VHF Spectra output drops slowly

[kj6ko]

Hello,
I have a D43 Spectra that slowly drops output power from about 30W to 5W in about 1min. Have replaced the PA, VCO and RF boards with no change. All voltages on the PA are steady, including the 9V on the driver chip. Don't have a spare working command board. Changed all bad caps out also. Everything else works fine on the radio.
Parts radio now??

Any thoughts??

Thanks

[Jim202]

You either have a bad votmeter or are not telling us something.

If the PA has been changed out, the only other thing that could cause a drop in power
is that the high current +12 volt feed to the radio is dropping down.

You might tell us what kind of an RF load your using. There is a slight chance that
the antenna output load is changing with time. Last thought is what ever your using
for measuring the RF output power is not telling you the truth.

Jim

Hello,
I have a D43 Spectra that slowly drops output power from about 30W to 5W in about 1min. Have replaced the PA, VCO and RF boards with no change. All voltages on the PA are steady, including the 9V on the driver chip. Don't have a spare working command board. Changed all bad caps out also. Everything else works fine on the radio.
Parts radio now??

Any thoughts??

Thanks

[kj6ko]

Sorry, but it's not the voltmeter, load or wattmeter. Believe me, I've built PA's for a living and am aware of all the problems you describe, but just for the record, used both a Bird43 and an HP 8920 and they jive. Used the load in the 8920 and TWO separate loads on the Bird43. Same meters and loads on other VHF, UHF and 900 radios work perfectly. The only thing I didn't do was check to see if the current draw drops with the power output, but judging by the temp of the heatsink,(none), there is no current draw. VCC on both devices during TX remained at 13.6V and the driver chip starts out at 9.6V and within 1 second, drops to 9.1V and stays steady. I may try cool spray to see if it is something in the ,or both, PA's but I'm ready to toss them in the "parts" box. I thought there was something else I could check like the VCO output maybe dropping?? I've spent way too much time on this radio for what I can sell it for, but I went this far, may as well try!

Greg

[Bill_G]

I'm guessing you'll find a corroded trace or plated feedthrough on the cmd brd from some old cap damage. Check near the large 10uf and you may have to pull the board out to look at both sides.

[Jim202]

Sorry for the questions. You have to understand that many of the questions that show
up here are from people that have minimal background and little or nothing for test
equipment. The info they provide kind of puts your answers in a testy position of just
how to pry the needed info out to even try to give a clue where to look without putting
them on the defensive mode.

I tend to agree with Bill, that there may be a poor connection. Trying to locate the
area of cause may take some careful snooping. My tact would be to use an insulated
item like a plastic tunning tool or a wooden stick. Start by pressing gently all around the
PA deck. Press on the different components one at a time. Something has to have a
poor connection or a bad part.

I use to die on the old GE Delta-s and Delta-SX series radios that had bad transistors.
At least they would give some hint of the problem. They used the large low Ohmage
resistors to help balance the power. If you had a bad transistor, the resistor would
be burnt. With the Spectra, you still could have a bad transistor. It might take an
RF probe to prove a bad transistor.

Good luck in the trouble shooting.

Jim

Sorry, but it's not the voltmeter, load or wattmeter. Believe me, I've built PA's for a living and am aware of all the problems you describe, but just for the record, used both a Bird43 and an HP 8920 and they jive. Used the load in the 8920 and TWO separate loads on the Bird43. Same meters and loads on other VHF, UHF and 900 radios work perfectly. The only thing I didn't do was check to see if the current draw drops with the power output, but judging by the temp of the heatsink,(none), there is no current draw. VCC on both devices during TX remained at 13.6V and the driver chip starts out at 9.6V and within 1 second, drops to 9.1V and stays steady. I may try cool spray to see if it is something in the ,or both, PA's but I'm ready to toss them in the "parts" box. I thought there was something else I could check like the VCO output maybe dropping?? I've spent way too much time on this radio for what I can sell it for, but I went this far, may as well try!

Greg

[Bill_G]

He says he's swapped out the rfpa, rf, and the vco brds. It sounds like a pwr set problem which would be the cmd brd. I haven't had one where the drive reduces over time, but it should be measurable on one of the three xters towards the back. It is possible his spare rfpa is bad, but he would know that. My long distance troubleshooting crystal ball says bad trace / bad power ctrl chip on the cmd brd.

[jackhackett]

I don't think you can tell much from measuring the voltage on the transistor collectors, they don't use those voltages to control the power. The control voltage varies the bias on the base of the Low Level Amplifier stage (the 8 pin chip type transistor) so you'd need to see if that voltage is dropping. You can find the control voltage on pin 3 of the 12 pin connector... on the top row pin 1 should be missing, the next one over is 3.

I'd tend to agree with the others about damage from leakage on the control board, especially since the large cap is right next to the power control chip. Sometimes just the residue from the caps is enough to cause problems as it can be conductive, make sure the board is cleaned off well.

[Will]

Jack and Bill are close on to the problem. This is not uncommon in Spectras and is related to leaking capacitors.


One question, did the OP replace the PA and heatsink as one piece? Or just the board?