Moseley PCL505 Receiver info needed

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kcbooboo
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Moseley PCL505 Receiver info needed

Post by kcbooboo » Fri Dec 05, 2014 1:57 pm

OK, now I'm working on the matching receiver. This particular unit has a 3-contact binding post at the rear for balanced audio output, not the BNC jack, although it does have two MPX BNC jacks but I'm not interested in those yet.

The audio board in this unit is quite a bit different than the one in the on-line manual. First, it has a switchable Noise Reduction feature. Second, it has a pair of TO-5 transistors with clip-on finned heat sinks that are apparently driving the two output leads (positive and negative phase). Neither of these features is in the manual I have, and of course that's where I'm having problems. The two outputs are distorted at any modulation level over 20 kHz. It's like flat-topping but the flat area is tilted, not flat like clipping due to hitting the power supply rail or ground. Something is clipping but without the proper schematic it's hard to trace. Each phase complements the other: on one it's the negative peaks that are flat; on the other it's the positive peaks that are flat. The Noise Reduction switch has no effect on the flat-topping.

The raw detected audio output is fine; goes right down to -60dB (0.1%) distortion at 400 Hz. The receiver itself seems to be plenty sensitive and so far everything else is working. It's just the balanced audio outputs.

So the big question: does anybody have a PCL505 receiver manual with this particular audio board in it? The most significant item is a mini toggle switch sitting upright in the middle of the board. Can't miss it. I would be interested in obtaining at least the schematic and component layout, but the parts values would be nice eventually, assuming it's something I can't readily identify without that.

Bob M.

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Deep Thought
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Re: Moseley PCL505 Receiver info needed

Post by Deep Thought » Fri Dec 05, 2014 3:57 pm

Tilted flat topping sounds like a capacitor is in the audio path before it gets to these, or your scope is in AC coupled mode and is distorting the waveform. What does the input side of those output transistors look like?
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com

kcbooboo
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Re: Moseley PCL505 Receiver info needed

Post by kcbooboo » Sat Dec 06, 2014 5:47 am

I would tend to agree. Since there's an output xfmr, AC/DC coupling didn't matter. The way the board is constructed, I can't get to the inputs of the transistors unless I dismount it, however the collectors are at +13.5V. I need to do more investigating and signal tracing. The manual I have shows a BNC for the composite output; my unit has the balanced output. There is one very large axial capacitor on the board (Sprague orange color) that might have been replaced; it's the only such cap there and it sure stands out. The boards in this unit are all green fiberglass with no component or connection point labels. Even the manual fails to call out the connection point numbers on the pictorials (it DOES for the transmitter).

I was surprised that the unit powered up and actually worked, given its expected age. No tantalums were shorted on this unit, as compared to its mating transmitter. It's also possible there's a manual on the shelf at the site it came from; I didn't think to look there when I picked it up Wednesday. Next time I'm in that town I will have a look.

In the mean time if anyone has the real manual that has the audio/output board with the Noise Reduction circuit and switch, please contact me here or via PM.

Bob M.

Update: one of those TO-5 collectors is at +13.5V; the other has the single-ended output signal, and it's bottoming out. The big orange Sprague capacitor couples that signal to the output xfmr.

I was scoping out the various op-amp outputs (all DIP-8 741s) and found a similar clipping on one of those outputs, but it was several volts above ground.

Something must have changed because all of a sudden I noticed the output level was at +20dBm, not +10dBm, so I reduced the output level and the clipping went away. I need to recalibrate the PGM and MPX metering, but at least it's taking a 400 Hz tone at 40 kHz deviation and producing +10dBm (terminated in 600 ohms) with under 0.1% distortion.

It would still be good to have a schematic of this particular board, but at least the clipping has disappeared. I'd feel better if I knew what changed. The audio board in the manual is for the 505/C (composite); I have the regular 505 (monaural).

Bob M.

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Re: Moseley PCL505 Receiver info needed

Post by Radio Ranger » Sun Dec 07, 2014 9:57 am

If this is the monoaural version of the 505, isn't the audio input on the transmitter preemphasized? That would explain why the audio was clean at 400Hz and really distorted at 20KHz, if you fed it the same audio level at both frequencies.

Steve B

kcbooboo
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Re: Moseley PCL505 Receiver info needed

Post by kcbooboo » Sun Dec 07, 2014 11:39 am

Yes, it is the mono version and it has de-emphasis somewhere on the audio board that I have no docs for. There are even some adjustments related to equalization so the de-emph matches the pre-emph in the mating transmitter.

The 20 kHz I was referring to was the deviation level in kHz, not the modulating tone frequency. That's been 400 Hz all along. The PCL505 spec is 40 kHz at full deviation.

Bob M.

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Re: Moseley PCL505 Receiver info needed

Post by kcbooboo » Wed Dec 24, 2014 11:04 am

I found the manual for the PCL505 system at one of the sites I tend to. Unfortunately it too only has the composite audio board for the receiver; nothing on the monaural board with noise reduction. Moseley had nothing either, nor did one of their gurus. So I'm still looking for some documentation on the PCL505 STL receiver mono audio board.

Bob M.

kcbooboo
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Re: Moseley PCL505 Receiver info needed

Post by kcbooboo » Wed Feb 11, 2015 5:42 am

After several emails with Mike Patton, I was pointed to Jim Jones at J-Squared Technical Services. He worked for Moseley and he actually HAD the mono audio output board schematic and parts layout that matched the one in my unit. He also had info for the RF preamp mine has. So multiple "thumbs-up" for him and his company. I printed the pages and added them to my manual.

Bob M.

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Dale H. Cook
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Re: Moseley PCL505 Receiver info needed

Post by Dale H. Cook » Wed Feb 11, 2015 7:00 am

Mike Patton and Jim Jones are at the top of the shops servicing vintage, especially orphaned, broadcast equipment, and I highly recommend both as service facilities.
Dale H. Cook, Contract Engineer, Roanoke/Lynchburg, VA
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