Quad 520f blowing fuse

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13-Jun-2019 04:57 PM

RC

RC

Posts: 2

Hello. I am a new member and was wondering if someone could help me. I am a valve amp fan and have an understanding of valve circuits but no nothing about solid state amplifiers, I have just purchased a Quad 520f but it is not working on one channel it is blowing the 6.3-amp fuse on its side of the power supply. I have disconnected that side from the supply and taken some measurements and it appears that the output transistors are shorted. If it turns out that this is the case are there other transistors and components that I should look at  and can they be measured in circuit.

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14-Jun-2019 11:49 AM

Stefaan

Stefaan

Posts: 613


Hello,

It is possible that only the power-transistors are broken but I would also check the driver- and other transistors with a multimeter as well as the diodes.

If this was not done yet you should also replace all the electrolytic capacitors as they have a lifetime of 10 years and the amplifier is much older.

All components you need are available in the Webshop.

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19-Jul-2019 04:42 PM

RC

RC

Posts: 2

Hello again and thanks for all your help and advice to my last question. Since my last post I have been having a lot of fun I purchased a second broken 520f from the same seller in the hope that I could make one working unit from two broken ones. Well I was able to find 10 good output transistors between both units and I am now the proud possessor of a fully working 520f the units have close serial numbers and all of the transistors are Motorola mj15003s so I am hoping they are similar if that matters. I realise the unit will need servicing cap changes, etc but so far it is sounding fine. My second question, I now have two 520fs is there any sound quality benefits to be had from mono -blocking of the amplifiers, I don’t need any more power or load driving ability. Originally, I wanted a pair of 510s but they are hard to find does a pair of 510s sound better than a 520f or a pair of 520fs mono-blocked? I wanted 510s because I like the idea of a transformer siting between an amplifier and my vintage hard to replace speakers. Thanks again for all your help.

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22-Jul-2019 07:23 AM

EJP

EJP

Posts: 1644

MJ15003 is fine in this amplier.

Unless you need more power or more load-driving capacity into 4- or 2-ohm loads there is no advantage to monoblocking, and indeed I would argue that there is little advantage anyway.

Using two 510s gives you two entirely separate PSUs, which may improve channel separation, and gives you the output transformers, which protect the speakers as you mention but also compromise the sound quality especially at LF. and HF. Note that it is quite high at 20KHz and isn't even quoted below 40Hz.

EJP


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