Spectra Squelch issues -- can any one help?

[swannie]

Hello,

I have a Dash mount VHF Spectra that I'm having issues with, and I'm not sure what else to do/try with it. It behaves like it has a loose squelch, and it's not really that sensitive. I hooked it up to my signal generator at home and it will start to break the squelch open right at -127dBm even though there is almost no usable signal at that level. In order to get it up to 12dB SINAD I need to pump the signal all the way up to -103dBm, I'm not an expert, but I'd think with a good Spectra I'd be getting 12dB SINAD at -120dBm or better?

Soooo... I appear to have two issues here, the loose squelch, and the lack of sensitivity -- and I'm not sure if they're related somehow? Can anyone point me in the right direction to try and troubleshoot this? Do those two issues point at a particular board being bad?

Before anyone asks, I already went through and replaced all the time-bomb caps, and I didn't notice anything that would indicate damage to any of the boards. Additionally, the radio exhibited this behavior prior to replacing the caps.

Any ideas?

Thanks!!
Brian

[kc7gr]

One thing I've found that can have a drastic effect on receiver sensitivity in the Spectras: The PIN diodes in the PA section, used for switching the antenna from receive to transmit and vice versa. I had a radio come in a while back with similar symptoms to yours. Replacing one of the PIN diodes (which had turned into a dead short) put everything back to normal.

Theory: The ultrasensitive squelch could be related to one or more bad PIN diodes creating all kinds of lovely noise, just from background RF fields being picked up through your antenna.

Happy tweaking.

[swannie]

One thing I've found that can have a drastic effect on receiver sensitivity in the Spectras: The PIN diodes in the PA section, used for switching...

Theory: The ultrasensitive squelch could be related to one or more bad PIN diodes creating all kinds of lovely noise, just from background RF fields being picked up through your antenna.

Cool, thanks for the heads up, I'll take a look at that when I get home tonight. I did a little googling around for how a PIN diode works, and I'm not quite sure how to test it. On a regular diode, I'd run a continuity tester one way expecting nothing to happen, and then reverse it expecting it to beep at me. If I understand how the PIN diode works, however, it sounds like I'd need to pump some RF into it and then put my meter across the PIN diodes to see if they show up as 0 ohms or not. If they show up as 0 ohms, then they are bad? Any advice?

Since the radio is at home, I haven't been able to open it up and look around yet -- any advice as to where the PIN diodes are on this radio, or is it fairly obvious?

Thanks for the reply! This radio had been a thorn in my side for a long time, so I hope this fixes it, otherwise if may go off to the junk pile...

Brian

[jackhackett]

You can test them pretty much the same as a regular diode, low resistance one way, high the other.
You're better off testing diodes with a meter that has a diode test function, that will give you a good idea of the forward voltage drop (around .6V for silicon, .2V for germanium, been a while since I've needed to check a PIN diode, but IIRC they're around .6V too).

You also usually need to take them out of the circuit to test.

[swannie]

Well, I cracked open the PA section and didn't see a big flashing neon sign that said "PIN Diodes HERE", so I poked around hoping to see if something was obvious (burned up, etc). No love.

Is there anyone I could send the radio to for service?

Thanks!
Brian

[Will]

I'll bet there is some hidden capacator leakage dammage in your Spectra. I can repair your Spectra.