Question about testing preamp/power amp

[waggs098]

I'm going to try again at troubleshooting a Quadraflex 650 I've had for a while. Has distortion when playing. I want to test the preamp first to see if the issue is there. My question is can I just unsolder the input wires to the amp section and solder on rca's to connect to a stand alone power amp?

Then the opposite can I solder rca's to the input of the power amp and connect a stand alone preamp to the power amp of the quad to test it?

 

[FileFixer]

I dont know how is preamp an amp connected between these two circuit but from your words it seems that preamp and amp inside receiver have own separate board and between these board is signal cable?

 

[waggs098]

I dont know how is preamp an amp connected between these two circuit but from your words it seems that preamp and amp inside receiver have own separate board and between these board is signal cable?

It looks to me like there are just positive and negative wires for each channel going to the power amp. Most of the other wires go to the power supply section.

There are also 2 other wires that are farther into the power amp section that go to a board that I don't know what it's for. That small board is connected to the tape monitor board and the tuner board.

 

[petehall347]

you will most likely need to add a coupling cap to block any dc .

 

[petehall347]

is it both channels ? if so power supply is common .. tape monitor switches are always suspect along with all other switches .

 

[FileFixer]

This is same problem what you have... Solved... http://www.diyaudio.com/forums/solid-state/100665-fixed-quadraflex-650fet-receiver.html

 

[waggs098]

is it both channels ? if so power supply is common .. tape monitor switches are always suspect along with all other switches .

Yes distortion is in both channels if I recall correctly. The biggest issue is no service manuals available for this unit. I'm going to hook it up again and test it so I can be sure it's in both channels. I'll report back shortly.

 

[waggs098]

Ok now that I have refreshed my memory I went farther into troubleshooting this than I thought. I have replaced some burned resistors on the amp board and some film caps that were by those resistors that got to hot.

Now that I have tested it again it definitely needs pots and switches cleaned but the issue is a constant hum in both speakers that is not volume controlled. As pete said I expect a power supply issue. Maybe the filter caps? Should I replace those next?

Also this unit does have mosfet finals in it. With no service manual can anyone tell me how to properly set bias for these transistors?

 

[petehall347]

some nice pics might help here and pointing out burned resistors .
if its anything like my mosfet amp it runs hot . 400w . 350ma idle .

 

[waggs098]

some nice pics might help here and pointing out burned resistors .
if its anything like my mosfet amp it runs hot . 400w . 350ma idle .

Ok here is an old pic of the amp board. The resistors and caps I replaced are circled in red. Just to note this receiver is rated at 65 watts I believe.