Separate names with a comma.
Discussion in 'CB and Export Equipment and Accessories' started by gogo_gadgit, Nov 10, 2010.
there isn't a relay in the rig, so what else can I check or replace,
To determine if the chip is working check pin 5 to see if the control voltage changes from RX to TX. If not you probably have a shorted cap on the control line. If the control line pin 5 changes and pin 8 doesn't have 8 volts on it in TX, lift the pin from the board and check the pin again. If you still have no 8 volts on the pin in TX the chip is probably dead. If there is 8 volts, you may have a short on the load side of the chip. Good chance it's a cap. They are getting old.
I take it the receiver is working? Just want transmit....
Lets take the easy route first. Check pin 8 of IC 5. This is your 8 volt regulator. You should have 8 volts there.
If the receiver is working then the TX supply should be ok, but we need to see if TR37 is pulling pin 2 of IC 5 low to switch it in TX mode.
The emitter of TR37 is the tx line to the mic plug.
I guess I did not word that good enough. If the circuit of IC 5 is bad, then when TR37 is switched to transmit, pin 2 will go low if there is a shorted circuit. If pin 2 (supply) goes low, probably a bad cap in the T/R switching circuit or an internal short in the IC itself.
you can confirm this by measuring the voltage on pin 9 of the PLL. it should read about 8 volts, but yours probably wont.
best not to operate the radio or leave it on longer than needed to do this quick test, as you could cause further damage to the radio.
if you measured the voltages on the MB3756 voltage regulator chip and found that they were off, you might want to check the electrolytic capacitors immediately surrounding the chip to make sure none of them are shorted and causing the bad voltage readings.
another test you can do when you find a pin on the chip with a weird voltage reading is to unsolder just that pin from the board and check the voltage on the pin with it disconnected.
if the voltage now reads normal, you know that the problem is external to the chip and is along the path connected to that pin.
do this with each pin with an out of spec voltage reading.
if the voltage on the pin(s) is still weird with it disconnected, then the chip is probably bad and needs replacing.
if you have trouble finding an MB3756 chip, NTE makes a sub, the NTE1271.
those can be real headaches.
best bet is to put things back to stock one by one and check operation at every step.