OMB PTX-20 test and repair help required

FM does it with frequency!
dicky96
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OMB PTX-20 test and repair help required

Post by dicky96 » Tue Nov 07, 2017 4:50 am

Hi guys
Just a quick bit about myself so you better know how to advise me

I am an electronics repair engineer originally from the UK now living on the sunny island of Gran Canaria. I have my city and guilds qualifications in TV Radio and Video repair and qualifications in microprocessor system design and programming. I'm a component level repair engineer working mostly on TV, laptops and DJ equipment these days. I don't know much about radio apart from when i used to fix AM CB rigs in the early 80s

OK so that's a quick background. I recently got involved with running an FM station here in Gran Can and I'm looking at the OMB PTX-20 exciter (or excitador as it says on it)

I need to test and repair if necessary. I can't test it on the station mast as it is currently live running another OMB exciter (which I think i is a newer model) and a OMB 1kW broadcast amp

So you can see I have good electronics experience but not specifically radio. I have the usual stuff in my workshop - meters, bench supplies etc, but my analog scope is only 20MHz and my digital scope is 50MHz

I found a link to the schematics for the PTX 20. Looking at it, I did power it up with a 160W current limiter (light bulbs) in the mains supply. the unit powers up, the green ENG PLL LED comes on which I think is the PLL locking and then it draws a bit of power (I can see this as the current limiting lamps light up a bit) and then cycles again. The meter jumps to the middle then back to 0 again. As it wasn't drawing much power (no short circuits) I powered it up directly from the mains - the PLL locks and stays locked, the meter goes to the middle then to zero and stays there.

I think this exciter is also a low powered transmitter? 5W/50W it says on the meter scale but I don't know how to operate the toggle switches. I think the audio input is the bnc marked ENT B.F? The cooling fan is noisy and I didn't power it up more than a few seconds as I recall CB radios did not like being turned on with no antenna. Also there is a metal cover missing from the PLL section

Basically can someone advise me what I should see on the meter display if the exciter is working, how I operate the switches and how I can test this in my workshop?

Thanks
Rich
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TPT
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Re: OMB PTX-20 test and repair help required

Post by TPT » Tue Nov 07, 2017 7:04 am

Poor thing looks like it got very wet.

OK. Understand that this is a 50 watt transmitter--the term "exciter" conveys that it is normally used to drive or "excite" a larger power amplifier, but the unit produces a complete FM signal at its rated power. So the first thing you need is a dummy load, or dummy antenna that will handle 50 watts.
Two places to look for used ones: RadioDan.Com and RFparts.com. What may be happening is that the transmitter comes up on your light bulb load--and promptly shuts down because of a bad impedance match. These critters like to see 50 ohms, not a common light bulb impedance. Looks like the meter is set to monitor forward and reflected power--if you had the meter on reflected power when it came up you would see a large indication there momentarily before the protective circuits shut down the RF amplifier. Of course--same indication on forward power for the same reasons. During normal operations the reflected power should be near zero, forward indicating the power out. Also looks like a "deviation" position which shows general level of modulation when fed with audio. Normally this is a composite stereo feed from something like an Optimod, but a conventional tone generator into the "MPX" or COMP" jack will also modulate the transmitter in mono.

But this may be progress!

I would plan on getting a replacement fan, and go through and shotgun the electrolytic capacitors. The fan may be the biggest expense, usually there are one or two filter caps that may be in the $5 range, the rest small values (like 30 cents US each).

dicky96
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Re: OMB PTX-20 test and repair help required

Post by dicky96 » Tue Nov 07, 2017 8:24 am

Hi and thanks for the reply

Yes it does look corroded in there which is really wierd as the station is in Puerto Rico the town in Gran Canaria (not Puerto Rico the carribean island)

Our climate is arid sub tropical so our annual rainfall is 80mm (3 inches) and temperature rarely goes above 30C (daily high) or below 20C (nightly low) at any time of year so we don't get much even in the way of condesation! It's on the coast but at the top of a hill so it didn't fall in the atlantic either!

OK so just to clarify, I wasn't using anything as an RF load - I had parallel 100W + 60W light bulbs in series with the 220V mains supply. It's a trick I always use on any unknown bit of kit when I first power it up - it stops things going bang and scaring the crap out of me lol. If the bulbs light brightly i know I have a short in the DUT. I don't know which position the switches were in but the meter went up and down.

It says 5W/50W on the meter scale - is there some way to set this exciter/transmitter to 5W mode? Then I could use a smaller load. I am going to the UK opn Friday for a week so can pick a dummy load up there as it is easier than getting one here

Capacitors I have a-plenty for TV and Audio equipment work. I also have ESR and capacitance meters. I should have what I need unless there are unusual values/voltages used in this sort of kit?

I ordered a fan already 230V AC 80mm. Was about £5 on ebay

Does it damage the unit to power it up with no RF load?

Is testing just a matter of feeding audio into the ENT B.F BNC connector and then tuning in an FM rado?

I have a sig gen / frequency counter still in the UK at my old workshop, I will bring it back with me as it isn't heavy

Cheers
Rich

vacuum tube
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Re: OMB PTX-20 test and repair help required

Post by vacuum tube » Tue Nov 07, 2017 8:46 am

I would guess the 50 watt scale is for forward power and the 5 watt scale is for reflected power. It looks like the toggle switches between the forward and reflected reading. Does one of the pots adjust the power output????

vacuum tube
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Re: OMB PTX-20 test and repair help required

Post by vacuum tube » Tue Nov 07, 2017 8:47 am

Guessing the POT AJ is the power out adjustment.

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Deep Thought
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Re: OMB PTX-20 test and repair help required

Post by Deep Thought » Tue Nov 07, 2017 10:39 am

This is the same unit as a Bext PTX-20. Unfortunately, it is rather old and Bext and OMB aren't terribly good about supplying manuals.

"AJ POT" is RF output level. "AJ DESV" is deviation (modulation) adjustment. The 50 watt scale is for measuring forward power while the 5 watt is for low forward power settings or reflected power. The top switch to the left of the meter selects which. The bottom switch to the left of the meter selects whether it is measuring power or deviation. You can turn the RF output CCW to near zero and get away with short periods without an RF load but you risk blowing out the PA transistors. Happily, it seems to be protecting itself which is a good sign but you really should not do that.
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com

dicky96
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Re: OMB PTX-20 test and repair help required

Post by dicky96 » Tue Nov 07, 2017 2:37 pm

Soooo... if I understand the replies right, from the sounds of things this is possibly functioning - once the PLL locks the output stage powers up, it draws a bit of power (just guessing from the brightness of my current limiting light bulbs maybe 30-50W) then shuts down due to no RF load, and the cycle repeats

I'll get my hands on a dummy load then and see what happens. Think I may be able to borrow one from a radio ham friend. It seems to me most if not all of the exciter is working

Cheers
Rich

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Re: OMB PTX-20 test and repair help required

Post by Deep Thought » Tue Nov 07, 2017 9:36 pm

Attach a proper dummy load and turn the RF level all the way down. If it stays on then gradually increase the power, watching the meter. This is only a 20 watt transmitter so a power draw of less than 100 watts would be expected.
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com

dicky96
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Re: OMB PTX-20 test and repair help required

Post by dicky96 » Wed Nov 08, 2017 10:43 am

OK will do

What type of RF Output connector is this?

Thanks
Rich
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NECRAT
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Re: OMB PTX-20 test and repair help required

Post by NECRAT » Wed Nov 08, 2017 12:10 pm

dicky96 wrote:
Wed Nov 08, 2017 10:43 am
OK will do

What type of RF Output connector is this?

Thanks
Rich
That is an "N" type connector. Very commonly used in RF applications.
http://www.necrat.us

"Arguing with an engineer is like mud wrestling with a pig. After a couple of hours, you realize the pig likes it"

dicky96
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Re: OMB PTX-20 test and repair help required

Post by dicky96 » Wed Nov 08, 2017 5:02 pm

OK Thanks to everyone who helped out even if some questions were very basic to you :D . On Friday I will going back to the UK for a week so will get a dummy load. Will be back a week later and let you all know how I get on.

I have some interesting (I hope) questions about why our 1kW transmitter does not propagate more than about 10-15 miles on this island - and how best to create an efficient beam pattern when half of our transmission power is wasted going out to the ocean - but I guess that is another thread and you need some photos of our current transmission mast/antenna configuration and google map location plus intended coverage area etc?

I'm looking forward to that discussion too if you guys are up for it

Cheers for now
Rich

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Deep Thought
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Re: OMB PTX-20 test and repair help required

Post by Deep Thought » Wed Nov 08, 2017 6:16 pm

10 to 15 miles is about right for a 1 KW station assuming that the antenna isn't very high off the ground.
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TPT
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Re: OMB PTX-20 test and repair help required

Post by TPT » Wed Nov 08, 2017 6:20 pm

Other factors: Type of antenna, actual radiated power (as opposed to the amount being fed into the coaxial cable), and, as mentioned--height of the antenna. In FM height is (almost) everything.

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Re: OMB PTX-20 test and repair help required

Post by TPT » Wed Nov 08, 2017 6:32 pm

Ah--Gran Canaria. I see the problem! A losing battle with geography. (Think Oahu--the islands are about the same size.)
600 square mile island with a 6,000 foot mountain in the middle.

dicky96
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Re: OMB PTX-20 test and repair help required

Post by dicky96 » Thu Nov 09, 2017 4:53 am

My Radio Ham friend in the UK has a suitable load for me to borrow, I'm seeing him while I am in the UK next week so will let you know how I get on with the PTX 20

Regards FM coverage and range question I will start another thread to keep this one on topic

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