Date   

Re: 7623A Storage problem

Colin Herbert
 

I have had some further observations, not good, I'm afraid.

I thought that I would leave the scope in "store" mode and see if any improvement happened. It didn't. When I powered down, left it a while and then powered up in "non-store" mode, I was hoping to see the scope working as it had been before, but the screen was totally blank; no beam, no beam-finder, no readout. I have checked all of the low-power voltages and they are all within spec. Also, I can see the saw-tooth output on the back of the scope, so some circuits are working. However, the -1475 V high voltage is absent. I assume this will have the effect of losing the beam, but what could have caused it's failure?

Another anomaly is that the voltage at the "collector" test-point on the storage board is -1.4 V, when it should be +150 V.

I would certainly like to get the "non-store" mode working again, possibly by repairing the -1475 V on the CRT, then assessing why the storage mode wasn't working properly.

TIA, Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Roger Evans via groups.io
Sent: 26 August 2020 15:38
To: TekScopes@groups.io
Subject: Re: [TekScopes] 7623A Storage problem

Colin,

I don't claim to be an expert on storage CRTs but I spent a surprisingly pleasant couple of weeks tracking down the several problems on a 7623A which not only had some real faults but also had the storage board wrongly wired up after removal by the previous owner. The performance of the 7623A was so good that when I spotted a 'spares or repair' 7633 at a bargain price I thought I would have another go. I did get it working but the storage tube was well used and its writing speed was not up to the (good) 7623A.

The methodology I used was to set up a repetitive store and erase cycle and follow the waveforms that the manual shows for each of the storage electrodes and each storage mode. The waveforms are complex and rely on some logic and 74123 monostables to set up the timing and then the logic switches different voltages into the op-amps that drive the relatively high voltages on the storage electrodes. Between the two scopes I must have replaced four of the 74123s and one or two 74xx logic ICs. The other failure common to both boards was (one or more of) the set of four transistors that control the 600V line (Q1728 ..Q1743) and resistors around these had drifted significantly in value. The 7633 also had internal tracking of the PCB traces (four layer board) in the vicinity of the high voltage area which I finally managed to fix by drilling and filing until I had cleared the leakage and adding jumpers for the tracks that had been removed in the process.

Having a digital storage scope to follow the erase and store cycle made life a lot easier than trying to remember what had just flashed on the screen and wouldn't return for a few seconds. A wild guess based on your symptoms would be that one of the 74xx logic ICs has failed but finding the culprit may take time.

Regards,

Roger


Re: 465B Horizontal Trace Issues - Help requested

 

Hi Gary,

The very first thing you need to do is to measure the low voltage test
points for DC level and ripple. They are marked on the board. These
scopes usually have bad electrolytic capacitors and you need to test for
that.

The service manual will tell you what to measure and what to expect.

The correct manual can be had here.

http://artekmanuals.com/manuals/tektronix-manuals/

Regards,

Tom

On 9/20/2020 7:41 PM, gary@pincus4.com wrote:
I have a 465B/DM44 that is new to me that I think has issues. I am a relative newby to scopes, but this one does not seem right. There is "compression" of the horizontal trace, but it varies. Sometimes the trace just shrinks to a width of about 1-1.2 major graticule (1/2" in the same spot), other times it only shrinks a bit, covering about 7.3 graticules, with the best it ever gets being about 9.6 (full screen = 10). This significant compression happens randomly, both when scope is relatively "cold' and after it is fully warmed up. It affects both channels. I've used different probes - no difference. I can impact this, by moving the sweep left to right with the horizontal position control. This sometimes moving the horiz position "fixes" it, other times not. If it is temporarily working OK, I can get it to shrink if I aggressively manipulate the horiz position knob. In addition the "timing" may not be correct - not sure I am saying this correctly, but the calibration square wave gets "smaller" horizontally as the trace moves from left to right. I would expect the length of the square wave to be the same all across the screen. I suspect this is related to the compression, but not sure - seems relevant to mention. The only thing I have done is removed the cover and made sure the connections to the deflection plates on the CRT were secure (they appeared to be). Everything looks clean and untouched inside, and overall the unit is clean and does not look abused. I have not cleaned any switches or controls, but that seems like a good place to start. If it matters, serial number is above 60xxx, which I believe makes it the later series. I have downloaded the manual from the TEK site, but they only have the low S/N version. If it would help I have photos and even short video of the bad behavior.
I come here seeking help, on how to proceed, possible causes, how to troubleshoot. Thanks in advance Gary.




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Re: 465B Horizontal Trace Issues - Help requested

@Sabrefencer
 

Note: this post was initially posted to Tekscopes2. That post has been deleted.


Re: 465B Horizontal Trace Issues - Help requested

@Sabrefencer
 

Thanks Shailendra, Cleaning the pots will be the first thing i do when i open it up again. My go-to is Deoxit-5.


Re: Looking for calculator keyboard for 7854

Szabolcs Szigeti
 

Hi,

How on earth did they lose the package?

Szabolcs


Martin Whybrow <martin@thewhybrows.co.uk> ezt írta (időpont: 2020. szept.
22., K, 10:49):

I'm after the external keyboard for my 7854; I found one at a good price
on ebay and bought it, but ebay's GSP managed to lose the package. All the
other units are either stupidly priced or the seller wants more for
shipping than the item itself is priced at. I have only found these in the
US, and shipping to the UK is prohibitively expensive in most cases. Does
anyone have one they want to part with, preferably in Europe or willing to
ship for a realistic cost from outside of Europe?






Re: 465B Horizontal Trace Issues - Help requested

Shailendra Krishan
 

I have faced a similar problem on my 465B. In my case, horizontal sweep used to shrink and expand at random. There was no linearity issue, as calibrator square wave looked uniform across the width of display, only shrunk randomly to anywhere from six to ten horizontal divisions. Fault was traced to an intermittent x1 gain setting potentiometer in the horizontal amplifier. This pot was replaced and gain adjusted as per service procedure, and scope has worked fine since then.

In your case, it may be a noisy horizontal position potentiometer, in addition to x1 gain potentiometer. Since you have linearity problems also, as you see shrinking of square waves at some positions of screen, there could be linearity problems in sweep area, or some limiting due to a failing component in one half of the horizontal amplifier section.

My two cents to share, hoping you can get some ideas to explore.

Shailendra


Re: Is my Tek 468 beyond repair?

SCMenasian
 

A fairly robust way of buffering would use a fast PNP transistor and a fast NPN transistor with the bases connected together and the emitters connected together; PNP collector goes to the Vcc supply; NPN collector goes to ground. Input to the (connected) bases and output taken from the (connected) emitters.
Shoot-through is impossible and current drive (both positive and negative) can be quite large. Voltage gain is unity, reduced by the EB drops. Efficiency is quite high. As long as you don't need voltage gain, this should work quite well. BTW, this also works very well for driving power mosfets.

SCM


Re: TDS 3000B-family 'scope (TDS 3014B) has problem with LAN interface

 

On Sun, Sep 20, 2020 at 08:03 PM, Raymond Domp Frank wrote:


A simple pin-converter is needed: A 24-pin socket for the DS1742W is placed
above a 32-pin socket + header with pins 7 - 28 of the DS1250W corresponding
with pins 1 - 24 of the DS1742W.
Pin 29 of the DS1250W is re-routed to pin 21 of the DS1742W and pin 32 of the
DS1250W is re- routed to pin 24 of the DS1742W. The original straight-through
connections of these two sets are removed, the remaining 22 connections go
straight-through.
I haven't yet looked at possible WE/OE/CE (timing) discrepancies in the
programmer but I'm confident things will be OK. Only the first 2K of the
"DS1250W" will be written.

I'll report back (or possibly, continue this monologue) with results.
In my ecstasy about the successful revival of my TDS3014B's Ethernet interface using the procedure described above, I forgot to thank those who helped me.

Thanks!

Raymond


Re: 465B Horizontal Trace Issues - Help requested

Roy Thistle
 

Hi Gary:
See,
http://w140.com/tekwiki/wiki/465
to download Tektronix's superb and easy to follow (compared to the stuff we get today.)
There is a lot of info on the Web about this very popular scope... just Google.
I suggest, read the 465B user manual to make sure you understand what the unit can, is supposed, to do. (Based on that understanding, you might have a better understanding of what issues (if any) the unit has.
A search on messages in this forum will also yield plenty of info/tips on these scopes.
Yes, post a link to your photos/video on Google Photos, Imgur... and video to YouTube... and people on the forum will take a look.
There are many very knowledgeable people on this forum, who generously share their time. Let us make best use of that.
Regards.
Roy


Re: Is my Tek 468 beyond repair?

Roger Evans
 

John,

A bipolar or JFET buffer is quite feasible but the collector (drain) load resistor has to be quite a small value. With the semiconductor turned off the resistor has to charge up 4 gate input capacitance plus stray capacitance in around 5nsec ( 1/4 period at 50MHz), 20pF total capacitance requires around 220R load resistance.

Roger


Re: Is my Tek 468 beyond repair?

John
 

Its probably a bit academic since I have ordered replacement parts, but funnily enough I was just wondering whether a transistor (or JFET) could be used to buffer the output and reduce stress on the crystal. Just needs one that can comfortably handle the switching speed I guess. Perhaps worth a quick test....


Re: Is my Tek 468 beyond repair?

satbeginner
 

If you don't want to wait that long, you can temporarily add a NPN switching transistor as a simple pulse inverter.

Input via 1k to the base, 10k between base and emitter, emitter to ground.
Pull up resistor of 1k from +5V to collector, collector as output to the 112's

This way you ignore the lower output level of the oscillator.

Quick and dirty, I agree...

Leo


Re: Is my Tek 468 beyond repair?

John
 

satbeginner, that is a good point, thank you, but yes, just to confirm 5V and GND were present at the oscillator and the 74S112 chips when I ran the tests.

Just another update on the crystal saga (the last one for now!), I ran the crystal test for about 10 mins yesterday but decided to do a further soak test this morning. Must have ran it for about an hour. Results initially were the same as yesterday: The top peak hit about 4.2V without load, about 3.5V when loaded down with a 470Ω resistor. However after an hour, the top peak was at just +2.6V. Not sure exactly how long it took to drop to that level, but wouldn't that be rather borderline for TTL?

In any case, replacement crystal ordered so will be able to compare when it arrives.


465B Horizontal Trace Issues - Help requested

@Sabrefencer
 

I have a 465B/DM44 that is new to me that I think has issues. I am a relative newby to scopes, but this one does not seem right. There is "compression" of the horizontal trace, but it varies. Sometimes the trace just shrinks to a width of about 1-1.2 major graticule (1/2" in the same spot), other times it only shrinks a bit, covering about 7.3 graticules, with the best it ever gets being about 9.6 (full screen = 10). This significant compression happens randomly, both when scope is relatively "cold' and after it is fully warmed up. It affects both channels. I've used different probes - no difference. I can impact this, by moving the sweep left to right with the horizontal position control. This sometimes moving the horiz position "fixes" it, other times not. If it is temporarily working OK, I can get it to shrink if I aggressively manipulate the horiz position knob. In addition the "timing" may not be correct - not sure I am saying this correctly, but the calibration square wave gets "smaller" horizontally as the trace moves from left to right. I would expect the length of the square wave to be the same all across the screen. I suspect this is related to the compression, but not sure - seems relevant to mention. The only thing I have done is removed the cover and made sure the connections to the deflection plates on the CRT were secure (they appeared to be). Everything looks clean and untouched inside, and overall the unit is clean and does not look abused. I have not cleaned any switches or controls, but that seems like a good place to start. If it matters, serial number is above 60xxx, which I believe makes it the later series. I have downloaded the manual from the TEK site, but they only have the low S/N version. If it would help I have photos and even short video of the bad behavior.
I come here seeking help, on how to proceed, possible causes, how to troubleshoot. Thanks in advance Gary.


Looking for calculator keyboard for 7854

Martin Whybrow
 

I'm after the external keyboard for my 7854; I found one at a good price on ebay and bought it, but ebay's GSP managed to lose the package. All the other units are either stupidly priced or the seller wants more for shipping than the item itself is priced at. I have only found these in the US, and shipping to the UK is prohibitively expensive in most cases. Does anyone have one they want to part with, preferably in Europe or willing to ship for a realistic cost from outside of Europe?


Re: Is my Tek 468 beyond repair?

satbeginner
 

"I hooked it back up to the board using jump leads, powered up the 5V rail but there was no output. I hooked up the sig gen again and got output from both 74S112s. "

Do you have the onboard 5V checked at the oscillator as well??
And the ground to the oscillator?

5V onboard does not mean it is 5V everywhere.

Just my 2cts

Leo


Re: Is my Tek 468 beyond repair?

John
 

Philips 74HCT112 in 16 pin DIL package are available from a UK seller on eBay at £2.99 plus 40p postage, not anyone I have bought from but I don't think I have had any dead or fake parts from UK sellers offering almost immediate delivery (2-3 days). Also in 16 pin DIL from France, six for 10 Euros, Texas Instruments, delivery in a couple of days so shouldn't be direct from China.
Roger, saw both of those. Decided to go for 74S112's as per original design. These were NOS and 10 pieces for a fiver and local to me.

Interesting that your crystal holds its voltage with a 390Ω load resistor across it. I have added a crystal to the shopping list. The total came to about £30 and included sockets, 74S112, 8085 processor, crystal and some caps. I know there are other faults but I hope it doesn't come to too much more. If this gets the computer running then all well and good. Of course, its not all about money but also the satisfaction of bringing a piece of vintage kit back to life, but I am not sure how much more will be worthwhile spending on it....

Parts will take a few days to arrive so in the meantime I will replace the caps that I can and remove the two 74S112s and clean the holes so the board is ready for the sockets and replacement ICs when they arrive.


Re: Is my Tek 468 beyond repair?

Roger Evans
 

Philips 74HCT112 in 16 pin DIL package are available from a UK seller on eBay at £2.99 plus 40p postage, not anyone I have bought from but I don't think I have had any dead or fake parts from UK sellers offering almost immediate delivery (2-3 days). Also in 16 pin DIL from France, six for 10 Euros, Texas Instruments, delivery in a couple of days so shouldn't be direct from China.

I also measured the output of a 50MHz can oscillator in 8 pin outline that I happened to have to hand. About 4.4V pk-pk with a 390R resistor either to ground or to Vcc. I thought the latter was a better match to sinking the TTL input currents.

Roger


Re: Is my Tek 468 beyond repair?

Harvey White
 

You may want to check the other variants of the "74" class, such as ACT, and ACHT.  Note that the T = TTL compatibility on the inputs, but TTL to CMOS (straight) does not.  2.4 volts minimum output is not 75% of VCC on the CMOS circuit, so logic 1 is iffy on straight TTL -> CMOS).  So for talking to CMOS (only) circuits, the HC, C, and AHC (etc) parts should be ok.  IF you're putting outputting to a TTL gate, or getting something from a TTL gate (most important) then you want HCT, ACT, etc., as long as the clock frequency is sufficient.

As for the surface mount stuff, IIRC the pinouts are identical, but the package size is not.  There are companies making surface mount to DIP adaptors that aren't too expensive if you can't design/make your own.  A single sided board is sufficient.  IF you design your own, and send it to a Chinese fab place, then duplicate the pattern and you're likely to have over a hundred for 5 dollars plus shipping.

Harvey

On 9/21/2020 8:02 PM, Brian Cockburn wrote:
Hi,

When I look at the Nexperia 74HCT112 data sheet I would guess that it will be fast enough. You may have to buy a few and hand select. But, and it's a big but, they look to only be available in surface mount. No DIP part. :-( Going for a 74S112 from a NOS vendor would be the best solution I reckon. And. just in case a reminder is needed. eBay and various Chinese vendors sometimes end up delivering re-marked parts (whether this is deliberate or inadvertent is a discussion for another day), so testing/validating the part on the bench is essential IMHO. You don't want to be installing a re-marked LS part that will fade out as the temperature goes up, if it even managed it at the start.

Cheers, Brian.





Re: Tektronix 464, 465, 466 Mod 157T

Edward Prest
 

The doc. doesn't seem to show where to tap the sawtooth from. i.e R1350 input.

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