UHF Receiver

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This page covers the Range 1 to Range 4 receivers. The Range 0 is a newer (and easier to modify) design.


The basic receiver is pictured below. Note the UHF R1-R4 receivers have a preselector that's flush with the front panel and only 3 adjustment. This is a dead give away that you have a UHF R1-R4 from the front. Unfortunately there is not an easy way to identify it further than by part number.

The receiver is a high side injection with the 1st Lo operating +73.35 MHz higher than the receiver frequency.

The SCM knows what type of module is inserted by reading voltage divider resistors on the u2600 A/D Converter. Some of these are on ports used for other things, Change Frequency and Lock and then A8 input is used for ID. Note the resistors have the same labels on the UHF boards but on other bands the same parts have different labels. The R2606 and R2607 form a voltage divider feeding the A8 input with the computed voltage in volts.

Range R2449 Chg Freq R2450 Lock SPACE R2606 R2607 A8 Volts
UHF R0 0 3300 5600 3300 1.9
UHF R1 0 0 1200 1200 2.5
UHF R2 0 3300 1200 1200 2.5
UHF R3 3300 0 1200 1200 2.5
UHF R4 3300 3300 1200 1200 2.5
VHF R2413 R2415 R2812 R2814
VHF R1 0 0 2700 1200 1.5
VHF R2 0 3300 2700 1200 1.5
800/900 R2422 R2414 R2816 R2815
900 0 0 1200 4700 4.0
800 0 0 1200 2700 3.5

The service manual excerpt is in PDF and below.

UHF Receiver Models TRE6281-TRE6282-TRE6283-TRE6284 Service Manual Excerpt

In PNG format.

Note on the schematic the VCO high and Low are reversed. This seems to be a recurring theme in the Quantar manuals.

Reversed VCO's

As the VCO runs at 73.35 MHz higher than the intended receive frequency the VCO will expect to lock over the following ranges. The ideal spot for the VCO to operate is between 2.5 and 7.5 Volts highlighted in pink below.

Range Lower VCO Upper VCO
Volts 0.8 2.5 5.0 7.5 9.2 0.8 2.5 5.0 7.5 9.2
UHF R1 462 467 479 491 499 478 491 499 506 514
UHF R2 498 511 519 527 536 514 527 535 543 552
UHF R3 528 543 549 555 564 541 555 561 567 575
UHF R4 553 567 574 580 589 568 580 587 593 601

The VCO is much like the VCO in the exciter, a sealed unit with no further diagram available from the factory.


Converting the boards

This is changing, I have a better way, old method.

R1 to R2

This is the conversion we're focused on as there are tons of 403-433 MHz (US Federal) Range 1 units on the market for cheap.

In general you should have the following to work on this:

  • Soldering iron with fine tip (Metcal suggested)
  • Hot air station
  • Under board heater
  • Small tools/tweezers
  • Silicone rubber stock cut to support the underside of the VCO.
  • Flux, and other small parts for rework
  • Quantar backplane extender cables (PCI extender cables cut down to fit)
  • A clear understanding of how the VCO's work and how the PLL operates
  • Quantar programmed up with the test code plug.

Almost needed, you really should have it, but can do without:

  • Voltmeter - You can use the WINRSS metering screen to view the steering level of the VCO. It's a pain to switch back and forth, but it can be done.
  • Service monitor/SINAD meter - If you don't have a meter you can use a known weak signal to do a 20db quieting test.
  • RJ-9 speaker to SINAD meter input cable.

Nice to have:

  • Bench DVM
  • Bench PSU
  • Spectrum Analyzer and probes

Parts needed

You will need the following parts for this. Note the capacitors which are critical in terms of value are noted. If you find that sensitivity is off, suspect that you got the wrong value of caps here, 1-2 dB will be noticeable.

Value Tolerence Part(s) Digikey partnumber Size
3.3 +-0.1pf 50v C2070 732-12216 0805
5.6 +-0.25pf 50v C2076, C2079 478-KGM21BCG2A5R6CT-ND 0805
6.8 +-0.1pf 50v C2054 478-KGM21BCG2A6R8DTCT-ND 0805
8.2 +-0.1pf 50v C2050 732-12219-1-ND 0805
13 +-2pf 50v C2051, C2059 478-10374-1-ND 0805
16 +-2pf 50v C2053, C2057 478-10377-1-ND 0805
22 5% 50v C2083 478-KGM21BCG2A220JTCT-ND 0805
3300 5% 1/8 watt R2450 RMCF0805JT3K30CT-ND 0805

Procedure Overview

In general the VCO (both) need to be moved up in frequency and the image filters from the mixer need to be moved up by changing some chip caps. The ID resistors of the board must be changed to identify the receiver as Range 2 as well. Prior procedures required a bunch of manual power supplies, sweeping the circuit, and monitoring with a spectrum analyzer. This procedure uses the Quantar to control the receiver as it's being worked on using an extender cable.

The VCO must be modified so that it's able to lock from 2.5 to 7.5v. The steering voltage actually is usable from .95 to 8.95v, but tempeture will affect the resonance of the VCO, and so you want to have the high and low frequency to be at 2.5 - 7.5 volts giving enough room on each side for environmental variations.


First lets prep the setup and get some notes on the receiver before we move it.

Setup the Quantar and install the extender cables.

Remove the receiver from it's housing.

Install the board in the work holder/vise.

Hook it up to the backplane and program in the Range 1 test codeplug.

Sweep and record the VCO steering voltage on each channel where it locks. If not using a voltmeter, you can use the measurement screen in RSS to see the steering level. This is very important as it gives a base line of performance on the receiver before modification.

Note that if you're using a volt meter on the VCO steering line, it will not receive properly. Ensure you do that separate from the SINAD testing.

Lower VCO Upper VCO
Channel 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
R1 384.5250 388.2250 391.9250 395.6250 399.3250 403.0250 410.5000 417.9750 418.0250 425.5000 432.9750 436.8000 440.6250 444.4500 448.2750 452.1000
R2 417.9000 421.9250 425.9500 429.9750 434.0000 438.0250 446.0000 453.9750 454.0250 462.0000 469.9750 474.1250 478.2750 482.4250 486.5750 490.7250
R1 Shield on UNLOCK UNLOCK 1.813 2.504 3.248 4.029 5.678 7.379 3.843 5.495 7.204 8.089 8.982 UNLOCK UNLOCK UNLOCK
R1 Shield Off UNLOCK 1.808 2.480 3.203 3.962 4.743 6.376 8.045 4.735 6.395 8.084 8.959 UNLOCK UNLOCK UNLOCK UNLOCK
Delta 0.667 0.699 0.714 0.714 0.698 0.666 0.892 0.900 0.880 0.870
R1 SINAD -120.1 -120.1 -120.0 -120.1 -120.1 -120.2 -120.0 -120.0 -120.0 -119.4 119.4
R2 Shield On UNLOCK UNLOCK 1.328 2.056 2.876 3.766 5.680 7.727 2.319 4.001 5.890 6.920 7.968 9.027 UNLOCK UNLOCK

VCO Modification

This is very critical circuit to work on and is a sealed ceramic unit. Motorola used unique VCO's for each band range, however the Range 1 can be modified using some "stub" lines of copper foil to bring it's frequency up. This is complicated as the unit is soldered in place and has a heavy plated steel cover soldered over it.

Prepare the VCO for rework

The VCO shield must be removed, but as the entire VCO will be reflowed at the same time, it will "fall" off the headers holding it off the board. This is solved by using some 1/4 inch wide 60 mil thick silicone stock. Fold the small pieces under the VCO and compress them to provide some slight pressure between the bottom of the VCO and the top of the PCB. This works as the headers will not be reflowed and release from the PCB.

After this is done, flip the board over and unsolder the 4 posts of the shield that go to ground. I find the best way to do this is lots of flux and solder braid with a pre-heated board, but chip-quick solder will work to lower the melting point so that when the top reflows the ground will too. Either way will work.

Now flip the board back and let it pre-heat. A bit of insulation over the VCO will help for this. I set my pre-heater to 550f and let it rip for 5-10 min.

VCO insulation while preheating

Set the hot air gun to 550F and 20 L/min of flow, slowly work the shield up in temp and test the solder melting around it. Once it's completely reflowed, lift up on the center of the shield and pull it directly up and clear of the PCB. Set it aside to cool. Careful you don't disturb any of the VCO parts, as they will most likely be reflowed. If you do, don't panic, just look at the before picture and put everything back to how it was on the VCO. It's a pretty easy circuit to figure out.

Before allowing the VCO to cool, tin the U portions of each VCO strip with solder and flux. Once this is done put the insulation over the VCO module and turn the pre-heater off, allow it to cool but not fast, as rapid changes can crack the ceramic PCB.

Modify the VCO

Power up the Quantar and the receiver. Ensure it's locked and you can read the steering voltages on the VCO. Now you need to program the Range 2 Test Codeplug into the quantar and let it reload. It should show up as receiver PLL unlocked when it boots, this is ok.

Prep two copper foil strips to 50 mil wide by .250 long. If the foil has adhesive on one side, remove it using some solvent.

Turn on the pre-heater and let it warm the VCO up, you will see the VCO voltage move as this happens.


  • Temp up = Volts up
  • Temp down = Volts down
  • Case off = Volts up
  • Case on = Volts down
  • Grinding the trim cap = Volts down
  • Move short towards U = Volts up
  • Move short away from U = Volts down

The concept is that we want the most tuning range from the VCO we can get, but that the 6-8 and 9-11 channels are all within 2.5-7.5v under normal operations once the VCO is reassembled.

First modify the lower VCO. This is best done on channel 7, as from solder hot temp to room temp, .75 to 1v higher is seen. Adding this to the +.7 to +.9v seen by removing the case, means we'll be adjusting the unit to almost 2v higher than it will be once reassembled. As we want to hit 7.5v at channel 8, we will run out of steering voltage trying to do that.

Set on channel 7 and solder the copper foil short over the lower VCO U bend in such a way that it shows 7.5v on the steering line. The VCO may unlock when hot, so solder it and then let it cool slightly to lock again. Blowing on it may help to lock back up. In practice it's better to have it slightly high as the stub can be ground with diamond grinder tool to shift the voltage down after it's assembled. Once you have the hang of it soldering it actually not hard.

VCO modification rules

Now to the upper VCO, and here we set to channel 9, and want 4.5v while hot. This may require bit more cooling to get it to lock between iterations, but it will lock easily.

Once the VCO is modified you can allow it to cool and test it more, or just reinstall the shield right now. If you allow it to cool, ensure you have the insulator on it while it cools, and record the shield off voltages before re-attaching the shield. If you do allow it to cool, use some solvent and remove all the flux from the VCO substrate. While flux residue shouldn't affect anything, it will look better and is more professional.

To reattach the shield, place the shield on the VCO and add a liberal amount of flux around it. Using the hot air gun to 550F and 20 L/min of flow reflow it and allow it to seat on the VCO. You may need to add some extra solder around the VCO shield, but typically there's more than enough to make it work. Once it's totally reflowed allow it to cool and solidify. As you need to solder the under side of the board, you can do that from the top, or flip it over and solder it.

VCO Shield Back on

Allow it all to cool using the insulator over the VCO again.

Test the VCO

Test the cooled VCO is locked across the channels 6-11, and note the voltage on channel 8 (Low VCO highest frequency) and channel 9 (high VCO lowest frequency), which should be 7.5 and 2.5 v respectively. If it's too high, you can grind the trimmer cap and bring it down, but within half a volt it is fine. If you'll be operating the station at tempeture extremums its more important, or if you intend to operate it out of the 438-470 MHz band.

Modify the image filters

Image filter under shield

The image filter under the shield needs to be modified, so the shield must be removed. This is done with the underboard pre-heater and 550f @ 20l/min of airflow. The shield will reflow and be easily removed.

It's best to prep the caps and have them ready as the parts under the shield will be reflowed when the shield is removed.

Remove C2050, 2051, 2053 and 2045 and replace them as follows:

C2050 8.2 pF
C2051 13 pF
C2053 16 pF
c2054 6.8 pF

UHF R1 to R2 RX Image filter case off R1 to R2

Once this is done, re-solder the shield in place.

Image filter in mixer can

The image filter continues into the mixer can, but it's a pop off shield which is much easier to expose rather than a sealed unit.

Replace the following parts

Remove C2057, 2059, 2070, 2076, 2079 and 2083; replace them as follows:

C2057 16pf pF
C2059 13 pF
C2070 3.3 pF
C2076 5.6 pF
C2079 5.6 pF
C2083 22 pF
UHF R1 to R2 RX First Mixer board

Change the Module ID

Change R2450 to a 3300 Ohm resistor. This will make the module be seen as an UHF R2 module by the SCM.

R2450 location by synthesizer IC

Clean board

I typically wash the board in some solvent/IPA or non-chlorinated brake cleaner and spray out the VCO. An acid brush or toothbrush might be needed to clean it up. After some rinses with clean solvent, bake it at 150f for 30 min and check that's it's clean.

Let it cool and move on to testing.

Test and Verify

Using the same method as before, note the final VCO voltage at each test frequency and the sensitivity. You should ideally see the same or within 1dB of performance depending on the stability of your test setup. Having the speaker connected helps as you'll be able to hear where it's at. The DSP does noise reduction, and with a steady weak signal, this can make it hard to test SINAD.

Useful Docs

Here is a complete list of the parts which are changed, color coded by section of the board.

Part number R1 R2 Notes
C2050 9.1 pF 8.2 pf +-0.1pf 5% is .41pf
C2051 16pF 13pf +-2pf 5% is .65 pf
C2053 18 pF 16 pf +-2pf 5% is .8 pf
C2054 8.2 pF 6.8 pf +-0.1 pf 5% is .34 pf
C2057 18 pF 16 pf +- 2pf 5% is .8 pf
C2059 18 pF 13 pf +- 2pf 5% is .65 pf
C2070 5.6 pF 3.3 pF +- 0.1 pf 5% is .165pf
C2076 7.54 pF 5.6 pf +- 0.25pf 5% is .28 pf
C2079 9.1 pF 5.6 pf +- 0.25pf 5% is .28 pf
C2083 47 pF 22 pf 5%
R2449 0 Ohm 0 Ohm ID Leave
R2450 0 Ohm 3300 Ohm ID Replace

The following image has all the R1 to R2 mods on it and can be printed on a 8.5"x11" or A4 page to have a reference on the desk while doing this modification.

UHF R1 to R2 all boards

R3 to R2

This is possible, the VCO is the hard part as the frequency needs to be lowered. This means adding length to the resonator strip, which is in practice harder than the lower R1 where we need to remove length. This actually wasn't that bad to do in practice.

In testing it doesn't have the same sweep in terms of volts vs frequency that the R2 or R1 conversion has.

Parts changed

Below is a colored copy of the parts changed. The * parts are the same between R3 and R2.

Part number R3 R2 Notes
C2030 6.8 pF 6.2 pF +- 0.25 pF
C2033 100 pF 56 pF 5%
C2038 56 pF 100 pF 5%
C2040 3.3 pF 5.6pF +-0.25 pF
L2030 4.7 nH 8.2 nH 5%
L2032 8.2 nH 12 nH 5%
C2050* 8.2 pF 8.2 pf +-0.1pf 5% is .41pf
C2051 8.2 pF 13pf +-2pf 5% is .65 pf
C2053 * 16 pF 16 pf +-2pf 5% is .8 pf
C2054 4.7 pF 6.8 pf +-0.1 pf 5% is .34 pf
C2057* 16 pF 16 pf +- 2pf 5% is .8 pf
C2059 9.1 pF 13 pf +- 2pf 5% is .65 pf
C2061 7.5 pF 9.1 pF +-0.1pF
C2070 2.2 pF 3.3 pF +- 0.1 pf 5% is .165pf
C2073 3.3 pF 2.2 pF +-0.1pF
C2076 2.2 pF 5.6 pf +- 0.25pf 5% is .28 pf
C2079 NP 5.6 pf +- 0.25pf 5% is .28 pf
C2083 10 pF 22 pf 5%
C2084 3.3 pF 1 pF +-0.1 pF
R2449 3300 Ohm 0 Ohm
R2450 0 Ohm 3300 Ohm

R4 to R2

No data on this yet, may be doable VCO will be a problem.