Unilab KL70 10m FM Converion - PA Matching schematic

Please find below the schematic showing what was changed. I ended up achieving 50W across the FM segment of 10m. Now I just need to set the deviation and the radio is ready for use!

Regards
Richard VK6TT

Unilab 10m FM Output Matching Network - Error in original radio corrected

You may recall from the post here where I first discussed the modifications needed to the PA that I modelled the output matching network as shown below:

With only 40W of RF out I removed one of the trim caps to see if I could learn anything that might explain why. What this revealed was that due to the pin-out of the trim cap it was not connected to the right point in the output network. Instead of the network above we had this:

Actual output network before error spotted

A combination of the trim cap being connected on the wrong side of the LC trap and using ideal parts when real might have been more appropriate mean the output network was matching to perhaps 32ohms, not 50 ohms.

So I connected the trim cap to the correct side of the LC tank. Now I was getting 50W and to be fair that is not a bad result. The 50W was flat across the 10m segment and the second harmonic was 50dB down. The third harmonic was over 60db down and I felt no need to measure it with more precision.

But as I was writing this I realised that an extra 39pF padding the very last trim cap might improve the matching. I tried this but the only benefit was the second harmonic content fell even further.

It appears I have to be satisfied with 50W unless I change the MRF247 to a transistor better suited to this application. If anyone manages to coax more than 50W from a MRF247 at 10m I would dearly like to hear how you achieved this.

Next time I'll publish a circuit for all the PA stage changes I made.

Regards,
Richard VK6TT

Unilab 10m FM conversion Transmitter PA mods - Low Pass Filter

Well it's been hot here, with two days over 40degrees Celsius now. I've used that time incorporating a pre-driver into the radio and considering the harmonic issue. It turns out that in Australia there is no allocation of the spectrum where the second harmonic would be an immediate problem. With that in mind I have stopped worrying about the exciter harmonic level for now.

The existing 7 pole LPF can be rebuilt. I have settled on a 7 pole Chebyshev filter as follows:

The suggested attenuation of the second and third harmonics is 37dB and 67dB. I would like more attenuation of the second harmonic but I will wait and see what I achieve in practise before redesigning the filter. If I achieve my targeted 50dB below the carrier for the second harmonic that will be less than 1mW of RF power being radiated which should keep me out of trouble.

I have some 110pF surface mount RF caps which has shaped the filter I have selected. I only need remove the two 33pF outside capacitors on the filter, to be replaced by 110pF capacitors, and pad the inside capacitors with 110pF.

LPF Board before modification

This morning I quickly wound the inductors, measured them to ensure they were close and put everything back together. And I removed that disc ceramic capacitor shown in the photo above.

Regards
Richard VK6TT

Unilab 10m FM conversion Transmitter PA mods - Revised Driver Stage

After a few cooler days I have been able to finish the driver stage. In place of the BLY32 there is now a two stage amplifier. It was all a bit of a squeeze but we managed to fabricate a PCB that would fit in the cut-out .

Schematic of replacement for BLY32 module

PCB of BLY32 replacement module before preparing to etch.

BLY32 replacement module installed

I made an error on the pcb layout which is why the collector of the 2SC1971 is floating. If you would like to see a revised circuit board as a pdf so you can build you're own please ask!

Using the 2SC1971 as a driver means we are limited to driving the final stage with around 6W. Which is enough for 100W from the radio if we have 13db of gain in the final transistor which I suspect we do have.

At present I am getting 40W out of the radio, current draw is around 5 amps. I think I have discovered why I am not getting 100W. It was to do with output matching. As built, the radio's output matching network is not as per the circuit diagram. It turns out the variable capacitors on my radio had a different pin-out with the result the matching network is different than I has assumed. I will alter my radio and let you know what happens.

Regards
Richard VK6TT

Unilab 10m FM conversion - Transmitter VCO Board

In the quest to get the harmonic content of the transmitter under control I returned my attention to the VCO board. Sure enough, when I measured the output of the mmic I found it was around 900mVpp and the flat topping of the waveform was clearly seen on the CRO. This equates to 3dBm for a 50 ohm load and is right on the limit for the mmic used. You may recall that the coupling capacitor from the VCO to the mmic was a parallel combination of 1p5 and 2p5. I removed the 2p5 and replaced it with a 1p5, giving 3pF in total. Did it make a difference?

Measured at the exciter output into my spectrum analyser (all readings in dBm):

	Before	After	Difference
Fundamental	22	20	-2
2nd Harmonic	13	3	-10
3rd Harmonic	-6	-7	-1
4th Harmonic	-7	-30	-23

So yes, it made a difference. While there was little change to the output or the third harmonic, the second and fourth harmonics were significantly reduced.

While the original mmic was fine when the VCO was running at 80MHz and the pre-scaler had plenty of sensitivity, the reduced pre-scaler sensitivity at 30MHz means the mmic has to be run at the limit to get enough signal into the pre-scaler for reliable counting to occur.

I see two alternative courses of action: substitute a mmic with a higher output rating and similar gain, or put a small amplifier stage between the mmic and the pre-scaler so the output level from the mmic can be reduced still further. A different mmic is perhaps the most expedient solution.

The existing mmic has a gain of 18dB at 30MHz and an output level of 3dBm. The suitable alternatives I know of are a MAR8 or a BGA616. Neither is a simple drop in replacement though.

Type Po -1dB Freq Range Gain at 30MHz uPC1651G (existing) 3 10MHz-1GHz 18 MAR8 12.5 DC-1Ghz 32 BGA616 18 DC-2.7Ghz 20

The BGA616 would be my first choice. It is capable of a higher output level then the MAR8 and this should result in even less distortion, hence harmonic output, than the MAR8 will give. The biasing of the BGA616 would also be easier given the 8V rail available on the VCO board. A 100ohm resistor and inductor will suffice. The MAR8 runs at a nominal 7.8V and I suspect the 5.6ohm biasing resistor it will need is too low in the event the temperature rises.

In some ways a small amplifier stage between the mmic and the pre-scaler is less critical. A voltage gain of say +4 would be ample and there is no need to re-jig the board for accepting a different mmic. And just about any small signal transistor could be pressed into service.

So I built this onto a piece of vero-board and mounted it as shown in the photo below:

And after removing one of the two 1p5 caps that coupled the oscillator to the mmic the results were:

Which confirms the mmic was definitely being over-driven. The relationship of fundamental to harmonic output levels now "appears" better. However, when I consider that these are measured after the tuned tank circuit I added on the main exciter board I think the results should be better.

I'm going to ponder this over the next few days as we go through a heat wave. I am going to rebuild the driver stage onto a small pcb which will fit in the cut-out where the BGY32 used to go. It will include a pre-driver stage to lift the transmit power from 20W to something closer to 100W. And since this involves etching I will also rebuild the buffer onto a small pcb which will look a lot neater. If you'd like either board just let me know!

Regards
Richard

Unilab 10m FM conversion Transmitter PA mods - 25W and rising

I finally got to smoke test the transmitter today. After some trial and error on the input matching for the driver stage I got around 25W out of the radio. I can't see any way to increase this without another stage so I am adding one more stage in the near future.

Harmonics will be a big issue. The second harmonic is around 10dB down, the third around 20dB down. While some improvement can be expected when the low pass filter is rebuilt, I don't expect it will be good enough in the current form. I know the harmonics are already present in the exciter output, so my plan is to put a band pass filter after the exciter and before the pre-driver that I will incorporate into the transmitter.

The good news is that the matching sections from the collector of the driver stage onwards all appeared close to correct. The output of the exciter did not appear to be a good match to 50ohms hence I had to re-jig the input matching to the driver on the run. Since I will have plenty of gain with another stage I will pad the output of the exciter with a 5dB or so attenuator to make it look like a better match to the filter I will build.

73's
Richard

Unilab 10m FM conversion Transmitter PA mods - Final Stage revisited

So I was sitting in front of the TV tonight and after a few comments about the show on TV I was banished to my shack. I started to read a few application notes on transistor matching and working the examples up in the smith chart to make sure I was getting the hang of how to use the Iowa Hills Smith Chart software. It dawned on me that I had not described how to determine if the matching network was suitable for the range of intended frequencies.

This is really easy and full credit to the author. Once we have our matching network at the design frequency we can select the option to "Sweep SC" on the right hand edge of the window and a black line shows how the load is transformed as the frequency is swept according the the span setting. I got the following:

Which intuitively is better than a big black line but by how much? Click on the Return Loss button and you get:

Which shows the return loss over the transmitting frequencies will be better than 20dB ie a good match

I can't wait to smoke test this!

73's
Richard

Unilab 10m FM conversion Transmitter PA mods - Driver Stage

I had a chance to rummage around in the junk box yesterday. If you're wondering why I would turn to a transistor I recover from a radio, instead of buying online, see my post at http://vk6tt.blogspot.com/2016/05/hf-transmitting-transistors-for-qrp.html

 Anyway, the first radio touched was a Philips 828 E band. They contain a BLY87 which would be a good candidate as a driver. However, they are stud mounted so that could be awkward.

The next radio was a RT85 for low band. They use a Mitsubishi 2SC1971 as a driver which is a nominal 6W VHF high band transistor. Not quite the 10W I was looking for but it forced me to rethink my approach.

The MRF247 final transistor has a gain of 8.5db at 175MHz. Adjusting for frequency at 6dB per octave that equates to 23.8dB at 30MHz. Much more than the 10dB I was allowing for. The full gain at 30MHz cannot be used if oscillations are to be avoided. However, if I shoot for 13dB of gain then the 2SC1971 would only be required to produce 5W, well within it's rating.

Adjusting the quoted gain for frequency suggests the 2SC1971 has 25dB of gain at 30MHz. Again, way too high. But the prospect of avoiding a pre-driver stage is a tease since a gain of 13dB from the driver stage dictates an input power of 250mW to achieve 100W from the PA strip. The exciter delivers 100mW. Quite close and maybe the gain per stage I have allowed is too conservative.

I'm going to try just a driver stage and dispense with the pre-driver.

I had to revise the input stage to the PA transistor for the increased collector load the driver stage required. After several iterations with the smith chart I concluded the higher collector load required meant the existing matching configuration could not be made to work.

My approach now is:

• C1, currently 100pF, is padded with a 10nF capacitor,
• removed the 2 turn coil L1,
• pad the input of the strip-line with an extra 860pF of capacitance using two capacitors, 470pF and 390pF.

That gives me potentially a good match between the base of the PA transistor and the collector of the driver stage. I will tackle this shortly and post a full recipe in due course.

73's
Richard

Unilab 10m FM conversion Transmitter PA mods - Final Stage

So, we now turn our attention to the final part of the modification. I suspect this will be the hardest part of the whole project. The existing PA needs a fair amount of work to get it running at 10m.

The challenges arises because:

• The BGY32 module is designed to work between 66MHz and 88MHz. This has to be replaced with a home brew alternative, and
• The MRF247 was never specified outside the 136-175MHz band, and
•  I will try to use transistors that are readily available.

That last point needs some clarification. I have a reasonable junk-box so I have plenty of devices I can recover.  However, I always try to build things that can be replicated. Which usually means I have more than 1 of whatever I used so someone else who is stuck can be helped. In this instance I don't know just what I have that could be pressed into service here so it will be a challenge.

If the MRF247 cannot be coaxed into service it will effectively mean a whole new PA strip.  Perhaps Murphy is elsewhere today.

Since the MRF247 is specified as a 75W transistor I decided that would be the target output power. However, I'd design the matching for each stage for a 30% margin for the output power i.e. almost 100W. It may transpire that I have more gain than the 10dB per stage I am allowing for and since the MRF247 could be driven towards 100W I wasn't going to say no to the extra 25W if it eventuated. There is no data that I could find on the series equivalent impedances outside the specified range of 136MHz to 175MHz so I had to make some assumptions for the matching networks. I assumed the following:

In an attempt to understand how the Unilab designers got the MRF247 to work outside it's specified range I looked closely at the input networks. I knew the BGY32 was a 50ohm output device. And the MRF247 was likely to have a series input impedance of 0.57+j0.3 at 77MHz. To test this assumption I tried a few software packages until I found one that helped me understand this: Iowa Hills Smith Chart software.

I found this software intuitively easy to use. It wasn't the first Smith Chart software that I tried but like the filter software from Iowa Hills this is clearly written by someone who uses the software. I do recommend you try this software.

With the values shown on the circuit I was able to confirm a match was possible at 77MHz between 50ohmns and the PA transistors input impedance of 0.57+j0.3. This was true whether the stripline was modelled and an inductor or as a stripline. In the end I left is as an inductor:

The next step was to see what I would have to change to get this existing input matching section to match a source of 9.4ohms. This would be the collector load required for a driver to deliver almost 10W. For a class C amplifier this is derived from the formula Po = Vcc * Vcc / ( 2 * R ).

With just an extra capacitor and a different inductor we can achieve a good match. An additional 180p capacitor at the BGY32 end of the stripline and an inductor of 86nH replacing the coil that is there is all that might be required.

So far, so good. I repeated the exercise for the output stage and decided that two 34.5nH coils and a 510pF capacitor to pad C603, or a 220p and 270p in parallel, would be a good starting point.

I'm going to make the changes outlined above and check my junk box for driver transistors before I progress any further.

73's
Richard

Unliab 10M FM Conversion - Exciter Board

If you have been following along we pick up from the last instalment and detail what changes are needed once the transmit VCO board is working. The big issue with this project is the output spectrum. We have taken a 80MHz broadband design and pushed it down to 30MHz. As a result, the output spectrum resembles that of a tripler!

The problem stems from the amplifier stage immediately after the VCO board. This is the final stage in the exciter and it really works well as a tripler. And it is quite good as a doubler too! What I did was to alter the circuit back to a narrow band design with some selectivity.

1. Pad C324, the 100pF input capacitor, with 1000pF
2. Remove C328 and C329, the two 1000pF emitter bypass capacitors
3. Remove L305, the tapped bifialar transformer wound on a ferrite bead.
4. Remove the resistor that was in parallel with this. Mine was a 680ohm resistor.
5. Clean out the holes and solder in 3 pins from a 0.1" header strip.
6. Take a T50-2 toroid, wind 16turns with a tap at 4 turns. (I have plenty of these if you're stuck)
7. Solder the toroid across the outside of the three pins. The cold end, that closest to the tap, goes to the pin towards the centre of the board. 
8. The tap goes to the middle of the three pins.
9. Solder a trim cap, say 40pF, across the two outside pins of the header.
10. Adjust FVR301 for maximum D voltage on TP303
11. Tune the trim cap for strongest output on 10m.

I'm still not entirely happy with the results. These changes are a vast improvement over where I started but the second harmonic is sill only 16dB down, and the third harmonic is 28dB down. There was a significant improvement with the higher harmonics and these are of no concern at present.

I now have a 100mW exciter but I will have to consider the output spectrum more carefully as I progress to rebuild the PA.

73's
Richard

Unilab 10m FM conversion - Transmitter VCO Board

The transmit VCO proved to be much more challenging. I started with calculated values for the capacitors and used the same biasing of the varactors as I discussed in the receiver VCO conversion.

This time I used a toroid in the tank circuit, a T50-2. I started with 12 turns but ended up removing 2 turns. I removed the resistor and thermistor and soldered a link on the source choke to ground. While I used a 91k resistor in the gate, 100k will be fine. The picture below still shows the varactor biasing diodes I put in place for testing. Remember to remove them before installation back in the radio!

Installing the VCO board back in the radio I found myself going in circles solving loop lock up followed by a poor output spectrum. Rather than bore you with the sorry tale let's cut to the chase!

The mmic buffer used is very susceptible to being over-driven and generating harmonics. Yet a small reduction in output means the pre-scaler does not get sufficient drive to work properly. I made the following changes:

• The 7p coupling capacitor is removed and I put a 2p5 and a 1p5 capacitor on the board, one on each side. This reduced the drive and the distortion on the output of the VCO board was much less apparent. 
• I then bypassed R407 with a wire link to get the pre-scaler working correctly. And finally, 
• I removed R411A to reduce the gain of this buffer and improve the output waveform still more.

If you haven't been able to find the circuit for this radio on the web here is a partial snapshot to help:

At this point I have a PLL which locks up and a nice waveform going into the remaining stages of the exciter.

Just like the receiver VCO board, I now looked at the variation in the control voltage as I locked at the top and bottom end of the 10m FM segment. The maths demonstrated that the existing loop filter was "awkward".  I ended up putting a second varicap in parallel with the existing one and changing a few capacitor values. Since making the initial mods using a toroid I had found a suitable variable inductor and I substituted that for the toroid. It makes adjustment for lock that much easier. I settled on a 3.5V variation in the control voltage and ignored the loop filter. I might revisit the loop filter after everything else is working.

The final(?) VCO component values are shown on the following schematic:

The balance of exciter board mods are in the next instalment.

73's
Richard

Unilab 10m FM conversion - Receiver VCO and PLL Stability

I noticed that while switching from receive to transmit and back again that sometimes the receiver PLL would drop out of lock. Here then is a brief discussion of how I resolved this issue.

The first thing I did was measure the VCO voltage at the minimum and maximum frequencies I had loaded in the eeprom. To achieve this I had to adjust the inductor and trim cap in the VCO until I had lock with a low control voltage. Otherwise, I couldn't get a lock at the high frequency. This immediately told me I had at least one issue to address.

Frequency         Tuning Voltage
50.74 MHz        0.1V
51.29 MHz        6.7V

Clearly that leaves little margin for things like temperatures change given the maximum tuning voltage under lock is approximately 8V. The control voltage swing needs to be much less than this, but how much?

Looking at the loop filter values left me nonplussed. I'd never seen a loop filter like this before. I have highlighted the relevant components on the schematic below.  I was tempted to re-design this part of the radio but went away and though about this for some time.It eventually dawned on me that there was an embedded lead lag filter, circled on the schematic below. Ignoring everything else I crunched some numbers which suggested that these values would be appropriate if the range in tuning voltage was more like 1V.

Looking at the component values used, below, the most expedient fix would be to change the 2p7 capacitor in series with the varicap.The first capacitor I tried was 15pF which gave a variation in the control voltage of 3.5V - 5.0V. Before I could measure that I had to get the PLL to lock again. I had seen this before with the transmit VCO. Despite the oscillator oscillating, the pre-scaler was giving an erroneous reading due to the input signal being too low. I added a 100 ohm resistor in parallel with R202 which fixed this.

Receiver VCO values before change to 2p7 capacitor

At this point the receiver synthesiser mods are almost complete. After putting the lid on the receiver I noticed the loop went out of lock on one channel. The channel either side was fine. Remove the lid and the channel locked up. Given the control voltage on the test point did not shift significantly for the frequencies above and below this troublesome channel I ruled out the lid modifying the oscillator frequency.

I cut a piece of blank circuit board to the size of the VCO enclosure. As I placed it into the enclosure as a temporary shield and I expected the lock issue to arise. It didn't. But putting the lid on while the temporary shield was in place still unlocked the loop. So the issue wasn't with the VCO.Which ruled out so many weird causes that I was stumped as to what it could be.

After sliding various assortments of metal around the top of the receiver enclosure I confirmed the issue is arising within the section housing the eeprom. I still hadn't got to the bottom of this. The frequency, 50.760 MHz isn't a harmonic of the crystal oscillator or even the 750kHz reference going into the PLL chip. But you can set the frequency 10kHz either side of this with no issue.

However, after another go at solving this I discovered the problem was due to incident light, and not an RF issue at all. It turns out that this one channel in the eeprom changed value if it was in darkness. I couldn't replicate this in my eeprom programmer but it definitely was due to darkness changing the data on the output pins of the eeprom. Anyway, I burned another eeprom and problem solved.

I have put the lid on, tightened the screws and next time I will start writing up the transmitter conversion.

Regards
Richard

Unilab 10m FM conversion - Receiver VCO

Apologies for the delay in posting this part of the project. I couldn't find my notes and my memory was failing me. In fact, when I looked at what I had done I was amazed it worked at all. So I decide to start from scratch. I am breaking this up into a few more parts than I expected. My logic follows and this time I have taken notes!

I assumed the receiver VCO was running on the high side based on a vague reference to this in the technical manual.

1. Standard radio covers 66-88 MHz. Use 77MHz as a starting point.
2. 1st IF frequency is 21.6MHz so Local Oscillator is running 21.6 MHz above 77 MHz, or about 92MHz.
3. For 29.4Mhz the local oscillator needs to be 21.6 MHz above this, or 51MHz.

I know from what I did previously that it wasn't as simple as padding capacitors to lower the frequency. I did a complete rebuild based on material in Solid State Design. I removed the inductor and capacitors from the oscillator board and rebuilt it to match the following circuit:

We need to add the resistor and diode somehow and the tank circuit is no longer tapped on the inductor. I attach a photo of my finished VCO below to help with how I placed the parts.

After soldering all parts except the inductor I used two 33k resistors (value is not critical as long as they are the same) as a divider to apply a voltage on the VCO line. With 8V to the oscillator board I tried a few inductors to see if I had one that was close to 0.44uH. I had trouble getting this circuit to oscillate so I changed C204 from a 68p to a 33p capacitor. Now I could get some oscillation! I held inductors in circuit by hand on the bottom side of the board and looked at the frequency counter. After a few goes I found an adjustable inductor that allowed for oscillations around 51MHz.

I soldered the inductor in place and adjusted the frequency for 51MHz. All going well this suggested the VCO control voltage would be around 4V when the loop locked up. After removing the resistors I plugged the board into the radio and tested.

Lo and behold, no lock. Hmnn....A bit of prodding with the CRO and it seems the output level was too low.
 
The first thing I did was add another 3k3 resistor in parallel with R201 to increase the standing current. I had noticed when I warmed the thermistor with my iron that the output level increased so this addressed the level issue.

But still no lock. Measurement showed the pre-scaler was clocking at twice the rate it should be. I speculate that this was due to insufficient drive, and not the presence of a strong 2nd harmonic, and padding C209 with a 10p surface mount capacitor resolved this.

I then changed the choke in the source to 82uH. At this point the local oscillator is on frequency and the PLL is locked.

If I was doing this again then on reflection I would start by bypassing the existing resistor and thermistor in the source with a 10n capacitor. If the pre-scaler was not clocking correctly I would then change the choke, then pad C209 if required. I'd be interested in hearing how your modification went and what you tried so please leave a comment below.

At this point I still need to check the stability of the loop and I will address this in another post.

73's
Richard

Unilab KL70 10m FM Conversion

The Unilab KL series were rugged FM commercial repeaters very similar, if not electrically identical,  to the Kyodo radio's. Converting KL150's to 2m and KL450's to 70cm is relatively straightforward, giving a 50W near indestructible radio. After successfully converting a KL70 to 6m FM operation I wondered if a 10m FM conversion was possible.

The KL70 was made to work between 66 and 88MHz. So a number of challenges arose or were foreseen:

1. The receiving band pass filters can't simply be re-tuned,
2. The local oscillators need a substantial amount of re-work,
3. The PLL loop filter may need modification, I'm yet to confirm this, and
4. The transmit harmonics will require filtering.

While still relatively straight forward, I've not found time to complete my conversion. However, writing about it is a sure way to elevate the project to a "finished" status so here goes.

In this post I will cover the receiver band pass filters. If you looked at my previous posts you will have noticed I'm a big fan of the Iowa Hills RF Filter Design software. Since the original bandpass filters on the receiver will not tune down to 29MHz I was forced to rebuild them.

Scouring the junkbox I found some nominal 160nH adjustable inductors. A few minutes work with the software and I had the following filter design:

My rejection of the image frequency of 72MHz is likely to be no better than 80dB for the two filters.At this point in time I didn't know about the series tuned filter transformation.  The same inductors used with series LC tanks would yield around 80dB for just one 3 element filter. If you knew of local transmissions on 72MHz then I'd use one filter like that shown, and one with series LC tanks. Let me know if you want more details on this alternative filter.

After etching a board and building two filters with the same pinout as the original filter I removed the filters from the radio, disassembled them, and put my new filter into the filter housings. Prior to putting the filter in the housing they looked like this:

 

With these two re-assembled filters soldered back into the receiver I turned my attention to the other parts of the project. More on that next time.

73's
Richard VK6TT

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