Tuesday, November 1, 2016

Drift on the BITX40


Many have noticed that the VFO of the BITX40 is a little 'drifty'. This is because the six bidirectional amplifiers (Q1, to Q6) have increased current for better signal handling, with each takes 22 ma and they heat up the board. Thse cause the board to warm up and the VFO to drift. You can see the board getting warm if you hold your finger on these parts. 

First thing you need to check is that the L4 is already glued to the board. This was originally kept unglued to allow you to change the VFO frequency coverage.  We assume that you have already glued the L4 (VFO coil) to the board. If you still feel your BITX40 to be drifty, Here is what you need to do :

What you need to do :
The resistors R12, R22, R32, R45, R55, R65 (six of them, all of them 100 ohms) control the current in the six bidirectional amplifiers. Replace these 100 ohm resistors with 220 ohm resistors.



The 100 ohms to be changed (in the circuit
How to do it :
Resistors as seen on the PCB

  1. Locate these resistors in the amplifiers around Q1,Q2,Q3,Q4,Q5 and Q6. These are marked as 101, black resistors. You need to desolder these. DONT DESOLDER THOSE MARKED AS '100' - THOSE ARE 10 OHM RESISTOR. Sorry, for the shouting, I thought it was important. 
  2. Remove them. My easy way to remove them is to use my regular 25 watt iron and gently press it's flat top over the resistor covering both ends as if it were a shirt that i was pressing with my iron. in a few seconds, the solder on both sides would have melted and you can simply slide the resistor away.
  3.  Replace. The pads on the PCB are large enough for you to solder regular 1/4 watt resistors vertically, Manhattan style. If want, you could solder SMD resistors as well, size 1206. If you don't have them, write to hfsignals@gmail.com and they will send you a pinch of these for free.
Why it works:
The board becomes cooler after this mod. It will still take 10 minutes for the drift to stabilize. But don't expect the board to be stable as soon as you have changed the resistors, the heat from all that soldering work has to dissipate before it can be stable.

3 comments:

  1. I have a different solution. It seems that the frequency determining caps in the VFO are just standard ceramic. I replaced the caps with NP0 surface mount caps, and I am very happy with the frequency stability. I believe that the caps are the Colpitts divider caps (C91 and C92) and the caps parallel with the variable cap (C95) etc. I had to buy a whole reel of the parts, so anyone who wants them can send a SASE to my QTH and I will send you a few sets. For my address I am good in QRZ.
    Joe W3TTT
    73, Joe

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