President Grant PC-406 -USB Problems

[EA5GU]

Hi guys, thanks for giving me access. I need to adjust an old president grant, which on USB does not work, only noise, but I can not find the service manual of this radio. You can tell me how to adjust the SSB well so that it comes out fine. Thank you Paco EA5GU

 

[nomadradio]

Is this problem in receive mode only?

Does the radio transmit in USB mode?

Do you have a way to listen to the radio transmitting SSB?

A radio that won't function on USB mode suggests that a crystal used for that mode alone may be the problem. If the two side-by-side 7.8 MHz carrier crystals are in sockets, you can swap them to see if this restores USB and causes AM and LSB to go dead.

There are other possible causes, but this test can be done without test equipment or a soldering iron.

73

 

[EA5GU]

Thank you for your quick answer.
It receives well in USB and LSB, but the modulation does not go well, only noise in both modes, in FM and AM TX and perfect RX, it is as if the modulator in SSB does not work well but I do not know how to adjust it.

Thank you

 

[nomadradio]

First, I want to thank you for including the number of the circuit board, PC406. This is usually a big help.

Just one problem. That number does not show up on the info I have for the original 1977 President Grant 40-channel radio. That radio has a 4-pin mike socket. The later version from the early 1980s has a 5-pin mike socket.

I'm fairly sure the one you have uses the large uPD858 PLL chip. This would make this radio about 40 years old. It's very common to find failed electrolytic capacitors in a radio that old. They can cause odd failures like this.

I would first check C59, a 47uf electrolytic cap. If it shorts, this will disable the SSB-only mike-audio amplifier TR16. Would not disturb other transmit functions. A voltmeter on the positive side of this cap should show around 7 Volts DC while in transmit for either USB or LSB, but NOT for AM.

Also, C58, a 1uf electrolytic cap will cause this if it becomes an open circuit. If it does, the mike audio from TR16 won't reach the SSB modulator chip IC2.

A radio this old is a candidate for having ALL the electrolytic capacitors replaced. Common name for this is to "re-cap" the radio.

I don't recommend this unless you're fairly confident about a project that ambitious. Eventually these radios will become old enough to make that remedy necessary, rather than just desirable.

73

 

[EA5GU]

Thank you very much for answering.
This radio has 5 pins in the microphone, uses the PLL MB8719.
I'm going to review the electrolytic capacitors.
I'll tell you.
Thank you
73

 

[EA5GU]

FYI
Hi guys.
I found in the network the scheme of the station, it is a Cobra 2000 gtl, the motherboard is the same. This is something else.
TNX

 

[nomadradio]

This circuit board is similar but not quite the same as the Cobra 2000GTL and original 1979 Cobra 148GTL.

Your radio has the double-conversion receiver for AM, but the PLL section is not the same as the 2000 and original 148.

Sams volume CB221 is where this radio can be found. Sams will sell you a reprint. They are aggressive about their copyright, so getting someone to make you a free copy may require some effort. And the quality may or may not be all that good.

I would first check VR11 located at the rear edge of the circuit board, about an inch to the right of the final transistor. If it gets turned too far, this can kill the SSB transmit.

It should be set to keep voice peaks just below the clipping level.

This radio is nearing 40 years old, so expect some of the electrolytic capacitors to fail soon if they haven't already.

Just can't identify one of those that would kill ONLY the SSB transmit without also affecting AM as well.

73