question on a 2950

[chris.s]

I have a 2950 that was given to me because it has a problem,when the top cover is off the radio it works fine nice and clear audio,when you put the top cover on the audio gets all messed up and fuzzy sounding,everything looks intact when it is compared to my good 2950,does'nt appear to have any parts missing,any ideas would be helpfull.


thank you and take care.

Chris.

 

[nomadradio]

Check to see that all of the 'ground' screws holding the circuit board to the chassis are tight. They can come loose and create some odd feedback gremlins.

If any of them are stripped, ream the hole in the aluminum to take a #4-40 screw from the hardware store. The nut to hold it to the hole in the aluminum should have an inside-tooth lockwasher under it.

73

 

[Hc714]

Seems like there was an issue with the case screws or the bracket screws when tightened causing the board to flex and cause problems too.

 

[chris.s]

problem seems fixed

I talked to a guy that does radio work and he told me to put the ceramic insulators back that go behind the finals instead of the thin plastic ones and the problem has gone away,he told me that he has seen the problem with other 2950s,does this make sense to anyone.

thanks for the replies and help.

Chris.

 

[Butcher318]

You might check the solder straps that wrap around the side of the board as well.

 

[nomadradio]

Hmmm. If you do decide to change the insulating washers under the finals and driver, make very sure you smear a thin layer of silicone heat-conducting compound on both sides of each insulator. This stuff is there to force out air gaps between the flat surfaces, and carry heat across to the chassis and heat sink. Sort of a "thermal gasket".

Without it, parts will overheat and fail.

The radio was built with white ceramic insulators.Not sure how much a thinner mica washer would change the picture. But plastic? Bad idea.


73

 

[Lazybones1222]

Re: problem seems fixed

the problem has gone away
Chris.

It works now. That WAS the problem.

 

[PLUTO]

Ok...I have to ask, how would the mica cause the problem? The only thing I can think of is if the insulating washers (on the bolt) that were used with the ceramics, being longer, were used on the thinner mica and not allowing the transistors to tighten to the heatsink. But that would be a heat issue that removing the top cover wouldn't fix. I've seen the board ground/board flex problem, but I don't understand the mica problem.

 

[Lazybones1222]

How about a short kindergarden explaination on what RF Bypassing does Justin.

 

[PLUTO]

How about a short kindergarden explaination on what RF Bypassing does Justin.

Yeah.....what he said

 

[Sonwatcher]

There may be another cause. My Yaesu did the same thing. After checking it out I found that a wire had a bare spot that touched the top cover. It would work fine when the cover was off.

 

[DTB Radio]

RF bypassing is basically filtering the transmit audio circuit so that RF does not get into it. Its generally done by adding small-value caps (.001uf or so) from certain circuit points to ground, which basically give the RF (but not the audio) an easy path to ground, taking it out of the audio circuit.

 

[Toll_Free]

RF bypassing is basically filtering the transmit audio circuit so that RF does not get into it. Its generally done by adding small-value caps (.001uf or so) from certain circuit points to ground, which basically give the RF (but not the audio) an easy path to ground, taking it out of the audio circuit.

Actually:

RF bypassing is using capacitors (usually) to electrically "short" a radio frequency to ground, while letting other frequencies pass.

IE, you put mica caps on your audio lines (as DTB was saying above) to limit the amount of RF (eliminate it, hopefully) on your mic line. Well, inside the radio, there is a lot of RF floating around.. Too much for good engineering practice. By bypassing the unnecessary, spurious and erroneous RF to ground, then you don't run the risk of having a signal where it isn't supposed to be.

A mixer takes freq a and b, adds (and subtracts) then together to get freq C and D. Well, you only want freq C, so they put a filter in, and it blocks filter D from taking the signal path.

Well, you have this problem, without RF bypassing (grounding for RF freq) freq D is allowed to "float" around the inside of the chassis. By "RF Bypassing" freq D to ground, there is no chance for it to get to, say, the mixer, or buffer of your TX signal.

Once that happens, all of a sudden, the TX is amplifying all kinds of "spurious" signals, causing additional dissipation, overloading components (they aren't designed for that frequency of operation), etc. I think you can see where this goes from here.

Anyway, you bypass by selecting a capacitor that has a limited amount of "reactance" (reactance is measured in "ohms"... but it is the measurement of a part or circuits ability to inhibit RF.... (susceptance is the opposite, but we won't talk about it here). If you have 0 ohms of reactance, your also called "resonant".

By picking the correct value of capacitance for the bypass cap, you eliminate (send to ground) the freq you DON'T want, while letting the desireable freq to ground.

A filter is nothing more than RF bypassing... You use the inductor to additionally tune the caps selective frequency.

Simplified, maybe too much so, but there ya go.

--Toll_Free

 

[PLUTO]

chris,

The clear mica insulators can cause the problem your describing in a 2950/70. Put the ceramics in. And make sure the main board screws are tight.

This is what I was asking about, I'm not implying that is wrong...just trying to understand how the mica insulators would cause the problem.

 

[Toll_Free]

chris,

The clear mica insulators can cause the problem your describing in a 2950/70. Put the ceramics in. And make sure the main board screws are tight.

This is what I was asking about, I'm not implying that is wrong...just trying to understand how the mica insulators would cause the problem.

A difference in thickness will cause a difference in the package substrate (which on that transistor is the collector, B+, which also makes it the one that is on the "output" side of the stage) will cause a difference in capacitance.

That, or a cold solder joint. Flexing the transistors around while replacing the insulators could have found and exploited a cold solder joint.

--Toll_Free