Help with CR-620 Blowing Main AC Fuse - Outputs Good

[clk1981]

I have a beautiful CR-620 that HAD bad outputs. I replaced all 4 outputs with MJ21193/94 and recapped the power supply section. I used a dim bulb and powered it on. The unit was repaired and worked well for a few weeks...

After running it for a few weeks, the main AC fuse (3.0A 250V) blew.

Hoping it was a fluke or a surge, I replaced the fuse and brought it up on the DBT and the bulb was glowing full force. After some troubleshooting, I found that if I removed both outputs for the RIGHT channel, the unit powers on, and I get a relay click. However both of the output transistors for the right channel test fine, and I even swapped them out with a different working pair, but the unit will still glow the bulb when the right channel outputs are installed.

Any ideas of even where to begin to start would be greatly appreciated!!

 

[avionic]

Probably another shorted component in the right channel. Start checking transistors and diodes. Starting with the output drivers and work your way back to the front-end of that amplifier.

 

[clk1981]

Probably another shorted component in the right channel. Start checking transistors and diodes. Starting with the output drivers and work your way back to the front-end of that amplifier.

Thank you for the reply. I guess what I am confused about is if there were shorted components other than the outputs, how does the unit power on and give a relay click with just the outputs removed? Wouldn't the other shorted components cause the bulb to dim?

 

[avionic]

how does the unit power on and give a relay click with just the outputs removed

The short /malfunction is holding the output transistor at rail voltage. With the outputs removed measure the voltage at the output transistors base connection at R750 and 752 on the circuit board. Should only see around a half a volt ie. 500mv

 

[clk1981]

The short /malfunction is holding the output transistor at rail voltage. With the outputs removed measure the voltage at the output transistors base connection at R750 and 752 on the circuit board. Should only see around a half a volt ie. 500mv

Ok thank you so much. I won't be able to get back to this until monday but I will do what you suggested and report back. Thanks!

 

[avionic]

how does the unit power on and give a relay click with just the outputs removed?

When outputs are held at rail. The outputs are being held in a "shorted" condition with reference to ground. By removing the output transistors. You removed the shorted to ground condition. In other words, the output transistor(s) are being switched on and the rail voltage at the collector is passing through the transistor to the emitter straight to the output of the amplifier. Where the protect circuit is detecting the DC voltage and preventing the protect relay from " setting". Removing the output transistors. Removes the DC voltage from the amplfiers output and the protect circuit operates as it should and allows the relay to "set" or "click".

 

[clk1981]

When outputs are held at rail. The outputs are being held in a "shorted" condition with reference to ground. By removing the output transistors. You removed the shorted to ground condition. In other words, the output transistor(s) are being switched on and the rail voltage at the collector is passing through the transistor to the emitter straight to the output of the amplifier. Where the protect circuit is detecting the DC voltage and preventing the protect relay from " setting". Removing the output transistors. Removes the DC voltage from the amplfiers output and the protect circuit operates as it should and allows the relay to "set" or "click".

Ok, I tested voltage on what I believe was R750 and R752 (service manual is blurry and the board is not labeled) and it looks like I had .3V

I did find that with the outputs taken out of the right channel, the socket for the NPN output had these voltages (40V, -17.6V, .1V) and the PNP output had (-40V, -40V, .1) Something seems off there but I am not sure where to look.

 

[avionic]

(-40V, -40V, .1)

Theres a problem..

 

[avionic]

Check the voltages around TR720 and 722 driver transistors.Please label voltages ie.

E--xx
C--xx
B--xx

Won't hurt to pull these two transistors and test them as well.

 

[clk1981]

Doing a diode check on my meter on TR722, I was getting some funky readings. .007V drop and 2.2V drop on the two junctions. I pulled TR722 and it oddly enough tested as a good PNP transistor. Weird. I went ahead and replaced it, and now the unit is powering on and working fine with the outputs back in (of course I DBT first). Thank you so much for your assistance in this!!!

TR722 was the culprit!!!

 

[avionic]

Just because it tests OK. Doesn't mean it will perform properly under load. Glad you got it squared away.