Ensoniq ESQ-1 debugging help

[dnigrin]

OK, here's what I found, interpreting your guidance above:

DUART

• (Rechecked) pin 35 [CS]: definitely goes high, virtually immediately

CPU

• (Rechecked) pin 7 [Vcc], using pin 1 [Vss] as ground: +5V

• Pin 37 [RESET] - gave me a steady waveform, relatively low voltage (0.5V?) square wave, see https://defectiverecords.com/temp/CPU-RESET.jpg

• Pin 39 [XTAL] - low

• Pin 38 [EXTAL] - waveform, 4 peaks then a pause - see https://defectiverecords.com/temp/CPU-EXTAL.jpg

EPROMS
I looked online to see what type of EPROMs they are, and believe they're type 27256. Assuming correct, I used this pinout description: https://ist.uwaterloo.ca/~schepers/ROMS ... /27256.png. I also assumed that [/CE, /PGM], "Chip Enable", was the same as "Chip Select" - if not please let me know! I also see an [/OE] which I presume is "Output Enable", let me know if should have measured that....

• OS LO, Pin 20 [/CE, /PGM]: some brief waveform activity, then high

• OS HI, Pin 20 [/CE, /PGM]: some brief waveform activity, then low

• OS LO, Pin 11 [D0]: waveform, see https://defectiverecords.com/temp/OSLO-D0.jpg

• OS LO, Pin 12 [D1]: waveform

• OS LO, Pin 13 [D2]: waveform

Although I'm not 100% sure what to expect on the various CPU pins (Reset, and the two types of Clocks), it does seem odd to me that one EPROM's Chip Enable pin went high, but the other low? Would this implicate those 74F139 chips you mentioned?

[Rasputin]

CPU

• (Rechecked) pin 7 [Vcc], using pin 1 [Vss] as ground: +5V

• Pin 37 [RESET] - gave me a steady waveform, relatively low voltage (0.5V?) square wave, see https://defectiverecords.com/temp/CPU-RESET.jpg

• Pin 39 [XTAL] - low

• Pin 38 [EXTAL] - waveform, 4 peaks then a pause - see https://defectiverecords.com/temp/CPU-EXTAL.jpg

That's very suspicious. And odd, as you said you measured the DUART before and got a waveform on X1-CLK and RES was going low. We'll have to dig into this more. I'll respond more when I have time to double-check the schematics and think about it all.

[dnigrin]

Thanks - as an aside, are you looking at any better schematic of the main board than the fuzzy ones at buchty.net? I also see the hand drawn ones there, but I have a tougher time interpreting those (and reading 'em in some cases)....

[dnigrin]

Also, I went back to double check my readings, so I don't lead us down the wrong path - all consistent w/ prior findings:

DUART

• Pin 32 [X1-CLK]: Perfect waveform, see https://defectiverecords.com/temp/DUART-X1-CLK.jpg

• Pin 34 [RESET]: Low

CPU

• Pin 37 [RESET]: Steady waveform, relatively low voltage (0.5V?) square wave, see https://defectiverecords.com/temp/CPU-RESET.jpg

• Pin 38 [EXTAL]: waveform, 4 peaks then a pause - see https://defectiverecords.com/temp/CPU-EXTAL.jpg

• Pin 39 [XTAL]: Low

[dnigrin]

I've been thinking about this without being able to make too much sense of things (albeit with essentially zero experience!), but one aspect where things maybe make a little bit of sense is when looking at the hand drawn schematics on the Buchty site, it seems that Pin 39 of the CPU [XTAL] is meant to be low (ground) if I'm reading things right? http://buchty.net/ensoniq/files/esq1-tg/15.jpg

I don't fully understand why the waveform though on the CPU [EXTAL] pin is a bit irregular, at least compared to the [X1-CLK] pin on the DUART....

Any further thoughts greatly appreciated!

[madtheory]

I don't fully understand why the waveform though on the CPU [EXTAL] pin is a bit irregular, at least compared to the [X1-CLK] pin on the DUART....

My thought is a filter cap on a nearby rail is faulty so the digital section is not getting low ripple DC, or the PSU itself needs new caps. Check ripple with a 'scope. However I don't know what amount is considered acceptable for the ESQ-1.

[dnigrin]

Thanks - the previous owner told me that the power supply was all recapped, and it looks to be so. My routine power measurements off of it seem OK (see earlier in the thread), but I don't know how to measure ripple with my scope; am I just monitoring the signal, turning up the resolution, and see how steady it seems I presume?

If there's other caps somewhere else I should be looking at, I'm all ears - making sense of the (blurry) schematics is still a challenge for my untrained eye....

[meatballfulton]

A steady stream of pulses on reset is suspicious. Reset is normally high when NOT in reset. In other words, if it's near zero all the time, it's stuck in reset. Should be close to +5V shortly after power on.

[dnigrin]

A steady stream of pulses on reset is suspicious. Reset is normally high when NOT in reset. In other words, if it's near zero all the time, it's stuck in reset. Should be close to +5V shortly after power on.

Thank you - so does this imply that my CPU is not functioning properly, or something else? I swapped it with the CPU for the other ESQ-1 I have and did not see any change in functionality, however that ESQ-1 is also non-functional so perhaps both of their CPUs are bad?

[meatballfulton]

No, the reset pin on a CPU is an input. If it's stuck low, it never wakes up. Check the schematic to see where the reset comes from.

Looking at the image of the scope, what's the voltage of he reset signal...0 or +5?

[dnigrin]

No, the reset pin on a CPU is an input. If it's stuck low, it never wakes up. Check the schematic to see where the reset comes from.

And so that's one of the challenges here - I've yet to find a mainboard schematic for the plastic ESQ-1, and furthermore even the metal ones out there I've found are very blurry and hard to read. There are hand drawn ones, but again, I believe these are also still for the metal version.

All that said, if I assume that something as basic as this would be the same for both plastic and metal versions, what I think is connected to the CPU [RESET] is Pin 7 of the 7665 chip seen in the top left here:
http://buchty.net/ensoniq/files/schemat ... digit1.gif. Furthermore, that's supported by the hand drawn schematic of the Reset Generator: http://buchty.net/ensoniq/files/esq1-tg/14.jpg

So I measured Pin 7 of that chip, using Pin 4 as Ground, and with Pinouts described here: https://datasheets.maximintegrated.com/ ... CL7665.pdf. Sure enough, I get that exact same funky waveform as on the CPU [RESET] pin. https://defectiverecords.com/temp/CPU-RESET.jpg

Looking at the image of the scope, what's the voltage of he reset signal...0 or +5?

0V is the 2nd line up from the bottom of the display, and it's scaled to 1V for major division, so the waveform you're seeing is close to +5V.

[dnigrin]

So could this be the root cause of my problems? The chips are not getting proper Reset signals, and using Rasputin's description of the startup procedure from earlier in the thread, the CPU is not "waking up", and then all the downstream activities after that never happen?

[dnigrin]

My thought is a filter cap on a nearby rail is faulty so the digital section is not getting low ripple DC, or the PSU itself needs new caps. Check ripple with a 'scope. However I don't know what amount is considered acceptable for the ESQ-1.

Also just following up on this - I looked at the +5V digital output of the power supply card, before sending on to the mainboard, and it's absolutely rock solid there, no discernible ripple that I can see with my scope.

[meatballfulton]

If the reset pin is at +5, then the CPU is out of reset. The 0.5V pulses shouldn't be tripping reset, the voltage would have to fall much lower, under 2V.

False alarm, back to the hunt.

[dnigrin]

False alarm, back to the hunt.

Understood, but bummer - suggestions for where to go next?