MCLR - Faulty? (URGENT)

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Bugger, bugger, bugger.

I lost the pad for the buffer out put when I was trying to desolder the pin ( I dont know how because i dont think it was heated excessively), but that doesnt matter.....

with the lost pad i connected my programmer up to the computer, still the LED remained lit, however much dimmer this time. with the collector of my ghetto inverter connected to the 10 K resistor that came off buffer 4 the LED was brighter again, but unresponsive to the programming softwares "commands" ( I have been using the chech feature)

what now?
 
Update, almost got it, i made a mistake with the pin, i accidently used clock pin 3, but i fixed that, now the led is dim when VPP is dissabled, and bright when its enabled (looks like i almost have it).

however it still is returning errors, and isnt programming, could this be due to the VPP voltage which now sits on 1.9V when VPP is disabled... what do you think?
 
eng1 said:
Hmmmm... what if you consider a small leakage current? I'd like to simulate that case... do you still think that the transistor would switch off with any resistance (base to emitter)?
Click to expand...

You're now moving from the realms of changing resistor values to the driver being faulty - the purpose of the resistor between base and emitter of the switching transistor is purely in case there is some slight leakage in the driver. In 99.99????% of cases you could leave that resistor out altogether (which I've seen done commercially sometimes) - however, it's good practice to have some sort of resistor there, and I would always do so.
 

Check all the voltages around the transistors, see why it's only dropping to 1.9V?. It 'may' need a resistor adding from Vpp to 0V?, try adding a 10K or so.
 
Someone electro had me tie VPP to ground via a 10K, that brought it down to 1.88 and i added a further 1K so now its sitting at 1.8V, almost there, but i dont know if thats a proper solution.
 
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What's the voltage on the input of the "ghetto" inverter when the VPP LED is dim? You may want to add a 10K resistor from the base of the "ghetto" inverter transistor to ground if this voltage is above 0.4V
 
Last edited:
kchriste said:
What's the voltage on the input of the "ghetto" inverter when the VPP LED is dim? You may want to add a 10K resistor from the base of the "ghetto" inverter transistor to ground if this voltage is above 0.4V
Click to expand...

It's already there, it was in my original suggestion for a discrete inverter.
 
Hi, sorry to ressurect such an old thread, but I'm facing pretty much the same problem as the original poster.

Whenever I turn my programmer on, the Vpp raises to 11.66 volts and sits there forever. I'm using a custom built tait-style programmer, using the 74HC14 chip. I've already tried changing to 74HC05 but got no luck.

Should I also try with the 74hc06 or 74hc07 chips as suggested back in this thread? Would that make a difference?


Thank you,
Cesar
 
Why would a design use 7406 or 74LS06 when so many new IC or CMOS inverters are available?

You think so too?

Because there is a reason. The circuit needs the open-collector feature of these "old" IC to properly drive the transistors. If you don't know that, don't try to change these IC types. Countless builders have fallen in the same trap.
 
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I originally thought about using cmos chips because I'm running on batteries and they consume less power while standing by. I'll try with the original LS chips then.

Also, I've seen the ozPic programmer using 74HC14 and thought it could work just fine, but maybe I missed something.


Thank you for the information!
 

You missed R1, Q4 and Q5 - which mean you don't require an open-collector gate.
 
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