Another radio falls victim of Bob's CB shop

[Low_Boy]

Got this magnum S3. Sealed with Bob's CB shop stickers. It was cranked to hell. Could not even get anything on the scope to look good. Parts must be changed somewhere I have to take a closer look. So like all other Bob's radios. After a few weeks one final blew.

 

[Handy Andy]

Document it well...

That may be your only recourse - to show work not done as OEM...

This is a classic example of Current Hogging...
And using only one part to "feed two or more" of non-matching (or even if matched).
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The current supply will always go to the hungrier one, being the one that turns on first and takes the longest part of cycle to produce power - this is what happens with thermal runaway...

When you use one part to bias two - the bias power goes to the one with the LOWEST turn on threshold. Taking all the current from the mains in the process and the other one just sits idle waiting for something that will never happen to it.
Sorry to see you get stung...

 

[Low_Boy]

It has some kind of erf2030 blue dot transistors in it. At one point it was discussed here palomar was selling these in matches sets but I do not see them anymore.

 

[TheRealPorkchop]

I have several Magnum radios, one of which that is 16 years old and never been fooled with or modded. I’ll have to hook it up to see what it’s doing on a scope.

I say that because if my dusty memory serves me correctly, with the TGM on the pattern does look bad, right? You tried it on and off? Not defending Bob here, just asking.

 

[Low_Boy]

I have several Magnum radios, one of which that is 16 years old and never been fooled with or modded. I’ll have to hook it up to see what it’s doing on a scope.

I say that because if my dusty memory serves me correctly, with the TGM on the pattern does look bad, right? You tried it on and off? Not defending Bob here, just asking.

Yes on and off the same. Can't say or show pictures at the present moment. It is only putting out 1/2 watt.

 

[144inBama]

I feel your pain!!!!! Hopefully you can get some satisfaction out of it.

 

[TheRealPorkchop]

Well hmm. Idk. You buy from Bob’s as a used unit or what?

 

[NZ8N]

It looks like it has been "volted" too, the center Test Point (to the Finals) is missing.

 

[TheRealPorkchop]

Magnum does that from the factory. Personally, I see it as no big deal because it’s exactly what happens when you put one of those radios into SSB mode. Of course the S3 was a sideband radio but it’s the same board as the sideband models.

 

[NZ8N]

I looked at the schematics (2SC1969 version, and the MOSFET change) and you are right, they do "volt" the finals from the factory, the AM Modulator goes to the Driver and Pre-Driver. It is an AM/FM radio, not an SSB, though.
http://www.cbtricks.com/radios/rflimited/magnum_s3/index.htm

And, the schematics for the 45 Watt MOSFET version does have only one EN369FN biasing part to both Gates on the Finals.

 

[Low_Boy]

I bought the radio used Puled the stickers and looked inside. It had a nasty pattern Sort of a V and triangle pattern. I could not get it to look good. Looked like a pattern when the limiter is removed on a Cobra 29. Not even boxcar. It has been on the shelf. I was ready for a change in my truck so decided to run that. But it did not work LOL. I will have to get some 2030 finals. It will live again.

 

[secret squirrel]

Does the radio have the Top Gun Modulator MD-1 and Compressor CP-1? Just curious I remember comments about bad scope patterns with the MD-1 and CP-1 installed with limiter removed and not tuned correctly? How does the audio sound with the V pattern?

 

[500kpot]

Could this be damage from a parasitic oscillation?

I ask because it looks like all the feedback was removed during the conversion process.

 

[Handy Andy]

Since 1969's are not used in CB's (MOSFET has taken this over) - an ORIGINAL S3 Final Driver section looks like this...

From CB Tricks Circa' 2007~14​

The above, whether matched pairs or not, they use Bias feeds for EACH transistor.

The Photo from @Low_Boy is the same board, but as with advances in technology, this board is the older dependable board design using Bipolars, versus the Pared-Down and Stripped like a Chevy Citation in Bad neighborhood - empty streets et al.. of the MOSFET version (S3's and S6's are their AM/FM only types) what's left is the burnt out shell as wreckage...

Parasitics? Possible - no neutralization - barren board - just pretty much direct feed...

It's the problem with "Drop in" MOSFET parts claimed to work as replacements. Not the "Best fit" in every case.

Their Gain as well as output can be a problem for older designs that use "Tank circuit" feeds wound like a Y-wye configuration (Multi-wind tri-tap coil that Galaxy/RCI pushed heavily - red enameled coil above) - not much room for adjustment. Hence the unequal sharing of the load and the subsequent thermal runaway problem.

More work needed to be done before it got sent out the door as "completed and ready for the Customer".

A simple thing like "testing" into a reactive (not a resistive) load would have shown it needed more work done to balance it out - BEFORE it left the shop.

If this had gotten into an actual customers' hands, the vehicle this was mounted in may have been totaled from the fire this thing could have made while in use. OR at least a nice smell of burnt electrical.

 

[Low_Boy]

Does the radio have the Top Gun Modulator MD-1 and Compressor CP-1? Just curious I remember comments about bad scope patterns with the MD-1 and CP-1 installed with limiter removed and not tuned correctly? How does the audio sound with the V pattern?

Yes it does have the top guns. I was told it sounded good.