Moseley PCL505 STL TX tuning

Gotta watch those Fresnel zones!
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kcbooboo
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Moseley PCL505 STL TX tuning

Post by kcbooboo » Wed Dec 03, 2014 6:02 pm

I was given a PCL505 STL transmitter on 949 MHz. It was quite dead when I got it, but three capacitors, two resistors, and one choke allowed it to come back to life. All three caps were tantalums that shorted, taking the other parts with them. The 1K front panel AFC pot was also shot but I replaced it with a 2K pot temporarily. So far it's working fine except little to no TX output.

Yes, I know these units are no longer type-accepted/approved for use on STL systems. But I like fixing things and this is real close to working the way it should. I've got the matching receiver but haven't delved into that yet.

All the metering values are as specified in the manual except almost zero PA current and the PA is only putting out about 1/4 watt, barely enough to move my Telewave wattmeter needle. Any tuning tool I use with a metal tip causes the TX output to go to zero as soon as I touch any PA tuning capacitor. I don't think I have any plastic alignment tools with a sharp yet strong enough screwdriver tip.

Any suggestions besides throwing it out?

Bob M.

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Dale H. Cook
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Re: Moseley PCL505 STL TX tuning

Post by Dale H. Cook » Thu Dec 04, 2014 7:57 am

You said "PCL505" so I presume this is the discrete channel version - the composite version is a PCL-505/C.

The transmitter metering values tell you little of what you need to know for troubleshooting. It has been 25+ years since I last did any major work on a 505, so it is difficult for me to offer any concrete troubleshooting suggestions without having one in front of me. I have the manual but that helps only so much. Have you checked the oven and the 9 volt supply?
Dale H. Cook, Contract Engineer, Roanoke/Lynchburg, VA
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TPT
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Re: Moseley PCL505 STL TX tuning

Post by TPT » Thu Dec 04, 2014 3:21 pm

Believe there were two models of the 505---you could tell them apart by which way the fins ran on the heat sink (vertical or horizontal). Believe the vertical fin version is still type accepted. I licensed one without any problem about two years ago (short hop across a college campus for an LPFM)

The output transistor may be the problem--when it goes you will get some signal leakage through it. Don't remember if it is replaceable.

kcbooboo
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Re: Moseley PCL505 STL TX tuning

Post by kcbooboo » Thu Dec 04, 2014 4:33 pm

This is the PCL505, monaural audio plus MPX, not composite. Heat sink fins run vertical.

I've got the on-line manual, such that it is. My PA module doesn't agree with the schematic; there are many more variable caps. It looks like two per stage plus two at the output. My load is good and I've always had it connected to a load whenever I enable the transmitter power. I wish there was a nice red LED on the front that indicated RF is being generated; that red flag in the push-button switch cover just can't be seen unless you look directly at it from the front.

The oven/AFC, modulated oscillator, multipliers, buffers, audio, PS are all working fine at the moment, just very little RF and it seems to move up and down all by itself. Frequency is stable, even at this low amount of power. I don't have any cables suitable to connect the individual modules to anything else, otherwise I could check for proper drive level. All I can go by are the meter readings, and they're all at or slightly above what the manual calls for.

Any of the PA transistors could be shot. I might have to just bite the bullet and connect it through my 50w 30dB attenuator to my spectrum analyzer and see what's coming out. The first stage of the PA module is a doubler; everything after that should be running around 950 MHz.

As soon as I touch any of the tuning caps with any piece of metal, even the short 1/4 inch blade of an alignment tool, the output and PA current goes to zero. I think, but am not sure, that the next-to-last metering position also goes to zero. Those push-button switches are quite dirty and I don't see a way to get any contact cleaner into them without submerging the entire assembly.

Bob M.

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Dale H. Cook
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Re: Moseley PCL505 STL TX tuning

Post by Dale H. Cook » Thu Dec 04, 2014 6:00 pm

kcbooboo wrote:Those push-button switches are quite dirty and I don't see a way to get any contact cleaner into them without submerging the entire assembly.
I have managed to get Deoxit into some mighty tight places using an insulin syringe. Although I now use insulin pens I hung onto my remaining stock of syringes just for that purpose. A 30 gauge needle can deliver a chemical into places that are otherwise unreachable.
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kcbooboo
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Re: Moseley PCL505 STL TX tuning

Post by kcbooboo » Fri Dec 05, 2014 7:58 am

Unfortunately I'm not at the point of needing shots; pills are holding my glucose level down to 110, so I have no syringes nor needles. I have a feeling that one needs a prescription to buy that sort of stuff at a legitimate pharmacy, although I can buy syringes on ebay and probably would find needles there as well. I don't even care if the sharp tips were cut off, or if they've been used.

It would almost be worth taking the switch bank off the front panel and using a hot needle or real tiny drill bit to poke or drill a small hole in the rear of the switch body, just so I can inject some contact cleaner in there. About the only other means of entry is around the plastic actuator shaft, and there isn't much clearance there either.

Why couldn't they just have used a 10-position rotary switch? There was plenty of front panel space. Easy to clean.

Bob M.

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Dale H. Cook
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Re: Moseley PCL505 STL TX tuning

Post by Dale H. Cook » Fri Dec 05, 2014 9:07 am

kcbooboo wrote:I have a feeling that one needs a prescription to buy that sort of stuff at a legitimate pharmacy ...
One does.
kcbooboo wrote:Why couldn't they just have used a 10-position rotary switch? There was plenty of front panel space. Easy to clean.
Yes, but needing cleaning because the contacts are exposed.
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Re: Moseley PCL505 STL TX tuning

Post by TPT » Fri Dec 05, 2014 11:26 am

I recall taking the switch bank out and dunking it in a bowl of alcohol--(rubbing, not drinking), which loosened up a bunch of crud.

kcbooboo
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Re: Moseley PCL505 STL TX tuning

Post by kcbooboo » Fri Dec 05, 2014 1:48 pm

I'm close to pulling the switch modules and dunking them. There aren't that many wires to unsolder. I unmounted the entire assembly and sprayed everything but who knows how much got into the contact area.

At least if a rotary switch gets dirty, it can be cleaned! And there are sealed rotary switches that get dirty just like these sealed pushbutton switches. The rotary would have been much less complex and no buttons would fall off.

Now, back to the problem area: tuning the PA for something greater than maybe 1/4 watt output.

Bob M.

kcbooboo
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Re: Moseley PCL505 STL TX tuning

Post by kcbooboo » Sun Dec 14, 2014 12:44 pm

After replacing the defective 1k AFC adjust pot and a 10uH choke that was taken out by a shorted tantalum cap, I got back to this unit. Determined to tune the PA, I connected the output through a 30dB attenuator to my spectrum analyzer. I also used the alignment tool I had with the shortest metal tip. Beginning at the PA output, I started tweaking for maximum 948 MHz signal. After noticing a 10dB improvement, I connected the unit back to my wattmeter and dummy load and had about 2 watts. I repeated the tuning, working from the output to the input and managed to get it up to 8.75 watts. The manual claims 5w minimum, 8w nominal. I reinstalled all the covers and let the unit run on the bench. Two days later it's putting out 9 watts and everything is warm and stable. I did one final tweak of the PGM meter and main deviation level pots so +10dBm input produced 40 kHz deviation, and it's finally done.

I suppose in a pinch it could be used as a tertiary STL system.

Bob M.

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