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Old 26th May 2008, 8:26 pm   #1
ppppenguin
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Default Quad 405 overheating

After 30 years of faithful service from new my Quad 405 was getting very hot this evening. I haven't done any serious fault finding yet - I don't think I've ever had the lid off before - so I thought I'd ask the experts first.

Amp was getting very hot so switched off in a hurry. Didn't even listen to see which channel wasn't working, if any.



Amp is marked Quad 405 but my booklet is marked 405-2. It has the earlier bronze heatsink and serial number is 25561. The channel PCBs are marked Quad 405 and M12368 ISS9. The Quad booklet doesn't have a full circuit diagram but I have Peter Walker's December 1975 Wireless World article which does. That article says the output transistors are BDY77. My amp has Toshiba 2SD424. First tests show that on one channel:
  • L3 (which doesn't appear on the schematic which has only 2 coils) has badly overheated but is still continuous.
  • FS2 (-50V rail) has blown
  • One of the output transistors is probably fried but I can be certain until I disconnect it.
While I've got it apart obviously I need to mend it but I'll also change any electrolytics and anything else that folks recommend. Does anyone have the correct circuit for my amp? What replacements would you recommend for the output transistors.

PS: Found a Quad service manual online which seems to have schematics for all the PCB variants.

Last edited by ppppenguin; 26th May 2008 at 8:41 pm.
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Old 26th May 2008, 8:41 pm   #2
ALANS ANITAS
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Default Re: Quad 405 overheating

Hi Jeffrey

Try Googleing BDY77. Quite reasonable price.

Regards

ALAN
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Old 26th May 2008, 8:55 pm   #3
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Default Re: Quad 405 overheating

RS have got the 2N3773G (the G means lead free) for £3.72+VAT. This seems to be a recommended replacement.
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Old 26th May 2008, 8:58 pm   #4
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Default Re: Quad 405 overheating

Change both O/P transistors! And check around them too for shorts and duff parts which caused them to fail.

Cheers,

Steve P.
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Old 26th May 2008, 9:06 pm   #5
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Default Re: Quad 405 overheating

My 405 is Serial 32XXX so of similar vintage, the O/P transistors are, as you say 2SD424. Your booklet must be much later than the amp. I have my original booklet and circuit diagram. Will look for it now, I can scan it if you like.

It's gone wrong because I looked at it!
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Old 26th May 2008, 9:07 pm   #6
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Default Re: Quad 405 overheating

Hi Jeffrey,
I've repaired several Quad 405's in a previous life... The small signal transistors in the early stages have a habit of dying (short) which takes out the output trannies as the whole amp is DC coupled. I'm sure you're already aware of this, but you need to run the amp up on test very gently on a current-limited supply after repair as they have a nasty habit of taking out your new o/p trannies if there's a rougue fault still lurking in an earlier stage.
Often, for speed, we would blanket replace all the transistors on the board.
Cheers,
Mark.
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Old 26th May 2008, 9:08 pm   #7
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Default Re: Quad 405 overheating

Quote:
Originally Posted by Steve_P View Post
Change both O/P transistors! And check around them too for shorts and duff parts which caused them to fail.
I'll second that. A failed component elsewhere in the circuit can pop an output transistor -- usually the power supply fuse saves the other one.

Also, on any amp where both the output transistors are of the same sex, you may have to adjust the biasing if you replace them.
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Old 26th May 2008, 9:26 pm   #8
mastermanx2001
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Default Re: Quad 405 overheating

Hi Jeffrey

Just looked at my circuit boards and they are marked M12368 ISS9 between the two fuses, seems to be same as yours.

Just sent you circuit diag. I can guarantee that this diag. was supplied with my amp. After fixing it, I suggest that a big improvement can be made by replacing the op amps with TL071. The original op amps should be in sockets, so its easy. Burr Brown op amps are supposed to the the best, I have tried them, did not really hear much (if any difference), but the 071s are way way better than original LM301A.
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Old 26th May 2008, 9:28 pm   #9
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Default Re: Quad 405 overheating

The remaining channel seems fine. I'm planning to check all the transistors on the dead channel and run it up on a bench supply with a very low current limit initially.

Do you think the 2N3773 is a sensible replacement for the 2SD424?

PS: I think I've got some TL071 in stock so I'll replace the LM301.
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Old 26th May 2008, 9:50 pm   #10
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Default Re: Quad 405 overheating

I don,t really know about the 2N3773, but Cricklewood Electronics have the 2SD424

http://www.cricklewoodelectronics.co...=search&page=1

Think I would be tempted to use those, not sure if they are Toshiba ones though. I think I remember that their can be oscillation problems with the 405, so I would play safe. Changing to the TL071 is safe, I have done it. It is used in the 405-2. I also have a 405-2. The improvement in sound quality between this and a standard 405 is quite large.

However, to me a 405 with TL071, sounds very similar to the 405-2, can't be bad.

Some info here, easy to get carried away though!

http://quadrevisionspot.blogspot.com/
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Old 26th May 2008, 10:12 pm   #11
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Default Re: Quad 405 overheating

I converted one of my 405s to a 405-2 using the Quad modules which came with 4mm sockts for the speaker connection (replacing those spring loaded things). I also took the opportunity to fit phono sockets for the input and an on-off switch.

These are great, making the amp more versatile. No drilling was required, there are four holes in the back panel with gromets fitted, Just took three gromets out and the two phono sockets and switch fitted perfectly. Perhaps I should have fitted a volume control as well.
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Old 26th May 2008, 10:24 pm   #12
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Default Re: Quad 405 overheating

Just noticed that on the "Quad Spot" it does mention the 2N3773, below is an extract:


* TR1: BC214C - BC559C BC560C BC415C
* TR2: ZTX304 - BCX32 BC682 BC546B MPSA06 2N5551
* TR3/TR4: ZTX504 - BC556B
* TR5/TR6: BC214C - BC560C BC415C BC559C
* TR7/TR8: 40872 - 2SA740 BD242C 2N6134 TIP42C BDX78
* TR9/TR10: 17556 - 2SD424 2SD676 2N3773 MJ15003
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Old 26th May 2008, 10:32 pm   #13
mastermanx2001
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Default Re: Quad 405 overheating

Quote:
Originally Posted by ajs_derby View Post
I'll second that. A failed component elsewhere in the circuit can pop an output transistor -- usually the power supply fuse saves the other one.

Also, on any amp where both the output transistors are of the same sex, you may have to adjust the biasing if you replace them.
No adjustable bias on the 405, I wont go into the design of it, because I don't fully understand it.

However, that is one of the points about the 405, no preset adjustment carried out in manufacture and no adjustments needed during its life.
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Old 26th May 2008, 10:34 pm   #14
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Default Re: Quad 405 overheating

If audiomm is around at any stage he's an expert on these things so can probably offer some advice.

The only thing I'd say (knowing nothing about Quad) is to remember to remove the compensation caps if you're replaing LM301 with an internally compensated op-amp like the TL071. Not sure what happens if you leave them, but you don't need them there.
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Old 26th May 2008, 11:35 pm   #15
mastermanx2001
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Default Re: Quad 405 overheating

I have done several conversions of 405 changing from LM301 to TL071 and never removed the 3.3 pF capacitor between pins 1 and 8. All amps have run like this for years, sounding good and proving reliable. I believe that pin 8 on the TL071 is not connected, so it is unnecesaary to remove the compensation capacitor. Anyway, in the unlikely event of wanting to change back to the LM301, it is just a case of putting the chip back in.

Also just noticed that the parts list supplied with my Quad 405 does show the 2N3773 as an alternative output transistor, see below:
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Last edited by mastermanx2001; 26th May 2008 at 11:54 pm.
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Old 27th May 2008, 10:47 am   #16
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Default Re: Quad 405 overheating

Just had the obvious thought, for those inductors to have got hot and a fuse blow, some heavy DC has probably been flowing. I hope that your speakers are OK, worth checking. There is protection circuitry in those speakers, though I am sure that you are aware of that.

Last edited by mastermanx2001; 27th May 2008 at 11:09 am. Reason: spelling
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Old 27th May 2008, 11:44 am   #17
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Default Re: Quad 405 overheating

I know the ESL63 has a crowbar circuit (as does the 405) and will find out whether all is well when I repair the amp and reconnect everything.

A helpful fellow, not a forum member, emailed me and commented on some aspects of the crowbar design which could give trouble. He also suggested that the cooked choke could have shorted turns and I will check this on an LCR meter.
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Old 27th May 2008, 11:58 am   #18
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Default Re: Quad 405 overheating

Yes the 405 protection has been known to cause problems, there is some info about that on the Quad Spot
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Old 27th May 2008, 4:08 pm   #19
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Default Re: Quad 405 overheating

I've had a closer look and it looks like it's just one 2SD424 output tranny and the fuse. All the other devices are intact, the electrolytics are still OK and the cooked choke is fine. I've ordered a couple of 2SD424 from Cricklewood so I'll report back when they're here.

I'll fire up the board on the bench first and be a bit wary of the output crowbar when I put the board back in the box.
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Old 27th May 2008, 4:48 pm   #20
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Default Re: Quad 405 overheating

Hmm! I wonder if the speaker crowbared causing the problem. I know it sounds obvious but how about putting some extra fuses in between the amp and the speaker, even just as a temporary measure until everything is proven. I guess that you will test the amp into something less valuable anyway.
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