02-19-2014, 07:36 AM
A few weeks ago I picked up this Fisher 4400 receiver:
[Image: http://www.philcoradio.com/images/phorum...r_4400.jpg]
It is a very unusual model - there is basically no info on it available online, not even at AudioKarma.
The right channel was dead, but the left worked fine.
After replacing some transistors in the right channel (many thanks, w4rtc
), I now have sound in the right channel but it is very low - if I crank the volume to max, the right channel is about at normal volume.
Here is a partial schematic showing the 4400's audio circuitry.
[Image: http://www.philcoradio.com/images/phorum...io_Amp.jpg]
I bought a transistor tester, and it confirmed that the original driver transistor was bad. According to the tester, the replacement GE-76 output transistors are good, as is the replacement GE-3 driver. So now I am looking at something else that is pulling down the 24 volts (shown as 22.5 volts on the partial schematic above). I arrived at the 24 volt figure by taking several voltage measurements in both channels and comparing the results of the working left channel to the non-working right - the left channel has 24 volts where 22.5 is shown at the outputs; the right channel has 19 volts. I have 45 volts where the schematic shows 46/46.2 volts.
I am suspecting capacitor C20 on the schematic (1500 uF, 50V) between the right channel output transistors and the right speaker output as leakage in this capacitor would pull the +24 volts down.
Any thoughts, comments, second guesses, etc.?
[Image: http://www.philcoradio.com/images/phorum...r_4400.jpg]
It is a very unusual model - there is basically no info on it available online, not even at AudioKarma.
The right channel was dead, but the left worked fine.
After replacing some transistors in the right channel (many thanks, w4rtc
), I now have sound in the right channel but it is very low - if I crank the volume to max, the right channel is about at normal volume.Here is a partial schematic showing the 4400's audio circuitry.
[Image: http://www.philcoradio.com/images/phorum...io_Amp.jpg]
I bought a transistor tester, and it confirmed that the original driver transistor was bad. According to the tester, the replacement GE-76 output transistors are good, as is the replacement GE-3 driver. So now I am looking at something else that is pulling down the 24 volts (shown as 22.5 volts on the partial schematic above). I arrived at the 24 volt figure by taking several voltage measurements in both channels and comparing the results of the working left channel to the non-working right - the left channel has 24 volts where 22.5 is shown at the outputs; the right channel has 19 volts. I have 45 volts where the schematic shows 46/46.2 volts.
I am suspecting capacitor C20 on the schematic (1500 uF, 50V) between the right channel output transistors and the right speaker output as leakage in this capacitor would pull the +24 volts down.
Any thoughts, comments, second guesses, etc.?
--
Ron Ramirez
Ferdinand IN
