Couple Spectra Questions

[Forts]

I recently picked up an analog VHF Spectra for use strictly as a receiver. Once I hooked everything up and programmed the radio I noticed there is a low, but noticeable hum in the background whenever the radio is receiving (reminds me of listening to an old AM radio).. Is this, or could this be, a symptom of bad caps that these radios are notorious for? Which brings me to my next question.. I was doing some reading and one post mentioned the radio would need a realignment after being recapped. Is this really the case? I have no problem recapping the radio myself, but I certainly don't have the tools to perform a realignment.

Thanks!

 

[gary123]

yes the symptom your hearing is a because of the caps. Some of the defective caps are in the audio section. Others are in the PA section.

I have looked at the diagrams for the radios and believe that realignment is not nessessary if you replace the PA caps. I think the only reason that alignment may be nessessary in some radios is because the radio may have been retuned to compensate for the failing parts.

 

[Forts]

Great, thanks for your info. I've opened the radio up and can't see any leakage from any of the caps thankfully. I think I will go ahead and do the replacement and keeps my fingers crossed that it won't need to be aligned afterwards.

 

[Forts]

Well I went ahead and pulled the caps from the RF board and the Command board. Once I got the caps off there was visible signs of light leakage from most of them (and the lovely stench you get once the hot iron hits them). Changed them out and the hum is now completely gone. I did have to use some 50v caps in place of 16v for the time being on the RF board and now I get a slight... almost squeal.. when a channel first comes active. I'm assuming this is due to the larger caps, and I will swap them back out with the proper values asap.

 

[gary123]

The voltage of the caps shouldnt make a differance. Its the value thats important. Double check your soldering etc. some of the traces are small ans close together. It is possible that the board coating arrond the changed caps has 'fallen' off and you have a low level short, either from soldering or the componant is touching something it shouldnt.

 

[Forts]

I wouldn't expect the voltage to have made a difference either but I wasn't getting that noise beforehand. Anyhow, I stopped by a local shop today and picked up the proper values. Swapped those caps in and wouldn't you know it, the damn hum came back haha. However, once I take the radio out to the car and run it on 'real' DC it's clean as a whistle. Must have some dirty AC on the power supply in my office. I also figured I may as well change the caps in the control head as well (even though the display was very bright and showed no signs of trouble). Glad it did as both caps were definitely leaking.

 

[cmdrwill]

Did you replace the 10 uF cap in the power amplifier? I use a mil spec 100 uF 25 volt radial leaded cap there, less chance of noise on the DC input.

 

[Forts]

Yes I did.... I replaced it with the same value though (10uF 50v).