Sinclair ZX80 / ZX81 / Z88 Forums

For the interest of anyone reading this topic, please note that the OP also opened a topic over on Spectrum Computing forums here.

Mark

Does it mean anything that TR4 has continuity between the center leg and lower leg?
Is between base and collector? Tried my best to find that

In terms of solder no short either up or down on the board.

With the board disconnected from the PSU, using the resistance range, test across C44.

ZTX650 / ZTX651 lead out: Test also TR4 (ZTX650 / ZTX651) on the 2kΩ and 20kΩ resistance ranges (don't use the 200Ω range) as below:

Red meter lead to the collector. Black lead to 0V/GND.

Red meter lead to the base. Black lead to 0V/GND.

Red meter lead to the base. Black lead to the collector.

Red meter lead to the collector. Black lead to the base.

Now switch to the diode test range. Red meter lead to the base. Black lead to 0V/GND.

Red meter lead to 0V/GND, black lead to the base.

Do you have a bench adjustable PSU? Or a regulated 12V current limited PSU?

Mark

Hey there!

C44 -> 2,80kΩ

Red meter lead to the collector. Black lead to 0V/GND.
7.10 and right after open

Red meter lead to the base. Black lead to 0V/GND.
4.8 and right after open

Red meter lead to the base. Black lead to the collector.
short

Red meter lead to the collector. Black lead to the base.
0.004 at 2k scale

Now switch to the diode test range. Red meter lead to the base. Black lead to 0V/GND.
1.108

Red meter lead to 0V/GND, black lead to the base.
.595

Do you have a bench adjustable PSU? Or a regulated 12V current limited PSU?
No, i only have that can use a pc power supply or a small dc-dc converter from lm2596.

Also i have updated the google drive with all pics asked, but also taking pics is clearly not a skill i have....

So I would say that TR4 is defective. I recommend cutting the legs and then desoldering each leg one at a time.

I've also looked at the photos. Me thinks you need to learn how to get control of the autofocus...

It's not the best soldering, but I've seen far worse.

Mark

Hi there Mark!

Well here we dealing with 20cm of snow
So yeah, i am going to pick a damaged board and work on my soldering skills. Is really important and normally it does not go so bad.... but with the zx i really had issues to do it right.
Do you work on electronics repairs? If so do you repair fpc connectors?

Also i just noticed my multimeter has transistor testing capacity, so while i wait for the new tr4 and 5 arrive i will test it and let you know.

Is there any other check i should do on the power delivery to the board while waiting?
And above all.... do you think i damaged the lower ram board by testing power on with it and damaged tr4?

Thanks in advance for your time and help!

Cheers
Fred

Ok...

After a shaming amount of time i managed to remove the TR4 as you told to do, remove the remain parts of legs, and solder something to it to test on the multimeter (something i never done before).
Yes the TR4 (ZTX650) give 000 on the test.

I had 2 BC549C that came with the recap kit and they gave 532 so now i learned that in TR datasheets there is the range of values that they have when they are good

As i also ordered the TR5 replacement should i also remove that one?

Cheers
Fred

Hello there!

So i replaced TR4 and now yes there is color in the image and first time it booted into normal sinclair screen but after some seconds started beeping and went to black center with some red stripes in the top of the square. I had the lower ram board plugged in.

Now i removed the ram board and went again to measure the voltage on the caps as you told me to do before:

Across C50 -> 11.73v
Across C34 -> 5.02v
Across C44 -> 12v
Across C45 -> 11.70v
Across C79 -> 13.68v


Across C80 -> 2.60v
Across C46 -> 15.63v
Across C74 -> 0.66v
Across C27 -> 4.90v
Across C25 -> 4.88v
Across C65 -> 0.60v

Also found in a site:
"An individual 4116 (Lower RAM) chip can be tested out of circuit with the following tests performed using a digital multimeter switched to the 20k ohms range (or equivalent range).
Test between pin 1 (VBB, -5V) and pin 16 (VSS, GND). You should get more than 11k ohms.
Test also between pin 8 (VDD, +12V). and pin 16. You should get more than 20k ohms. "

So i went to test this way all the original lower ram chips:
- 5 gave a lot more resistance values than indicated here but similar
- 3 gave same in pin1 and pin16 but open on 8 and 16

Ah and also i tested voltage to the lower ram and all seems correct.
At least i see -5v in pin 1, 12v in pin 8 and 5v in pin 9 in all lower ram sockets.

So, now we have all the supply voltages within specification.

The Z80 is crashing, most likely due to a fault accessing RAM.

Do you still have the 'upper' RAM disabled? If yes, please show me (photos) the connections where you attached the wire.

The resistance test of 4116 DRAM chips is meant to be a guide on if they are completely toasted, rather than indicating that they are okay. I suggest you stick with the 'lower' RAM replacement board.

It's also possible that there is a fault with a multiplexer chip.

Mark

So.....

Was without the upper ram disable.
Well.... so i got the package from Phil with new TR4 and TR5 replacements. TR4 was already done so i went to remove the TR5 and put the new one.
And it boots to the dandanator card with nice picture and all seemed fine. I reset power to press it to boot to test mode, it gives the usual ram fail 01234567 - with ram board in.

After that no more boot.... i just cant understand this....
Did not touch anything else but TR5 and had no issues to remove the existing and solder the new as best as i can say....

Now if i power on without ram board and with upper ram disabled i get a i think purple color with white rectangle....
If i put lower ram board in and power it gets black border and center rectangle full of colored squares with like strange chars on them.