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MICOR Low-Band Receiver Conversion 42-50 MHz to 50-54 MHz By Robert W. Meister WA1MIK

The MICOR low-band receiver (TLB6850 series) normally covers four ranges: L: 25-30, M: 30-36, H: 36-42, HH: 42-50 MHz. (The last digit of the model number will be 1, 2, 3, or 4 respectively, depending on the range.) The receiver board with the extender (TLB6860 series), normally only used in mobile radios, can be modified following the same procedure. (None of the low-band receivers have AFC (Automatic Frequency Control) and the K1003 channel element doesn't have an AFC input pin.) Raising the coverage range up to 50-54 MHz requires reducing the value of a bunch of capacitors by about 2/3rds of their HH value. This means you must remove the original part and replace it with a smaller value part.

The TLB685X receivers have a 5.26 MHz IF. The TLB696X receivers have a 5.36 MHz IF. The different ranges also use different crystal multipliers and have different injection directions (low-side vs. high-side). Figure out what you've got by examining the crystal and operating frequency of the channel element. For the HH range, the formula is:

F_carrier = (F_crystal times 3) plus 5.26 MHz (= low-side injection)

Not everyone has access to the appropriate service manual for the MICOR products. I always like to show the actual work being performed with photos and diagrams, rather than just list the parts and leave it up to the user to figure out where they are.

There was already sufficient information available from other sources that described which parts to change. None of these had photos or diagrams; they all required that you have the proper manual for your equipment. The following table summarizes the component values that need to be changed and who's list they came from. All capacitor values are in pF. All resistors are 1/4 watt, 5 or 10%, and values are in ohms.

Comp.	Stock HH	KB6MIP	W3KKC	Motorola	W1GPO	WA1MIK
C107	1.5			1.2	1.2	1.2
C108	47	22	33	36	36	33
C109	90	62	68	68	68	68
C110	90	62	68	68	75	68
C113	27	18	18	18	22	18
C114	160	120		100	130	100
C115	1.2			0.82	0.62	- - -
C116	24	18	18	16	18	18
C117	0.82			0.56	0.62	- - -
C118	24	18	18	16	18	18
C119	0.75			0.51	0.62	- - -
C120	24	18	18	16	18	18
C121	0.82			0.56	0.62	- - -
C122	5	3	3.3	3	* 15	3.3
C125	43	33	33	33	36	33
C127	51	33	43	40	39	39
C128	1.2			1.0	1.0	1.0
R103	3000			3300	3300	3300
R109	100			120	120	120

* Note: for W1GPO's C122, L109 is replaced with L105 or L106 from another receiver.

What You'll Need:

I was unable to purchase capacitor values below 1.0pF, so I left those original components in place. This reduces the task to just 13 stock value capacitors and two resistors. I had the 1/4-watt resistors in my parts bin. The table below shows the Mouser part numbers that I ordered. These are all 50V or 100V C0G Murata capacitors. Values are in pF.

Value	Qty	Component Designation	Mouser Part Number
1.0	1	C128	81-RPE5CH1R0C2P1B03B
1.2	1	C107	81-RPE5C2A1R2C2P1B03
3.3	1	C122	81-RPE5C2A3R3C2P1B03
18	4	C113, C116, C118, C120	81-RPE5C1H180J2P1Z03
33	2	C108, C125	81-RPE5C1H330J2P1Z03
39	1	C127	81-RPE5C1H390J2P1Z03
68	2	C109, C110	81-RPE5C1H680J2P1Z03
100	1	C114	81-RPE5C1H101J2K1A3B

John W1GPO discovered that you can buy the exact low-value axial-lead caps from Surplus Sales of Nebraska. They're listed as "Molded Ceramic Temperature-Compensating" on their main capacitors index page and cost $2 each. They have a $10 minimum order.

Preparation:

First, check out and tune the receiver to a frequency between 42 and 50 MHz to verify that it's operating within spec. It's important that you do this first, because if the receiver doesn't work well when you finish the conversion, you won't know if it was bad to start with, or if you did something wrong while modifying it. I know someone who converted an unknown receiver, and it had poor sensitivity when he finished. It took a while to figure out what was wrong. A lot of work had to be undone to resolve the problem. Turns out it had nothing to do with the conversion work he did, but we both still spent several days trying to find where the missing 30dB of sensitivity went.

I had previously purchased an assortment of channel elements. In that group was a K1003 channel element on 45.000 MHz with the proper crystal frequency for this board, so I plugged that into the board and tuned the receiver in my link receiver chassis. It breaks squelch at -128dBm, and according to the squelch circuit (short vs. long squelch tail), 20dB quieting is at -118dBm. My own ears tell me that the crackles in the signal were gone at -116dBm, which is probably a better indication of the true sensitivity. For meter readings, I got M3: +28uA, M4: -2uA, and M5: +3uA, all with no signal. (There's no M1 or M2 on this receiver, but they might be present on the models with Motorola's "extender" noise-blanker circuit.) This receiver strip is operating very nicely and should easily convert to 50-54 MHz.

Doing the Conversion:

We'll be changing capacitors inside the multiplier and front-end coil assemblies. There are several different styles of coil shields: small square, larger square, those with through-holes for the tuning core, and those without. You will need to get to the small capacitors inside the shields, so ten of them have to come off.

I wrote most of the article before actually performing the conversion. Once the parts came in, I printed the parts list above as well as the X-ray view of the board below, and worked from those. I started with the resistors and worked my way up the list, but there's no best way to change the parts. It just takes time to locate the part on the X-ray view, locate it on the board, locate the two solder pads, unsolder the part, verify the value, find the bag with the new part, and install it. Heathkits were a lot easier.

Remove the receiver board from the enclosure. Remove two screws that secure the bottom shield over the front-end coils and remove the shield. Refer to the photo below. I'd also strongly recommend that you remove the black plastic pull-handle above the shield, as you'll probably hit it accidentally with the hot soldering iron more than once during this procedure. Twist one end and push it through the hole, then repeat that process with the other end. You'll thank me for it in the morning.

I suggest that you do number the coil shields, using any convention you want, in one corner with a pencil; you can rub them off later. Remove the ten coil shields marked with red blobs or circles in the photo below. One shield over the mixer doesn't have a coil under it, but you must remove it to get to one resistor.

Each shield is held in with two metal tabs soldered through holes to the ground foil of the circuit board. The locations are circled in red the photo below, and the shields are outlined in yellow. One hole has two tabs in it. After removing the solder, the tab can be wiggled slightly to free the remaining solder bond and the shield can be removed from the component side of the circuit board.

Unsolder the appropriate capacitors and resistors, and install the new components according to the table above. Do them one at a time so you don't get confused. Refer to the X-ray view of the circuit board below. Make sure the leads of the new parts don't short out to nearby ground foils, and keep the leads as short as possible, no more than 1/4 inch. Many leads are attached to ground points, and they take a lot of heat to melt the solder on both sides of the board. I had to turn my iron up to 700F, but I'm sure all the components survived.

This piece of the schematic diagram below shows the various components that you'll be working with. Every part in the multiplier, front-end, and mixer areas, without a value next to it (except C115, C117, C119, and C121) gets replaced. Click on the image for a larger view.

After you've replaced all the parts, reinstall the coil shields in their original locations and thoroughly solder them to the circuit board. Clean the flux off the board with an appropriate solvent (it just looks better that way), replace the bottom shield, install a new channel element, and retune the receiver according to the manual, following the procedure for moving the frequency a large amount. The complete alignment procedure can be found in a 195kB PDF file here.

Results:

It took me two hours - from start to finish - to do a proper job replacing all 15 parts. I'm a stickler for details and double-checked that the value of each part I removed agreed with the parts list. I also made sure the new parts were close to the circuit board and would not touch any of the coil shields. If they're all nice and straight, they're probably good to go.

After reassembling the receiver, I tuned it up following the procedure shown in the PDF file above. The receiver had previously been tuned to 45.000 MHz. I installed another K1003 channel element that was on 53.330 MHz, also with the proper crystal frequency for this board. I had to turn the multiplier coils two turns counter-clockwise to get them to peak. I had to turn the front-end coils five turns clockwise to get them to peak. (Your experience may vary depending on the starting frequency.) As far as I can tell, the receiver is operating just as well as it did before I performed the modification. It took me only a few minutes to re-align the receiver. The following table summarizes the various meter readings and performance indications:

Parameter	Before Mod	After Mod
Frequency	45.000 MHz	53.330 MHz
Open Squelch	-128 dBm	-127 dBm
20dB Quieting	-118 dBm	-118 dBm
No Crackles	-116 dBm	-117 dBm
M3 Meter (Idle)	+28 uA	+35 uA
M4 Meter (Idle)	-2 uA	-2 uA
M5 Meter (Idle)	+3 uA	+3 uA
M5 Meter (+20uA)	-115 dBm	-115 dBm

Acknowledgements and Credits:

The component values came from articles posted on Repeater-Builder and elsewhere on the web.

Schematic diagrams, alignment procedure, and circuit board X-ray views came from the Spectra TAC Voting and Satellite Receivers Instruction Manual, 6881039E45.

MICOR, Spectra TAC, and a bunch of other terms are trademarks of Motorola, Inc.

Thanks go to Mike WA6ILQ who offered suggestions and additional information.

Photographs were taken by the author and are copyright by him.

Contact Information:

The author can be contacted at: his-callsign [ at ] comcast [ dot ] net.

This page was created 29-Nov-2008

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Article text © Copyright 2008 and date of last update by Robert W. Meister WA1MIK.

This web page, this web site, the information presented in and on its pages and in these modifications and conversions is © Copyrighted 2007 and (date of last update) by Kevin Custer W3KKC and multiple originating authors. All Rights Reserved, including that of paper and web publication elsewhere.