quad 405-2 fuse blower

[Douglas M]

405-2 had been delivering usual Quad sound via FM4 and 44. Powered down system. Restarted system 1/2 hour later, popping sound, 405-2 blew its mains fuse. Unplugged everything, installed new mains fuse, plugged power cord in. Mains fuse blows as soon as amp is plugged in. Power light never lights. Mains fuse appears scorched. Fuses on boards aok. Anyone else with similar headache? Anyone else with similar headache with cure?

[tunneldiode]

Oh dear.
If the fuses on the boards are ok (all four of them), sounds like either the mains TX (hopefully not) or the bridge rectifier (hopefully)

[CopperTop]

Douglas

There's quite a bit of information about the 405-2 out there, such as this site which contains the schematics.

Quad 405 Information Page

If I'm interpreting the diagram correctly there's not a lot between the mains fuse and the on-PCB fuses - just the transformer, rectifier and smoothing caps. I'm not an expert, but I would guess that either one or both of the the smoothing caps is done for. Have they ever been replaced with 'fresh' ones? Or maybe the transformer has developed a short somewhere - don't know how often that happens, if ever.

You could disconnect everything from the transformer output and check to see if the fuse still blows which should eliminate the transformer from your enquiries.

Then you could replace the smoothing caps, which would be a good thing to do, anyway, and see if that cures the problem. Replacing the rectifier should be cheap and easy too.

You could also disconnect the amp PCBs from the PSU to prove it isn't one of them causing the problem - unlikely if their fuses haven't blown.

A recommendation I can make is to use old unwanted 'sacrificial' speakers when doing your testing, as there was once an error on the 405 PCBs that could blow your tweeters if the ground was not continuous between the signal input and speaker outputs. I know this because a Quad 405 'clone' I was playing with, also faithfully reproduced the error...

[Dennis4SB]

Talk with Stephan at Dada Electronics:

Home

Very nice about giving free advice and if you want will sell Quad specific parts and kits.

Unless the caps have been replaced within the past 10 years I would bet it is them.

[Tajzmaj]

Hi
I have 405 myself too and I have repaired some too...So advice
Remove fuses on both amp modules. Install new mains fuse and observe. There can be only 4 things defective if fuse blows. 1 gretz rectifyer, 2,3 one of smoothing caps, 4 transformet. But transformer is on my opinion very unlikely. There is hot environment in 405 closed housing so caps are most common. Rectifyer is possible too because is somehow underrated ad suffers high peak currents when you switch on amp....there is enough space to install some 30-50A 400-1000V unit. Someone gives good and wise advice. Use some cheap speakers when testing the unit....
It will be also nice idea to replace all electrolytic caps on amp boards too. They are only few of them and all are standarf values. And really cheap.
Be careful
Regards
Taj

[Douglas M]

Thanks for info. Always spooky when device goes from operational to non. Amp sounded so fine, then silence. No event, just non operation. Would have expected degraded sound quality then something. Puzzling.

[Mooly]

Originally Posted by Douglas M

Thanks for info. Always spooky when device goes from operational to non. Amp sounded so fine, then silence. No event, just non operation. Would have expected degraded sound quality then something. Puzzling.

Puzzling or not you are going to have to do some basic measurements Check the bridge rectifier, and also see if you can read a short across the main rails. Also check the output transistors for reading short circuit collector to emitter.

Also if you can not find anything definite then I would recommend a bulb tester to power the unit and do some live voltage measurements. That consists of a 100 watt mains filament bulb in series with the incoming supply which will limit current and prevent damage/fuse blowing etc.

[Tajzmaj]

And one more suggestion. Because you will be forced now to repair quad you can also upgrade it. There is just enough space to install 22000uF/63V caps from epcos.....an upgrade of overheated and old output transistors with something new(MJ15024) and replace those nasty op amps........And you will get realy nice sounding amp. There is a lot of information about such things on net...
Regards