ways to stop bleedover

[N_Jay]

Didn't some early synthesized radios also have an issue where the step was not dead on and there was a small error that walked across from one way to the other as you went up the band. (Or are a few brain cells misfiring today?)

 

[rfradioconsult]

As pointed out earlier "bleedover" usually denotes adjacent channel interference. What you are experiencing is plain old intermod. Intermod can usually be expressed as Frequency A , Frequency B and desired receive frequency C. So lets do some simple math A 157.86 x 2 = 315.72 MHz - B 159.51 MHz = C 156.21 MHz. or 2A-B=C.

So if Transmitter A and Transmitter B are on the air at the same time the signals are probably mixing in your receiver and generating a third frequency of 156.21 MHz which is what you are trying to receive.

A notch filter tuned to the paging TX should reduce that signal enough that intermod shouldn't be a problem.

 

[kb2vxa]

From Jack J:
"I have repaired a few of them, Warren. In fact, I used to own one until a few years ago when I gave it to my son (still worked but had a few bad memory locations). I'd have to look but I think I have the blueprints around here somewhere. Surprised me when I ordered a service manual and that's what I got. The 101 used a few proprietary IC's and I think the memory chip was one of them. But I don't remember the IF being offset by band."

Ah, looks like I have found a fellow tech and 101 fan. You reminded me of that proprietary static memory chip and the trouble I had with Electra over it, one of the ports blew and wonder of wonders they sent me a replacement with the very same blown port. It made me wonder what kind of idiots they had working for them, it came wrapped in foil and the pins jammed into a piece of Styrofoam! The least they could have done was wrap bus wire around each one individually in turn and so on down the line, the holes in the foil were large enough there was no contact with them. Now isn't it customary to chop off a bit of the anti-static stock tube and ship the IC in it???

Unfortunately the weakest point in the whole thing was the slide switch (channel/programming switches) assembly. To save a buck the whole thing was an open printed board that collected dirt and had to be cleaned frequently. To make matters worse eventually the plating wore off and the slides broke contact necessitating replacement of the entire sub assembly.

I must give credit to the one who designed the receiver board, it was the hottest I have ever seen. The Cushman told the tale, it had such sensitivity and a low noise figure it made a lot of commercial gear look sick. The specs in the manual were a bit deceptive, usually they give threshold sensitivity but as I measured them they corresponded to the 20dB down or full quieting level. If I remember right they were something like <.1uV @ 160MHz and <.25uV @ 450Mhz.

Quite right N_Jay, single conversion receivers in particular were prone to compound errors. The VHF Lo Band was pretty much dead on, Hi Band wandered a bit but when you got up into UHF it really became noticeable. Extreme accuracy is difficult to achieve in the best crystal controlled master reference oscillators so when you get into the PLL controlled VCO and its inherent errors little things are compounded enormously and become noticeable. Thanks for jogging my memory, I think we just rediscovered the reason for that IF shift I wrote about earlier.

Oh and BTW has anyone noticed Trax has disappeared? I'll bet he's left the thread to the techies and went off shopping for a new scanner like he said he would. Heh, that kind of leaves the I triple E guy beating a horse that died way back on the last page now huh? (;->)

 

[N_Jay]

How about them early synthesized FM broadcast exciters where you had to break the PLL loop once a week and dial in the free running frequency of the oscillator, (and note any error in the logs) because the FCC would not approve a synthesized exciter (forcing you to run it as if it was crystal controlled).

 

[w2rea]

You are right about the techies. LOL. I reprogramed the scanner as suggested from 156.210 to 156.205 and that combined with ajusting the squelch seemed to work pretty good. I don't pick up the distant patrol cars and portables like I did before but the feed sounds a lot better.

 

[kb2vxa]

Well at least you have a partial fix until you save your pennies for a better receiver. Yeah, that N_Jay is something, iddnee? (;->)

No, but I heard about it from a broadcast engineer. The only PLL I had to untangle was the chroma reference oscillator in the occasional TV when it unlocked and the crystal had to be trimmed. OK, burst amp grid grounded, now where did I put that diddle stick? Oh what a pretty rainbow!

 

[jackj]

I think you might be right about single-conversion receivers N_Jay. Most double-conversion receivers that I've seen also used the 2nd oscillator as a frequency reference for the phase lock loop. This would compensate for any frequency drift and you would end up with the proper 2nd IF frequency regardless. This cheapened up the circuit which made the bean counters happy and had the side benefit of actually improving the receiver's performance.

I think the FCC's problem with PLL circuits in broadcast exciters was the amount of phase jitter they had. The jitter could be reduced with proper filtering of the DC control line but it would always be there at some level. The FCC used to care about quality, not so much anymore.

Warren, a lot of chip manufactures would ship their ICs with the pins inserted in a piece of conductive foam. I don't know how they are shipping them today but if that was really Styrofoam then the IC was probably blown when you pulled it out of the Styrofoam. The static charge that the Styrofoam would build up would be fantastic. But what the hey, you sell more chips that way.

I meant to ask earlier Warren, was that Cushman you used a CE-3 or a CE-6? Since we were a Motorola owned shop, we used the service monitor with the Bat Wing on it.

 

[prcguy]

The BC-101 probably has a fairly narrow VCO range and they use both hi and low LO injection and the IF offset was probably not a design "feature" (poor design) but probably the LO or master osc was way off frequency. Its common for even current scanners and receivers to use both hi and low LO injection in the same radio and with the master osc off freq some bands will have the IF low in frequency and other bands it will be higher. If you zero out the master osc or LO osc the high and low off freq IFs will eventually converge. I see this when working on older wide coverage military radios.
prcguy

"But if you are getting a different IF frequency when you program in a UHF frequency than what you get with a VHF frequency then you have a problem with the phase-lock-loop system or the programmable divider."

Hey, I didn't design the thing so it's not my problem. (;->) Now if you're familiar with the 101 then you should be familiar with the frequency step programming sequence using the front panel channel switches. Talk about primitive! Remember this was before CPU control, numeric keypad entry and display, I had a couple of ham rigs that were rather complicated dialing up frequencies too being the steps were selected by rotary switches. The problem wasn't in the PLL and VCO circuitry (that never changed) but rather the divider chips in use at the time and BTW still in use in CB rigs since all they have to do is come up with 80 frequencies (40 ch T/R pairs) in 10KHz steps with the Class C channels locked out.

If you can come up with the BC-101 manual you'll find the programming formula from which you can easily figure out which switch controlled which frequency step in the divider. At one point I had it down to a science because I was listening to federal frequencies not listed in the channel chart but being so long ago memory fails so please don't ask. (;->) Hmmm, too bad I disposed of all my old service manuals and data sheets when the archives became too much to carry around in a box. The original full size factory blueprints are probably a bit of a collector's item these days, oh well.

 

[kb2vxa]

I never bought directly from a manufacturer* but rather from distributors and whether over the counter or shipped they were properly packaged either in a bit of the tube or inserted in conductive foam. It was Electra that screwed up, whether they blew the port or I did really doesn't matter, ordinary Styrofoam is great for demonstrating static electricity to a class.

* You cannot buy from a manufacturer unless you want to receive a 50gal drum full of ungraded, unbranded chips or other solid state components. That's how a sorting house receives them, they go down a line getting one test after another some being pulled off at each station until at the end of the line those that fail every test are ground to powder and recycled. That's the grading process, then comes branding and numbering according to each "manufacturer's" specifications. You may gave used a cross reference guide and/or noticed how the generics are direct replacements for more than one brand, that's how it's done, half a dozen brands and part numbers all the same. I have no idea who actually made the components, all I did was calibrate the testing stations.

When you have shelf upon shelf of test gear in front of you somehow fail to notice which Cushman you grab, I could have used a Marconi but somebody else had it out. All I remember was a big metal box with a 'scope, meter and a bunch of knobs on it. A bat wing eh? Are you sure the shop wasn't run by the Wayne Foundation?