Tek 475 scope

[Corri]

Just bought this scope on ebay, just for something to do. Never been in a scope before. I got interested in electronics a couple of years ago, built several solid state guitar amps, changed to tube amps, all the while trying to learn something. Got a looong way to go. Anyway, I pulled this baby out of the box and discovered it is deader than last weeks news. I checked the ac power through switch, fuse and so on and it is there. DC, nothing. No low voltage, no high voltage. I was considering removing transformer to check it but really hate to tear things apart unless I am pretty sure of what I am doing. So I thought maybe someone with experience on scopes might give me some suggestions.

 

[dorke]

This scope is an excellent 200MHz analog CRT of times passed(early 70's),was the top of the top at it's time.
I own a 465 which is 100Mhz and was the standard scope for pros in the 70's.

It is also a very complicated device to fix even for Pros.
You may be lucky and the problem might be a power supply failure.
Be warned that a CRT scope has deadly voltages inside it !!!

What you should do is use the 475 service manual and be very very careful!
Good luck.

 

[Corri]

Thanks dorke, I did buy a download manual and it had been scanned nice but the original owner had felt a need to scribble notes on the schematic and then draw long sweeping arrows to whatever he/she was referring to. Being that the scribbles covered up the very thing I needed, it caused a temperature rise in my cranium but I will work my way through it.

 

[hevans1944]

I did buy a download manual

The manual at the link @dorke provided is free from Tektronix. I downloaded it and didn't see any markups.

There are still a few (old) people around who make it a hobby, of sorts, to repair "vintage" Tektronix oscilloscopes. The problem is finding them, and finding one willing to have a go at repairing yours for a reasonable cost. It would have to be a labor of love, because these 'scopes are a real bitch-kitty to work on and are filled wall-to-wall with custom made or specially selected and matched components likely to be unavailable anywhere. As @dorke said, even pros try to avoid repairing these.

The model you have was a real jewel, the crème de la crème of portable o'scopes back in the day. Like @dorke, I too have a model 465, which has half the bandwidth of the 475. I purchased two of them, used or pre-owned, over the past forty-something years. Both have provided good value and many hours of use on my workbench as well as in the field. However, there are shortcomings that I now find unacceptable, so I recently "upgraded" to a digital storage oscilloscope (DSO). a Hantek DSO5202P. I still often use the Tektronix o'scopes side-by-side with the Hantek, but if either of the Tektronix o'scopes ever fails it will be discarded rather than repaired, Or perhaps offered, gratis, to someone who still repairs and/or collects vintage analog oscilloscopes if they are willing to pay for the shipping.

I would suggest you download the manual that @dorke linked to and perform some real troubleshooting before yanking out the power transformer. The power transformers hardly ever fail. Check that the line-voltage selection switch is set to correspond to your line voltage. Gain access to the secondary windings feeding the full-wave bridge rectifiers and verify that AC voltage of the appropriate magnitude is present on each of the secondary windings. If so, look at the raw DC output of the bridge rectifiers to see if any of the outputs are shorted. It is also likely that the aluminum electrolytic filter capacitors are marginal at best. Good luck finding replacements that fit within the narrow confines of the original parts. Also be aware that the most expensive replacement part, the cathode ray tube (CRT), is likely to be unobtainable at this late date.

 

[Corri]

Thanks hevans1944, every bit of advice helps. Also, I am 86 and with ALS and REALLY like to restore old things, does that qualify me for the old folks?

 

[hevans1944]

@Corri" Yar, 86 sorta gets you in the ball park of old folks, at least down here in southwestern Florida where anyone less than eighty is considered to be a youngster. I'm not there yet at 73 going on 74 in June, but I do aspire to becoming old. It looks like you are "beating the odds" with respect to life expectancy for someone diagnosed with ALS. I wish you success in getting your 475 oscilloscope operational again. It's a fine old warhorse and a shame to put out to pasture.

What I will never be is proficient at repairing Tektronix oscilloscopes, at least the "modern" ones based on semiconductor instead of vacuum tube electronics. I did dabble. in the 1960s, at repair and alignment of the older vacuum tube oscilloscopes that used distributed differential amplifiers for the vertical deflection. That was a nightmare that required purchasing from Tektronix matched sets of vacuum tubes to get the best performance. But, once our lab started using the more modern transistorized 'scopes, repair was pretty much out of the question.

 

[dorke]

Hi Corri,

As always,good notes and advise from @Hop

Well there are a few members here over 80.
I'm not one of them,but am not a "spring chicken" either

I'm willing to help you fix the scope "on-line" (it will take time),
Since If the scope is totally not working it is most probably a power supply failure.
Should be possible to fix.

I understand you are a novice,therefor,
I warn you again of the dangerous High Voltages in that CRT scope !
Please don't try to test (obviously not touch) any voltage above the Line-Level voltage in the scope !

What you should have as a Minimum is a reasonable DVM(If you have one please post it's photo).
A soldering iron and soldering stuff(what is your soldering skill?).

In any case this may be a good learning experience for you.
Are you willing to start?

 

[Corri]

Yeah dorke, I'm ready. Also, I didn't mean to trojan horse the forum. I am learning and have limited electronics knowledge, but I do have most of tools needed to check things out. I also have a 475 scope that works fine though I have never took the time to really learn how to use it. I know, that sucks. Anyhow, I do like to hunt out electronic problems and would really like to find out if this old girl is salvageable. With what I have traced so far I am convinced that there is nothing on the trans secondary side.

 

[dorke]

O.K.
I see you have a Fluke DVM, which is excellent.

If you are convinced there is nothing on the secondary of the main transformer(hope you checked AC voltages),we should check the primary:
Please refer to ch. 11-power supply , under Diagrams and Circuit boards.

The first thing to do is to check the resistance between the live and neutral of the scope power plug while it is disconnected from the mains and the scope power switch is set to ON.
It should be low resistance,
The expected resistance can be calculated from the scope power consumption which is 100W(I will leave that as a drill for you).

You can compare it with what you get on the working unit.

I'm guessing you will get an open circuit/high resistance.
If you do,look at the diagram(attached) ,and trace it for the faulty device .

 

[Corri]

Sorry it took so long to reply, had company. Ohm readings: through power cord 6.4O, through on off switch, 115v230v switch through medium switch, all 6.4 O. AC v same switch's, 122 v.
low v dc. -15-.015
-8-.061
+5-.053
+15-.015
+50-.015
+15-.015
+110-100v
high v-0
f1318-26.34
Using ohms law I thought ohm reading to be higher but good machine measured 6.6 ohm. I don't know if this helps any dorke but here it is. I am getting a little worried because everywhere I go in this machine I find phillips screws with the head messed up. I think probably a good repairman would at least have the proper screwdriver. Also, fan does not run.

 

[dorke]

The Primary looks fine.
I'm assuming the voltages you have tested are all on TPs(Test points).

To check the transformer secondary,please check the DC voltages on :
C1412,C1414,C1442,C1452,C1462,C1472 (both probes on each capacitor,careful not to cause a short!).

Please don't check HV noted voltages.
I can't find f1318.

 

[Corri]

Yes, I was using test points. F1318 is on left corner of board, just above transformer. I hate to appear dumber than I am but the way these solder pads are laid out I am not sure which pads are the caps. Any help there?

 

[dorke]

Well,
there is a layout photo on the manual,it isn't clear enough.
The caps are connected to the rectangular bridge rectifiers "+" and "-" pins.
If it is easier you can check on these instead.
Look at the photo attached.

If you can,please take a clear photo of the actual board, it will be of great help.

 

[Corri]

Will be back to you in an hour or so. Thanks

 

[Corri]

Having a problem up loading picts. Will check caps and be back.

 

[Corri]

I cannot be sure which one of these solder spots are the caps you need. Anyhow dorke, don't you think seeing as how we have no voltage on most all test points that we have a supply problem? Whoever was in here before obviously didn't find an easy fix and probably didn't want to go to the trouble of removing the trans. I believe I will go to the trouble of doing just that. At least then I will be sure of one thing. I will post my results when I get them. In the meantime, thank you for the help.

 

[(*steve*)]

I believe I will go to the trouble of doing just that

I believe I would follow @dorke's advice.

 

[dorke]

No, I wouldn't remove the transformer before making sure it is faulty in-circuit.
The fact that you don't have most voltages doesn't point to the transformer itself as the fault.

That is so since most voltages are regulated in the 475 scope,
they are created using IC's which work from some unregulated voltages(+15V and +50V).

As a learning experience for you , I wanted to guide you step-by-step in a logical general way of finding the fault,it requires patience.
We should advance by checking the Primary ,then the Secondary ,and then the unregulated voltages before going into the regulators.

Another way to check the Secondary is by checking the AC voltages on it's windings .

In general,
I would't remove any part before checking it in-circuit and having an almost certain "proof" it is faulty.
That is because removing parts can be a lot of work ,and can by itself cause damage to the part itself ,to the PCB, and even to other near by parts.

Re-instaling it may also create lot's of problems and further damage:
Wrong installation may be fatal to the instrument's "health" or the device itself.
A device may be put back in a wrong orientation,wrong order wiring etc.
Soldering shorts,cold or bad solders,solder "drip" shorts, overheating damage to the device or PCB etc.

In our case,
there isn't enough evidence to proof the transform's fault,
not yet anyways ....

 

[73's de Edd]

Sir Corri . . . . .

After initially finding that you are Amerikanski . . . .and will be using 120VAC power. . .along whth that ohms reading across the primary of the power transformer . . . .is looking as normal.
MY next step would be to switch DVM to AC metering mode and measure the line voltage to initially confirm that you DO have a good working AC mode DVM.
Then you move over to the secondary of the transformer for those 4 windings shown there on post #10 and check for the lowered AC voltage presence there on all of them, and then, if all being present, move to the raw DC filters that they feed and confirm DC presence.

All are marked up and board placements are being shown. You just need to confirm against PCB stampings.

If having all voltages present . . .the reg supplies follow.

Thassssssit . . . .
Currently in my test equipment stable . . . of just this series . . . (2) - 465's (1) - 466 Storage scope
(1) - 475 . . . .WITH the DVM pack on its top.


A9 CARD PARTS PLACEMENT REFERENCING . . . . .



73's de Edd
.....

 

[Corri]

Sir Corri . . . . .

After initially finding that you are Amerikanski . . . .and will be using 120VAC power. . .along whth that ohms reading across the primary of the power transformer . . . .is looking as normal.
MY next step would be to switch DVM to AC metering mode and measure the line voltage to initially confirm that you DO have a good working AC mode DVM.
Then you move over to the secondary of the transformer for those 4 windings shown there on post #10 and check for the lowered AC voltage presence there on all of them, and then, if all being present, move to the raw DC filters that they feed and confirm DC presence.

All are marked up and board placements are being shown. You just need to confirm against PCB stampings.

If having all voltages present . . .the reg supplies follow.

Thassssssit . . . .
Currently in my test equipment stable . . . of just this series . . . (2) - 465's (1) - 466 Storage scope
(1) - 475 . . . .WITH the DVM pack on its top.


A9 CARD PARTS PLACEMENT REFERENCING . . . . .

View attachment 41338

73's de Edd
.....

Thank you73's de Edd, for an intelligent and very helpful post, with even a touch of humor. I was confused as to which solder post were the lytic +and- post so I removed trigger gen. board to see what was what. Of coarse I found the caps were four pin which were new to me. Now I know where I stand and I was also able to see where the sec. wires go. It has been a hectic week for us, hopefully will get better soon. I will then replace pcb ( I will try not to destroy the whole scope) and then do your testing. Thanks again.