as a subject) so I need a scope to work on that one as well (which is either my father's old 2213, or the 2215A). The 2213 could also be on the bench, but I've given up trying to diagnose its intermittent channel 1 flakiness, so the 2215 is the primary scope for the moment, and everybody else is either a patient or a donor.

-- Jeff Dutky