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HP25 repair - need advice (TESTING POINTS PICTURED)
04-06-2019, 03:43 PM
Post: #6
RE: HP25 repair - need advice (TESTING POINTS PICTURED)
Hi everyone, given the bad weather we have started to work again on this project.
We attached the Vbatt to the board and we have noticed that on pin1 of the Toroid there was no activity at all.
On a working HP25 with Vbatt attached we have found a square wave which means the the oscillator works.
We suspected then of a transistor, the 0550 which, has we learned from our PSU prototype forms the basic oscillator.
(if interested in this you can find more about the PSU we built, in this post
However, both the transistor : the 0550 and the 071 had a good HFE.
We striped then all of the PSU components and tested them and all of them are good.
Diodes, capacitors, resistance, all of them, even the Toroid, that we tested with our PSU prototype !
Nevertheless all of Vdisp, Vgg and Vss have values equal to Vbatt ....
What could we possibly check ?
Remember the ACT, Rom and Ram have been removed, and only the cathode and anode/rom drivers are present on the PCB
Thank you very much !!!

Edoardo & Alberto
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RE: HP25 repair - need advice (TESTING POINTS PICTURED) - albertofenini - 04-06-2019 03:43 PM

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