Date   

Re: Is my Tek 468 beyond repair?

John
 

Thank you. I have downloaded some information on the 8085 to review, but in the meantime you have given me some things to look at. I should perhaps have been more specific when I said signals are absent. In fact, ALE is stuck HIGH but definitely not pulsing.

I did find a marking on the circuit diagram that says "LO REMOVES INTERRUPTS" on pin 13 of U255 (74LS74). Pin 9 of same is connected to INTR pin on the 8085.Grounding pin 13 of U255 brings INTR LOW and ALE LOW, but still nothing happens. I performed a reset with pin 13 held LOW but it made no difference. I am not sure how one can tell whether data is being read, but I was intending to research that aspect.

RESET_IN is high unless I move the jumper at which point it goes low while grounded. I will check the status of READY, SID, HOLD, TRAP and the various RST pins to see whether that reveals anything. I see RST 5.5, 6.5 and 7.5 on the diagram.

I will do some further investigations and report back.


Re: Is my Tek 468 beyond repair?

Roger Evans
 

John,

I don't have any direct experience of the Tek 468 or the 8085 processor that it uses but I am fairly familiar with some of the other 8 bit micros, so this is gleaned from the 8085 manual:

If there is no ALE signal (it should pulse once for each byte to/from memory) then the processor is not stuck in a loop. There are three possibilities 1) it has executed a HALT instruction, in which case pins 29 and 33 on the processor will be at logic zero; 2) one of the interrupt sources is permanently active. The 8085 is the opposite of what I am used to, RST5.5, RST6.5, RST7.5, INTR and TRAP are all active high. If any of these inputs are stuck at logic high they will prevent the program continuing (without reading the 8085 manual more carefully I am not sure if this condition can stop the CPU trying to fetch the next instruction, in which case ALE should be switching) 3) the processor is stuck in Reset, in which case pin 36 will be stuck low, eg due to C174 being short circuit or the Run/Reset jumper being in the wrong position (there is also a jumper on TRAP that needs to be in place).

If there are any 8085 experts around then please correct any errors in the above.

Tarnishing of pins could well be the cause of errors and also if you search this site you will find references to a particular type of IC socket (I think made by TI) that is generally a poor design. Do you have any Deoxit?

Regards,

Roger


Re: 585A -- time base A linearity

Dave Wise
 

Hmm. There's no single component that's used only on that setting. 0.5ms/div, 0.2, and 0.1 all use C160G/M, while all 5's from 50us/div to 0.5s/div use R160F, all 2's use R160E, and all 1's use R160D. See the schematic for "TIME-BASE A TIMING SWITCH" in the manual, available at w140.com .

What pot are you referring to? R348 SWP CAL? Note that series resistor R349 was changed at S/N 9300.

Dave Wise
________________________________________
From: TekScopes@groups.io <TekScopes@groups.io> on behalf of Tom Lee via groups.io <tomlee=ee.stanford.edu@groups.io>
Sent: Thursday, September 17, 2020 12:00 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] 585A -- time base A linearity

That symptom could be caused by a leaky timing cap for that sweep
setting. If it starts off at about the correct speed on the left, but
slows down as it moves to the right (e.g., square waves get
progressively squished together), that would be a further indictment of
the cap.

Try subbing it with a replacement cap and see if the problem goes away.

--Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 9/16/2020 23:52, sdturne@q.com wrote:
Hi all,

Some of you may recall the 585A I got earlier this year that had a short. I was able to get it up and running. Ended up replacing a pair of totally short diodes in the power supply with new 1N4007s which cleared the "blow fuse violently when you flip it on" problem. :o) I also ended up having to replace a trimmer capacitor that was totally broken and spinning free, which was really borking the fastest time/div settings. Luckily I found a seller on ebay with OEM original replacements for it, sold in fours.

Anyway, there is one outstanding problem that is left, and that is time base A does not quite meet up to the linearity specifications in the manual, mostly at 0.5 msec/cm. One step I took which helped a lot was to replace V161 in the Miller runup circuit with a NOS 6CL6 I was able to get. This actually brought most of the time/cm settings into specification. However, the 0.5 msec/cm setting is the outlier here. The settings on either side of it are in spec, but this setting has a very apparent linearity problem; the trace is getting shorter the further right it goes. You can really see this with a square wave input. The pot that is supposed to adjust this out is all the way at the extreme already.

I figure a good first try would be to clean the time base A control, as it is rather hard to get at. Beyond that, does anyone have an ideas about why this might be the case? I realize that these are really 3% at best instruments and will not be perfect. However, this particular thing shouldn't be this bad I think.


Thanks!

Sean






Re: Is my Tek 468 beyond repair?

John
 

BTW, I should also mention that all the pins on the two ICs that I removed were very tarnished. I tried cleaning (very carefully since they seemed rather fragile) with both IPA and Servisol switch cleaner but it made very little difference. I would have added this to the previous post but can't see a way of editing it?


Re: Is my Tek 468 beyond repair?

John
 

Roger, that's a kind offer which I may take you up on. I will e-mail to ask particulars. However, first, here are the results of the test procedure on page 330:

All voltages are within normal parameters.

Tests 1 - 6 were successful. All timing pulses were present and PRDONE was present on U472-16 (140).

Test 7 thru 11 had no signal present, so no signal on ALE, RD or WR or wavefore on TP477 or U255-5.

I'm not sure what 12 and 13 are about and need to look them up, but U255-9 is "stuck" high.

Since the above tests were not completed satisfactorily, I didn't do on to the Trap Test Procedure or the ROM Checksum. I have yet to go through the process on P228. but I don't understand what Service/Options Switch X10000000 means? I am presuming this refers to the row of DIP switches on the top board but am I right in thinking its simply the configuration of the individual switches from, left to right? So first switch ON and all the remaining ones OFF?

I have noted your point about pin 27 on the 28C64 EEPROM.


Re: TDS 684A needs recapping?

 

On Wed, Sep 16, 2020 at 02:02 PM, Ray wrote:


Roger,Thanks for clarifying that.I was getting worried there for a moment
Weird. Yeah, if I go online I have to click on "view quoted text" to see what you wrote...
For some reason, your emails to the list appear as quoted text.


Re: Is my Tek 468 beyond repair?

Roger Evans
 

John,

Since you are in the UK, I can offer to program a couple of 28C64s for you, please e-mail me (rdot gdot evans at talk21 dotcom). There are ','s between my initials. If you would find having a device programmer is useful then please don't let me stop you buying one. Beware the 28C64s are EEPROMs and really need the WEn pin 27 tied to VCC to prevent inadvertant writes in circuit.

Regards,

Roger


Re: Is my Tek 468 beyond repair?

John
 

Roger, thanks for the reference to KO4BB.com site and references to the manual pages. I will check that out and report back.


Re: Is my Tek 468 beyond repair?

John
 

Miguel, thanks. This is what I suspected. I think unless anyone has a better suggestion I might buy one of these anyway but with all the cables, adapters and bits:

https://www.ebay.co.uk/itm/TL866II-Plus-Minipro-Auto-Programmer-Kit-10-Adapters-BIOS-Flash-Motherboard-USB/363091405640

I don't have a parallel port on my PC so the Wilhelm is out and the Martin Everhards 2700 is rather too specialized.

I am in Leicestershire in the UK BTW. It seems that to read the existing ROMs I will need to build an MKB36000 to 27C64 adapter and then reverse to fit the 27C64 into the MOSTEK slots. From what I have read it seems very likely that the ROMs will be "rotted" by now but it would be interesting I think to check them anyway.


Re: Is my Tek 468 beyond repair?

Roger Evans
 

Page 328 of the Tek 468 Service Manual at KO4BB .com (linked to from Tek Wiki) shows how to isolate the analogue and digital portions of a 468 to verify that the analogue section works correctly 'stand alone'.

Page 330 lists a series of basic tests on the processor board unfortunately it is not specific about which of these rely on the ROMs being correct. You would need another scope to do the testing and it would be useful to know how far down the list you can go before you stop seeing the expected signals. Signals like the 5MHz and 2.5MHz clock, ALE and some activity on RD and WR would indicate that the processor is at least operating even if it doesn't have the correct op codes in the ROMs.

Regards,

Roger


Re: Is my Tek 468 beyond repair?

Miguel Work
 

No, this programmer is for atmel AVR ony

Where are you located?

https://www.youtube.com/watch?v=Gdmsni8cGvY

If can program eeproms for you



-----Mensaje original-----
De: TekScopes@groups.io [mailto:TekScopes@groups.io] En nombre de John
Enviado el: jueves, 17 de septiembre de 2020 10:31
Para: TekScopes@groups.io
Asunto: Re: [TekScopes] Is my Tek 468 beyond repair?

Thank you for the pointer to the TekWiki resource. I have found and downloaded the two ROM files. I do have an AVR programmer and a ZIF socket board that can take up to a 40pin DIL chip but I'm not sure whether this can be used to read the MOSTEK ROM or write the 27C64:

https://www.ebay.co.uk/itm/AVR-Minimum-System-Board-ATMEGA16-ATmega32-USB-ISP-USBasp-Programmer-For-ATMEL/202959381564

I am happy to invest in a suitable programmer provided it is versatile enough to read/write a range of vintage EEPROPM chips.


Re: Is my Tek 468 beyond repair?

John
 

Thank you for the pointer to the TekWiki resource. I have found and downloaded the two ROM files. I do have an AVR programmer and a ZIF socket board that can take up to a 40pin DIL chip but I'm not sure whether this can be used to read the MOSTEK ROM or write the 27C64:

https://www.ebay.co.uk/itm/AVR-Minimum-System-Board-ATMEGA16-ATmega32-USB-ISP-USBasp-Programmer-For-ATMEL/202959381564

I am happy to invest in a suitable programmer provided it is versatile enough to read/write a range of vintage EEPROPM chips.


Re: 585A -- time base A linearity

Albert Otten
 

On Thu, Sep 17, 2020 at 09:01 AM, Tom Lee wrote:


That symptom could be caused by a leaky timing cap for that sweep
setting. If it starts off at about the correct speed on the left, but
slows down as it moves to the right (e.g., square waves get
progressively squished together), that would be a further indictment of
the cap.
Try subbing it with a replacement cap and see if the problem goes away.

On 9/16/2020 23:52, sdturne@q.com wrote:
Hi all,
However, the 0.5 msec/cm
setting is the outlier here. The settings on either side of it are in spec,
but this setting has a very apparent linearity problem; the trace is getting
shorter the further right it goes.
I second that, especially since at 0.5 cm/div the high value 5 M of R160 is switched in. The cap voltage increases when the sweep goes further to the right.. Any leakage then causes nonlinearity.

Albert


Re: Tektronix 475

Simon
 

I now have it working properly, but still don’t know what the original problem was. Maybe resocketing transistors and the CRT did some good. The Y output amplifier was bad, but I found a new one on eBay. After getting a decent trace, the Y attenuators were not working properly. I brushed the leaf contacts with IPA, removed the attenuator blocks one by one, cleaned the pins, wiped them with IPA and plugged them back in. The AC-GND-DC switches also needed similar treatment. I have rational progression in Y gain on both, so all in all I think it had been in storage a long time and many contacts had gone bad or intermittent. The X position occasionally wanders to the right, but comes back after a while, maybe another poor contact to track down.
Thanks to everyone for useful advice.
Simon


Re: Is my Tek 468 beyond repair?

Steve Dench
 

I believe the ROM images are on Tekwiki, I think that is where I got them
when I needed them. You will most likely have trouble finding an original
24 pin Eprom, but I used (as i understand most do) make up an adaptor board
to use a standard 28 pin Eprom like 27C64 etc.

On Thu, 17 Sep 2020, 4:44 pm John, <subs@qcontinuum.plus.com> wrote:

Thank you for the part numbers and checksums for the ROMs. Unfortunately I
do not have an EEPROM programmer so am unable to verify the checksums or
program new ones. I would need a bit of help determining what programmer
would be required to read these MOSTEK ROMs? Alternatively, perhaps someone
could advise what programmer to get to program 27C4s? I might be willing to
invest but then I would also need the code files, which, despite a bit of
Googling I have not yet found anywhere. My Googling has turned up the
possibility that either Motorola MCM68764 or Cypress CY7C264-55WE are pin
compatible, but again, I have no idea what programmer would be compatible
with those and still need the files. It seems that the feathertech archive
is no longer available.

I did also come across this resource which suggests they may sell them:

https://vintagetek.org/468-mostek-mkb36000-rom-repairs/

I have sent them an e-mail asking whether the service is still available
and at what cost.

In the meantime, if anyone has the images, a copy would be greatly
appreciated.






Re: 585A -- time base A linearity

Tom Lee
 

That symptom could be caused by a leaky timing cap for that sweep setting. If it starts off at about the correct speed on the left, but slows down as it moves to the right (e.g., square waves get progressively squished together), that would be a further indictment of the cap.

Try subbing it with a replacement cap and see if the problem goes away.

--Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 9/16/2020 23:52, sdturne@q.com wrote:
Hi all,

Some of you may recall the 585A I got earlier this year that had a short. I was able to get it up and running. Ended up replacing a pair of totally short diodes in the power supply with new 1N4007s which cleared the "blow fuse violently when you flip it on" problem. :o) I also ended up having to replace a trimmer capacitor that was totally broken and spinning free, which was really borking the fastest time/div settings. Luckily I found a seller on ebay with OEM original replacements for it, sold in fours.

Anyway, there is one outstanding problem that is left, and that is time base A does not quite meet up to the linearity specifications in the manual, mostly at 0.5 msec/cm. One step I took which helped a lot was to replace V161 in the Miller runup circuit with a NOS 6CL6 I was able to get. This actually brought most of the time/cm settings into specification. However, the 0.5 msec/cm setting is the outlier here. The settings on either side of it are in spec, but this setting has a very apparent linearity problem; the trace is getting shorter the further right it goes. You can really see this with a square wave input. The pot that is supposed to adjust this out is all the way at the extreme already.

I figure a good first try would be to clean the time base A control, as it is rather hard to get at. Beyond that, does anyone have an ideas about why this might be the case? I realize that these are really 3% at best instruments and will not be perfect. However, this particular thing shouldn't be this bad I think.


Thanks!

Sean





585A -- time base A linearity

Sean Turner
 

Hi all,

Some of you may recall the 585A I got earlier this year that had a short. I was able to get it up and running. Ended up replacing a pair of totally short diodes in the power supply with new 1N4007s which cleared the "blow fuse violently when you flip it on" problem. :o) I also ended up having to replace a trimmer capacitor that was totally broken and spinning free, which was really borking the fastest time/div settings. Luckily I found a seller on ebay with OEM original replacements for it, sold in fours.

Anyway, there is one outstanding problem that is left, and that is time base A does not quite meet up to the linearity specifications in the manual, mostly at 0.5 msec/cm. One step I took which helped a lot was to replace V161 in the Miller runup circuit with a NOS 6CL6 I was able to get. This actually brought most of the time/cm settings into specification. However, the 0.5 msec/cm setting is the outlier here. The settings on either side of it are in spec, but this setting has a very apparent linearity problem; the trace is getting shorter the further right it goes. You can really see this with a square wave input. The pot that is supposed to adjust this out is all the way at the extreme already.

I figure a good first try would be to clean the time base A control, as it is rather hard to get at. Beyond that, does anyone have an ideas about why this might be the case? I realize that these are really 3% at best instruments and will not be perfect. However, this particular thing shouldn't be this bad I think.


Thanks!

Sean


Re: Is my Tek 468 beyond repair?

John
 

Thank you for the part numbers and checksums for the ROMs. Unfortunately I do not have an EEPROM programmer so am unable to verify the checksums or program new ones. I would need a bit of help determining what programmer would be required to read these MOSTEK ROMs? Alternatively, perhaps someone could advise what programmer to get to program 27C4s? I might be willing to invest but then I would also need the code files, which, despite a bit of Googling I have not yet found anywhere. My Googling has turned up the possibility that either Motorola MCM68764 or Cypress CY7C264-55WE are pin compatible, but again, I have no idea what programmer would be compatible with those and still need the files. It seems that the feathertech archive is no longer available.

I did also come across this resource which suggests they may sell them:

https://vintagetek.org/468-mostek-mkb36000-rom-repairs/

I have sent them an e-mail asking whether the service is still available and at what cost.

In the meantime, if anyone has the images, a copy would be greatly appreciated.


Re: Is my Tek 468 beyond repair?

tgerbic
 

John,

I have a 468 as well but have not used it in a very very long time.

Unless a near impossible part to find part has burned out it should be repairable.

Since this is a processor controlled scope, you need to make sure the processor part of the scope is working to properly troubleshoot. The ROMs could, over time, lose their programming and need to be replaced. A couple of bits here or there could stop the scope from working. This is what people may refer to as "ROM rot". ROM code is available and the ROMs were originally MK36000s, but can be programmed in a 27C64, since finding and programming MK36xxxs ROMs can be a challenge for most people. I seem to remember that you may need to make some wiring change to use the 27C64 chips or use an adapter, if it fits, in place of the original ROMs.

If you have an EPROM reader/programmer that can read the old 36xxx ROMs, the checksums and part numbers should be:

MK36693J-5
160-0459-01
Checksum C2AC

MK36694J-5
160-0759-01
Checksum EF01

If the checksums don't match, the ROM is probably bad. If you are going to replace one, it is best to replace them both.

Hope this helps.
Tony


Is my Tek 468 beyond repair?

John
 

I recently acquired a Tektronix 468. When powered up (with digital mode turned off) it will occasionally display a good trace but usually there is no horizontal sweep. The time-base is working but the scope is behaving a bit oddly. For example, CH1 shows up OK but CH2 shows on both CH1 and CH2!. All power rails are spot on voltage. Having checked through all the signals on test points within the Horizontal Amp, Sweep Logic and Trigger circuits, most signals are present, but one or two are missing. There is no horizontal signal to the input horizontal amp. Various signals are present as illustrated in the service manual in the Sweep display logic and trigger section, but with some notable exceptions. For example, there is no pulse feeding the 74LS02 from the 74LS74 which I suspect is the reason for no or intermittent output. There is no start A Swp pulse at TP49 although all other waveforms in the trigger sections are present. There is a possibility that there may be a problem with the sweep logic, possibly the 74LS74 IC on the main board, but I suspect this may not be the full story because the the digital section does not work at all.

On power-up, the 468 is meant to start with a power-on self-test. The first test is the ROM checksum followed by the Lamp Test and then the RAM Verification Test. The manual indicates that if the ROM checksum fails then it will "neither respond to the front-panel switches nor will it operate". It also says that "Front Panel indication of a ROM failure is that the Lamp test is never started".

Well none of the switches on the storage module respond regardless of whether of whether storage mode is turned on or off, however the main oscilloscope function switches do seem to be working. Nothing appears on the 7-seg LED display except 3 dots. The Lamp Test where all the segments of the display light up does not happen which would seem to indicate that the ROM test has failed.

My question is whether the oscilloscope should still operate in non-storage mode? Or is the failur of the storage module the reason why the scope is behaving strangely? I also have read that a ROM can succumb to something called "ROM rot". I don't know much about this, but can the ROM be re-programmed or replaced? Is the ROM code available anywhere?

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