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[dead tek 485] Can i fix it ? (newbie here)


marcosjl31@...
 

Hi guys

Happy New Year to all.

I'm new in the group.
I've bought a working (supposed to) Tek 485 for a few euros here in France : received it and tried it for ~10mn. I powered it off and cannot power it on again...
I know this is an old beast but I'd really like to have it up and running on my hobbyist electronic bench.

=> I'm not an electronics expert but know how to read schematics, use a multimeter and a soldering iron to replace faulty components. I'm in my fifties and thus understand perfectly the dangers of dealing with High Voltages....
Do you think fixing this 485 scope is within my reach, with my basic knowledge ?

I've got the Service Manual and probably my first steps will be follow Reed Dickinson "Troubleshooting tips" from this topic : https://groups.io/g/TekScopes/topic/20933356

I'll be glad to ear from you, any advice will be really appreciated !
Thanx in advance.

Regards
Jose L. Marcos


Bob Albert
 

Seems like a power supply issue.  Measure the voltages and go from there.
Bob

On Wednesday, January 6, 2021, 02:12:13 PM PST, <marcosjl31@gmail.com> wrote:

Hi guys

Happy New Year to all.

I'm new in the group.
I've bought a working (supposed to) Tek 485 for a few euros here in France :  received it and tried it for ~10mn. I powered it off and cannot power it on again...
I know this is an old beast but I'd really like to have it up and running on my hobbyist electronic bench.

=> I'm not an electronics expert but know how to read schematics, use a multimeter and  a soldering iron to replace faulty components. I'm in my fifties and thus understand perfectly the dangers of dealing with High Voltages....
    Do you think  fixing this 485 scope is within my reach, with my basic knowledge ?

I've got the Service Manual and probably my first steps will be follow Reed Dickinson "Troubleshooting tips" from this topic : https://groups.io/g/TekScopes/topic/20933356

I'll be glad to ear from you, any advice will be really appreciated !
Thanx in advance.

Regards
Jose L. Marcos


Roy Thistle
 

On Wed, Jan 6, 2021 at 02:12 PM, <marcosjl31@gmail.com> wrote:


I powered it off and cannot power it on again.
Can you say more about what they told you when you bought it. (They said it was/was not working? They thought it was a T.V.?)
Can you say more about what happened when you powered it on the first time? (What did you see, smell, hear? Did a fuse blow?)
Do you have another oscilloscope, to use for troubleshooting?
Do you know how to properly use a 400 series scope?
Did you read the user manual? (or at least look through it?)


Jim Ford
 

Happy New Year to you as well, Jose'!  Yes, Reed is the resident 485 expert.  Lucky you, finding one for dirt cheap!   Jim FordSent from my Verizon, Samsung Galaxy smartphone

-------- Original message --------From: marcosjl31@gmail.com Date: 1/6/21 1:02 PM (GMT-08:00) To: TekScopes@groups.io Subject: [TekScopes] [dead tek 485] Can i fix it ? (newbie here) Hi guysHappy New Year to all.I'm new in the group. I've bought a working (supposed to) Tek 485 for a few euros here in France :  received it and tried it for ~10mn. I powered it off and cannot power it on again...I know this is an old beast but I'd really like to have it up and running on my hobbyist electronic bench.=> I'm not an electronics expert but know how to read schematics, use a multimeter and  a soldering iron to replace faulty components. I'm in my fifties and thus understand perfectly the dangers of dealing with High Voltages....     Do you think  fixing this 485 scope is within my reach, with my basic knowledge ?I've got the Service Manual and probably my first steps will be follow Reed Dickinson "Troubleshooting tips" from this topic : https://groups.io/g/TekScopes/topic/20933356I';ll be glad to ear from you, any advice will be really appreciated !Thanx in advance.RegardsJose L. Marcos


Roy Thistle
 

On Wed, Jan 6, 2021 at 02:12 PM, <marcosjl31@gmail.com> wrote:


Do you think fixing this 485 scope is within my reach, with my basic knowledge
?
Well... I won't give my opinion on that... but I observe... many of us posting here will spend many posts/hours directing you by remote control. In that regard much depends on your ability to follow directions and make good observations.
Someone already suggested to start doing stuff to the power supply... but you haven't yet told us whether you've checked to make sure there is mains power, the proper fuse is in the scope, and the correct value, the scope is set to the proper voltage, and whether the line cord is good, or if the power switch is good or not.
Maybe you already did that.


J. L. Trantham
 

Jose,

Happy New Year to you as well.

I've resurrected several of these over the years and they all had shorted tantalum capacitors on various boards that bring down the power supply.

There are some 'combs' that you can remove to isolate the various boards. If you find the pin for the various voltages and check resistance to ground you can likely identify the board with the shorted tantalum. Once fixed, don't be surprised to discover that it failed again. That leads to further checks for shorted tantalums. The ones that survive the 'resurrection' not infrequently fail on the reapplication of correct voltage and fail, thus starting the process all over again.

It's doable and, compared to some of the issues, relatively simple.

Start with getting a manual.

Good luck.

Joe

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of marcosjl31@gmail.com
Sent: Wednesday, January 06, 2021 3:02 PM
To: TekScopes@groups.io
Subject: [TekScopes] [dead tek 485] Can i fix it ? (newbie here)

Hi guys

Happy New Year to all.

I'm new in the group.
I've bought a working (supposed to) Tek 485 for a few euros here in France : received it and tried it for ~10mn. I powered it off and cannot power it on again...
I know this is an old beast but I'd really like to have it up and running on my hobbyist electronic bench.

=> I'm not an electronics expert but know how to read schematics, use a multimeter and a soldering iron to replace faulty components. I'm in my fifties and thus understand perfectly the dangers of dealing with High Voltages....
Do you think fixing this 485 scope is within my reach, with my basic knowledge ?

I've got the Service Manual and probably my first steps will be follow Reed Dickinson "Troubleshooting tips" from this topic : https://groups.io/g/TekScopes/topic/20933356

I'll be glad to ear from you, any advice will be really appreciated !
Thanx in advance.

Regards
Jose L. Marcos


Tom Gardner
 

If it is just capacitors, you can probably fix it.

I've had startup fail due to a marginal C1841.

Use google to search for C911. That, and two others, are 15V tants on a 13V PSU rail, and they aren't shown on the "PSU" schematic.

A working 485 is a delight, particularly since it has a proper 50ohm input, not 50ohm//15pF.

On 06/01/21 21:02, marcosjl31@gmail.com wrote:
Hi guys

Happy New Year to all.

I'm new in the group.
I've bought a working (supposed to) Tek 485 for a few euros here in France : received it and tried it for ~10mn. I powered it off and cannot power it on again...
I know this is an old beast but I'd really like to have it up and running on my hobbyist electronic bench.

=> I'm not an electronics expert but know how to read schematics, use a multimeter and a soldering iron to replace faulty components. I'm in my fifties and thus understand perfectly the dangers of dealing with High Voltages....
Do you think fixing this 485 scope is within my reach, with my basic knowledge ?

I've got the Service Manual and probably my first steps will be follow Reed Dickinson "Troubleshooting tips" from this topic : https://groups.io/g/TekScopes/topic/20933356


 

Hi Jose,

I was in almost exactly your circumstance a few months ago: very little experience with electronics, but I can read a schematic and a multimeter (and I know what Kirchhoff's and Ohm's Laws are, even if have trouble spelling "Kirchhoff"), and I have been successfully fixing a small pile of similar vintage scopes (from the 70s and 80s, several 475s, a 475A, a 2215A, and a 2236) since then. It's really not that hard if you are patient and careful.

You've gotten off to a good start by getting the manual, and the troubleshooting tips document. First thing to do (as you've already been told) is to check the power supply rails, and fix each fault in turn.

A working 485 is a wonderful scope, and certainly worth your time and effort to fix up. It sounds like you got a really good deal on it, even if it requires repairs. I can tell you from experience that there is nothing like working on one of these old scopes to teach you about electronics. You will learn a lot, and probably enjoy the process.

-- Jeff Dutky


Harvey White
 

In most of the manuals of that era (and they got less detailed and less "nice" as time went on....

You'll find that Tektronix published a lot of theory of how their scopes worked.  They also had troubleshooting information, check A, then B, then C, and so on.  They had performance verification, and also calibration procedures.

It's going to be worth it to get a good manual (I can recommend ARTEK, for one) for the appropriate serial number range for your scope, they cost (typically) under 20 USD, and the ones from ARTEK are particularly nice.  A problem with some other scans is that they are low resolution, which means that when you zoom in to get the value or designation of a part, those numbers are just blocks.  Check around, and if there's a good high resolution scan, it's preferable, perhaps even if you need to spend money. However, you can start with the free stuff and spend money if you need to.

With any piece of electronic equipment, it naturally floats or sinks depending on the power supply voltages.  TTL chips, for example want 5.0 volts + .25, or -.25, but won't be guaranteed to function properly at all conditions if the supply voltage is outside those limits.  Tektronix was very good at telling you the voltage ranges, and (almost equally important) the ripple of those voltages.  Once you start looking at the power supplies, you may well need another oscilloscope for measurements that a multimeter just won't do.

First check the supplies, then go from there.

Harvey

On 1/6/2021 4:02 PM, marcosjl31@gmail.com wrote:
Hi guys

Happy New Year to all.

I'm new in the group.
I've bought a working (supposed to) Tek 485 for a few euros here in France : received it and tried it for ~10mn. I powered it off and cannot power it on again...
I know this is an old beast but I'd really like to have it up and running on my hobbyist electronic bench.

=> I'm not an electronics expert but know how to read schematics, use a multimeter and a soldering iron to replace faulty components. I'm in my fifties and thus understand perfectly the dangers of dealing with High Voltages....
Do you think fixing this 485 scope is within my reach, with my basic knowledge ?

I've got the Service Manual and probably my first steps will be follow Reed Dickinson "Troubleshooting tips" from this topic : https://groups.io/g/TekScopes/topic/20933356

I'll be glad to ear from you, any advice will be really appreciated !
Thanx in advance.

Regards
Jose L. Marcos





marcosjl31@...
 

Hi *,
it's really a pleasure to receive so many answers on my first uncomplete post !

Ok. Tried again this morning to power the 485 up and it came alive !!
Here is a status of what i can see on the general status of this unit :

=> The scope was supposed to be in working conditions but needed checking and calibration. No tek probes provided.
Following calibration procedure found on the group, what I can see is :
. several light indicators are not working : x1 led on channel 1, 1Mohm/50ohm indicators on both channels,
. intensity control (top most left) does not work properly : at CCW position I already get a brillant trace on the CRT, at CW position the trace is totally blurrish, focus and astygmatism controls seems to be working (but I don't know if the resulting trace has a normal decent thickness...)
. slope level (B triggering) is not working : it does not change the triggered signal.

=> Concerning the cal signal display (BNC cable used here) :
* channel 1 (1Mohm/50ohm selector pushed, no light indication)
I do not get a full 5 div display - 4,8 div instead. I do not get 1 horiz div per half square wave - 1st half wave is 1div, but others are more in 1.4 long (I did not find a proper way to attach a picture ? :( ).
If 1Mohm/50ohm is in released position, i only get a "line" on the CRT.
* channel 2 (1Mohm/50 ohm selector in released position ??)
As mentioned, Slove level switch has no effect on the signal (triger is done on positive no matter the swich position).
I get almost a 5 div vertical display and the same horiz behaviour as channel 1.
If 1Mohm/50ohm push button is "in" (pushed) : the square wave is not correctly displayed (bottom part of the square wave ??)

=> Concerning the power Off/on not working
If I power off the unit, I'm able to power it on after a delay : probably the time capacitors get discharged ?

=> Concerning Instruction/service manual, I got my copy from : http://bama.edebris.com/manuals/tek/485

I'm affraid that this unit has multiples issues...

Will probably start by the PSU one.

Thanx again, and look forward reading your comments.

José


Simon
 

Is this the 485 that was on Le Bon Coin? I think sellers of this type of equipment are honest up to their level of knowledge. Some do no even know what an oscilloscope is.
Most of this equipment has been stored for a long time and this one may be from IBM originally.
The advice to check power supplies first is sound. I bought a 475 from Le Bon Coin that had been stored for a long time and basically all it needed was thorough servicing. If the transistors are on sockets it is a good idea to reseat them using a dab of isopropyl alcohol, check the pins for corrosion at the same time. Then attend to each circuit in an orderly manner, once you are certain that the power supplies are within specification.
Simon
Gers


marcosjl31@...
 

Hi Simon,
No : i bought it from EBay (but I think it's the same one that was proposed on Le bon coin).
Will start by PSU as already suggested. But my concerns are on the other observed behaviours...
Jose
Toulouse


Stephen
 

If it’s the one from Damartin-en-Goelle, I met the guy at his house some time ago. He is very knowledgeable, and has/had many tek scopes. He works for the European Space Agency.
The one I bought from him at the time (475A), died less a month after. That’s why I pass on his stuff now.

I still have to find time to fix it.


Simon
 

I have seen quite a few scopes from this location and assumed he was a dealer.
Simon


Stephen
 

On Thu, Jan 7, 2021 at 01:26 AM, Simon wrote:


I have seen quite a few scopes from this location and assumed he was a dealer.

Simon
No he’s not a dealer. I guess he’s just selling his collection....


marcosjl31@...
 

Hello guys

here is a follow up...

I opened the scope and had a look around. The only relevant things I've seen is that PSU board has been repaired at least once :
* C2018 has been replaced by an electrolytic cap 100uF 100V
* c2021 has been replaced by an electrolytic cap 47uF 5V
I also noticed that R2021 had problems : the external coating is gone. The resistance measures 11ohms (in circuit)

I measure DC resistance to ground for the 13 low voltage tests points, and there is not shorts (0ohms). Troubleshooting tips suggests that "low" resistance need to be check but the lowest I get is on +15v with 94 ohms to GND.
Other lower values are : 109 ohm (+9v) and 139 ohm (-15v).

I powered the unit and measured the 13 voltages, they are all in specs except +9V that is under the tolerance - reading 8.57V.

May be first thing should be replace R2021 with a proper 10 ohm, 0,25W 0,5% ?

Jose

Jose


Roger Evans
 

Jose,

Don't worry about the apparent failure of the B timebase controls. Tektronix use the terms Channel 1 and Channel 2 to refer to the two vertical inputs and Sweep (or timebase) A and Sweep B to refer to the main and delayed horizontal signals. So the B trigger controls will only have an effect when you are using the delayed timebase (pull out the time/div knob and then turn it, and even then only if you are using the 'trigger after delay' setting. The manual will make things clear.

The 465 has a very different power supply to the earlier 400 series, and one of the changes is that there is a 'balance node' which checks that all the major low voltages are correct and if not it turns off the power.

If you look at schematic <14> Power Inverter and find U1910, you will find eight resistors connecting the low voltage power rails to pin 2 of U1910, labelled 'balance node'. If all the low voltages are correct the voltage on pin 2 will be close to zero and the inverter will continue to run.

You will notice that one of these voltages is +9V (R1954) and this is the supply rail that you find is not within spec so this may be the cause of the unreliable starting of the power supply (if it is close to the limit value then a small change in +9V can make the difference between OK and not OK.

If you look at schematic <15> Power Supply, then you see that the +9V is derived from +15V (which should be OK) and an operational amplifier U2052 which forces the output voltage (on pin 2) to be very close to the voltage at the junction of the precision resistors R2051 and R2052. It is possible to check the values of these resistors but to be safe you should carefully unsolder one end of each before making the measurement. Before you do that I would recommend measuring the voltage across R2056, 15R. The manual shows a current rating of 0.3A for the +9V supply so the voltage drop across R2056 should be less than 0.45V, if it is more than that then Q2056 may not be able to control the output effectively. C2055 or C2056 may be leaking, or there may be a fault further downstream. Also measure the voltage at the collector of Q2056 with respect to ground and the voltage on pins 2, 3 and 6 of U2052.

I don't think there is an urgent need to replace R2021 but it is your call.

The 'low' resistance you measure from the supply rails to ground are not a cause for concern. It is very rarely a real 'resistance' but is the sum of the currents through many parallel paths, mostly through PN junctions in the various semiconductors. If the supply voltage is OK that is the acid test.

The calibrator should produce its nominal output into 1Mohm and one tenth of that (0.5V) into 50R. The voltage level is adjustable and in a well used scope there is no guarantee that it is still accurate. It is also switchable between 1kHz and 1MHz so you should tell us which frequency you used for the timebase observation.

Let us know how you get on.

Regards,

Roger


Roger Evans
 

Just spotted an error, the voltage drop across R2056 should of course be less than 4.5V.

Many apologies!

Roger


Ron Rossi
 

In case you don't have a PDF version of the service manual I had found a searchable version on the web somewhere. I put a copy at http://kk1l.com/TEK485_r485_text.pdf.

Good luck with your project. I have a 485 as well which recently went dead similar to yours. We are following the same path :)


Ron Rossi
 

Trying to post this a second time. Not sure what happened to the first.

In case you would find it easier to use a searchable service manual for the 485, I have put a copy on my website...

http://kk1l.com/TEK485_r485_text.pdf

73 es God Bless de KK1L, Ron <><