VHF Spectra Receiver dead

[k4tdo]

Hey fellow Motorola users. I have on my bench a VHF 146-170 45 watt spectra. The raddio worked fine a week ago. Today the receiver is dead. I dont even get a busy light on the front of the radio injecting an on freq. signal full tilt form my Service Monitor. I get the volume and channel change beeps and tones, just no receive activity at all. Suggestions?...Thanks Tim

[Cowboy]

Hey fellow Motorola users. I have on my bench a VHF 146-170 45 watt spectra. The raddio worked fine a week ago. Today the receiver is dead. I dont even get a busy light on the front of the radio injecting an on freq. signal full tilt form my Service Monitor. I get the volume and channel change beeps and tones, just no receive activity at all. Suggestions?...Thanks Tim

Have you tried reprogramming the drawer?

[kc7gr]

And have you...

--Checked for the infamous electrolytic cap leakage problem?

--Checked the PIN diodes in the antenna switch network?

--Ramped up your signal generator until the thing DOES receive? (or until the generator doesn't go any higher than, say, 0dBm?)

Happy tweaking.

[Zack08]

Also, you could remove the RF boards and reinstall them. I was helping somebody fix a spectra and this was all he needed to do to fix a FAIL001. The pins must have oxidized and were no longer making good contact.

[Will]

Receiver dead, no busy LED with strong signal.

Common problem. Probably too late as the leaking capacitors have already done their damage.

I am able to repair most of them. Then, I work on lots of Spectras.

Bruce, even with bad PIN diodes in the antenna switch, most Spectras can still hear a 'wide open' signal generator.
I agree, " the infamous electrolytic cap leakage problem" will "kill" a receiver, or make it really sick.

[kc7gr]

(snippage)

Bruce, even with bad PIN diodes in the antenna switch, most Spectras can still hear a 'wide open' signal generator.
I agree, " the infamous electrolytic cap leakage problem" will "kill" a receiver, or make it really sick.

(hitting self on head) I actually knew that. What I didn't realize was that I hadn't read through to the part where he said he already ramped up his generator.

This is what I get for speed-reading first thing in the morning.

Happy tweaking.

[Will]

OK.

(hitting self on head) I actually knew that. What I didn't realize was that I hadn't read through to the part where he said he already ramped up his generator.

This is what I get for speed-reading first thing in the morning.

[Cowboy]

(snippage)

Bruce, even with bad PIN diodes in the antenna switch, most Spectras can still hear a 'wide open' signal generator.
I agree, " the infamous electrolytic cap leakage problem" will "kill" a receiver, or make it really sick.

(hitting self on head) I actually knew that. What I didn't realize was that I hadn't read through to the part where he said he already ramped up his generator.

This is what I get for speed-reading first thing in the morning.

Happy tweaking.

Keep talking gents - the information you two provide, along with the info from the many other resident gurus, is simply invaluable

[kcbooboo]

A friend had a "dead" UHF receiver. Wouldn't hear a thing, but it had squelch noise and control head beeps just like yours. We swapped front ends; nothing. We even injected a signal directly into the RF board, bypassing the front end; nothing. Somehow we tried one of the IF freqs and it heard that, so we backed up a bit ahd checked the 2nd oscillator. Found it was running near 100 MHz but it wasn't stable, nor was it on the right frequency. We then swept the input signal over a wide range and the radio heard it somewhere with proper sensitivity. So it wasn't really dead, just way off frequency.

To make a long story short, there's one, and only one, physical potentiometer in the whole radio, and it controls the 2nd oscillator frequency which is phase-locked to the main oscillator. That pot went bad. It's very tiny, surface-mount, and soldered under itself, making it hard to replace. My friend ended up extracting it and replacing it with a pair of fixed resistors. Once it locked up, the radio started receiving just fine again.

The service manual makes no mention of this pot, but it does explain the 2nd oscillator circuit slightly. There's more information about it elsewhere on the web, but as it's already in print, I won't waste the bandwidth by posting it here.

Bob M.

[jackhackett]

there's one, and only one, physical potentiometer in the
whole radio, and it controls the 2nd oscillator frequency which is
phase-locked to the main oscillator. That pot went bad.
Bob M.

IIRC that pot actually adjusts the negative bias voltage to the VCO, in any case it is in an area of the board that is frequently affected by the leaking caps. There are also some chip resistors and some plated through holes in the board right near the pot that are often damaged.

[k4tdo]

OK Guys and Gals......I have this sick radio on the bench......
The Transmitter makes full power and is dead on Freq. Has good audio etc......
I have swept the receiver about 10 mg either side of my programmed RX freq. Srtill no RX. Not even a busy light.
I have inspected the 2nd oscillator pot mentioned above. I can not see anything wrong with it. Adjustments of it make no difference to the radio.

Now what?

[racerman1cars]

Like Bruce & Will said probably Caps..