Twin Tube GFX-1

[bunkyloo]

Is there anything else worth trying on this thing? Any thoughts would be great.
Steve.

[MarshallJMP]

Well got too excited, as soon as I tested the unit powered up battery installed it read pin 1- 7.18 and 3-4.99 and then it crashed while testing so I tested battery in and 1 was5.15 and 3 was 3.642.
I then tested no battery and 1 was 5.54 and 3 was 4.04 and now won't boot with the 3.6 volt battery. :facepalm: .

The input voltage is too low, it needs to be at least 7.5V, with an absolute minimum of 7V. So 7.18 is already quite low, 5.15 won't work as you can see the output is only 3.64V, not nearly enough for all the 5V logic to work. Could be a few things, transformer, bridge rectifier BR2 or somethings is drawing too much current on the 5V line. Can you measure the AC voltage coming from the transformer? From what I can see it should be the 2 yellow wires on the connector next to BR1 and BR2. you should also be able to measure it on BR2 pin 3 and 4. BTW don't forget to put the meter on AC voltage!!

[bunkyloo]

Ok across the the yellow wires unplugged I get 10.71 volts and plugged in I get 8.16 volts.

[MarshallJMP]

Ok that looks good to me, and you still have only 5.15V on the input of the 7805?

[bunkyloo]

Ok the 1 and 2 pins read 5.98 with battery out and 7.62 with battery in.

[MarshallJMP]

Strange that the voltage changes with the battery in or out?

Do you have have the old ic that you changed out?

[bunkyloo]

No I don't but i did buy an extra one I believe.

[MarshallJMP]

I checked the whole schematic and that ic is the only one where the 5V and the battery come togheter. So if you have a spare try to replace it.
Also before you put the new one in (and the old one is out) measure the voltages again. There is something very weird going on here.

[bunkyloo]

I swapped it out again and its the same old thing. This is really frustrating that we are running out of things to try.

[MarshallJMP]

Did you measure the voltages before you put the new one in?

[bunkyloo]

Well no I forgot to do that. :facepalm: I had been busy with work and when I had a moment I just did it.

[MarshallJMP]

Do you see any signs of modifications done to the unit?

[bunkyloo]

The only thing i see is the input and output pots have been replaced and wired to th board instead of pcb mount and they work fine but besides that no.

[MarshallJMP]

Ok the 1 and 2 pins read 5.98 with battery out and 7.62 with battery in.

I just can't figure out why the voltage rises when you put a battery in it.

Can you send (by email) a few high res pics of the inside of the unit? Maybe I can see something.

[bunkyloo]

Will do  this weekend . :bow:  :thumb-up: