AM Transmitters – Page 3 – Engineering Radio

Middle Aged Iron: Cetec/Bauer 701B transmitter

In service as a backup unit at WALL 1340 KHz in Middletown, NY:

WALL 1340 KHz, Middletown NY AM1A on air, 701B into test load

I believe the Cetec transmitter is from the early 70s. I wouldn’t really call it old, we have much older units in the field that are still in backup service. WALL itself has been on the air since 1942 from this site. The tower out back was replaced in the mid ’90s and is 147 degrees tall. It broadcasts the “True Oldies Channel” and is currently owned by Cumulus, soon to be Townsquare.

The site is also home to sister station WRRV (92.7 MHz) which has a side mounted antenna near the top of the WALL tower. We are currently reconnecting the CCA transmitter as the backup for WRRV. That unit is also from the early 1970’s.

WRRV WALL transmitter site, Middletown, NY

WRRV WALL transmitting tower, Middletown, NY

Well designed circuit boards

Sometimes it is the little things that catch the eye. When I was installing a Nautel transmitter recently, I was admiring the circuit boards used for the transmitter controller. I have seen a few circuit boards that are functional, but leave a little to be desired in the form department. Does it really matter? Perhaps not, but often times those tiny, almost insignificant details come back to bite you. Little things like having the voltage regulator pins correctly placed or putting a toggle switch on the correct side of the board. I have seen both mistakes from another, well known transmitter manufacturer.

Anyway, these are a few photographs of some well designed, well laid out circuit boards.

This is the main controller board.

Surface mount components.

Logic chips.

Nautel XR harmonic filter, part back part is the circuit board

Part of the harmonic trap for the XR series transmitters.

It really is the little things that make big differences. A circuit board under a cover that few people will ever see may seem like a very small and insignificant detail, but I notice and admire these things…

Proper drive levels for a Harris SX series transmitters

I was working on a Harris SX5 the other day and snapped a picture of the scope while measuring RF drive levels. There are still quite a few of these units out in the field, judging from my search engine results. I thought it would be helpful to post something about it. The RF power amp boards for the Harris Gates solid-state AM series transmitters are the same design, I believe.

In order to fully drive the RF MOSFETs in this particular series of transmitters (Harris SX1, 2.5, and 5 including A models) at least 26.5 volts peak to peak is required. Less than that and the device will turn full-on and internally short. To measure RF drive, the transmitter must be in local with control voltage on, with the rear door interlock defeated (this can be safely done if the transmitter is wired with separate AC feeds for control and RF power supply). Make sure the RF power supply is defeated and will not turn on. Measure across the input of each of the toroids that feed the gates of the RF devices.

Harris SX series transmitter drive level test, 27.45 volts, 1,110,000 Hz

It should measure between 26.5 and 29.5 volts. This one measures 27.45 volts peak to peak. Each input toroid on every PA board should be measured as the toroids themselves have strange failure modes and may pass resistance and continuity tests, yet still not provide proper drive voltage to the attached devices. This has to do with core permeability. Each toroid feeds two RF MOSFETs, replace part is IRF-350.

As always when dealing with an SX transmitter, good luck.

Oh damn, La cuarta parte

Today there will be a quiz. Ready? Look at this picture and see if you can spot the problem:

If you said “Hey, that green wire seems a little odd; it disappears behind the heat sink next to that screw then reappears again at the top,” you are correct. What really sucks is the green wire is the transmitter off connection to the remote control. So, when the PA board was secured to the heat sink, the wire was trapped between the board and the heat sink. Since the components were cold, they did not pinch through the insulation right away, no. Rather, after the transmitter ran for several hours at full power, it got hot enough to displace the wire insulation and cause a short. Doh! The transmitter is off and it won’t come back on!

This is a picture of the wire after it was removed:

Haste makes waste. Unfortunately, it was someone else’s haste that ruined my Saturday afternoon when I was supposed to be taking my son to little league practice. I am sure that some not-so-kind words will be exchanged very soon.

Always double-check your work.