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Motorola 555 ssb base tx issue

Retro CB Guy

Well-Known Member
Jun 22, 2014
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Got a friend who has multiple of these Motorola 555 base am/ssb and he wanted me to ask, to see if anyone has a known fix for TX dropping out... on Motorola SSB rigs ?

Thanks in advance much appriciated, Retro
 

b2cooll

Member
Feb 21, 2015
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Got a friend who has multiple of these Motorola 555 base am/ssb and he wanted me to ask, to see if anyone has a known fix for TX dropping out... on Motorola SSB rigs ?

Thanks in advance much appriciated, Retro
Thanks Retro, yeah I have a few Motorola SSB CB rigs, they all do the same thing. After 5 min, the TX just fades away to milliwatts. I thought it was a heat issue but cold spray would not confirm that. But leave the radio off for 5 mins and it works again, for a couple of mins. I have seen maybe 6 of these and they all do it. The design is different than mainstream CBs, so there is just no info on them on the net, Really hoping someone out here remember the "Fix" on these Motorola's
 

nomadradio

Analog Retentive
Apr 3, 2005
5,314
7,634
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Louisville, KY
www.nomadradio.com
someone out here remember the "Fix"
Yep. Soldering iron and fresh solder. Might be able to get by with just a dab of rosin flux and a hot iron tip.

Pretty sure this is the one that had trouble with the double-sided circuit boards. Circuit paths that get completed from foil on one side to the other side would come loose.

Not so different from chronic intermittent issues in the GE Superbase.

73
 

Jay Mojave

Sr. Member
Nov 17, 2015
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I have a couple Motorola 550 mobiles and a Base Motorola 555, it seems like the Motorola Mic's use a extremely small wire that makes up the Dynanic Mic coil, that breaks resulting in no audio to the radios.

Kens Electronics has or had microphone elements that replaced the orginal Motorola mic element. I ordered a few but had to modify the mic plastic case to make it fit. See: http://www.kenselectronics.com/index.html

Please let us know if you find circuit board traces lifted up or needing to be resoldered.

Jay in the Great Mojave Desert
 

b2cooll

Member
Feb 21, 2015
10
6
13
62
I have a couple Motorola 550 mobiles and a Base Motorola 555, it seems like the Motorola Mic's use a extremely small wire that makes up the Dynanic Mic coil, that breaks resulting in no audio to the radios.

Kens Electronics has or had microphone elements that replaced the orginal Motorola mic element. I ordered a few but had to modify the mic plastic case to make it fit. See: http://www.kenselectronics.com/index.html

Please let us know if you find circuit board traces lifted up or needing to be resoldered.

Jay in the Great Mojave Desert
Thanks for the feedback, I will offer some more details as I have a good 40 years of repair experience, just retired at Fluke and love working on these old rigs. These do have a odd mic, a condenser if I remember powered by the radio and they tend to get damaged. I have a good mic on mine.
This radio has me stumped, and every Motorola CB555 I've seen. The radio will give a good carrier on AM and mod on SSB for maybe 10 min, then the RF fades away to nothing. Turn off the radio for 20 mins and it will work for 2-3 mins.

This says its heat or cold solder joints. Yet I have re-flowed the board in all spots that i see the RF path and others.
No pressing on the board will let it temp work. I used a can off cold spray to cool things down on the finals / IF mixers and i could not get the cold spray to make it work only after its been off for 20 mins ? so odd

I replaced the electrolytic in the RF chain and put new grease on the finals.
Yet this board is not used in any other CB and the BIAS is not clear, seems to be fixed.

Now I have 5 or 6 of these in base and mobile form, they all work great, nice RX all the time, but if you cant talk on them.. what are they good for.
So I thought for sure other people out here have seen this ?
even the ones on for sale on eBay will also say the TX fades out...

Sorry about the long message, I hope it got the point out, appreciate any and all help on this, I am thinking its BIAS or Coupling or dis caps ?
Seems like it has to be a one part type fix like change C104 it would be nice
 
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b2cooll

Member
Feb 21, 2015
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Posted Pictures, and I just want to say thanks again, Their just is no info on this mainboard anyplace. They are nice rigs, very good specs and a nice look, need to find a fix > See attached pictures of the mainboard these are double sided and that makes troubleshooting harder, yet I can not find a poor solder connection that would cause this 73s Bruce
 

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b2cooll

Member
Feb 21, 2015
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Sams Photofact CB292 has the service manual for the CM555, which is the mobile version.
Thanks for that Info, I do have a copy of that, I will post a picture of the finals. I do not see any Bias adjustments and the manual does not speak of any bias adj there.
The final is a C1817 so my experience with that is limited,
The mixer has a couple of electrolytic and I replaced them thanks 73s
 

b2cooll

Member
Feb 21, 2015
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The microphones used on Motorola CBs have a self powered preamp powered by the audio line, as I recall.
Good Memory... you are right, I called it a condenser DOH but it is a dynamic mic with a built in pre-amp, no battery as it is powered by the radio.
 

b2cooll

Member
Feb 21, 2015
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Shoot, with my multiple units all sharing the same fault, I thought for sure there was a known fix for these Motorola CB555 rigs... they are nice and when working as kind of special as they use a true Motorola design. Worth fixing.
 

b2cooll

Member
Feb 21, 2015
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I will post a picture of the schematic / finals where you can see there is no bias adjustments, Really my troubleshooting has led me to some type of coupling or decoupling of the RF signal
An odd radio, I am not aware of this chassis being used in any other radio.
no help on the web really, Mike has a nice video on them, yet did not show this fault.
 

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Handy Andy

Do Your Research First, Then Decide...
Apr 23, 2018
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Your condition, get those Flukes out, because you may need to monitor several spots.

The RF amp strip you posted...

...has this...
1644962110009.png

Check the bias.
Yes, just measure and if bad...
Test and - Replace R305 and or R311 as needed
and INSPECT and TEST each part in the chain for failure to cause this.​

But the Datasheet shows something different - and a BIG difference at that...

1644962006363.png

The Difference is, the DUT is supposed to be run Grounded "grid" setup nearly into class C (RFC Choke on Base Lead)

Your Radio has a different BIAS scheme - So this can cause the part to heat up REAL FAST and fade...

It's also why R305 is directly on the BASE lead of the Final - PAST the RF choke network - for stability - and you can measure the BIAS direct on the BASE for test purposes.

This "sag" can also come from the on-board regulator - might want to change it from the older Motorla barely 0.75 amp capacity to a 1A upgrade - won't hurt and will keep the voltage from sagging.

So, note the RED Arrows for TESTING - make sure the Diodes are good, and you are NOT pumping 5V or even 3V - has to be 0.6 to 0.7V - that is why you may need to fix the resistor mess and DON'T USE WIREWOUNDS use Carbon Composition types for best results.

It is my fear that you have a TX part in overheat even the TX mixer is suspect considering you can make it recover so something in there is getting awlfully hot and causing the shutdown.
 

b2cooll

Member
Feb 21, 2015
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Nice reply,
"Thank you" and did I hear the word DUT :^) a common term used in my calibration background.
Really this is a very detailed and thought out observation with some good suspects to look over.

I will go thru this CB and measure, measure and measure some more to tackle this issue until I find a solution to this.
Having several units that show the same symptom had me thinking this has to be a well known fault and this form would lead me right to it :^)
I understand, not an easy ask just not many Motorola CB experts.

I will not forget to post the results back to this thread as I will not stop my quest.

PS I also thought that the heat had to be the culprit,
But if true I would have thought the can of cold spray would have shown a faster recovery or a temporary OK condition as I got things Frosty

All my tricks up my sleeve could not get the RF to go active until the power was removed for 20min or longer. Room temp 75 degrees

Anyone Else ? that can dust off the old brain cells on this one.......
73s
 
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