HP-35 Power Supply Waveforms

[albertofenini]

(04-09-2019 10:15 AM)albertofenini Wrote:  

(04-09-2019 12:49 AM)cruff Wrote:  If the transistor you selected for Q1 is out of tolerance for its operating parameters, as compared to the original type, that might be enough to keep it from performing the regulation function in the way the circuit was designed. If that is the case, then selecting a lower value zener diode would suffice to adjust the voltages down. You could check to see what the waveform on the base of Q1 is doing, perhaps there is not quite enough drive current to switch Q1 on more fully to reduce the Vss voltage, and indirectly the other two voltages. Also verify that R1 is close to the listed value on the schematic.

Thanks for the idea, Q1 is actually an old original 0550,
I'll check the HFE and I'll check the R1 value but I'm pretty sure is close to what it should be, and this R1 is also from an old logic board
I'll also check the waveform on the base of Q1
thx !!

We did few tests, and we re-designed the layout, now the position of the components is more close to the one on the electric circuit and this helps in troubleshooting.
We have now good Vcc and Vss, very close to the desired values, but still Vgg about -15,6 V
In the previous message I have erroneously indicated Q1 as a 0550.
This is wrong, Q2 is a 0550, Q1 is a 0094
We have three different Q1 0094, one have an HFE of 1, another one have an HFE of 2, the third one have an HFE of 16.
I think the first two are almost dead, the third one could be good.
We have tested the three in combination with both Q2 0550 HFE 16, and Q2 2n3904 with HFE > 100
The result is the some, Vgg equal to -16
No matter which Q1 or Q2 we use
R1 is 1470 ohm, which is a good approximation of the 1500 ohm requested.
Attached is the waveform from Base of Q1 (0094 with HFE=16)
What do you think ?