Hello
I have a Quad 405-2 with one channel down. One channel works but the other gives a loud mains hum. It is taking about 300 watts from the mains so I am keeping it off.
Does anyone have experience of what are the common faults on the 405-2. I have circuit diagrams and test gear like multimeter oscilloscope and signal generator and have repaired the preamp a few times now.
Help would be appreciated with common faults that give loud mains hum on one channel.
I have a Quad 405-2 with one channel down. One channel works but the other gives a loud mains hum. It is taking about 300 watts from the mains so I am keeping it off.
Does anyone have experience of what are the common faults on the 405-2. I have circuit diagrams and test gear like multimeter oscilloscope and signal generator and have repaired the preamp a few times now.
Help would be appreciated with common faults that give loud mains hum on one channel.
The QUAD 405 that I repaired decades ago had a shorted triac in the protection circuit and the electrolytic capacitor of the DC feedback was shorted. Someone else had already attempted to repair it, I don't know what was replaced.
It may be a good idea to build a dim bulb tester (crude current limiter) before going any further.
It may be a good idea to build a dim bulb tester (crude current limiter) before going any further.
A crude current limiter consisting of an incandescent lamp connected in series with the equipment under test.
I think you should also check the driver and output transistors and follow up whatever advice you get from others on this forum; there are people here who have much more experience repairing QUAD 405s than I have.
At some moment when you have tested and fixed everything you can without applying power, you will have to turn it on and check the bias points. A crude current limiter can then hopefully prevent damage if there is still something seriously wrong.
I think you should also check the driver and output transistors and follow up whatever advice you get from others on this forum; there are people here who have much more experience repairing QUAD 405s than I have.
At some moment when you have tested and fixed everything you can without applying power, you will have to turn it on and check the bias points. A crude current limiter can then hopefully prevent damage if there is still something seriously wrong.
Last edited:
If your caps haven’t been replaced then that’s your first suspect. I recently restored a 405 and all the electrolytics on the boards were shot. It’s probably also worth replacing the big caps too — Keith Snook has a good recommendation for ones that fit that don’t require lots of padding. They do have screw terminals though, and you Might add a bit more insulation between them and the top cover…
prior to restoration my 405 was starting to develop a hum. After replacing the caps it’s basically silent now.
prior to restoration my 405 was starting to develop a hum. After replacing the caps it’s basically silent now.
I built a clone of the 405 back in 1976, Still have it.
You need to test every transistor and diode and confirm they are OK before you turn on power. This is the same procedure I would use for ANY amplifier repair. You are just wasting time and guaranteeing destruction of replacement parts if you haven't replaced EVERY failed part.
If you don't screen or "triage" semiconductors in circuit for obvious shorts and out of circuit for subtle issues like leakage you are just wasting your time. The 405 is a very simple amp design compared to many from the 80s/90s.
One of the best pieces of gear for checking in-circuit semiconductors is a "component tester" AKA Octopus. Since you own an oscilloscope, you already own the main piece required. You want to test in circuit using say +/- 5 volts with current limited to under 10 mA.
You need to test every transistor and diode and confirm they are OK before you turn on power. This is the same procedure I would use for ANY amplifier repair. You are just wasting time and guaranteeing destruction of replacement parts if you haven't replaced EVERY failed part.
If you don't screen or "triage" semiconductors in circuit for obvious shorts and out of circuit for subtle issues like leakage you are just wasting your time. The 405 is a very simple amp design compared to many from the 80s/90s.
One of the best pieces of gear for checking in-circuit semiconductors is a "component tester" AKA Octopus. Since you own an oscilloscope, you already own the main piece required. You want to test in circuit using say +/- 5 volts with current limited to under 10 mA.
Last edited:
In the 405-2 the triac is located on board and can be isolated by removing the solder link next to the output terminal. I would check the output transistors first as these are often destroyed if the triac fires. With working output transistors and the triac disconnected you can power up the amplifier with no load, then check the voltage on the output connection, if it is a few mV the amp should work and you can apply a signal and measure the ac output voltage which should follow the music, if it does you can connect to a speaker, if its more than a few mV it will require further investigation. As a matter of course I would replace all the electrolytic capacitors
Stuart
Stuart