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Re: TDS794D, alleged shorted power supply?

Richard Knoppow
 

Do you have or have access to a metered Variac? If you do connect it and start with low voltage and watch the current meter. Be ready on the Variac power switch. Current can come up pretty fast even if there is no short but a short will generally draw a lot of current right away. I would not be too concerned with the seals since this is a "reject".  How do you know the line cord is good under load if you haven't powered it up?
   Either put it on a metered Variac or dim bulb tester or stick it in a display case.
   Of course, I would open it up first and look for any signs of something shorting. Have someone else follow you up, its easy to miss even fairly obvious things.

On 7/28/2020 2:30 PM, Roy Thistle wrote:
I've got this TDS794D in skookum cosmetic shape... that is just daring me to plug it in, and boot it up. Only... the... assumed... "dumpster diver" I got it from claims... "It has a shorted power supply." Further query directed to the dd, did not elucidate significantly.
Fuse has continuity.
Line cord is good, under load.
No toasty smell to it.
I can't see as it has ever been opened (It still has an intact SIMCO, anti-tamper/cal sticker on it... and none of the usual clues a thing has been opened, since SIMCO had it.)
I am kinda chicken to try to power it up... because, I so want it to work... or be an easy fix.
The only explanations... for the "explanation" that it has a shorted power supply...that have crossed my mind... is that that is what SIMCO reported... or documented... if the 794D was indeed sent to them for cal/repair... or even came from them... maybe from an equipment auction?
Another thing that I was thinking it that if the scope boots, at least to error checking, would it report and error indicating a shorted (low?) power supply?
Any suggestions/recommendations, on how to proceed? Any "gotchas" to watch out for?
Anyone got a screen shot of one booting?
The service manual (from the Tektronix Website) has a reasonable service flow chart... so if I dare to plug it in... and it starts to boot... I can work from there. (I don't see anything about error reporting... or explanations/table of error codes.)



--
Richard Knoppow
dickburk@ix.netcom.com
WB6KBL


Re: TDS794D, alleged shorted power supply?

amirb
 

On Tue, Jul 28, 2020 at 08:48 PM, Jay Walling wrote:


A likely candidate if the scope is cycling on and off is the CRT assy. Easy to
test for - just disconnect the ribbon cable to it.
the OP has not even plugged it in yet so it is not clear if it will cycle on and off

if you have a auto transformer or isolation transformer, just start the scope at something like 50-60Vac
and increase it gradually and check the current draw. If not, just put a light bulb in series and plug it in

another option is to open the thing and unplug the power supply output connector (you need to lift the cpu board and perhaps some
option boards to reach it and then plug it in and check the power supply alone (even unloaded is OK)
this way you can visually check the psu as well. I would imagine you will see burned components right away


Re: TDS794D, alleged shorted power supply?

 

A likely candidate if the scope is cycling on and off is the CRT assy. Easy to test for - just disconnect the ribbon cable to it.


Housings to make custom TM500 plug ins, plus some good salvage

 

I have had many requests from stuff day visitors about junk TM500 plug ins to use to make custom test items, extenders, and so on. I am now out of free TM500 giveaways, and scrap metalwork to make TM500 items, but I went on a deep search and found something just as good, maybe better!

I have a big pile of mint looking Argo Systems plug ins (complete with pull tabs and covers), they are made from Tek provided TM500 parts, so they are prefect for making your own TM500 widget. Just keep in mind these two things:
1. they ARE NOT electrically compatible with a TM500 frame, even though the connector is the same.
2. The connector has no slots, so if you are going to try and use it after stripping the other stuff out, be aware it has NO SLOTS, you will have to file those in yourself.

These are FULL of great salvage parts, one even has a Trilithic 1-32dB step attenuator (wow), the other type is loaded with socketed logic dips. In any case, it's the best solution I could come up with, and they are just $25 each. you can see them all here on the stuff day page:
https://www.sphere.bc.ca/test/stuffday.html#tekfans

There are even some double-wide ones in the Argo Systems listing below these plug ins, I can offer them for $25 as well.

scroll down almost to the end of the Tek items, and you will find the Argo as210 stuff.
all the best,
stay safe,
walter (walter2 -at- sphere.bc.ca)
sphere research corp.
https://www.sphere.bc.ca/test/index.html


Re: Just got a 475, nothing showing on display

Stephen
 

I’m almost as new as you are to this, and here. But it sounds like the same problem I have with a 465. I have a dead HV Multiplier. Experts here will guide you and tell you more. I hope I’m wrong.


TDS794D, alleged shorted power supply?

Roy Thistle
 

I've got this TDS794D in skookum cosmetic shape... that is just daring me to plug it in, and boot it up. Only... the... assumed... "dumpster diver" I got it from claims... "It has a shorted power supply." Further query directed to the dd, did not elucidate significantly.
Fuse has continuity.
Line cord is good, under load.
No toasty smell to it.
I can't see as it has ever been opened (It still has an intact SIMCO, anti-tamper/cal sticker on it... and none of the usual clues a thing has been opened, since SIMCO had it.)
I am kinda chicken to try to power it up... because, I so want it to work... or be an easy fix.
The only explanations... for the "explanation" that it has a shorted power supply...that have crossed my mind... is that that is what SIMCO reported... or documented... if the 794D was indeed sent to them for cal/repair... or even came from them... maybe from an equipment auction?
Another thing that I was thinking it that if the scope boots, at least to error checking, would it report and error indicating a shorted (low?) power supply?
Any suggestions/recommendations, on how to proceed? Any "gotchas" to watch out for?
Anyone got a screen shot of one booting?
The service manual (from the Tektronix Website) has a reasonable service flow chart... so if I dare to plug it in... and it starts to boot... I can work from there. (I don't see anything about error reporting... or explanations/table of error codes.)


Re: DC503 Not Working

Stephen
 

I’ve been quite busy today and I haven’t had the chance to check what you guys suggested yet.
However, I did notice something that may or may not be worth noting: When I input a signal to input A,
let’s say 1Khz, and “if” And only if the only digit lit is the one in the middle (the 3rd from the left I think), when I’m in the Totalize A position, and press the start button, it counts from 0 to 9 repetitively. And this is all this unit does.


Just got a 475, nothing showing on display

Jake W
 

Hey guys,

I just got a Tektronix 475 from ebay. The fan spins, lights come on, but there’s no trace and no dot when I hit “beam finder”. I measured the power supply test points and got these results:

+5: 4.99v, 48.5 ohm to ground, 0mv ripple voltage
-8: -7.96v, 40.8 ohm, 0mv ripple
+15: 14.98v, 68 ohm, 4mv ripple
+50: 50.00v, 2.78k ohm, 36v* ripple
+50 unregulated: 67v, OL, 31.9v* ripple
+105/160: 139.4v, OL, 13.8v* ripple
+110: 111.1v, OL, 20.9v* ripple

* where I measured high ripple voltages, my multimeter switched back and forth from That voltage to zero in about 1 second intervals. Not sure what that means.

The resistance on the +5, -8, and +15 volt rails seems very low, is that normal? I didn’t see any expected values in the service manual (although I haven’t read too deep).

I also noticed some kind of corrosion around where the giant caps(?) are.

It might be worth noting that the beam finder seemed to work on the ebay listing, but I haven’t been able to make it work.

I’m new here and fairly new to electronics in general so I’d appreciate any help. Thanks!


DO NOT REPLY: Apologies for the SPAM

 

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Please do not reply to me or to TekScopes regarding THIS email.
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I apologize for allowing a spammer to become a TekScopes member.
He has been banned.
His posts have been removed.
All replies to it from TekScopes members have been removed.

Thank you to Siggi for alerting me to this spammer.

Dennis Tillman W7pF


Re: 11801 questions - funny fan noise and light

Perdad
 

I have an CSA 803C and I also have this Fan-problem and the Light.

The Fan in my instrument works fine for about 2 minutes after turning it on and after that it starts beeing noisy.
I have now changed to a Noctua Fan (Noctua NFA12x25, PWM/FLX does not matter) which I really can recommend.
The air flow is good and it is wisper quiet to start with and gets a bit louder when the error hits the Fan so to say :o)

I have tried to remove the PSU but the cables are very short and entangled with the boards surrounding it and the
conectors are hard to reach, but I will try to give it a go some day.

Regards, Per


TAS455 TAS465 SMPS 119-5024-00 (Astec AA18020) repair

Jon Harrison-Hughes
 

Hi everyone,

I'm trying to save this 'scope from landfill after a previous owner connected it to 230V with the power supply switched to 110V ! It's the earlier model (pre S/N B020099) with the switchable Astec AA18020 power supply. I have two initial requests

1) C5 and C6 , VDR1 and VDR2 have been removed. Please could someone tell me what was originally fitted in these positions.
2) Does anyone have a schematic for this power supply ? It's omitted from the Tek 070-8524-02 service manual.

Many thanks,

Jon


Re: Tektronix 465: Weird Trace behavior

Colin Herbert
 

A few questions:

Are there any internal cables that might be getting caught between the cabinet and something else?

Is the mu-metal shield on the crt OK. Mu-metal can lose its magnetic screening properties if it is bent or otherwise worked; it's a bit like work-hardening and can only be fixed by annealing, which isn't going to be easy with a scope.

Has the cabinet got magnetised some way while it was off? You could try taking the cabinet off and move it around the scope when it is turned on. If it has got magnetised, that might show it up. It shouldn't be too difficult to de-magnetise the case, but you will need a fairly strong de-magnetiser.

Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Jose Alfredo
Sent: 28 July 2020 18:46
To: TekScopes@groups.io
Subject: [TekScopes] Tektronix 465: Weird Trace behavior

Hi,

I had just replace my tekscope capacitors, and when testing everything
was ok. But, when I closed the cabinet, the trace, in ground position,
for both channels, get curved down, in the left end of the trace. If I
remove the cabinet, it gets fixed. Can anyone of you help me with this
issue?

Thanks

Jose Alfredo


Tektronix 465: Weird Trace behavior

Jose Alfredo
 

Hi,

I had just replace my tekscope capacitors, and when testing everything was ok. But, when I closed the cabinet, the trace, in ground position, for both channels, get curved down, in the left end of the trace. If I remove the cabinet, it gets fixed. Can anyone of you help me with this issue?

Thanks

Jose Alfredo


Re: DC503 Not Working

SCMenasian
 

If the voltage at pin 6 turns out to be LOW, measure the resistance from pin 6 to ground two ways - with the plugin installed in the mainframe or not. This will indicate whether the problem is in the plugin or the mainframe.


Re: DC503 Not Working

Stephen
 

On Tue, Jul 28, 2020 at 02:19 AM, Roger Evans wrote:

Interleaved.


When you measure R331 in-circuit there is a parallel current path through U330
and R330 (1k) so you might well see a low value of resistance and it would
probably be different if you reversed the polarity of the probes.
I measured it in circuit, but with the unit off. If it’s of any significance.

R331 being
low wouldn't be a problem since it is only required to establish a logic 1
level at pin 6. The problem would arise if for instance there was some damage
to the PCB giving a low resistance from pin 6 to ground or one of the
'optional' connectors on the TM506 backplane that someone had previously set
Apparently not. No optional jumpers or anything on the connectors themselves, but « Option 2 » is installed. However, nothing is connected to it.

for a different plugin. As long as the voltage on pin 6 is more than about
2.5V that is read as logic '1'. It will probably measure very close to 5V.
I will check all that.

The regulator for the -22V supply is not an IC, it is the discrete circuitry
around Q540, Q545, Q548 and the PNP pass transistor in the mainframe.
Maybe I didn’t express myself correctly. What I meant is that I also replaced U500 and U251.

Roger


Re: DC503 Not Working

Stephen
 

On Tue, Jul 28, 2020 at 02:12 AM, <scm@menasians.com> wrote:

Interleaved

According to my diagram, R331 should be 5.6k. Actually, if it's 1k, there
should be no problem (unless someone changed it in the past to try to cure the
real problem).
It appears to be stock. And I measured it in circuit. I’ll lift a pin and see what it says...

Move the test lead to pin 6 and measure the voltage there. If
it is logic HIGH, the multivibrator should run; logic LOW inhibits the
multivibrator.
Will do.


Re: DC503 Not Working

Roger Evans
 

When you measure R331 in-circuit there is a parallel current path through U330 and R330 (1k) so you might well see a low value of resistance and it would probably be different if you reversed the polarity of the probes. R331 being low wouldn't be a problem since it is only required to establish a logic 1 level at pin 6. The problem would arise if for instance there was some damage to the PCB giving a low resistance from pin 6 to ground or one of the 'optional' connectors on the TM506 backplane that someone had previously set for a different plugin. As long as the voltage on pin 6 is more than about 2.5V that is read as logic '1'. It will probably measure very close to 5V.

The regulator for the -22V supply is not an IC, it is the discrete circuitry around Q540, Q545, Q548 and the PNP pass transistor in the mainframe.

Roger


Re: DC503 Not Working

SCMenasian
 

According to my diagram, R331 should be 5.6k. Actually, if it's 1k, there should be no problem (unless someone changed it in the past to try to cure the real problem). Move the test lead to pin 6 and measure the voltage there. If it is logic HIGH, the multivibrator should run; logic LOW inhibits the multivibrator.


Re: TM506

Stephen
 

For those who have been following and are interested, the FG502 is now up and running smoothly.
I will check if a calibration is needed.
It turns out the entire output stage was fried. If I follow the brown discoloration of the board from the BNC output backwards, I’d say someone must have injected some nasty signal in there.


Re: DC503 Not Working

Stephen
 

BTW, it may be worth mentioning that R331, which is connected to pin 6 of U330(B), reads 1K in circuit.

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