Substituting the SRF7043
in the Yaesu FT-100

Before undertaking any replacements described, please be certain to read the warnings at the bottom of this page.

A "relatively" common problem with the Yaesu FT-100 is the failure of the VHF/UHF final transistor.  Exactly why this is is open for debate - and the causes may be multiple.  An incomplete list of possible causes may include:
On the above points, I make no judgments and consider them all equally to be potential problems.  To be sure, several of these points seemed to have been addressed in later production runs of the FT-100/D - but whether or not these modifications reduced the likelihood of device failure is unknown - to me, at least.

When my FT-100 (non-"D") version failed, it did so somewhat insidiously.  Transmitting occasionally on 2 meter SSB into a reasonably good antenna (but not the best - the measured VSWR on 2 meters was 2:1) everything seemed to be going well - until I smelled something "hot" - and it turned out to be the FT-100.  Later checking showed that the idle current of the transceiver was on the order of 10-20 amps - and fluctuated wildly.  Another quick check showed that there was no longer any 2 meter or 70 cm output power.

Popping the cover, I quickly found the source:  The VHF/UHF final transistor - the SRF7043.  Investigation using a jeweler's loupe showed a tiny hairline crack in the ceramic case of the transistor - a sure sign that something was very wrong.  A quick check of availability of this part also showed that I was in for sticker shock:  A new transistor - if available - was typically on the order of $200 - more or less.

Trying to dig up data on the SRF7043, I noticed several things:
Exactly what other device would substitute for an SRF7043 isn't clear as I have yet to dig up a datasheet specifically for this device.  Rummaging around on the net, I found that PolyFet makes a number of similar devices - but not having data on the SRF7043, I couldn't be sure what would work.

This shows the installation of the SK702 in the FT-100.  Note the removal of the capacitors across the output terminals of the transistor and the reconfiguration of the  "output" side of the balun - see text.
Click on the image for a larger version.
Close-up view showing the installation of the SK-702 and the configuration of the 1:4 coaxial balun

It has been suggested by others that a possible substitute for the SRF7043 was the MRF166W, which is equivalent to the PolyFET SK702, so I ordered two, as the minimum order was $200 with each device costing just over $100 each when shipping was included - still cheaper than just ONE SRF7043 - if you could get it!  Note:  Contrary to other information, the MRF166W is NOT equivalent to the SRF7043!

Results:

Simply dropping the SK702, a 28 volt device, in place of the SRF7043 resulted in rather poor amplifier efficiency:  At saturation, there was about 10 watts out on 2 meters with about 6 amps of Drain current and 5 watts on 70cm with about 4 amps of Drain current.

Removing all of the capacitors on the output of the device (namely C3553, C3555, C3607, C3608, and C3609) improved things somewhat:  At 13 volts of drain voltage, the unit saturated at just over 30 watts of RF output with 6.5 amps of Drain current on 2 meters and almost 20 watts of RF output with the same current on 70cm.

Another recommended modification is one made to the coaxial 1:4 output transformer using the yellow coax cables.  These cables (which are appear to be pieces of 17 ohm coax - yes, 17 ohms) transform the fairly low output impedance of the transistors' drain circuits up closer to 50 ohms and do so in a balanced manner - hence the "push-pull" circuit.  While testing with the SK702, I observed that these coaxial transformers seemed to slightly affected by their routing and proximity to nearby components.  To eliminate this tendency (which may be related to circuit instability - but this is only a guess) I reconfigured the transformers by unsoldering the "non-transistor" end completely, connecting the center conductors of the coax to the outputs as shown in the picture, then connecting the shields together, leaving them floating.  After this was done, the 1:4 transformer appeared to be completely immune to routing of its coax.  It is worth noting that the "new" configuration is functionally identical to the "old" one in terms of impedance transformation.

For the time being - and to preserve linearity of the output (extremely important if one operates CW, SSB or AM) I set the "Maximum" output power on the adjustment menu to provide a maximum of 15 watts output on 2 meters and 10 watts output on 70 cm - this, taking into account the need to be able to operate linearly with a power supply voltage of just 10.5 volts as might be expected with wire losses and using a weakening battery.

Comments:

While this device will work, it falls somewhat short of the original device's output capability - about 6 dB or 1 S-unit.
Trying the LK722's - an expensive experiment:

After first trying the SK702, I ordered a pair of LK722's to try out.  In speaking with an engineer at PolyFET, it was determined that, for maximum power in a broadband amplifier circuit, this would be a better choice than the LK721 as it was designed for 12.5 volt operation and appeared to be more rugged.  They were $104 each and with a $200 minimum invoice, I obtained two of them.

During the initial testing, it seemed to be fine in that I could obtain about 45 watts of saturated output power on 2 meters and about 30 watts on 70cm.  Like the SK702, it, too, seemed to operate with best efficiency and power output without the output loading capacitors (C3553 etc.)

Everything seemed to be going fine:  There appeared to be no obvious sign of instability and the devices seemed to be operating with good efficiency and were not getting unduly warm - and suddenly, the output power dropped by about 6 dB.

Investigation first revealed a loss of the gate bias supply, dropping the devices into Class-C operation and lowering their gain.  Further investigation revealed that they had developed a low-resistance Gate-Source resistance - about 15-20 ohms - which swamped out the gate bias.  All this time, I had been observing the output of the amplifier on the analyzer and I saw nothing that had caused concern, and output power versus drive seemed to be stable - a reasonable indicator of device stability.

Carefully re-checking the output circuitry for any obvious failure (and finding none) I put the 2nd device in:  It, too, operated flawlessly for a few minutes before the same thing happened.

Hmmm...  (Actually, I said other words - none of which I will repeat here!)

So, I put the original SK702 back in and it worked exactly as before.  Being somewhat worried about ruggedness, I did several things to try to kill it such as unterminated/short the output at full output - but nothing seemed to faze the device, so I put everything back together again.

So what happened with the LK722's?  I don't know, exactly.  The most likely cause was some sort of spurious oscillation, but I saw no evidence of such in all of the testing.  I will leave it up to someone braver than I to conduct further experiments along this line...


Additional Comment on the use of the LK722 or similar devices:

I have seen claims that an LK722 (or other devices) will simply "drop in" as a replacement and give full power output, but these sources, when asked for any specific details about the replacement procedure and about the performance, have not responded.

 If you have successfully used the LK722 (or any other device) as a replacement for the SRF7043 (even if it is at lower power) please let me know, along with some details and your experiences so that they can be shared here!

After device installation:

Once the device has been installed, some modifications are recommended to the "Alignment Menu" items.  These modification are required as you have reduced the amount of output power that the radio is capable of on VHF/UHF.  Failure to do the steps involving F-21 through F-23 will likely result in a badly distorted and "splattery" signal on 2 meter and 70cm sideband.

These steps are:


Datasheets:

PolyFET LK721
and PolyFET LK722 - These are 12.5 volt devices:  See comments on the LK722 in the sidebar.

PolyFET SK702
- This is the device that I tried and describe above.  It works fine and seems to be quite stable and rugged - if you willing to settle for lower output power than with the original device.

PolyFET SK701 - This device will also work - but it produces LOWER power than the SK702 - see comments below.

Comments about the SK701:

The SK701 is often found on Ebay for a reasonable price and is often shown as a "Substitute" for the SRF7043.  It should be noted that the SK701 is a LOWER POWER device than the SK702 mentioned here - a fact usually NOT mentioned by the Ebay sellers!.

In correspondence with others who have used the SK701 instead of the SK702 as a replacement, here are a few things that you can expect:

MRF1570 Data Sheet:

Here is an annotated copy of the MRF1570 Data Sheet for your reference, with thanks to the person who sent it to me.
Other possible replacement devices:

According to Bob, KC3VO, it is possible to use an MRF1570 device (after minor mechanical modifications, but little or no electrical modification) to replace the SRF7043 and achieve excellent power output and stability:  For more information, contact him directly.


Warnings
Please read the following:

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This page last updated 20100727

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