Twin Tube GFX-1

[bunkyloo]

Well I replaced r19 and its back to the same stuff. Would my next course of action be replacing the ram chip as it is affected next in line? Any ideas where to look next would be cool.
Thanks Steve.

[MarshallJMP]

Can you measure the voltage on pin 8 en pin 2 (4 is ground) and let me know. You must insert the battery for this and turn the unit on, even if it doesn't boot up.

[bunkyloo]

On U19 pin 2 reads 4.71 volts and pin 8 reads 4.73 volts.

[MarshallJMP]

Ok here's your problem, the difference between these 2 inputs must be between +- 10mv and 50mV. Since you only have 20mv it switches on the battery input, which means it shorts the capacitor C3 of the reset circuit to ground, which causes the µP to go into reset mode and it makes CE2 of the RAM low which is active high. That's why if you insert the battery it shuts off.

It seems your 5V supply is a bit on the low side. Most 7805 5V regulators have 4% tolerance, so it's below that (should be at least 4.8V), what I would do is replace the 7805 with a 7805ACT which is 2% and replace cap C85 (470µF/25V) and C86 (10µF) while you at it. Or use a battery of 3.6V.

[bunkyloo]

Great I appreciate the help I will order those up and will let you know the results thanks again Steve.

[MarshallJMP]

ok keep me updated.

BTW this the mouser part number  863-MC7805ACTG

[bunkyloo]

Well just got my last capacitor today, got it put back together and bam, same thing any ideas where to go next?
Thanks Steve.

[MarshallJMP]

What about those voltage son pin 2 and 8? Can you measure these again?

[bunkyloo]

Pin 2 reads 3.51 and pin 8 reads 4.10 .

[Iperfungus]

Hey guys!

I ended up selling my second unit with its pedalboard to a guy who will do the repair job.
I hope Fabrizio will join the Depot Family soon!

Phil, prepare yourself for other questions.  :lol:

[bunkyloo]

@ Iperfungus
Looks like these units have quirks of there own. :facepalm:

[Iperfungus]

@ Iperfungus
Looks like these units have quirks of there own. :facepalm:

That's the funny side, Steve!  :lol:

I like mine a lot, now.
Happy to have a 100% working unit, I don't need a second one.

But I'm also happy that my second one went on the hands of someone who will give it new life!

@rnolan: Rich, Fabrizio is attempting to register to the Depot...but new users registration is suspended. I suggested to use the NEED HELP button and to shot you an email.

[Iperfungus]

Update.
Fabrizio found the problem and the damage, due to a wrong connection of the transformer to the motherboard pins: not upside down but moved to one side, leaving one pin free, so that the whole pin connection was wrong.
Some diodes are shorted and need to be replaced.
But he'll explain it himself, once he joins the Depot family.

[MarshallJMP]

Pin 2 reads 3.51 and pin 8 reads 4.10 .

Wow why are the voltages so different now? pin 8 should read 5V? and why did the battery voltage drop by almost 1V?

[bunkyloo]

I have no clue but I did swap all 3 components. Also that was without the battery installed.
Installed the battery and  pin 2 is 4.67 and pin 8 is 4.85. Sorry for the misunderstanding. Steve