How many DC volts for power supply?

[johnl88]

Hello all,
Im working on a Zenith preamp model 973-4016 its a 1992 so its quit a bit older. I dont know much about this stuff although a know some basics, anyway i would like to know what output voltage DC I should be getting from the OUT TO PREAMP side of the power supply,WITH the preamp connected to it. When the preamp isnt connected i get a few mV's DC,when its connected i get just about one volt,.880V DC, and about 15 v AC. I notice without the preamp the volts AC are about 17 V. The only markings on the power inserter as far as output...is a marking "secondary 19 V AC" and thats it. I had the case off the power inserter to take the readings...the model no. for it is ZP-0316 also a zenith. I would just like to know what the DC volts should be for this so that I can determine better... whether its good or bad. It worked great until we got a heavy rain storm,then it stoped working.
All the uhf channels went completly out then the few vhf channels would slowly weakin over a course of an hour every time we pluged the power supply back in. Its my uncle's preamp and he really likes it and doesn't want to buy a new one so i would like to try and fix the problem IF i can. Thanks so much

 

[KrisBlueNZ]

I did a web search and most of the hits were you asking this same question on Yahoo Answers.

So this is a radio receiver or a preamp or what? Start by uploading some pictures. See https://www.electronicspoint.com/resources/how-to-take-photos-of-circuit-boards.6/ Add markings showing where you measured those voltages.

 

[johnl88]

Hmmm i would think it was obvious when i mentioned that the uhf channels went out that i was refering to tv....and as i mentioned its a preamp. So its an outside tv antenna preamp with power supply. And,with the casing off the power supply, i measured the voltages with the positive lead on the center pin and with the negative lead on sheilded ground side of where the coax comes in...on the inside of the power supply...on the side that supplys power to the preamp.

 

[(*steve*)]

Aaaah, so its a masthead amplifier then?

They are often powered from a low voltage AC source. I was wondering why you'd be measuring AC to a preamp.

The 0.88V DC you see when the power supply is connected is due to the uneven loading caused by (presumably) half wave rectification. If it says 18V and you measure 15, it's probably fine. There may be some series resistance to prevent damage in case the power supply is shorted.

I'd see if the voltage from the power supply drops as the signal fades.

These devices are very easily damaged by lightning (even if it strikes nowhere near your antenna). This on might be trying to tell you to buy a new one.

 

[johnl88]

Ok, thanks so much for your insight there. Couldn't post any pics...compatibility issue with my pc. And i sure couldn't find any schematics online for this old thing. Read a new article that stated that these older preamps made prior to 1993 were rectified to -12 v dc ,the newer ones were +12 v dc....anyway looks like it needs replaced. just have to tell my uncle that he'll just have to wait and get a new one.