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VCO Problems, Won't Receive on Some Frequencies by W1TO

Problem: I have a problem that just started on my MP. I have no received signal from 7.5 to 14.5 mHz. on the main receiver. I can receive signals on the second receiver. I get zero power output if I try to transmit on 20 or 30 meters. If I try split operation I also get no transmitted signal.

As I tune from just below 7.5 the rx goes from working to not working and as I tune from just below 14.5 to above the receiver comes to life. I can always hear some background noise but no signal in the 7.5 - 14.5 range.

Solutions: Hi Tom. This one has come up before. Sounds like your VCO 2 has stopped.. If you're lucky, it may just need a " tweak"... If not, then a component has died. BTW.. I've come across five other rigs with similar problems in the last month.. 73 John EI7BA ----  More From John, EI7BA Hi Tom.. I reckon it's VCO 2 on the Local Unit. I reckon a tweak should fix it. You'll need page 3-4, and 3-5 of the Technical Manual, a DVM, and a tuning wand to tweak the VCO trimmer capacitor. The Local unit, is accessed from the top of the rig.. Remove the unit behind the loudspeaker...i.e the one with the two SO239 antenna sockets on it. Underneath that unit is a flat plate, remove it, and the Local Unit is underneath.. At the back of this board is a metal screening enclosure with 4 holes on the top. VCO 2 is the second hole from the LHS.. Refer to the manual and set it up as per instructions.

When all is well again (hopefully..!!) reassemble the units. Before you put all the screws back in, rewire the two SO239 sockets, by replacing the stiff wire from the SO239 centre pins to the PCB with multistrand ( flexible) wire.. Because... If they haven't given trouble yet... They WILL in the future. The stiff wire pushes and breaks the solder joint on the PCB when you're pushing on the PL259 plugs. Trust me..!John EI7BA http://www.iol.ie/~bravo/

 

From Carlos SM0KCO - I didn't open the MP until I got the Service Manual...To check the local unit: remove the top cover
with the MP looking at you lift the card which have the 2 UHF PL-259 lift again a metal protection, the local unit is under it
The 4 VCOs are located under a screened enclosure with four holes on the top VCO 2 is the second beginning from the left

Comments: Thanks for all the suggestions. I am happy to report I am now receiving on 7.5 - 14.5 mhz, and transmitting on 20 and 30 meteres. When I checked the voltage on TP 4003 in the range from 7.5 - 14.5 I got a constant 13 volts (approximately) versus the 8.2 I was supposed to get at 14.5 and less below. Changing TC 4002 had no effect.

I turned the Local Unit board over and there were two wire leads in the area of TC 4002 that appeared to have marginal solder joints. These were not surface mount parts but were wire leads coming through the board. I touched these joints up and then measured the voltage on TP 4003 and it was close to where it was supposed to be. I adjusted it a little higher and it is working
now. I hope I did a good enough job on the two leads resoldering, I hear a little popping now and then, I may take one more critical look at those joints. Thanks to all that helped. Very good, Tom. I'm sure there are a lot of MP's out there with the same fault. Credit for this fix goes to w6xa who fixed the other radio I mentioned earlier. I'm glad I could forward the information to the readers of this reflector too. Esa OH7WV

Dear friends I also fixed the VCO trouble yesterday. 3 times I worked through the possible bad soldered points until it worked OK.I took the occasion to check the 4 VCOs and got the recommended voltages in all of them. sm0kco carlos

 

 

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