I'm having some difficulty with a Fosgate 500m I recently purchased. I knew it was bad, but I have fixed at least 100 Fosgate amps over the years. This is the first 500m I've had on the bench, so I attacked it like everything else. I have replaced:
All source resistors
All gate resistors
TL072 opamps in the gain stage
Q301, 302, 302, 304, 305, 306, 307, 308,
Q106, 107, 108, 109, 110
All electrolytics
All P and N channel output FETs
I can set bias, and it will play at low volume cleanly. Once any power is called for it will let a pop out through the speaker, and open up most of the gate resistors on the P channel FETs.
This amp came with IRF640 and 9640 FETs, and 10 ohm gate resistors. All appeared OEM. The schematic calls out 100 ohm gate resistors.
Due to IRFP640/9640 supply issues right now, I substituted FQP12P20 for the P channel, and FQP19N20 for the N channel outputs, using the original 10 ohm gate resistor value.
Could this just be an incompatible FET, or should I try installing 100 ohm gate resistors per schematic and see how it goes?
All source resistors
All gate resistors
TL072 opamps in the gain stage
Q301, 302, 302, 304, 305, 306, 307, 308,
Q106, 107, 108, 109, 110
All electrolytics
All P and N channel output FETs
I can set bias, and it will play at low volume cleanly. Once any power is called for it will let a pop out through the speaker, and open up most of the gate resistors on the P channel FETs.
This amp came with IRF640 and 9640 FETs, and 10 ohm gate resistors. All appeared OEM. The schematic calls out 100 ohm gate resistors.
Due to IRFP640/9640 supply issues right now, I substituted FQP12P20 for the P channel, and FQP19N20 for the N channel outputs, using the original 10 ohm gate resistor value.
Could this just be an incompatible FET, or should I try installing 100 ohm gate resistors per schematic and see how it goes?
Rockford amps can be sensitive to FET changes. For example, in the amps that use the IRF540, the IRF540N will cause strange problems, including problems like you're experiencing.
If the gate resistors are opening, it's very likely that the FET has failed, even if not shorted D-S.
A similar amp that I have photos of uses 100 ohm gate resistors.
https://www.newark.com/vishay/irf640pbf/n-channel-mosfet-200v-18a-to-220/dp/63J7355?CMP=AFC-OP
https://www.newark.com/vishay/irf9640pbf/p-channel-mosfet-200v-11a-to-220ab/dp/63J7436?st=irf9640
Datasheets, Electronic Parts, Components, Search - Octopart
If the gate resistors are opening, it's very likely that the FET has failed, even if not shorted D-S.
A similar amp that I have photos of uses 100 ohm gate resistors.
https://www.newark.com/vishay/irf640pbf/n-channel-mosfet-200v-18a-to-220/dp/63J7355?CMP=AFC-OP
https://www.newark.com/vishay/irf9640pbf/p-channel-mosfet-200v-11a-to-220ab/dp/63J7436?st=irf9640
Datasheets, Electronic Parts, Components, Search - Octopart
Last edited:
I have personally experienced the 540 to 540N issues. It does feel familiar. What's really interesting is that when removed, the P channel FETs still test good in my component tester. I can replace the gate resistors, and repeat the failure. I have seven new 9640s with the same lot code. I'll just have to pay the premium at Newark for the 640s. I think the best path forward would be to install 100 ohm gate resistors to match your pic and the schematic I have. Thanks Perry.
With a multimeter, what's the resistance between the gate and other terminals of the FETs that blow the 10 ohm resistors?
Will the FETs blow (or blow the resistors) if you drive the output to near clipping with no load?
Will the FETs blow (or blow the resistors) if you drive the output to near clipping with no load?
Q112 through Q124 all exhibit roughly 26kohms between the gate and drain, and 8kohms from drain to source with the red lead from the meter on the drain. Fluke 189 DMM. The gate resistors read as follows:
R152 - 49.52
R158 - 10.15
R164 - 10.76
R170 - 20.29
R176 - 10.12
R182 - 10.16
R188 - 15.56
These were new resistors just installed prior to the last test. It let out one pop before I shut it down. I can't turn it up enough to drive the output to near clipping. It will pop and damage the gate resistors at very low volume. I have gotten it to make the same pop probing the 337 regulator when testing the +/- voltage for the opamps. I will try a test run with no speaker connected tomorrow. I found two new IRF640s in my stash. I will probably install those two, and a corresponding two 9640s with 100 ohm gate resistors tomorrow and see what happens. I have attached the schematic I am referencing.
R152 - 49.52
R158 - 10.15
R164 - 10.76
R170 - 20.29
R176 - 10.12
R182 - 10.16
R188 - 15.56
These were new resistors just installed prior to the last test. It let out one pop before I shut it down. I can't turn it up enough to drive the output to near clipping. It will pop and damage the gate resistors at very low volume. I have gotten it to make the same pop probing the 337 regulator when testing the +/- voltage for the opamps. I will try a test run with no speaker connected tomorrow. I found two new IRF640s in my stash. I will probably install those two, and a corresponding two 9640s with 100 ohm gate resistors tomorrow and see what happens. I have attached the schematic I am referencing.
Last edited:
You need to isolate each gate by either removing the gate resistors or by lifting the gate leg clear of the board to know what each FET gate reads. Then check the resistance gate to drain and gate to source. All should read OL on ohms.
I'm assuming that you measure the gate resistors out of the circuit.
I'm assuming that you measure the gate resistors out of the circuit.
The P channel FETs are 3.05megohms drain to source with the red lead on the drain. The N channel FETs are 2.7megohms drain to source with the black lead on the drain. All FETs are OL from the gate to drain and source. All FETs also test good on my component tester. Gate resistors were measured out of circuit.
This appears to be another case of incompatible substitution. I have two IRF640 and two IRF9640 installed with 100 ohm gate resistors, and no instability issues are present. I'm ordering 10 of the 640s from Newark today, and hopefully I get at least 7 from the same lot. I'll update when all 14 drivers are installed and the amp is either functional or not.
This appears to be another case of incompatible substitution. I have two IRF640 and two IRF9640 installed with 100 ohm gate resistors, and no instability issues are present. I'm ordering 10 of the 640s from Newark today, and hopefully I get at least 7 from the same lot. I'll update when all 14 drivers are installed and the amp is either functional or not.
Last edited:
I've never done this with these amps (speaker tapped from transformer) but in the larger Rockford amps of this vintage, I jump the delay transistor collector to emitter to eliminate the delay.
With the Rockford amps, they charge the rail caps then enable the output. If there is a problem, the outputs are sometime forced to dump all of that energy which can cause them to fail, often one after another when trying to troubleshoot.
Eliminating the delay allows you to use a current limiter to see if there is a problem well before the caps fully charge.
With the Rockford amps, they charge the rail caps then enable the output. If there is a problem, the outputs are sometime forced to dump all of that energy which can cause them to fail, often one after another when trying to troubleshoot.
Eliminating the delay allows you to use a current limiter to see if there is a problem well before the caps fully charge.
Definitely an incompatible substitute MOSFET in this case. 7 new IRF640 and 7 new IRF9640 with 100 ohm gate resistors per schematic and it is up and running properly. Thank you for your help Perry.