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Info on "page 75" by electronics unlimited
- Thread starter fastfreddie1269
- Start date
It is the same amp as many others, that use a different name.
Attachments
the pre-amp is burned out since somebody connected the RF out
from their radio to the "antenna" of that amp to see if it works
like that. it did not work, so then they started flipping switches,
and it still did not work. so then they swapped the coax cables
on the back of that amp to see if it would work that way.
now how many times have I seen this...
from their radio to the "antenna" of that amp to see if it works
like that. it did not work, so then they started flipping switches,
and it still did not work. so then they swapped the coax cables
on the back of that amp to see if it would work that way.
now how many times have I seen this...
Thanks for your response. Do you know which parts are most likely bad then? Or how to go about repairing it?the pre-amp is burned out since somebody connected the RF out
from their radio to the "antenna" of that amp to see if it works
like that. it did not work, so then they started flipping switches,
and it still did not work. so then they swapped the coax cables
on the back of that amp to see if it would work that way.
now how many times have I seen this...
It could be the preamp transistor or a relay You need to figure that out. If you looking at the amp from the front, the preamp relay is on the left and you'll see a small transistor and a metal can that is your preamp transistor.
I am just joking, and assuming this. but most likely,
that is what happened.
to burn that
RX pre-amp transistor out.
that is what happened.
to burn that RX pre-amp transistor out.
Biggest single assassin of the preamp is the SSB switch. Sooner or later someone keys it with that switch on. When you key the mike with that switch off, it takes the relay a short moment to wake up and move the contacts. During that moment, your transmit power is being pushed upstream the wrong way into the output of the preamp circuit. A protection diode usually absorbs this safely so long as it's really brief.
The SSB switch delays the relay from dropping out to keep it from chattering while transmitting sideband, since there's no carrier to hold the relay closed. But it also delays the relay's pull-in time. That brief burst of AM carrier is now a lot-longer burst. Usually long enough to fry the preamp transistor fairly soon. The preamp transistor will have a type number marked on it. And if you see a small glass diode next to the preamp transistor, it should get checked, or just replaced along with the transistor. They're cheap.
There's a chance that this is what keeps the preamp from working. Assuming, of course, that the 50 year-old preamp relay is working properly. You can put the amplifier in the coax line with the power off and test each relay by flicking your finger against the side of the relay. You may hear a brief interruption the receiver's noise level. Or it may just cut out and reveal a relay problem.
73
The SSB switch delays the relay from dropping out to keep it from chattering while transmitting sideband, since there's no carrier to hold the relay closed. But it also delays the relay's pull-in time. That brief burst of AM carrier is now a lot-longer burst. Usually long enough to fry the preamp transistor fairly soon. The preamp transistor will have a type number marked on it. And if you see a small glass diode next to the preamp transistor, it should get checked, or just replaced along with the transistor. They're cheap.
There's a chance that this is what keeps the preamp from working. Assuming, of course, that the 50 year-old preamp relay is working properly. You can put the amplifier in the coax line with the power off and test each relay by flicking your finger against the side of the relay. You may hear a brief interruption the receiver's noise level. Or it may just cut out and reveal a relay problem.
73
Some variants of that amp, have the ability to turn the preamp on, when the power is off. Some don't.
Usually requires a third relay. It gets used to sense the radio's carrier, and either tickles the antenna changeover relay for transmit, or unkeys the preamp relay when the preamp is used by itself.
Unless you're Texas Star. They do it with 4-pole switches for the power and preamp switch. Just one drawback. You can't use a remote standby switch.
73
I have two of these that preamp works with power off. One is the DSS, the other is Firebird. Both have two DPDT relays.