Building A Cheap 27MHz Receive Pre-Amp
By Jason Reilly.....
You’d like to give your rig a bit of a boost on receive only, but looking at the available add on receive amplifiers, you decide that they are either too fiddly or too expensive? Then this project is for you!
Many modern CBs have a 2SC1674 transistor as the front end receive amplifier. While this is a good choice for the job, you can replace this transistor with one that has lower noise and higher gain characteristics. If you look through the Dick Smith catalogue, you will find the 2SC3355 transistor for the princely sum of $3.00. This transistor has a suggested upper frequency limit of 6.5 GHz, as compared to the 2SC1674s 600 MHz. While this might seem a bit of an overkill, it isn’t; here we are taking advantage of the fact that a transistor that has a certain gain at frequency X, at a lower frequency it will have a higher gain.
Fitting the new transistor is not as simple as removing the old and installing the new. The pinouts of the 2SC3355 are different from the 2SC1674, and you will need to bend the pins around for it to work when it is placed into the circuit. (Looking top down, with pins facing away from you)
| Original 2SC1674 | New 2SC3355 | Connect As |
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The easiest way to do this is to bend leg C so that it comes around behind and goes to the outside of leg A.
Once installed, you should find that signals will now read with quite a higher signal level. If you like, you can adjust the RX level pot inside the CB to return the signal meter reading to normal.
Are there any situations where this wouldn’t work or not be suitable? Some CBs use a FET front end amplifier, of which this modification is not suitable. Also, some CBs use the front end transistor in a common base configuration, rather than the more normal common emitter configuration, and again, if this is used, the 2SC3355 would not work. Some Midland and Lankar CBs use this configuration, for example.
I would also not recommend installing the 2SC3355 in CBs that do not have a RF gain control. The reason for this is simple. With the added gain that this modification provides, strong signals may cause receiver overload, and cause ‘splatter’ as it is commonly known. With careful use of the RF gain control, you can help avoid this from happening.
While I have used the 2SC1674 as a candidate for replacement, I’m sure that others, if used in a common emitter configuration would equally benefit from being replaced with the 2SC3355. As a cheap, easy and effective way of boosting your receive signals, you can’t go past this modification.