Hello everyone!
I recently bought a broken Ampeg Portaflex PF-350 bass amplifier head in unknown condition. As it turns out, it's completely dead. Tried powering it on with the dim bulb tester, dead short.
I have attached the schematic below.
What I found:
TH1 is cracked visibly but measures 6 ohms. Will replace it but should be okay for testing for now.
Either Q1 or Q2 was dead shorted. Don't remember which one but one of them. Both are FQPF12N60C. I replaced both with SiHP18N50C to test as I had these on hand. They are the same package, pinout, mosfet type. Slightly lower Vds but lower RDSon and higher current.
One of the output mosfets (IRFB4227) was dead shorted. Removed both for now. Also removed IRS20957S as it's most likely bad as it's VCC resistor R143 4R7 was also blown.
No other visible damage.
Checked also bridge rectifier, diodes D15 and D18, checked for shorts across +-55V rails and low voltage rails, nothing.
With this done, I powered it up with a dim bulb tester again. Low power PSU comes on. Fault light is on on front panel. High current PSU makes a slow ticking noise and fails to start. 100W bulb in series, bulb is dim and every time the PSU ticks, it glows very slightly for a fraction of a second. I tried 250W worth of bulbs as some Behringer/Turbosound PSUs I have repaired in the past with the same KA3525A controller did not want to start with a 100W current limit. Heatsink with Q1, Q2 and bridge rectifier gets quite hot, both with 100W and 250W limit. Eventually, Q1 and/or Q2 will fail shorted again.
I have ordered new IRFB4227s, new IRS20957S and new FQPF12N60Cs as well as a few other things.
Any ideas what to check? Could the problem have been the SiHP18N50Cs? Bad KA3525A? I have one on hand and I can replace it. But it seems unlikely to fail as it's isolated from Q1 and Q2 via T2?
Thank you in advance if anyone has any ideas!
I recently bought a broken Ampeg Portaflex PF-350 bass amplifier head in unknown condition. As it turns out, it's completely dead. Tried powering it on with the dim bulb tester, dead short.
I have attached the schematic below.
What I found:
TH1 is cracked visibly but measures 6 ohms. Will replace it but should be okay for testing for now.
Either Q1 or Q2 was dead shorted. Don't remember which one but one of them. Both are FQPF12N60C. I replaced both with SiHP18N50C to test as I had these on hand. They are the same package, pinout, mosfet type. Slightly lower Vds but lower RDSon and higher current.
One of the output mosfets (IRFB4227) was dead shorted. Removed both for now. Also removed IRS20957S as it's most likely bad as it's VCC resistor R143 4R7 was also blown.
No other visible damage.
Checked also bridge rectifier, diodes D15 and D18, checked for shorts across +-55V rails and low voltage rails, nothing.
With this done, I powered it up with a dim bulb tester again. Low power PSU comes on. Fault light is on on front panel. High current PSU makes a slow ticking noise and fails to start. 100W bulb in series, bulb is dim and every time the PSU ticks, it glows very slightly for a fraction of a second. I tried 250W worth of bulbs as some Behringer/Turbosound PSUs I have repaired in the past with the same KA3525A controller did not want to start with a 100W current limit. Heatsink with Q1, Q2 and bridge rectifier gets quite hot, both with 100W and 250W limit. Eventually, Q1 and/or Q2 will fail shorted again.
I have ordered new IRFB4227s, new IRS20957S and new FQPF12N60Cs as well as a few other things.
Any ideas what to check? Could the problem have been the SiHP18N50Cs? Bad KA3525A? I have one on hand and I can replace it. But it seems unlikely to fail as it's isolated from Q1 and Q2 via T2?
Thank you in advance if anyone has any ideas!
Comment