Tuesday, April 21, 2020

A little work on the Heathkit SB-200 Amplifier

The Heathkit SB-200 is a very popular HF amp, and very dependable. This one belongs to a friend, and had begun to vent the filter caps from time to time. It was time for a little work, so here we go....


Case removed, ready to remove inner covers...

The amp was in very good condition, just had a bit of dust to get rid of :-)

With the upper inner cover removed, it exposes the tubes, filter caps, etc.

I removed and cleaned the dusty tubes. This will help the tubes run a little cooler...

It has the original Cetron 572 tubes installed from the build sometime around 1980. Here they are, clean and ready to be reinstalled...

I also used compressed air to clean the RF compartment, and the rest of the amp.

Note the filter caps. The originals were much larger and only 125uF vs. the newer, smaller caps in blue that provide 330uF. This will yield a better filtered DC supply, and take up less space than the originals.

Here, you can see the power supply board refitted with new capacitors and reinstalled. All bleeder resistors were in tolerance and good condition.

While in the amp, I included a few updates. If you notice the two diodes attached to the back of the plate current meter, this provide protection in the event of a tube short or arc. I also installed HV glitch protection, and bias protection.


All finished, tested, and ready for re-assembly.

Here she is, headed back to her owner. This amp is extremely well cared for, and should last many years to come!

Monday, March 2, 2020

Tri-Band Repeater Antenna Build, cont'd

This is a few more photos of the new repeater antenna build, and some observations.While tearing down a 4-bay antenna on 154mhz for parts & materials, I measured things with my MFJ Antenna Analyzer, and took note of some dimensions...

First, the larger dipoles are from the 154mhz antenna. I made a little pigtail connector for the MFJ, and took some readings. Resonance and impedance of these types of antennas is impacted by proximity to the mounting mast and the width (these are 3") of the loop. They also require at least a 1/4 wl of mast above and below for proper operation. The larger units were resonant at about 154mhz, where I expected. These dipoles are 17" per side, from the mount to the end of the loop, total 35" tip to tip (counting the 1" dia mount in the middle). The loop is 3" wide, edge to edge, and they are 3" from the mounting mast to the edge of the dipole loop. These are the dipoles I will modify for 2 meters.

The smaller dipoles are 15" per side, from the mount to the end of the loop, total 31" tip to tip (counting the 1" dia mount in the middle). They were resonant at about 168mhz. I altered the formula to calculate the proper dipole size, and it clearly doesn't track as I alter the dimensions, so I'll be doing a bit empirical testing (read trial and error, hahaha) to work the bugs out of the 220 and 440 antennas.

Another interesting observation...using DB Products RG-11 cable, the matching sections were pretty much the same length as the tip-to-tip dimension of the folded dipole elements. We'll see how this works out when I get these tuned and installed on a test mast...

I plan to use the single mast section on the right (with the rain bonnet) to install a pair of folded dipoles for 2m, 220, and 440. My plans are to keep this a compact antenna. I realize it'll be a compromise antenna, but I need to keep size down for a DC grounded tri-bander. 

RG-11 in the original installation...

154 mhz vs 168 mhz...

This pair will be modded for the 147.39 repeater.


Wednesday, February 26, 2020

ARR RX Preamp Failure, 442.400 repeater...

Shortly after getting the 442.400 repeater back on the air in test mode, I discovered the receive preamp was not working. This is an ARR P432VDG GaAsFet rx preamp. It has worked well for many years, having originally been installed back around 1994. This repeater took a lighting strike when it was installed at the Zebulon site, and I had never tested the preamp. I had to replace the PA at that time, so I'm guessing that's when the preamp failed. 

I put it on the IFR and spectrum scope, and discovered that it was a "deamplifier" :-) About 34db of loss when powered up, the GaAsFet has failed. This unit is about $100, so it's certainly work repairing. A new FET is ordered, so I'll post more about this during / after repairs. In the photo below, I set the sig gen for 0dB direct, and with the preamp inline, you see the 34dB or so of loss. 


Tuesday, February 25, 2020

Converting a UHF Motorola Micor to Repeater Service

I posted info on my old Motorola Micor UHF repeater on my blog, and people asked questions about how to do it. This is a fairly easy task, though it's far easier to stack two of these radios (one for receive, and one for transmit), but I enjoyed the task of repeating within a single radio. 

There's adequate isolation to do this, even when using a receive preamp, with a good set of cavities. These radios are cheap as dirt, often costing less than the crystal required for conversion to the ham bands. The nice thing, being crystal controlled, is you have no phase noise to deal with as with synthesized rigs. They are TOUGH AS NAILS! I've run them hot, it's been hit by lightning, and has operated an horrible physical conditions at one of the repeater sites. Users have locked it up for hours on end while cross-banding HF operations through it. The repeater in my previous blog has been in operation since 1994, so 26 years is nothing to sneeze at.  I'll snap some pics of the internals that I've modified, when I go back into it, to help illustrate the process. 

As I said, it's much easier to simply tune up a couple, one on the RX frequency and one on the TX frequency, wire a controller, and let'm fly. But, if you want to modify one for full duplex repeat, here's the info. I wrote the compilation of notes I accumulated and included my experiences in Microsoft Word. You can download the file, and if you don't have MS word, there's a host of online converters you can open it with for free. 

Go to my website at http://www.WB4IUY.net , mouse over the toolbar across the top of my webpage to [WORKSHOP], and click on "Schematics Manuals". You'll find Motorola Micor in the listings. Have fun!!


Monday, February 24, 2020

WB4IUY 442.400 Rptr back on!

After a long period of inactivity, I'm in the process of getting all of my repeaters cleaned up, checked out, minor repairs made, and readied for going back on the air. Here's a couple of photos of the 442.400 repeater, online in my work shop. I originally built this back in 1993. I've made a few adjustments, and so far, all is running good. The PA is running at low power (15 watts), and it is carrier access. I do have it set up for a sub-audible tone of 88.5, but that is disabled at the moment. It's running on a test antenna out behind the shop, only about 20 feet off the ground. 

This will be going back on in the stack of repeaters: 29.620, 53.07, 147.39, 224.80, & 442.400. I'll post more of the permanent installation, once the antenna work is completed. 

This repeater uses a 6 cavity Sinclair duplexer on top (light brown enclosure on top of the radio), a homebrew bandpass cavity (gray in color) stacked on top of the duplexer. The Sinclair duplexer was originally used in the commercial service a few mhz above the 70 cm amateur band. They were repaired and retuned for the input frequency of 447.400 and the output frequency of 442.400. The bandpass cavity was built from a scrap 800mhz analog cell site cavity. The repeater was built from a Motorola Micor mobile radio used by the NC Forestry Service on about 452/457 mhz.

The controller is mostly homebrew, using a Communications Specialists CW ID'r to handle identification chores. All timing (time-out, COS, tx hang timer, ID front porch timer, etc.) is analog and built from 555 timers and discrete components. Audio processing is also homebrew using LM324 op-amps. All audio mixing and controller circuitry is self-contained within the Motorola Case, making for a compact UHF repeater. A pl of 88.5 is re-encoded on it's output signal during COS activity using Selectone encoders. This is for linking purposes.