STL Overshoot

Gotta watch those Fresnel zones!
Post Reply
fmrfman
Posts: 66
Joined: Sun Aug 17, 2014 1:43 am

STL Overshoot

Post by fmrfman » Tue Sep 06, 2016 11:20 am

I have noted high modulation on the modulation meter of my FX50 exciter. Somewhere in the neighborhood of 10 to 40% over. I am doubtful of the meter as I am not hearing any evidence of this much over modulation on-air. I believe it is a Moseley 505 STL receiver. In the past I was able to power cycle the unit and it would be fine for a while. Not so anymore.

I don't have this problem when feeding the exciter directly from an Orban 8100A on site.

Mark

TPT
Posts: 656
Joined: Mon Dec 03, 2007 3:18 pm
Location: St. Marys, WV

Re: STL Overshoot

Post by TPT » Tue Sep 06, 2016 12:55 pm

A 505C receiver is IRU, and has a black face. The later model 606C has a silver face and is 2RU high.

If it is a 505C, I would suspect electrolytic capacitors, and possibly corroded IC's. Replace the filter caps and any coupling or bypass electrolytics, then look carefully at any Teas Instruments 8 pin DIP IC's A number of the TI IC's made in the 70's (era of the 505) had improperly plated leads when fabricated in Malaysia. The leads tend to corrode--leading to some interesting problems. Such as the intermittent problem you describe.

A further possibility is an output pot on the composite drive--getting dirty, failing.

ChuckG
Posts: 916
Joined: Tue Mar 11, 2008 11:12 pm
Location: Moo
Contact:

Re: STL Overshoot

Post by ChuckG » Tue Sep 06, 2016 1:48 pm

I'm pretty sure the Composite "c" version is no longer legal for use (although the mono version may be). That aside, that unit dates to 1975 or so and could probably use a complete rebuild if it hasn''t thusfar been touched. I"d forgotten about the IC pin problem. Didn't they tarnish or rust?
<><><><><><><><><>
Chuck Gennaro
Central Wisconsin

fmrfman
Posts: 66
Joined: Sun Aug 17, 2014 1:43 am

Re: STL Overshoot

Post by fmrfman » Tue Sep 06, 2016 2:06 pm

Moseley's out of business, no? Anyone still rebuilding these?

TPT
Posts: 656
Joined: Mon Dec 03, 2007 3:18 pm
Location: St. Marys, WV

Re: STL Overshoot

Post by TPT » Tue Sep 06, 2016 5:04 pm

Both. I've seen chips with the pins completely rusted off. Also was a problem with MS/MX-15 exciters. And yes, I don't think most of the 505C transmitters are type accepted except as a back-up, but I've come across the receivers being used with newer STL transmitters....which is interesting because apparently the receiver was the problem with the system. Also--there were two versions of the power amplifier circuitry on the 505C. Some have vertical fins on the heat sink, some were horizontal, I believe the horizontal version was an earlier, more troublesome design.

ChuckG
Posts: 916
Joined: Tue Mar 11, 2008 11:12 pm
Location: Moo
Contact:

Re: STL Overshoot

Post by ChuckG » Tue Sep 06, 2016 8:04 pm

fmrfman wrote:Moseley's out of business, no? Anyone still rebuilding these?
Last I dealt with a Moseley STL- and it was many years ago- Jim Jones at j-squared in Oregon was the guy to call.
<><><><><><><><><>
Chuck Gennaro
Central Wisconsin

User avatar
Shane
Posts: 753
Joined: Fri Feb 01, 2008 12:08 am
Location: Omaha
Contact:

Re: STL Overshoot

Post by Shane » Tue Sep 06, 2016 9:33 pm

Moseleysb.com. Same trademark font. I reckon they wouldn't want to mess with a 505 or 606 anymore but would be happy to sell you their latest, greatest "solution."
Mike Shane, CBRE
---Omaha---

W2XJ
Posts: 89
Joined: Mon Dec 03, 2007 12:15 pm

Re: STL Overshoot

Post by W2XJ » Wed Sep 07, 2016 11:33 am


kcbooboo
Posts: 448
Joined: Sat Dec 10, 2011 4:53 pm
Location: East of the Mississippi

Re: STL Overshoot

Post by kcbooboo » Thu Sep 08, 2016 5:17 am

I worked on a dead 505 system. Several shorted tantalum capacitors. Every one blew up and was easy to see. Every one also took out the component in series (10 ohm resistor or a small inductor). I also found a few 1/4w carbon resistors that had changed value drastically, in both directions. I just went at it module by module until it worked at the end. It didn't help that the manual I had was for the composite (BNC/baseband) input model commonly used at FM stations and these units were the analog (balanced pair xfmr) input model as this was used at an AM station. I don't know why the mis-match. I was able to get info for the analog boards I had from J Squared.

Also, that TX On/Off pushbutton switch was very intermittent. I wish they had used an actual LED to indicate that it was truly transmitting, rather than the flag inside the switch button. But other than that, when I got finished, the unit was meeting all specs. So they're not impossible to fix, they just take time to get the circuit boards out to replace defective components.

Bob M.

Post Reply