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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!


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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
 

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.


Re: DC503 Not Working

Stephen
 

On Tue, Jul 28, 2020 at 12:39 AM, <scm@menasians.com> wrote:


I finally printed the manual out and figured out what scan clock disable (pin
6) is. It is an input from the backplane connector (pin 27A). This should not
be low resistance to ground, either on the DC503 or on the backplane. It
probably won't be, however. I suspect that since there seems to be some
intermittancy involved, that something is marginally disrupting the
multivibrator. Check the driven circuits as Roger suggested. Otherwise
suspicion returns to the multivibrator, itself. Resolder all the connections,
clean the socket and IC pins carefully and, possibly for good measure, replace
the capacitors (I know they look OK DC wise).
I will do and check all what you guys very kindly suggested, and report back as soon as it’s done.
I will replace those two 0.1uF capacitors for good measure. All I have readily available at the moment are rated at 600V instead of the 100V the schematic calls for. Overkill and way much bigger as you can imagine, but that’s all I have. Will try to make them fit as the top board needs to sit on top without shorting them.


Re: DC503 Not Working

SCMenasian
 

I finally printed the manual out and figured out what scan clock disable (pin 6) is. It is an input from the backplane connector (pin 27A). This should not be low resistance to ground, either on the DC503 or on the backplane. It probably won't be, however. I suspect that since there seems to be some intermittancy involved, that something is marginally disrupting the multivibrator. Check the driven circuits as Roger suggested. Otherwise suspicion returns to the multivibrator, itself. Resolder all the connections, clean the socket and IC pins carefully and, possibly for good measure, replace the capacitors (I know they look OK DC wise).


Re: DC503 Not Working

Stephen
 

On Mon, Jul 27, 2020 at 10:52 PM, Roger Evans wrote:

Interleaved answer.


Stephen,

You have (at least) two distinct issues with comments coming in which are
relevant to one or the other and are randomly interleaved.

1. The LCD display does not scan so typically only one LED is lit.
You mean that when the unit is on, only 1 “zero” should show?
I’ve never had one of these. I thought 4 should be show (from picts I’ve seen).

This you have traced to U330. One thing that hasn't been mentioned here is that the
output from U330 pin 10 also goes to U332 pin 6 so a fault in U332 (which I
don't think you have changed)
I have changed ALL the IC’s on that board but the 2 that I mentioned in an earlier post.

would stop the multivibrator from working. If
U332 is socketed, just remove it and see if you can get the square wave on
U330 pin 10.
Will do.

2. The -22V supply is somewhere around 0V. This completely changes the bias
of the analogue input stages and makes it less likely that any inputs will
actually trigger the digital circuitry.
When I measured 0V across that capacitor, the top “Trigger Board” was removed. Hence the cap was not
connected to the coil.

Here, I at least, am confused as to
whether there is too much current draw or a fault in the voltage regulator.
Both Voltage regulators were also changed. I really changed EVERY IC’s on that board but 2.

If you can measure the voltage across R546 it should help to narrow down the
problem.
Will do and report back.

If the voltage across R546 is more than about 0.6 - 0.7V it will
turn on Q540 and shut down the regulator as intended - then you have to track
down what is drawing too much current. If the voltage is much less than this
then there is likely to be a fault in the -22V regulator and you could measure
the voltages on the base, emitter and collector of Q548. It is also worth
checking that R546 is close to its nominal value, if it has drifted high it
will shut down the regulator prematurely.

Roger
Thank you Roger.


Re: DC503 Not Working

Roger Evans
 

Stephen,

You have (at least) two distinct issues with comments coming in which are relevant to one or the other and are randomly interleaved.

1. The LCD display does not scan so typically only one LED is lit. This you have traced to U330. One thing that hasn't been mentioned here is that the output from U330 pin 10 also goes to U332 pin 6 so a fault in U332 (which I don't think you have changed) would stop the multivibrator from working. If U332 is socketed, just remove it and see if you can get the square wave on U330 pin 10.

2. The -22V supply is somewhere around 0V. This completely changes the bias of the analogue input stages and makes it less likely that any inputs will actually trigger the digital circuitry. Here, I at least, am confused as to whether there is too much current draw or a fault in the voltage regulator. If you can measure the voltage across R546 it should help to narrow down the problem. If the voltage across R546 is more than about 0.6 - 0.7V it will turn on Q540 and shut down the regulator as intended - then you have to track down what is drawing too much current. If the voltage is much less than this then there is likely to be a fault in the -22V regulator and you could measure the voltages on the base, emitter and collector of Q548. It is also worth checking that R546 is close to its nominal value, if it has drifted high it will shut down the regulator prematurely.

Roger

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