Deciphering the worst manual ever: Ortec VT120 repair

[ChristofferB]

Hi! I've recently bought two Ortec VT120 "fast preamp" for channeltron timing experiments. I can't get them to work..

This is not necessarily the amplifier's fault, since they're 10-350 MHz bandwidth, and I don't think I have anything that produces rising edges that steep.

Either way, there's only 3 transistors in them, and no other semiconductor components, so I thought swapping them would be an easy attempt at a fix. The two shows the same characteristics with a slow pulse input: an output about half as big as the input; they're supposed to be 20x volatge gain.

now, the manual from Ortec: https://www.ortec-online.com/products/electronics/preamplifiers/vt120 is fine, but the resolution of the schematics.. jeez.

Can anyone tell the model numbers on the transistors?? I'm at a loss. See below.

Thanks in advance!

[ChristofferB]

Oh, and the input clamping diode package as well! I think those might be bad. Could they be swapped for something like normal back to back 1n4148's?

They're BAV99's.

[ChristofferB]

Here's a picture of the board, the right is input, left is output. I havent gotten much luck with the smd part numbers.

[xrunner]

Well, I played with a bunch of transforms and maybe it's clearer. Best I could do is attached.

Does it look like the transistors begin with BF? I can make out some numbers. Maybe if you use that you can find something. I played with Irfanview to get that much. 

[ChristofferB]

Thanks for your effort! Yes, BD or BF.. the rightmost one looks like it sais BD912, which does exist but is a TO220 power BJT.

[Messtechniker]

BF is most likely. BF transistors are commonly used in high frequency applications like this one.

A BF 812 does exist. Maybe 8 is being misread as 9.

[picburner]

Judging by the application and the SMD code they could be:
BFR93
BFR93
BSR12
I would definitely not replace the BAV99 with two 1N4148.