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Thanks for getting back to me. Looking forward to your results.

Have a great vacation. talk to you when you get back.

Thanks very much. Take your time. I appreciate your willingness to help. Maybe I can get to the root of this problem.

Hi Mr. Dokken. Thanks for your response. If it's not too much trouble, could you give me a few readings from the chip U430? I'm looking at either pulses or voltages from pin 27, pin 17, pin 24, pin 26, pin 22, pin 16, and pin 20. This would go a long way in trying get this chip working. Thanks...

Thanks for the info, Dwight. I'm learning a lot working with Dave.

I hope you find something. This seems like a bigger problem than I anticipated. Thanks.

Here are the Photos you requested from pin 19, pin 25, pin 26, pin 27 in that order. The voltages were low, high, high, and low in the same order. Hope that helps. The last 3 photos look like noise to me. I was using a x1 probe.

Didn't get to the computer today. To many honey dos. Tomorrow for sure.

I'm back and recuperating nicely. Got some dizziness, but that's another issue. Fixed the U446 inverter problem. U449 was bad and caused the low readings on RP4(5). U446 doesn't heat up anymore and works as it should. Tomorrow I'll get the readings on /MREQ and pins 25, 26, and 27, on the CPU...

Thanks for your great explanations on RP4(5). I'll try to solve this U446 problem by looking at the other gates. Whatever is causing it is shorting out Pins 12 and 13 and there was a burning smell. I know Z80 is the major problem we're trying to solve, so if I can solve this problem, I can get...

Your right, Dave I've been going in too many directions instead of working on the main problem. I mentioned that chip U446 was getting super hot and shorting out between pins 12 and 13. It's not the Chinese Chips fault. I traced the problem down to RP4(5). This resistor reads 1.5k ohms. I...

You're so right on RP4 pin 2. Just for kicks I removed chip U448 and U447 pin 4 shot up to 5 volts. The only thing I could get off of RFSH was noise, no real signal. It read low at 3.78 volts the same as M1. I happened to notice that Chip U446 gets super hot. This was not the Chinese chip but...

The reset gives a momentary pulse when the computer is turned on as well as the chirp in the speaker. The wait pulse is still high. The refresh pulses and is low. The reset pulse when first turned on and is high. I found something interesting, U447 which gives the interrupt to the CPU always has...

I'm sure you're right, Dave. I'd still like to know why the wait doesn't go to 0 when I press a key, though. No pulse or anything. Maybe I should be concentrating more on the interrupt and see where it goes. I agree I must get the M1 pin to pulse. I did change out the Z80 chip with a known good...

Loved the computer when I first built it back in the 80's. It was all new and fun to operate. It sat unused for at least 30 years and I'm trying my hardest to get it working again.

I found something interesting, Dave. The wait key is stuck at 5 volts no matter if I press a key or not. According to the Heath manual this puts the CPU on hold . Normal for this pin (24) is 0 volts. When I press a key, I get an instant 5 Volts on U433 Pin 12, and 13. This gives a pulse out of...

Wish I could buy this terminal in good condition.

This is the Z80 on the terminal board. The terminal board is lying flat on the bench. CPU's have been swapped. I'll try lifting the M1 pin and checking it on the scope. Thanks.

Went back and rechecked the pins with the scope. M1 low with no pulse at all. MREQ pulse low. IORQ pulse low. RD pulse low. WR pulse low. Wait no pulse High. INT no pulse low Clock pulse low. NMI pulse low. Reset no pulse High. Hope that helps a bit. I have a question on chips from China. Do you...