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Motorola CM-550 POOR SSB AUDIO.

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canuck7

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May 29, 2014
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worcester , uk
hi All, pete in england here. newbie to this site. i just recieved a motorola cm550 i bought off ebay usa which was advertized at good working order. i plugged it in and it recieves ok and AM tx is clear enough. but when i transmit on usb/lsb and self monitor my own audio is is poor and scratchy and distorted. the motorola "power mike works but is'nt in the best of condition.. i cannot return rig to seller cuz i got hit this end for V.A.T [UK stealth tax] and import duty as well as a hefty delivery charge. i've seen this same rig discussed on this site and one owner talks of "restoring" his. i am not a rig repairer but has anyone familiar with this radio any idea why the tx ssb audio is poor? after rave reviews on this rig i am left disappointed so far.. i am wondering if the mic element is worn out. i shall try another.. thanks in advance, p
 

canuck7

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May 29, 2014
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Location
worcester , uk
hi again

Folks, i see there's been quite a lot view my post. i did discover afterwards that running the motorola amplified "power" mic t ohigh leads to distorted ssb audio. 9 is max, 6 or 7 instead seems to make the audio better. i thing i notice is unless the mic setting is at 9 then when i speak on ssb there is no output indicated on the meter in the rig and also very little output power showing on the meter on my ham ATU. turn the mic up to show tx whne speaking and it distorts. some Motorola guys here on this website talked about wiring up another non amplified motorola mic. i like the idea of trying that. i'm worried that voltage comes out the mic socket in the rig to power the standard mic and i might do damage to rig or mic i'm trying to fit. i have an astatic 104mb6 which i's love to try but need advice off the motorola experts on here. i will try the rig today by speaking to a friend on a ssb handheld nearby. i'll post results of reported audio..
 

WA0CBW

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Shawnee Kansas (Kansas City)
Since the same microphone is used for both AM and SSB and it sounds fine on AM then you most likely have the mic turned up too much for SSB. In SSB mode there should be NO RF output unless you speak. There is no "CARRIER" in sideband so there would be no RF output when you key the mic. Whistle into the mic and adjust the mic level until the RF power meter indicates the rated SSB power output. Normal voice peaks should be between 1/4 and 1/2 the rated peak power output. I have several of these radios and they work very good.
BB
 

canuck7

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May 29, 2014
Messages
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Location
worcester , uk
motorola CM-550 advice.

Hi, thanks for reply and advice. all noted. i do understand about ssb and the absence of any carrier. i realize to the PEP or peak envelope power is not what you get a contant of in speech , only whistle and that ordinary speech in say 100w pep rig is only a fraction say 10w average on the voice. i've tried that what you say, reducing mic main on ssb on a whistle until it just starts to drop.. thing is then when i speak the rigs meter and my power meter on the atu show next to nothing in rf output. althrough self monitoring there is no problem at all hearing my own audio. not suprising being as the recieving rig without any antenna plugged in, [kenwood ts430] is only 4" away. so too sum things up, it seems the rig shows no actual rf output at all on any sort of low power metering When i reduce the mic gain enough to make
audio distortion go away on ssb. i noticed too that the mic element is soldered directly to the PCB in the mic. so for now i will not bother trting another mic element. good to here you have several of this type of rig.. i wonder if they are all cm-550's or other models. there's a "as new" mint base version of this rig on ebay USA- $700... out my price range at the moment. have you any schematics or servicwe manuals for this rig? one other thing, when i key the mike on am, the meter needle flys to the end momentarily then comes back down and settles half way,but the atu is showing a steady 4w output on the power meter. on ssb voice peaks, only with the mic gain at 8+ setting!, it it showing about 10-15 on a whistle. i wish i could get this rig serviced..! i love it though so far and will be looking for other motorola's ... P
 

prcguy

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So Cal - Richardson, TX - Tewksbury, MA
I used to sell the Motorola CB line in the late 70s/early 80s and the CM-550 usually has great audio fidelity on AM and SSB at all mic settings. It could be the rig needs a little alignment after all these years. The best mic I have used on these is the matching Motorola desk mic, which was the same as used on the commercial Motorola rigs like Spectras but factory wired for the CB series.

I have a mint CM-550 in the box but it has a PLL problem where it goes out of lock randomly and an alignment did not fix the problem. Otherwise it sounds fantastic on TX when it works. Someday I'll find someone familiar with this model to get it back in working condition.
prcguy

Edit:
I just realized my radio is the CM-555 base, same radio but in a larger box with power supply.
 
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JayMojave

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Dec 13, 2007
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712
Location
Mojave Ca
Hello C7: Good advice here. It would be a good call to try turning down the Mic Gain down on the front panel. Like CBW said.

These Motorola Radios have a bad microphone, as some, the wire wrapped around the magnet is super thin and will break over time and normal use, and not repairable. My experience.

A good Technician can make any Mic work on any radio. I used a Commercial Motorola Mic modified to work on the Motorola CB Radios, a lot of modification. But will worth it.

These Motorola CB Radios are well worth having a good tech repair.

Hello PRC Guy: Were did you sell these Motorola Radios at???? I bought 3 from Eagle Electronics in Glendale Ca and loved them.

Jay in the Mojave
 
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prcguy

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So Cal - Richardson, TX - Tewksbury, MA
I lived in CO during the mid 70s/ early 80s and we sold the Motorola CB line along with CPI and other fun stuff of the day. I still have a couple of 40ch AM Mocats along with the CM-555 that needs PLL work.

The mics on most of these were dynamic but the radios like most commercial Motorola rigs were designed for carbon or electret condenser and uses a bias voltage to the mic. The hand mics had a little preamp and the desk mic was an electret with a preamp and those sound the best with the Motorola CB line.
prcguy

Hello C7: Good advice here. It would be a good call to try turning down the Mic Gain down on the front panel. Like CBW said.

These Motorola Radios have a bad microphone, as some, the wire wrapped around the magnet is super thin and will break over time and normal use, and not repairable. My experience.

A good Technician can make any Mic work on any radio. I used a Commercial Motorola Mic modified to work on the Motorola CB Radios, a lot of modification. But will worth it.

These Motorola CB Radios are well worth having a good tech repair.

Hello PRC Guy: Were did you sell these Motorola Radios at???? I bought 3 from Eagle Electronics in Glendale Ca and loved them.

Jay in the Mojave
 

canuck7

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Joined
May 29, 2014
Messages
4
Location
worcester , uk
hi guys, thanks for you've comments and advice. i've found some tune up info on the cm555 on "cb secrets" online. some board as the cm-550. i tried adjusting the pot for ssb output- it did nothing. after the comments on here so far about the mic element capsule going faulty, it seems to resonate with what i was already thinking. the mic only seems to drive the rig properly on ssb with a load whistle with the mike gain on 8+. but when you speak the audio is distorted but not driving the rig -ie, only putting out milliwatts. so i turn the mic down to 5-6 and audio ssb becomes clear and the rig is STILL showing very low output, ie= The ATU milliwatt setting and STILL no movement on the rig meter. i am convinced the mic is bad. i mean the mic element and for all i know maybe the cap and even the tranny in the mic have gone bad?. i am going to desolder the mic element of the pcb and experiment with other elements. i have crystal, heil hc 4, dymamic high output and even a electret condenser element. the mic potentiometer ha gone bad too, it "works" but makes a terrible scratching sound when adjusting it at all. guys, the advice about using a motorola desk mic, would it be the HMN1050D, i like the idea of that, they are "not available here in england but i see many of them cheapish even new on ebay usa. i think this cm550 "power mic" may have worked well back in the day but it is shot now and i want rid of it. so the hmn1050d is electret condenser? so the 8v that powers the cm550 mic will power this desk mic? all i need is to know how to wire it if i get one..!? i do not want to damage or blow anything on the cm550 cuz then i would be stuffed getting it repaired here......p
 
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