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Re: 500 series Calibration

Christopher Hilton-Johnson
 

Chuck

As ever you are correct - I had overlooked the sneeky addition to the SW10 switch on schematic page 5 of the 545B manual, which clearly connects V125 to the preset adjust pot when in the Auto position. Mea cupla!

Having said that discretion etc works for me - I calibrate to good functionality bu not necessarily to the absolute!

Even so the scopes (both 545B & 549) work well within their published performance specs - except perhaps the 549 whose tube is a bit tired, but then aren't we all!

Good to hear from you & thanks

Christopher

On 06/01/2021 16:20, Chuck Harris wrote:
Hi Christopher,

Fundamentally, I believe it is because there are a lot of
other circuits controlled by the many poles of switch
10B (DC, AC, AC LF REJECT, AUTO).

Check out Schematic #4 and #5's SW10B wafers.

The calibration specifies that TRIGGERING MODE control
should be in the AC position, and the STABILITY control
should be in the CW position.... that would cause SW110
to be in the position shown in the schematic, and would
put the wiper of the STABILITY control at -150V, and
connect it to the grid circuit of V125.

But if the TRIGGERING MODE switch was in the AUTO position,
the switches would connect the wiper of the PRESET ADJUST
trimmer to the grid circuit of V125.

My guess is that would affect the centering adjustment in
some way that isn't exactly repeatable.

-Chuck Harris


Christopher Hilton-Johnson wrote:
hi Guys

Am spending the first few days of the newly instigated UK national lockdown
calibrating several 500 series scopes – a 545B and a 547.

In each case there is an intriguing point where the manual, as part of the A and B
timebase trigger level centering calibration asks you to:

for A Time Base Cal- ground the junction of R19 & R20 – both 545B (Change
instructions) and 549 (manual)

for B Time Base Cal – ground the junction of R69 & R70 - both 545B (manual) and 549
(manual)

Now here is the issue. Finding and connecting to the required junctions can be/is
hazardous particularly for those of us who have something of the shaky hand syndrome
and a well developed sense of self preservation when working with high voltages in
cramped spaces.

But wait: there appears to be a simple solution in every case. With the timebase
trigger switch in the *Auto* position there is a direct path between those resistor
junctions and chassis – the schematic says so and a simple continuity test with the
scope off confirms it. Interestingly my version of the 545B manual uses this method
in the manual, before changes brought in by the change information.

So am I missing something here? Does the suggested 'jumper lead' provide magic
continuity properties? Is there some hidden feature not shown on the circuit diagram?

Any ideas why the obviously easy way is not followed, indeed is specifically
disavowed it the case of the 545B change information.

Thanks in advance








Re: Intermittent power on problems with 2465B

Laura
 

The pre-regulator gets start up current via R1020. If it goes

high in value the scope will be on the raw edge of starting.

Correct value should be 270K.


Re: Intermittent power on problems with 2465B

Alex
 

Here is the "bible" of 2465 teardowns and what not. Very useful thread, all 69 (and going) pages are worth reading, specially if you are new to these scopes, or are even considering buying one off ebay, etc.

https://www.eevblog.com/forum/testgear/tektronix-2465b-oscilloscope-teardown/


Re: Intermittent power on problems with 2465B

Alex
 

On Tue, Jan 5, 2021 at 07:34 AM, Mark Hatch wrote:

I was hoping not to have to recap.... But it sounds like that is where this is
going.
If you want some peace of mind, I would consider a full PS recap for any 2465x scope a must do (sooner than later). Let alone take a quick look at A5 as well as you might actually be able to stop additional ongoing board damage on its tracks.

But you could of course just go about testing every cap inside the power supply, but that generally entails removing each cap to be tested. Once you find the bad one(s) just replace those and put the other old ones back in, and just wait until the next one fails and you have to go over the process all over again. And the cost of ordering a full set of caps from Mouser or Digikey should be around $30-40, so why not order all of them at once, and then while you have the new ones on hand, replace all of them.

Been there done that, trust me you want to get this out of the way and end up with a power supply that you know will no longer fail for a long time due to faulty caps. Besides, some of the caps that have been in there for 30+ years might still be working and not causing any problems, but at the same time they might already be leaking and you want to correct that as well. It usually takes pulling the cap from the board to discover leaks. So I guess by now you get my drift.


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

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


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

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....


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

Simon
 

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


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

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.


Re: Tek 7904 In Pulse Mode Related to C1511 on LV Regulator

 

Oops I mentally cross wired 485 and 7904 in my reply below, but as the PSU topologies and implementations are almost identical my comment still stands.

The most common tick mode culprits in the (early) 7904 PSU are C1314, C1318, C1353, and C1354 (all on the Cap. Rectifier board at the top of the board).

If the bulk filter caps go OC, then they can also cause tick mode, easy to check by measuring the voltage across them - they should both read about the same voltage (within 10-20%). Beware that they run at line (mains) voltage (in USA). Problem is that it's almost impossible to find replacements for these ☹, so you're reduced bodging in a modern snap-lock to cap to replace (unless you know of a 1000uF 200V 3/4" pitch part).

In the inverter board, VR1246 (110V in the early 7904, and IRC 120V in later ones) can go low voltage, and C1249 can go leaky or short which also causes problems, but neither of these are common problems.

On very rare occasions the Controller IC (U1275) can also go bad, only way to be sure on this is to swap parts.

Its also possible for faults in the VR circuits on the LV regulator board to cause tick mode because all the LV supplies contribute via a resistor array to a "summing junction" that should sit at 0V when the supplies are at their nominal values - these can be a pain to track down.

HtH
David

-----Original Message-----
From: TekScopes@groups.io <TekScopes@groups.io> On Behalf Of David C. Partridge
Sent: 07 January 2021 10:06
To: TekScopes@groups.io
Subject: Re: [TekScopes] Tek 7904 In Pulse Mode Related to C1511 on LV Regulator

For sure don't replace the metal cased tantalums in the PSU. It's highly unlikely they have failed.

If the power supply is in burst mode (ticking), then my experience of fixing 485s strongly suggests looking for a shorted dipped bead tantalum hanging off one of the power rails as the first probable culprit. Sure there are other possible causes like the 120V Zener going low but that's not very common fault.

Is the PSU ticking?

-----Original Message-----
From: TekScopes@groups.io <TekScopes@groups.io> On Behalf Of David Campbell
Sent: 07 January 2021 00:59
To: TekScopes@groups.io
Subject: Re: [TekScopes] Tek 7904 In Pulse Mode Related to C1511 on LV Regulator

This scope is 40 ( +) years old.* It is not a repair. It is a restoration.*

Replace ALL the caps in the PS. Try again.


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

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


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

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


Re: Peter Keller's CRT book

Jean-Paul
 

Bonjour cher Dennis, HAPPY NEW YEARS!

Yesterday, I have received the book, I am amazed by the hard cover binding, quality of printing and size and depth of the work.

BRAVO to Peter for this excellent tour de horizon of CRTs, with special focus on the Tek developments.

BRAVO to you for all your hard work on behalf of the group!

I am very appreciative,


All the Best and keep safe!


Jon


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

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


Re: Tek 7904 In Pulse Mode Related to C1511 on LV Regulator

 

For sure don't replace the metal cased tantalums in the PSU. It's highly unlikely they have failed.

If the power supply is in burst mode (ticking), then my experience of fixing 485s strongly suggests looking for a shorted dipped bead tantalum hanging off one of the power rails as the first probable culprit. Sure there are other possible causes like the 120V Zener going low but that's not very common fault.

Is the PSU ticking?

-----Original Message-----
From: TekScopes@groups.io <TekScopes@groups.io> On Behalf Of David Campbell
Sent: 07 January 2021 00:59
To: TekScopes@groups.io
Subject: Re: [TekScopes] Tek 7904 In Pulse Mode Related to C1511 on LV Regulator

This scope is 40 ( +) years old.* It is not a repair. It is a restoration.*

Replace ALL the caps in the PS. Try again.


Re: Grid Bias Adjustment on 475A

 

Michael,

Oh, I know exactly what you are talking about. It looks like and empty pad or via, and it IS partially hidden under a bundle of wires. I saw it and wondered about it when I took the picture. Okay, my course is clear. Tally ho!

-- Jeff Dutky


Re: Grid Bias Adjustment on 475A

Mlynch001
 

On Thu, Jan 7, 2021 at 12:14 AM, Jeff Dutky wrote:


've checked all the wires in that vicinity to see if any are connected to
ground, but found nothing. If you could indicate anything that you think might
be a good candidate for the HV multiplier ground wire I would appreciate it.
Otherwise I guess I'm taking off the vertical board (I can see the aluminum
case that houses both the HV multiplier and (I think) the transformer) and it
does look like I can get to it with the vertical board removed.
Jeff,

The ground wire comes in from the top side of the board and connects to the ground plane. It is behind the vertical board. You do not have the service jumper that is present on some 465 and 465B models. The vertical board will need to come out to disconnect the Ground conductor. I have uploaded two additional pictures to the file in order to indicate where this connection should be located.
--
Michael Lynch
Dardanelle, AR


Re: Grid Bias Adjustment on 475A

 

Michael,

I don't see a ground connection for the HV multiplier. I have posted an image of the HV section of my 475A in the photo album entitled "475A DIY Calibration Adventures" https://groups.io/g/TekScopes/album?id=258453

I've checked all the wires in that vicinity to see if any are connected to ground, but found nothing. If you could indicate anything that you think might be a good candidate for the HV multiplier ground wire I would appreciate it. Otherwise I guess I'm taking off the vertical board (I can see the aluminum case that houses both the HV multiplier and (I think) the transformer) and it does look like I can get to it with the vertical board removed.

-- Jeff Dutky


Re: Grid Bias Adjustment on 475A

Mlynch001
 

On Wed, Jan 6, 2021 at 11:48 PM, Jeff Dutky wrote:


It appears to be mounted on the reverse side of the main interface board by
two nylon nuts, but I don't see any electrical connections. I haven't found
any discussion in the service manual of the HV multiplier.
Jeff,

There are two connections to the HV multiplier on the back side of the board. The input comes from the Transformer, through a diode along with a conductor. and the ground through the wire that I previously described. The manual that I am referring to has several mentions of the multiplier. Page 3-26 describes the HVM. Page 4-16 describes Troubleshooting the circuit. A bad multiplier will cause a reduced width, dim and fairly unfocused display. If the transformer happened to be bad, I believe that you can access it in the same manner, although I have never had to do that job myself. I am not strictly focused
on the HVM, but I have seen several failures of that part. It would pay to eliminate it as a suspect.
--
Michael Lynch
Dardanelle, AR


Peter Keller's CRT book

ken chalfant
 

Greetings,

I live in Colorado Springs, Colorado and received my copy of Peter Keller’s book today.

I want to extend my complements to Peter for his efforts to create such a wonderful and informative book.

Additionally, I wish to express my gratitude to Dennis Tillman for his efforts, pain and suffering to make this work available to us, as well as all the other work he does on our behalf.

I really hadn’t given it much thought but I guess I was expecting a “perfect bind” or soft cover book. When I opened the mailer and was met with a beautiful “hard case” or hardcover gold debossed text book I was delighted. After spending some time with the book I am so very glad I got on the list to get a copy.

Dennis and Peter - Thank you so very much!

Regards,

Ken


Re: Grid Bias Adjustment on 475A

 

Michael,

I just re-read your earlier post detailing how to disconnect the ground for the HV multiplier. I will try that straight away.

Thanks.

-- Jeff Dutky

P.S. Also thanks for pointing out that I can get at these parts by removing only the vertical amp board. That's still a fair bit of work, but well less than half as much effort as I was anticipating.

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