CMB PET 3032 ( 2001N BOARD )

[Desperado]

Hi guys for your happiness ( I hope ) I'm back with a new pet!
I have black screen on this Cbm, i also checked voltages on the four voltage reg. and they are fine with -5, +5, +5 and +12 V.
I checked cpu pin 7 and i have high signal....i also checked Cpu pin 40 but i have always low signal
I tried to bridge c68 but nothing happens....
Maybe bad 555??

Thanks so much!

 

[daver2]

Another one!

It may not be the 555... That is a guess isn't it?

Check the schematics - it could be A2 (555), A3 (74LS04) and/or A10 (7417). The A3 and A10 gates are also in series between the 555 and the CPU. You can't say it is the 555 unless you have tested the 555...

Check (with your logic probe (after you have powered up the machine) at the following points:

A2 pin 3 (output from the 555).
A3 pin 2 (output from the 74LS04 inverter).
A10 pin 12 (output from the open collector buffer).

Then short out capacitor C68 (0.1uf) and check the three (3) pins again.

Let's play the game "see which pins do not change state"...

Dave

 

[Desperado]

Another one!

It may not be the 555... That is a guess isn't it?

Check the schematics - it could be A2 (555), A3 (74LS04) and/or A10 (7417). The A3 and A10 gates are also in series between the 555 and the CPU. You can't say it is the 555 unless you have tested the 555...

Check (with your logic probe (after you have powered up the machine) at the following points:

A2 pin 3 (output from the 555).
A3 pin 2 (output from the 74LS04 inverter).
A10 pin 12 (output from the open collector buffer).

Then short out capacitor C68 (0.1uf) and check the three (3) pins again.

Let's play the game "see which pins do not change state"...

Dave

Hi Dave, yes Dave another
I'm writing a sort of step-by-step guide for the checks to be carried out on each PET ... I will always need it in the future!

 

[daver2]

Yes, I've been meaning to write a guide for years - and get it included as a 'sticky post' on the Commodore forum. Yet another job that I have still to do. Oh well, there's always retirement...

That would be a good guide for you to have.

Dave

 

[Desperado]

I haven't any signal on A2 pin 3..... same if i short C68...

A3 pin2 : always LOW
A10 pin12 : always LOW

 

[daver2]

By "any signal on A2 pin 3" do you mean it is floating (i.e. not a true TTL signal)?

Can you also check pins 1, 4 and 8 using your logic probe.

Dave

 

[Desperado]

By "any signal on A2 pin 3" do you mean it is floating (i.e. not a true TTL signal)?

The logic probe stay off on A2 pin3....

 

[Desperado]

A2 PIN 1: LOW
A2 PIN 4: HIGH
A2 PIN 8: HIGH

 

[daver2]

Your guess was right this time...

Time to warm up the soldering iron and swap A2 (the 555) - it is duff.

Pin 1 is the GND power supply.

Pins 4 and 8 are directly connected to +5V - so should indicate HIGH.

I just wanted to make sure we had a power supply on A2 !

The output on pin 3 should either be HIGH or LOW. Neither is not an option for an operational 555 device.

Dave

 

[Desperado]

Your guess was right this time...

Time to warm up the soldering iron and swap A2 (the 555) - it is duff.

Pin 1 is the GND power supply.

Pins 4 and 8 are directly connected to +5V - so should indicate HIGH.

I just wanted to make sure we had a power supply on A2 !

Dave

Unfortunately i haven t 555 spare part now

 

[daver2]

A guess a couple are on order now?!

Dave

 

[Desperado]

Yes, i order 4 pieces now! Can i try to use the 555 of the other pet....

 

[Desperado]

I tried with 555 of the other pet and now i have correct reset signal on Cpu pin 40!
Unfortunately i have always black screen and no pulse on Cpu pin 7....
Time to ROMULATOR?

 

[daver2]

Yep, get the old (new) ROMULATOR out.

Dave

 

[Gary C]

, there's always retirement...

Dont you dare, we need you at work !

 

[daver2]

Everyone else is retiring around me... Last man out switch off the lights!

Heading up your way on Monday. Outage training...

Dave

 

[Desperado]

here we go again sadly...i am desperate!
I have black screen also with romulator

 

[daver2]

Do you have SYNC pulses on the CPU though?

Dave

 

[Desperado]

Do you have SYNC pulses on the CPU though?

Dave

No...i have High signal...

 

[Hugo Holden]

I haven't any signal on A2 pin 3..... same if i short C68...

A3 pin2 : always LOW
A10 pin12 : always LOW

A3 pin 2 always low, implies A3 pin 1 is always high as would be A2 pin 3. So the output of the 555 timer is stuck high right ?

Now when you power the computer up (or fit a reset button), a low trigger pulse is generated at A2 pin 2 because the 0.1uF capacitor is not yet charged up. This sets the output of the 555 timer A2 on pin 3 high.

It stays high until C67, the 1uF Tantalum capacitor on pin 6 & 7 charges up via the 1 Meg resistor, to the threshold voltage for the 555 or about 2/3 x 5v, which takes about 1.1 seconds because it is 1.1 x RC, then the output of the 555 pin 3 falls low. But it is not falling low is it ?

The 555 timer IC is extremely reliable, Tantalum caps are not, possibly the 1uF timing capacitor has shorted out or is leaky, rather than the 555 being defective.