FT225RD - SM7OVK

Introduction

I have owned my FT225RD since 1984. This was my first transceiver for 144MHz.
It is a practical transceiver, since all boards are easy to change and it is easy to make own PCB's to replace the originals. There is plenty of space so you do not have to be a surgeon to be able to operate your soldering iron inside the receiver.
The receiver is far from perfect in its originally state, compared with new designs. Several improvements can be made and I will try to show some of the improvements that I have made on my receiver. These ideas are not only my own and if you have any other ideas please do not hesitate to let me know!

Maintenance

With a radio as old as the FT225 it's important with maintenance and here are some hints presented.

Measurements

I have made several measurements and some are found on this page. I would like to point out that this page contain many pictures and will take more time to download than most other pages.

Front end

Originally the noise figure is as DJ4GC mention in a good article in VHF-Communications, 3/86; "The receiver noise figure lies uncomfortably close to a two-digit number".
In this article DJ4GC describes some modifications that can be made on the original input-PCB;

  • Change the first stage from a 3SK51 to a BF981 - this gives a better noise figure. I am not sure about how many dB's, but it is a few.
  • Change the cap-diodes, used for fine-tuning of the input stage and the mixer, to fixed 10pF capacitors. The Q of the input and output circuits will increase. The loss in these circuits will decrease and they will be more narrow band, but - I only use the first 500kHz of the 2m-band.
  • DJ4GC also show some changes of filtering the control-voltage to the local oscillator. A better approach for filtering the control-voltage is shown by SM5BSZ on his eminent homepages! These homepages are a great source for inspiration and I recommend a visit!
I started to make some modifications according to SM5BSZ's article in R&T 1981, (R&T was a swedish monthly magazine now called Ljud & Bild). I did also change the input transistor and replaced the cap-diodes as suggested by DJ4GC. The receiver is better but it will still not be able to handle all strong signals present on a region1 contest! One big reason for this is the mixer. Originally a dual-gate FET is used.
MuTek manufactured, and I think they still does, a good front-end PCB. Another approach was made by SM0KFJ. This was merely a "copy" of the MuTek-PCB. After what I know, this PCB was not sold, only distributed as a schematic and a layout. (I do not know if this PCB-layout is still obtainable in some way, (?).) These constructions both used a BF981 as input stage, a 7dBm diode-ring mixer and a single 15kHz 6-pole crystal-filter.
If you are about to build a front-end by yourself here are some ideas .

Other modifications

Here are some of the modifications I have done. I have tested all modifications carefully, but when writing these documents there is always a possibility that something is forgotten. If so please inform me and I will change this as soon as possible, comments are always welcomed!

Before you do any modification, be sure to unplug the AC-cord! There will be lethal voltages inside the transceiver! Do not try to do any modification if you do not have the proper soldering-skills! All modifications are made at your own risk! Take care and isolate all not used wires carefully.

RX/TX-split;

Very early I split the TX and RX since it is more practical when using pre-amplifiers and/or power-amplifiers. When I did this modification I also changed the "UHF"-connector to a N-connector. This will not improve the receiver or put more power to the antenna, but N-connectors are much more reliable than UHF-connectors and I am sure that after some practice you will get better results with the N-connectors.
It is VERY hard to get a really 100% contact with UHF-connectors and they need a lot of heat when soldered. Normally these connectors, or the coaxial cable, can't stand this heat and then you will not have a good result.

IF output;

I first made a IF-split where it was possible to put filters etc in the IF-chain by using a IF-loop on the rear-panel. I realized that this wasn't used very often and when I needed a BNC for the dual tune I reconsidered an IF-output. If one is interested in direct-conversion designs or other high performance designs, this will be a very nice way to make the first tests and in this way only the demodulator has to be built for initial tests.

LF-filter;

Some time ago there was a short article in QTC, the "swedish version of QST", where a simple LF-filter was shown. Since there originally is not any CW-filter in the FT225 I decided to test this simple LF-filter.
After several tests I also began looking at the original AF-filtering and this modification is found here.

Volume control repair and modification;

When my volume control potentiometer got problems I felt that I had to exchange it. Since the volume control is quite sensitive and you can't change it much without getting a lot of more soundlevel I modified the amplifier board.

Clipper circuit;

One of the SM5BSZ-modifications was to take the audio-modulation from the FM-part and feed the SSB-part since the FM-signal has a nice clipper-circuit. This gives a very loud modulation and for nearby stations the modulation is quite strong. Since I never bought a memory, I re-use the switch on the front to switch the clipper-circuit on/off. I just cut the PCB tracks where the switch is mounted and use the switch. This switch can originally not be locked in position, but on my FT225 I found this "locking thread" on the "Burst"-switch. By gently replacing the "locking thread" the "memory"-switch now locks in position. The "Burst"-switch now get a millimetre longer out, but who cares...
After all these years there could be oxide on the switch-terminals. If so, it is very important to clean the terminals before soldering and use good soldering-wire. I recommend you not to use any of these contact-spays obtainable at several petrol stations. With these spray-cans you might get a better result for a couple of months, then the contact begins to collectdust…

Noise-blanker

Many users of the FT225, including myself, are not happy about the noise-blanker performance. I therefore made some tests and a modification of the NB-circuit.

AGC

The AGC in fast mode is way to slow for high performance meteorscatter, here is an easy way to make the AGC circuit faster.

Extended frequency range and crystals

It's possible to extend the frequency range of the FT225 from 144-146 to 144-148MHz. (This is only for European models, US will have this already!) In many countries this will be illegal! It's also possible to have fixed channels by adding crystals.

Other accessories

Computer interface

When using the FT225 with WSJT or similar computer software then it is nice to have a interface capable of keying the PTT automatically.
Some information about meteorscatter is found here.

Biastee

I feed my preamplifier with a biastee. In this way I do not need any extra cable to the preamplifier mounted at the antenna.

Cavity filter

Here are some hints when modifying a cavity filter. A single cavity gives a very narrow bandwidth and could be good for the DX-station with nearby high-power stations!

Things for sale!

Here are some things for sale. All is ham-related and several items are FT225-related.


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