Problem with QUAD 44 hifi preamp.

[Rob Flinn]

I have an issue with a QUAD 44 Hifi preamp in that it is violently motorboating. Where I am I have limited test gear (i.e a dmm) but I opened it up and noticed one of these known dodgy Rifa caps across the power switch. It definitely looks a bit ifffy since it has a crack in the case and some black residue possibly from arcing. Rather than blow up the IMF transmission lines that the system is hooked up to can anyone confirm that these Rifa devices can cause that type of issue ?

 

[radardoug]

Rifas across the mains generally let the smoke out. This type of cap is supposed to be designed to be safe. They dont catch fire, just smoke. But that wont be your motorboating issue. That would be dry power supply caps in a very old peice of gear.

 

[Rob Flinn]

Rifas across the mains generally let the smoke out. This type of cap is supposed to be designed to be safe. They dont catch fire, just smoke. But that wont be your motorboating issue. That would be dry power supply caps in a very old peice of gear.

Thanks Doug. The only problem with the dry caps analogy is that I have compeletely replaced the electrolytics in this unit & verified the connections. Motor boating may not be quite the right description , but it creates a massive thump at about 0.5Hz. This goes if the volume is turned down & increases if bass boost is applied. I have removed the Rifa & there is no ripple that I can measure with my DMM. I have plugged it into another amp & speakers and it appears to work ok. Will have to try it into QUAD 405.

 

[Tubetec]

Those Rifa caps can go out violently , Ive seen it happen , the thin acrylic moulded case cracks at the sides , moisture gets in , causes leakage ,then it either heats up and smoulders or goes bang . Maybe check the diodes in the PSU are good , check the rails to make sure nothing is getting dragged down . If the noise is gone with the volume down it should help you narrow the search , try disconnecting the input to the pot , if the noise is gone your issue is pre pot , if it remains its after pot .
The logic switching can also get funky on these , maybe spend some time researching other peoples fix ups on the 44 .

 

[Rob Flinn]

Those Rifa caps can go out violently , Ive seen it happen , the thin acrylic moulded case cracks at the sides , moisture gets in , causes leakage ,then it either heats up and smoulders or goes bang . Maybe check the diodes in the PSU are good , check the rails to make sure nothing is getting dragged down . If the noise is gone with the volume down it should help you narrow the search , try disconnecting the input to the pot , if the noise is gone your issue is pre pot , if it remains its after pot .
The logic switching can also get funky on these , maybe spend some time researching other peoples fix ups on the 44 .

Thanks for the tips. The power rails seems good. I did think about the logic switching but it seems stable. Unfortunately I don't have the 405 that was being used with this pre amp here, so I don't know if the 44 was becoming unstable when the power for the 405 was being taken through the preamp. I also did some research on other peoples fixes but nobody ever had this problem & none of the problems they did have were close to this. When I took the Rifa out it did seem to work fine with an HH amp I have here, so maybe that was the problem, but as Doug says the x2 type Rifas are designed to fail open circuit & therefore should not cause this sort of issue.

 

[Tubetec]

The pre only acts as a mains switch for the power amp which has its own transformer/psu .
They may well fail open circuit but Ive seen those things spit fire and carbonised remains all over the place in the process .

 

[Rob Flinn]

The pre only acts as a mains switch for the power amp which has its own transformer/psu .
They may well fail open circuit but Ive seen those things spit fire and carbonised remains all over the place in the process .

Yes, I realise that the pre only acts as a switch for the Power amp. However the Rifa is across the mains switch in the pre, therefore there is a lot of added load when the power amp is attached. Although the added load is not actually passing through the Rifa, so it's working state should remain the same. This Rifa is definitely split & shows carbon too. I was wondering whether it could somehow break down to short the power so it slowly oscillated. I've replaced Rifas before but never when a unit has had this motorboating problem.

 

[Mixermend]

Clearly the first step is to check the power supply voltages - and see if one, or both of the rails is ramping up and down.

The Quad 44 is modular - there aren't really any high gain stages that might be unstable, apart from, perhaps, the disc input - which can easily be unplugged to check if it is the problem.

Just disconnect the RIFA capacitor to confirm that this could not be causing your problem.

 

[Rob Flinn]

Clearly the first step is to check the power supply voltages - and see if one, or both of the rails is ramping up and down.

The Quad 44 is modular - there aren't really any high gain stages that might be unstable, apart from, perhaps, the disc input - which can easily be unplugged to check if it is the problem.

Just disconnect the RIFA capacitor to confirm that this could not be causing your problem.

Thanks, if you read above I've already done all this. It seems to be stable without the Rifa.

 

[Mixermend]

Even very experienced repair people would never dare to say that they have seen it all!

 

[ruffrecords]

If you still have motorboating after replacing ALL the electrolytics then look to the negative feedback loop - possibly an open circuit capacitor.

Cheers

Ian

 

[Rob Flinn]

Thanks Ian, I have replaced all the electrolytics. I think it seems to have resolved itself since removing the Rifa cap across the power rails but I need to check it out with a 405 amp though. I have a 405 in England to check it with but the amp I'm using in France (where this system will live) is a different one that seems to have come from a broadcast facilty so may have been modified. The French 405 is stable without the 44 connected.

 

[guy_4]

Hello,
These X2 RIFA caps are famous for blowing in Revox and Studer units.
With lots of white smoke and terrible smell...:-((
I have replaced tens on their tape recorders.
Best,
Guy