A drop of 0.2V is good for using a transistor. What transistor is used on your programmer? Usually a BC557 is used and it's max collector current is 100mA. Some use a BC327 which has an max Ic of 800mA. The dual powered PCB3 I was using for testing was using an A733, which is spec'ed at 150mA.

 

All transistors on the PCB are S8050.

 

All transistors on the PCB are S8050.

S8050 are NPN; overkill to use them on a Willem. What is used for the PNP transistors? If they used the complement for S8050, that would explain the low saturation voltage you observed.

 

Others are S8550. All near the step up regulator and 7805 IC.

 

Thanks for the info.

Now you should check VPP for voltage drop issues. Put a 50mA load on VPP at each of the programming voltages (21V and 25V in particular). What you want to verify is that VPP remains stable for the amount of time required to program a 2732 (4 minutes). The problem here is the step up regulator, not the transistor switch. VPP should remain within a volt of the programming voltage.

Once voltage drop issues are addressed, you need to address VCC options. For blank verification, you need 4.2-4.45V (lower is better). For programming, you need 5.0, 6.0, and 6.25; I believe these need to be within 5%.

Then you need to make sure you select the correct programming pulse width. For devices that are programmed open loop, it's 50mS. For algorithms that include program margin checking, it's 10uS to 1mS; depending on the algorithm used. I usually simplify things by using a 100uS pulse and VCC=6.25V.