QUAD 405 output offset /dead channel

[yestertech]

Looking at a 405 at the moment
One channel is U/S with around a -5v offset on the LS output.
There's no audio output at all.
Other channel is fine.
The rail fuses are intact and there's no obvious signs of distress.

Any ideas where to to start digging ?


Andy

[Craig Sawyers]

First port of call - the input op amp. I've had one fail on my 405 with no other fault. Remove, put in a turned pin socket and put in a TL071 - which is what the original was or ought to be (if the 405 is a later one; early ones used an LM301A). TL071 is still manufactured, and available easily.

Any major failure would have taken out one or both rail fuses.

Craig

[nutteronthebus]

Hi have a look at the DADA page for the service manual ( both 405 1 & 2 )

http://www.dadaelectronics.eu/downloads

Dave

[KeithsTV]

Also check R7 & R8 both 3.3k. These should be replaced with higher wattage rated parts.

Normally if one of these fails to output is sent to either +55v or -55v unless the output crowbar catches it.

Keith

[fetteler]

I seem to recall there is a tantalum cap (C3?) in the DC servo circuit. That would be aspect for causing offset.

Steve.

[Craig Sawyers]

I think you mentioned C2 failure if it was a tant here http://www.vintage-radio.net/forum/s...ad.php?t=85728 .

Craig

[yestertech]

I've subbed the LM301 temporarily for NE5534 - no change to symptoms at all

Next stop is C2

I can't see the DADA info yet - I have to create an account to log in.

A.

[DangerMan]

There is a wealth of Quad 405 information including circuit diagrams of the different versions buried in the links here [Keith Snook's excellent site]:

http://keith-snook.info/quad-405-mods.html

[fetteler]

Originally Posted by Craig Sawyers

I think you mentioned C2 failure if it was a tant here http://www.vintage-radio.net/forum/s...ad.php?t=85728

Craig you are quite right I did! Your memory is much better than mine

So it's C2 not C3... I wonder if there are non electrolytic types small enough to use in that position these days?

Cheers,
Steve.

[Craig Sawyers]

Farnell part 1902028 is a Panasonic 100uF 4V radial. 5mm diam, 2mm lead spacing and 7mm high.

But given that the offset can be of either polarity, I'd be tempted to use two 220u 4V from the same range 1902029 in series + to + or - to - in series to mimic a bipolar cap. They are a bit bigger (6.3mm dia).

Or go for 2326003 , which is a bipolar capacitor by design, again Panasonic. 100u 10V. 6.3mm dia, 2.5mm lead spacing and 11.2mm high. Five for a slightly more than a quid.

Craig

[stevehertz]

For whatever reason, when changing ICs on Quad 405s, 34s and 44s, be aware that the original TL071 ICs are bog standard, very lo-fi, 'pound-a-piece', industrial ICs. A decent upgrade - and noticeable improvement - is to use a dedicated audio quality, drop in replacement such as the OPA134 which is designed specifically for high quality audio use and was not available when the aforementioned Quads were designed. Here's the datasheet: http://www.ti.com/product/OPA134 .

Now I don't claim to hear a difference between cables so make no mistake, I'm no audiophool. But I could quite easily hear the difference when I replaced the ICs in my 34, 44 and 405-2. It is a worthwhile upgrade to perform at any time on such a Quad, but particularly if you have it apart for another reason, just swap em.

[Whaam68]

It's worth registering with Dada as the downloads section has all the quad service manuals for free. You can also view their quad specific forum and all their refurb kit manuals. I used their kits on a 405 mk1 a 44 and a 33 and the difference was obviously better so worth a look at what components they are replacing if nothing else.

[stevehertz]

Good advice in general, but with the caveat that you take some of the upgrades with a pinch of salt - use common sense and electronics theory in other words - preferably a mixture of both. Not sure if what I now say is relevant to Dada, but in the past I've seen so much about 'improving the sound of Quad equipment' that is questionable. For example, does using thicker wire over a 6" stretch really improve the sound, or does it just satiate the idea that beefier, thicker wires must be better? And why change perfectly good, original sockets? stuff like that. I mean, if anyone wants to do those things in the pursuit of enjoyment and happiness, I cannot argue with that. Some people would say there's a fine line between being an audiphool and a serious hifi enthusiast. I disagree, it's a chasm.

[evingar]

Originally Posted by stevehertz

Now I don't claim to hear a difference between cables so make no mistake, I'm no audiophool. But I could quite easily hear the difference when I replaced the ICs in my 34, 44 and 405-2. It is a worthwhile upgrade to perform at any time on such a Quad, but particularly if you have it apart for another reason, just swap em.

Thanks for the tip. My 405-2 is due for a cap inspection / replacement. I may try changing the OP-Amps while it's in bits.

[Whaam68]

Just to clarify The Dada refurb kits don't contain boutique parts they just aim to replace the components that are likely to have worn out with basic modern equivalents. Their philosophy is very much to preserve the original quad sound and the pricing is fairly reasonable given the level of support and step by step manuals. Apart from the option to change sensitivity to better match modern sources and use of contemporary opamps it's all basic stuff, No wire rolled on the thighs of virgins. I've been really happy with the ones I've done and my 405/44 are now my weapon of choice, but horses for courses & all that.

[stevehertz]

Sounds good to me. I did an overall of my 405-2 a few years back involving changing under-rated blackened resistors and potentially faulty caps etc - nothing voodoo-like or smelling of snake oil . I have since bought a kit of parts to reduce the gain, but haven't got around to implementing it yet. Only a 1001 jobs to do.. On my 44 I have changed the ICs for the audio quality ones that I mention above.

[Craig Sawyers]

In the Quad 405 service manual it says:

"Transistors - on M12368 issues 5, 6 and 7 BDY77 or BDY74 may have been used for TR9 and TR10. BDY77 is a suitable replacement for both, but faster transistors may cause instability"

BDY77 has an fT of 800kHz, so is really really sluggish. Far slower than any self-respecting power transistor nowadays.

On later issues, they used either 2SD424, 17556 or 2SD676, which they say are "interchangeable" . These have an fT of 5MHz.

But the detailed circuit in series with the dumpers changed several times, so I guess Quad had a few problems with stability depending on transistor choice.

Which is all to say that those who radically change the 405 circuit (and there are those who do) are doing something which could be catastrophic at some point.

Craig