Dead Left channel on my Quad 405 MK1

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22-Sep-2017 11:14 PM

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Posts: 8

Hello Gents


I'm new here but I have been a Quad solid state user for many years. Apologies for asking for some assistance in my very first post.

I am a phono cartridge maker and don't know too much about amplifier electronics but I can help myself in a basic way.

Some years ago when I bought the 405, I tidied-up its casing, fitted an on/off switch, fitted proper good quality binding posts, added a pair of rca jacks and replaced its two big capacitors with a pair of Kendeils which you can see in my pics if I can upload them here. At the time, I also replaced some smaller capacitors where I could reach those.

During the past several years, my 405 worked perfectly without any issues at all until about a week ago when I discovered its left channel had no sound.

I removed the top cover and discovered one of the sand filled? 4A fuses had blown. After looking carefully, I can see no other overheated part or burnt wire anywhere.

I did not have a replacement 4A fuse in stock and replaced the blown item with a similar sand-filled 3A fuse which also blew immediately.

What could have gone wrong?

Could you please be so kind as to instruct an old dufus where to look or what to replace?


Thank you

Mr Pye




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23-Sep-2017 11:41 AM

EJP

EJP

Posts: 1412

You have definitely blown one or both of Tr9/10. Check with a multimeter across collector (the case) and the emitter pin. So you will need to replace there, with MJ15003. But the next question is why? which probably involves Tr7/8 (BD242 or equivalent), and possible upstream issues.
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23-Sep-2017 03:49 PM

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Posts: 8

Thank you EJP.


I'll have a look and do some measuring.


Regds


andre/Mr Pye

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23-Sep-2017 03:59 PM

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Posts: 8

Btw, I don't think I have blown components. The amplifier is well-ventilated with nothing on top. The only unusual thing is the 405 is more or less permanently switched on with the pre-amp's volume set so low one can barely hear music is playing.

The reason being that I now live alone in a big house after the death of my wife in January this year and I leave the system switched on due to previous house robberies here, hoping the system's sound will deter more break-ins at my house.


IMO the problem may rather be age-related - some component/s which came to the end of its life.


Regds


andre

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24-Sep-2017 12:29 PM

EJP

EJP

Posts: 1412

Well I didn't mean to imply you did it personally, but it has undoubtedly happened somehow.
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28-Sep-2017 11:05 PM

PHD

PHD

Posts: 6


I had a similar problem on a 405 earlier in the year it took a fair amount of time to fix.  Whereas amplifiers like the 303 are AC coupled the 405 being DC coupled can be much more difficult to repair. TR9 and TR10 were certainly blown but also the protection circuit (T1 T2) was faulty in that T2 had blown.  Even replacing all these caused TR9 and 10 to blow again. Eventually the fault was traced to R8 being open circuit thus putting an incorrect voltage on IC1. Joost's recommendations to use a Variac when testing 405's is good advice as one can bring the voltage up slowly. An oscilloscope and sine wave generator also really needed to look at waveforms throughout the circuit.

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29-Sep-2017 10:59 AM

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Posts: 8

Thank you PHD!

I have a variac and your recommendation to to look at R8 is very useful.

I have to buy some 4A fuses today and will post here re the diagnosis.

Kind regards

andre

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29-Sep-2017 11:50 AM

PHD

PHD

Posts: 6


My problem happened to be R8 but my point was that any component failure in the initial stages could manifest itself with further component failure at the outputs.  Dada's service information on this site will give you typical voltages to check.

Best of luck!

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06-Oct-2017 11:58 AM

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Posts: 8

Hello Gentlemen

A friend helped me to diagnose the dead channel on my Quad 405 MK1 and the problem was traced to one of the Toshiba 2SD424's.

He suggested I replace all 4 on the two boards (dead channel board # M12368 ISS.9), together with replacements also for the 4 40812's.

My friend researched some replacement data sheets for me and recommended the following replacements: 

2N3773 or MJ21194 or possibly MJ15024.

Will these work? Which of his recommendations are first choice, if any?

Thank you & best regards to all.

Mr Pye






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07-Oct-2017 02:29 AM

EJP

EJP

Posts: 1412

I use MJ15003 or 2N3773, both good.
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07-Oct-2017 09:49 AM

hummph

hummph

Posts: 29

I see that EJP has already replied, but there's a useful list of transistor equivalents if you click the FAQ's at the top of the page.

Good luck!

Regards

Hummph

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07-Oct-2017 10:53 AM

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Posts: 8

Thank you both!


Regds


Mr Pye

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27-Oct-2017 03:22 PM

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Posts: 8

Hello again gents

Apologies for not keeping you up to date about my 405MK silent left channel. I have been unwell for many days with all kinds of flu nasties.

I have tracked down 4 pieces  2N3773 which I will use to replace the presently fitted Toshiba 2SD424 on both boards.

The same local (ZAR) source was supposed to get back to me about replacements for the 40872's since the 7th October but she never did. 

I also gave the source the alternative (2SA740) but she says these items are 'very old'.

Is there perhaps another alternative I can use as replacement for 40872 or 2SA740?

Thank you and best regards to all assisting me here.

Mr Pye









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28-Oct-2017 02:58 AM

EJP

EJP

Posts: 1412

See the list of equivalents in the FAQs linked from the menubar.
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06-Nov-2017 08:52 PM

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Posts: 8

Fail.Some 2N3773's and BD242C's arrived today and I worked slowly and carefully, removing the old and replacing them with the new.

I then replaced the 4A fuse which kept blowing, connected the 405 to its pre and switched on.

The same thing happened - the same fuse blew again.

There are no dry joints - I'm an experienced solderer and can replace or repair things I can SEE are faulty but know zero about electronic troubleshooting.

Where else can I look or measure, please?

Cheers

Mr Pye








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