old school rockford punch 40dsm messes up fm reception horribly.

[thebestofall007]

hello diyaudio, i was wondering what would cause an old-school rockford punch 40dsm (1992-93 vintage) amplifier to mess up fm reception horribly. i got this amp from a friend who had something wrong with it, where it wouldnt power on because of a blown diode. the good news is that replacing the diode made it work (and BOOOOY does it sound oooh so good), but i have a big problem with the reception. what happens is that when i turn up the volume, static interference from the amplifier gets worse, even on my strong local stations, and does it regardless of what head unit i have it hooked up to. i REALLY want to use this amp but i cant put up with my amplifier wreaking havoc with my radio reception. if this will help, i have included a pic that shows the diode i replaced. the 1n4003 diode (the one labeled "cr7" on the board closest to the power terminals with the burn marks) by the metallic green electrolytic cap is the one that was burnt and i replaced. any info would be great.

thanks

[Perry Babin]

CR7 is supposed to be a 1N5366, not a 1n400x. This 'may' make a difference since the 5366 is used to clamp spikes on the power supply. It's likely that you have other power supply issues. You would need an oscilloscope to determine if the waveforms are OK or excessively noisy.

It's also possible that you have a problem with your antenna cable. It should be shielded which should prevent the amp from causing problems with reception.

[thebestofall007]

thanks a bunch, sir/madam. i didnt know that it needed that kind of diode; it was burnt so bad that i could barely make out the reading. about the antenna cable: i have my pioneer deh-p4900ib stereo installed in a honda crx where i have a very cheap made plexiglass vr3 wally world amp installed in the center console right under the headunit (its a bitch finding good mounting locations for amplifiers in those small cars where i dont have to do any modifications) and i have absolutely no problems with noise. this rockford is an awesome amp and i love how they underrate the watts of this gem; THEY MAKE AMPS LIKE SH** TODAY!! the quality of these amps is the reason why i want to fix it.

[thebestofall007]

you also mentioned oscilloscope. i havent had any experience using one; how would i go about hooking one up to the amp?

[AKHeathen]

Have you tried an amp of similar power level, or checked your grounds yet? Engine compartment as well. Another thing is if it has a toridal or not one of the reasons they switched to torroids is that they don't emit the electrical noise as a traditional... It may be old enough to be non-torroid, in which case you would need to place the amp further away from any antennae run....

[Perry Babin]

I've never seen those diodes blow unless the amp was connected with reverse polarity (which almost always destroys the FETs also) or if the amp had a problem with the power supply.

You use the scope much like you would a voltmeter but the scope can show you things that a multimeter cannot.

If you remove the cover and measure the voltage on the first leg of the power supply FETs, you should read ~5v on each of them. If one is significantly different than the other, that would indicate that there is a problem. Do NOT slip when measuring the voltage and insert a 10 amp fuse in the B+ line feeding the amp to 'help' protect it if you do slip. Place the black meter probe on the amplifier's ground terminal. Place the red meter probe on the point where you need to measure the voltage.

[thebestofall007]

Originally Posted by AKHeathen

Have you tried an amp of similar power level, or checked your grounds yet? Engine compartment as well. Another thing is if it has a toridal or not one of the reasons they switched to torroids is that they don't emit the electrical noise as a traditional... It may be old enough to be non-torroid, in which case you would need to place the amp further away from any antennae run....

the vr3 amplifier i have also uses a torioid as well as the rockford, but the vr3 outputs no noise at all even under the console (as mentioned, its a doosy to find other spots to mount an amp in that small car). the ground is good (wire less than a foot; i put the dremel with a sanding wheel on it to clean off the paint for a good connection).

[thebestofall007]

Originally Posted by Perry Babin

I've never seen those diodes blow unless the amp was connected with reverse polarity (which almost always destroys the FETs also) or if the amp had a problem with the power supply.

You use the scope much like you would a voltmeter but the scope can show you things that a multimeter cannot.

If you remove the cover and measure the voltage on the first leg of the power supply FETs, you should read ~5v on each of them. If one is significantly different than the other, that would indicate that there is a problem. Do NOT slip when measuring the voltage and insert a 10 amp fuse in the B+ line feeding the amp to 'help' protect it if you do slip. Place the black meter probe on the amplifier's ground terminal. Place the red meter probe on the point where you need to measure the voltage.

gotcha. i priced an oscilloscope and it was pretty high and i dont have that kind of money to invest in something i will only use occasionally (i have a digital voltmeter, though). does a bad power supply also run excessively hot; i ask because when i bench tested the amp on an old computer power supply. it barely got warm, even though i cranked the hell out of it with 2 ohms into both channels and bridged to a 4 ohm sub (tri mode). the amp sounded awesome and played stably (and still ran cool) when i hooked it to my ipod or when playing a cd but not the fm, and interference was less on the am, what a surprise (YES i listen to good ol' am too).

[thebestofall007]

which ones are the power supply transistors, the ones to the left or to the right if i was looking at the amp with the power terminals facing me?

[Perry Babin]

Q1 and Q2.