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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

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

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

Comparison with Tony’s “quick once around the processor”. Confirmed. Confirmed. Confirmed. Confirmed. Confirmed. 500Hz signal with 400μS pulse. Cycle width is 2ms. 8 = LOW, 9 = HIGH Pin is

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

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

The +12V and +5V values you report below seem pretty OK to me. You didn't say anything about the -5V line ...

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:

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

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

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

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:

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

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

I also did a compilation of all this with a conversion socket and it was on Bluefeather and maybe on Ko4bb site. I can send you both if need be. On this site in the files section there should be a

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

Status of control pins on 8085: READY H (5V) SID L HOLD L RST6.5 L RST5.5 H (4V) RST7.5 500kHz pulse as indicated in manual INTR H (4V) RESET_OUT L (1V) RST_IN H

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

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

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