Date   

Re: Tek 4654M; No Trace, No +5V, No +95V; Help

Harrison
 

Hi Bert
I have pulled a lead on both the C582 (2.2 mF) and the C584 (2.0 mF) caps in the 95V circuit. Both are +50% -10% Spec. The C582 read 2.40 mF Good would be between 3.3 mF and 1.98 mF. The C584 read 2.46mF. Good would be between 3.0 mF and 1.8 mF. So I considered these good and reinstalled the lead. I can not seem to find a new or used U550 anywhere so I was thinking of constructing one. Unfortunately on the schematic I have I don't see values for the diodes or any spec information on the 3X multiplier. Plus I am not sure if it is good, i.e. safe and wise to construct a HV module.
So for right now I am at a loss on what else I can try or test and/or determine a good way to determine if the U550 module is good or bad.
Harrison N1FAM


Re: Is my Tek 468 beyond repair?

John
 

tgerbic, thank you for the confirmation on those 3 pins.

So do I understand from the replies below that the processor shouldn't halt even with a corrupt Mostek ROM and I should still see pulses on ALE regardless? I found a book online that discusses the 8085 in detail and I got the same impression but this level of diagnosis is something rather new to me.

If I have the time tomorrow I will hook up the LA to some of the pins. Maybe that will reveal something. I need to prepare my laptop though with some software.


Re: Tek 7854 intermittent issues

Nick Corvid
 

Sorry about that David, to be honest I completely forgot it existed I'll get a value as soon as I'm able.

On another note, yesterday on a whim I pulled out all the digital the boards, which I had never done before on this machine. Nearly everything looked perfect except for on the MPU board where there was an unhappy looking cap.

Board in question: https://groups.io/g/TekScopes/photo/252782/1?p=Created,,,20,2,0,0
Capacitor I mentioned: https://groups.io/g/TekScopes/photo/252782/0?p=Created,,,20,2,0,0


Re: Is my Tek 468 beyond repair?

tgerbic
 

Roger, you are correct.

Looking at my notepad, I penciled in L and L for 38 and 39. Not sure how this turned into both High. Must have got distracted. Pin 37 should be 2.5Mhz as it is one half the 5Mhz clock.
Pins 37, 38 and 39 are ok on John's 468.

Regards


Re: Is my Tek 468 beyond repair?

Chuck Harris <cfharris@...>
 

The 8085 doesn't halt unless it executes a 166 octal instruction.
000 octal is one of the many NOP instructions, and 377 is a RST7
instruction, which calls the 7th interrupt vector location,
and starts executing... so typically, it will just maraud through
memory forever, never finding a HLT instruction.

Those of us that hoped for better behavior learned to salt unused
EPROM and RAM space with 166's. We also typically stuck a HLT
instruction in the 7th interrupt vector location, so executing a
RST7 would stop the marauding processor cold in its tracks..

Note, the 8080 is an octal machine. Its structure makes sense
in octal, and nonsense in hex.

The boot sequence is pretty simple, it loads the program counter
with zero, and starts to execute whatever it finds there.

-Chuck Harris

John wrote:

Roget thanks.

Yes, I was using an analogue scope and watching for any flicker no matter how slight. Might be time to break out the Rigol and try single shot mode. On the other hand without any boot code to read from the ROM the processor would just simply halt so which would presumably happen very quickly. It have been trying to find a description of the exact sequence of events the processor follows at power up, i.e. the boot-up sequence if you like, but have yet to find something on that subject. I am wondering whether your book contains such information?

I tracked the GPIB interrupt input to a connector on the memory board, but couldn't fathom where it went from there. I agree with your conclusion. BTW, I realized later that the buffer in non-inverting. Pins 1 and 19 were low so the buffer should be operating. I didn't expect that 1.5V on the input would be sufficient to cause a HIGH on the output side, but as you say, leaving it open is not good practice. Perhaps there needs to be a pull-down resistor to GND, but it might work as you suggest, that is the detection fails (as the GPIB board is not present) and the interrupt is masked off.

I just tried the procedure on page 28 bearing in mind the comment by "Tom" posted by Daveolla. The scope powers up without the fan running but I do get a trace, however, only CH2 seems to be displaying regardless of the input switch position so there seems to be no chopping. CH1 does not appear and its position control does not seem to do anything. Only CH2 works. On the other hand this does at least demonstrate that the failure of the digital mode is preventing a trace from running in normal operation and that in principle at least the analogue side does work. It is not entirely unexpected that there may be problems to resolve once the digital side is fixed. I have restored it back to normal operation for now.

Thank you for getting the ROMs in the post. I should be able to sort out some adapters.

Really appreciate the comments from everyone as well.






Re: Is my Tek 468 beyond repair?

Harvey White
 

The processor goes to location 0 and starts executing code on a power on reset.  Typically, the only instruction that should be at that location should be a jump to a routine that performs power on reset functions.  While the processor *could* continue executing other instructions immediately after location 0, the lowest locations are used for other interrupt conditions, and you lose the ability to use those interrupts.

A well populated 8085 system should have a series of jump instructions starting at 0.

If the processor sees nothing from the ROMS during power on startup, and reads all 0xFF, then you'll be executing an RST 7 instruction, which puts the current address (0), on the stack, and transfers execution to location 0x38.  You'll read 0xFF from that, so you'll have a continuing series of repeats of RST 7 if you can read nothing.

Data from your memory should be valid at the rising edge of the RD pulse (active low).

You should get ALE (address latch enable) pulses as well.

Harvey

On 9/18/2020 11:11 AM, John wrote:
Roget thanks.

Yes, I was using an analogue scope and watching for any flicker no matter how slight. Might be time to break out the Rigol and try single shot mode. On the other hand without any boot code to read from the ROM the processor would just simply halt so which would presumably happen very quickly. It have been trying to find a description of the exact sequence of events the processor follows at power up, i.e. the boot-up sequence if you like, but have yet to find something on that subject. I am wondering whether your book contains such information?

I tracked the GPIB interrupt input to a connector on the memory board, but couldn't fathom where it went from there. I agree with your conclusion. BTW, I realized later that the buffer in non-inverting. Pins 1 and 19 were low so the buffer should be operating. I didn't expect that 1.5V on the input would be sufficient to cause a HIGH on the output side, but as you say, leaving it open is not good practice. Perhaps there needs to be a pull-down resistor to GND, but it might work as you suggest, that is the detection fails (as the GPIB board is not present) and the interrupt is masked off.

I just tried the procedure on page 28 bearing in mind the comment by "Tom" posted by Daveolla. The scope powers up without the fan running but I do get a trace, however, only CH2 seems to be displaying regardless of the input switch position so there seems to be no chopping. CH1 does not appear and its position control does not seem to do anything. Only CH2 works. On the other hand this does at least demonstrate that the failure of the digital mode is preventing a trace from running in normal operation and that in principle at least the analogue side does work. It is not entirely unexpected that there may be problems to resolve once the digital side is fixed. I have restored it back to normal operation for now.

Thank you for getting the ROMs in the post. I should be able to sort out some adapters.

Really appreciate the comments from everyone as well.





Re: Is my Tek 468 beyond repair?

John
 

Roget thanks.

Yes, I was using an analogue scope and watching for any flicker no matter how slight. Might be time to break out the Rigol and try single shot mode. On the other hand without any boot code to read from the ROM the processor would just simply halt so which would presumably happen very quickly. It have been trying to find a description of the exact sequence of events the processor follows at power up, i.e. the boot-up sequence if you like, but have yet to find something on that subject. I am wondering whether your book contains such information?

I tracked the GPIB interrupt input to a connector on the memory board, but couldn't fathom where it went from there. I agree with your conclusion. BTW, I realized later that the buffer in non-inverting. Pins 1 and 19 were low so the buffer should be operating. I didn't expect that 1.5V on the input would be sufficient to cause a HIGH on the output side, but as you say, leaving it open is not good practice. Perhaps there needs to be a pull-down resistor to GND, but it might work as you suggest, that is the detection fails (as the GPIB board is not present) and the interrupt is masked off.

I just tried the procedure on page 28 bearing in mind the comment by "Tom" posted by Daveolla. The scope powers up without the fan running but I do get a trace, however, only CH2 seems to be displaying regardless of the input switch position so there seems to be no chopping. CH1 does not appear and its position control does not seem to do anything. Only CH2 works. On the other hand this does at least demonstrate that the failure of the digital mode is preventing a trace from running in normal operation and that in principle at least the analogue side does work. It is not entirely unexpected that there may be problems to resolve once the digital side is fixed. I have restored it back to normal operation for now.

Thank you for getting the ROMs in the post. I should be able to sort out some adapters.

Really appreciate the comments from everyone as well.


Re: Is my Tek 468 beyond repair? Trap and Kernel test procedure

Harvey White
 

The 8085 has certain address locations it goes to under certain hardware conditions.  (without me looking them up, it's in the data sheet), there's one for power on reset, the hardware traps (rst 7.5, etc), interrupts and so on.

If you monitor the address lines on the processor (fortunately, 16 of them), then (for example) when the reset line goes active, the processor should show that the address lines go to zero (IIRC) and start to execute there.  In the 8085, you don't have vectors (the processor doesn't fetch and address from location 0, then use that to show where to execute (as in the 6800 or the 6500 series), but starts executing at that location.  Thus, the location typically contains a jump instruction which goes to actual code.

once you see that the location is being jumped to, you may be able to use the higher bit of the address to monitor the triggering event if you wish.

For an 8 bit microprocessor, a 32 bit is more of a minimum requirement, with 16 address, 8 data, and 8 other signals (ALE, R/W, etc).

Still, you could take the highest 4 bits of the address, run them through a 74LS20, then again to invert the answer, and that would give you some indication of the address if you start to trace further.

Harvey

On 9/17/2020 11:34 PM, Heitor Lima via groups.io wrote:
Hi group

I have 2 TEK 468 with problems in digital circuits.
First I would like to inform the group that I never worked with the 8085 microprocessor.
In the page 5.36 of TEK 468 Service Manual, to initiate Trap test we must to momentary unplug P262 and reconnect it . Microprocessor will be interrupted and the Trap Test routine will begin.

My question is, how to know if the routine was performed correctly? I have a 16 channels logic analyzer
and I can record the signature for the whole routine. However, I don't know how to check if it
was performed correctly. Could you ot somebody tell me how to check?

The same applies to the Kernel Test. How to check if the routine was fully complied with by the microprocessor?

I would like to have a step-by-step of the trap and Kernel routines to check if the routine was done correctly.
The same applies to the Kernel Test. How to check if the routine was fully complied with by the microprocessor?

Many thanks in advance

Heitor Lima ( From Rio de Janeiro - Brazil )





Re: Is my Tek 468 beyond repair?

Roger Evans
 

I don't understand Tony's measurement of 38,39 HIGH, on my schematic pin 39 is connected directly to ground.

Tracing back the input for the RST5.5 pin, in the absence of the GPIB board there is precisely nothing connected to pin 14 of U472. Leaving a TTL input unconnected is normally not good practice, I am surprised if Tek would have done that, but unconnected TTL inputs float high and the conclusion must be that in normal operation the CPU would check for the presence of the GPIB board and if not found it would leave the RST5.5 interrupt masked off.

John, when you said you had looked for a burst of activity on the cpu pins immediately after reset were you using a normal analogue scope or a scope with single shot storage? The processor activitiy could only be for 10s of microseconds and easy to miss on a normal scope.

I noticed the RAM on the 8085 board is made up of 2114s, I seem to remember a thread on one of the UK forums where these were regarded as failure prone but I have no personal experience with these.

The two programmed 28C64s are in the first class post (don't hold your breath). I hope making the adapter boards is not too much hassle, a lot depends on the room available.

Roger


Re: Is my Tek 468 beyond repair?

John
 

Comparison with Tony’s “quick once around the processor”.

1 5Mhz Squarewave
Confirmed.

2 5Mhz Squarewave, half size from 5V down to ~2.5V
Confirmed.

3 Low, short high pulse on power up
Confirmed.

4, 5, 6 Low
Confirmed.

7 500Hz digital signal with 2ms positive pulse
Confirmed. 500Hz signal with 400μS pulse. Cycle width is 2ms.

8, 9 High
8 = LOW, 9 = HIGH

10 100Hz digital signal with 10ms positive pulse
Pin is high. No pulse.

11 High, occasional negative pulse
Pin at approximately 0.8V. No negative pulse observed.

12 to 19,k and 21 to 33 Address/data/control constant traffic
12 - 19 = HIGH
21 - 31 = LOW
32 = 1.5V approx.
33 = LOW

20 Gnd
Ground connection confirmed.

34, 35, 36 High
34 = LOW
35 = HIGH
36 = HIGH

37 132Hz squarewave
2.5MHz quare wave

38, 39 High
Both LOW

40 +5V VCC
Confirmed.


Re: Is my Tek 468 beyond repair?

John
 

Reed, thank you for the note about the switch on top of the board. I will check it out. Power supplies are OK and all voltages present. I read in the service manual about the "service ROM" and wondered whether that would still be available. I will bear you in mind should I need it.

Tom, thank you for the description of the error in the troubleshooting of the analog section. Will be carrying out that procedure soon so will bear that in mind.

Daveolla, thank you for the description of the files. I contacted Bruce and he has granted me access to the server. The files are now hosted on ftps and you have to e-mail Bruce for the access details. I did find the 468.zip file but not the bigger Tek468.zip. I did find a PDF of the adapter pinout in the files section on the TekScopes group.


A couple of tests for next time you are looking at the scope:
Check that RST7.5 is 50kHz not 500kHz. 500kHz would indicate that U440 is dead and would probably saturate the processor with interrupts.
Sorry, my mistake. Its at 500Hz which is as per point 5 in the test procedure, not kHz.

Check that pin 14 of U472 is also high so that we are not looking at a failed U472
Yes, pin 14 (gate output) is high, hence RST7.5 is high. Pin 13 (gate input) is at about 1.5V. Grounding either makes no difference. G1 (P1) and G2 (P19) are low.

Measure S0 and S1 (pins 29 and 33), that might show that the CPU had executed a HALT instruction and had at least done something after Reset.
S0 and S1 are both LOW which would seem to indicate a halted state. I tried monitoring both during reset/power up thinking that there should be some activity, particularly on S1 as the CPU boots and attempts to read the ROM but nothing happened - it just stayed low.


Re: Is my Tek 468 beyond repair?

Roger Evans
 

Just correcting my misreading of the schematic for the RST7.5 input. The _input_ to U440 is 50kHz so the RST7.5 frequency should be 500Hz as measured by Tony.

Roger


Re: Tek 7854 intermittent issues

 

The +12V and +5V values you report below seem pretty OK to me.

You didn't say anything about the -5V line ...

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Nick Corvid
Sent: 17 September 2020 22:26
To: TekScopes@groups.io
Subject: Re: [TekScopes] Tek 7854 intermittent issues

About to test where Dave said but are the barely reachable points on the power supply for the digital section alright? I could snake some tools down and measure from a 5 volt and 12 volt point. 12 volt was 12.02 volts and had a small amount of high frequency noise but was mostly negligible. The 5 volt was 5.08 and had about 30ish mV of relatively high frequency ripple.


Re: Is my Tek 468 beyond repair?

Roger Evans
 

John,

I was initially surprised that Reset out was low but that is correct since 8080 peripheral devices have an active high reset pin!

A couple of tests for next time you are looking at the scope:
Check that RST7.5 is 50kHz not 500kHz. 500kHz would indicate that U440 is dead and would probably saturate the processor with interrupts.
Check that pin 14 of U472 is also high so that we are not looking at a failed U472
Measure S0 and S1 (pins 29 and 33), that might show that the CPU had executed a HALT instruction and had at least done something after Reset.

The GPIB interrupt stuck high does need tracking down.

Regards,

Roger


Is my Tek 468 beyond repair? Trap and Kernel test procedure

Heitor Lima
 

Hi group

I have 2 TEK 468 with problems in digital circuits.
First I would like to inform the group that I never worked with the 8085 microprocessor.
In the page 5.36 of TEK 468 Service Manual, to initiate Trap test we must to momentary unplug P262 and reconnect it . Microprocessor will be interrupted and the Trap Test routine will begin.

My question is, how to know if the routine was performed correctly? I have a 16 channels logic analyzer
and I can record the signature for the whole routine. However, I don't know how to check if it
was performed correctly. Could you ot somebody tell me how to check?

The same applies to the Kernel Test. How to check if the routine was fully complied with by the microprocessor?

I would like to have a step-by-step of the trap and Kernel routines to check if the routine was done correctly.
The same applies to the Kernel Test. How to check if the routine was fully complied with by the microprocessor?

Many thanks in advance

Heitor Lima ( From Rio de Janeiro - Brazil )


Re: Is my Tek 468 beyond repair?

tgerbic
 

Thought I would pull out my 468 and see if I could find anything useful to post. Well no trace on power up :( However it passes self test. Had to replace the 1200uf cap in the power supply to bring it back to life. First repair on this scope since new. Will probably replace all the big PS filter caps in the future in this and my 465B scopes, since I bought them from Tek in the 80s and they use the same parts. Cheap insurance, maybe... The 465B has never been touched. Guess I should go fire that one up too. They have been sitting for a decade.

Self test runs with 8's and dots first then a bunch of zeros flashing around on the display, then it goes blank when the test passes.

I did a quick once around the processor to provide some general signals for a running unit.
1 5Mhz Squarewave
2 5Mhz Squarewave, half size from 5V down to ~2.5V
3 Low, short high pulse on power up
4, 5, 6 Low
7 500Hz digital signal with 2ms positive pulse
8, 9 High
10 100Hz digital signal with 10ms positive pulse
11 High, occasional negative pulse
12 to 19,k and 21 to 33 Address/data/control constant traffic
20 Gnd
34, 35, 36 High
37 132Hz squarewave
38, 39 High
40 +5V VCC

Regards
Tony


Re: 585A -- time base A linearity

Sean Turner
 

Sorry I'm too tired to reply to anything individually tonight...long day at work (things are busy when you aren't normally in the office due to covid, and then suddenly you are in the office and you have everything to do), but noted on the proposed directions to take troubleshooting...thanks!!!

I'm crack it open this weekend and take a closer look.

Sean


Re: 585A -- time base A linearity

Chuck Harris <cfharris@...>
 

I would clean the capacitor and the 5M resistor with IPA
before I condemned it... but you are right about the list
of usual suspects.

-Chuck Harris

Dave Wise wrote:

I agree.
And it's unlikely for a polycarbonate (C160G) or ceramic (C160M) to go leaky, but it does happen now and then. Unless R160F has become nonlinear, the caps - and the switch insulation - are pretty much the only suspects.

Dave Wise
________________________________________
From: TekScopes@groups.io <TekScopes@groups.io> on behalf of Albert Otten via groups.io <aodiversen=concepts.nl@groups.io>
Sent: Thursday, September 17, 2020 9:50 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] 585A -- time base A linearity

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 .
That's correct Dave, but when C160G/M is leaky then this will show the most effect on non-linearity when R160F with the highest value 5M is switched in, so at 0.5 cm/div.
Albert













Re: Tek 7854 intermittent issues

Nick Corvid
 

About to test where Dave said but are the barely reachable points on the power supply for the digital section alright? I could snake some tools down and measure from a 5 volt and 12 volt point. 12 volt was 12.02 volts and had a small amount of high frequency noise but was mostly negligible. The 5 volt was 5.08 and had about 30ish mV of relatively high frequency ripple. Pictured here: https://groups.io/g/TekScopes/photo/252782/0?p=Created,,,20,2,0,0

Here are the points I mentioned:
5V: https://groups.io/g/TekScopes/photo/252782/1?p=Created,,,20,2,0,0
12V: https://groups.io/g/TekScopes/photo/252782/2?p=Created,,,20,2,0,0
Sorry for the picture quality on those I couldn't get a conventional camera to see in.


Re: 585A -- time base A linearity

Tom Lee
 

I agree with Dave. The cap, or possibly some contamination across its terminals, is the highest-probability cause of the problem.

The timing resistor has more or less a constant voltage across it, as well as a constant current through it (the circuit is the classic Miller integrator that Tek favored until the 7B92's limitations stimulated an overdue change in tradition), so it's hard to imagine what pathologies it could possess to cause the observed behavior. If it does ultimately turn out to be the resistor, I'd be very interested in some curve tracings.

Good luck with your debug!

--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 9/17/2020 10:30, Dave Wise wrote:
I agree.
And it's unlikely for a polycarbonate (C160G) or ceramic (C160M) to go leaky, but it does happen now and then. Unless R160F has become nonlinear, the caps - and the switch insulation - are pretty much the only suspects.

Dave Wise
________________________________________
From: TekScopes@groups.io <TekScopes@groups.io> on behalf of Albert Otten via groups.io <aodiversen=concepts.nl@groups.io>
Sent: Thursday, September 17, 2020 9:50 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] 585A -- time base A linearity

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 .
That's correct Dave, but when C160G/M is leaky then this will show the most effect on non-linearity when R160F with the highest value 5M is switched in, so at 0.5 cm/div.
Albert










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