Crown D-75 Amp

[Army25R]

I have a question about Crown D-75 Amps. I have two of them and they both have lost audio. Someone suggested that it could be the IC chips in the audio path. I have replaced both of them without any success. There is an orange/red light near the power swith that I am not sure if it is a power light or an error light. I have checked the fuse already and it is not blown. Has anyone had this issue before and been able to fix it?

[indianajo]

get a DVM multimeter. I find non-autoranging easier to read without all the jumping around while to try to get the probe on the pin. Take off all your jewelry. Put meter on 200V scale. Look at the IC diagram you can download from datasheetcatalog.com. Figure out which are the power supply pins. Being careful not to short pins to other pins, check the two power supplies. If you go from + supply to - supply, correct answer is sum of the two numbers. Wrong answer, debug the power supply. Common problems are outdated electrolytic capacitors (over 20 years old high quality, over 5 years old generic quality) and bad solder joints. aikenamps.com has some useful basic info on line. For basic debug techniques,like how to use a meter and what the readings mean in transistor circuits, I like the community college text "Electronic Devices, Electron Flow Version" Thomas L. Floyd, Prentice Hall publishing ISBN 0-13-363599-6. I got mine for $2 at a charity resale shop because it is obsolete. Your local community college bookstore might have something just as good. A lot of devices have been invented since I took physics lab in 1969. Good luck.

[djk]

Check the zener diodes that regulate the supplies for the opamp.

Also check the input coupling cap, and the cap in the feedback loop to ground.

[Evenharmonics]

I too have Crown D-75 and am wondering if it's defective or not if you don't mind me slightly derailing this thread. The sound comes out but the amp itself (not from speakers) has a slight hum. Is it normal for this amp? Also, it produces "pop" from the speakers couple seconds after turned off. Is it also normal for this amp?

[Frank Berry]

Don't know about the slight hum. Possibly a loose power transformer mount.
It's normal for the amplifier to "pop" when it's turned off.

[Evenharmonics]

Originally Posted by Frank Berry

Don't know about the slight hum. Possibly a loose power transformer mount.

I traced the hum with my ear and it comes from the left side of amp which is where the circuit board is mounted and the opposite side of PT.