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Old 10th Jan 2019, 5:30 pm   #1
soundray
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Default Quad 405-2 help

Hi, new subscriber here.
Iím looking for help on a Quad 405-2.

I occasionally do a little bit of tinkering on vintage amps, fitting new caps, etc, and agreed to look at a 405-2 for a friend. It has quite loud hiss on the left channel and hum on the right. Audio output from left seems normal level, albeit with hiss, output from the right is low level and gets very distorted when the preamp volume is raised a little. There is also a large splat when powered down.

I changed all the electrolytic capacitors, including the power caps, hoping that would cure the problems, it didnít. My next guess was the main transistors TR9/10 so these were replaced on each board, still no change. Iím now out of ideas.

Iím very much an amateur with only a basic knowledge of electronics but I can use a soldering iron and read a digital meter, which is all Iíve needed so far. Iím hoping you guys with more knowledge and experience can offer advice on where to look next.

Many thanks
Ray
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Old 10th Jan 2019, 8:07 pm   #2
stevehertz
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Default Re: Quad 405-2 help

With respect, that's the kind of thing that happens when people just change loads of components in nothing more than the hope that they may be the cause of a problem. Try googling Quad 405-2 problems... https://www.google.com/search?ei=1JY...71.aMjEhk-gXQE
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Old 10th Jan 2019, 8:33 pm   #3
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Default Re: Quad 405-2 help

Hi
There is a ton of info on 405 problems and refurb on the Dada electronics site forum. You can register for free. Their refurb kits are also very good with step by step manuals.
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Old 10th Jan 2019, 10:45 pm   #4
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Default Re: Quad 405-2 help

R7 & R8 will need changing as they are underrated 3K3 1w offset from the PCB ( the old ones are .25w and they draw .37w )
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Old 11th Jan 2019, 12:55 am   #5
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Default Re: Quad 405-2 help

Thanks all for the replies.

stevehertz

I have been a sound engineer/recordist for many years and have always had a maintenance department to do the electronics and sort out any problems. Now that Iím semi retired I have started to take the top off and look at these things as a bit of a hobby.

Iíve only looked at vintage Quad amps so far, a couple of 33s and 303s and a 405, all of which have had original caps, trimmers, etc, so I considered changing them to be a sensible starting point anyway rather than just a random act.

My experience so far has been that changing these old caps, trim pots and the occasional transistor and checking/setting some adjustable voltages has cured most problems. Lucky I guess. This one is clearly beyond that and beyond my experience, hence asking for any pointers.

I did a Google search for 405-2 problems and arrived here as, having read a few posts, it seemed to have knowledgeable contributors and might be the most helpful to an amateur like me.

Whaam68

I will check out the Dada electronics site in more depth, thank you.

nutteronthebus

Thanks for info, I will check and replace them.

Ray
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Old 11th Jan 2019, 5:16 am   #6
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Default Re: Quad 405-2 help

In common with most solid-state audio power amplifiers, these designs consist of several stages all DC coupled together. Each stage is only correctly biased if the whole lot is working correctly.

This means that if one part fails, there is a high risk of a chain-of-dominoes sequence taking out a number of active devices. This may sound bad enough, but it has a nasty consequence in that the chain of dominoes can easily form a ring. There is now no way to fix it if the method is to replace one part at a time and then apply power to see if that one fixed it. Each part you fit is immediately killed by its neighbours when power is applied and the repairer thinks 'that new part can't be bad, I've just fitted it'.

Transistor power amps have acquired the reputation of being little devils to fix and have driven some people to refuse to touch anything other than valves ;-)

The way to fix them is to take the long way round and not be tempted by quick fix shortcuts. With power off the amp you need to find ALL the bad parts and replace them ALL before applying power. In a commercial environment where time costs money, the most efficient fix might be to pull all the semiconductors, measure all the resistors and fit a known good full set of semiconductors - then maybe check the removed set to see if any of the higher value parts can be recycled.

There are some quick fixes which work, but you only find out about them by learning from seeing which parts pop most often.

I've designed a few audio power amps from scratch. Not copied a mix of circuits from here and there or done some mods, I mean starting with the maths and trying novel architecture. When firing up a prototype you lack the comfort-blanket of knowing it once worked! Techniques involve lighting up one stage at a time with temporary feedback arranged to control the bias before the output stages are put on, and using current-limited bench power supplies. This would be tediously slow for doing a single repair, but on a prototype it allows checking the performance of each stage against the design intent.

Douglas Self's website is quite illuminating regarding solid state amps, it might go deeper than you want, but it does explain what's going on.

David
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Old 11th Jan 2019, 7:45 am   #7
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Default Re: Quad 405-2 help

"output from the right is low level and gets very distorted when the preamp volume is raised a little. There is also a large splat when powered down." Sounds like it's oscillating, do you have a scope? Or possibly DC on the output. When testing best to have the amp connected to a dummy load or test speaker, IE one your not bothered about loosing.

The standard procedure when fixing stuff is to do a thorough visual inspection of all components and for power amps check for shorts, EG check transistors before you apply power then slowly power it up using a variac and or lamp limiter, then check voltages. there's a lot more to it than this though.

Your amp as David says is DC coupled, these can be a nightmare to fault find and fix and to be honest not a job for the inexperienced, I'm worried you'll get in deep doo doo with this amp and potentially strain your friendship with your mate. If it were me I'd think carefully before offering to fix a friend's expensive amplifier if I wasn't too sure what I was doing.

That said it's possible to fix such a beast with little knowledge and plenty of guidance; the Quad 405 has a good deal of info written about it.

Good luck, Andy.
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Old 11th Jan 2019, 9:56 am   #8
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Default Re: Quad 405-2 help

Keith Snook has a lot of info on updating 405/405-2 on his web page http://www.keith-snook.info/quad-405-mods.html

have fun Dave
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Old 11th Jan 2019, 11:03 am   #9
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Default Re: Quad 405-2 help

I sympathise with your challenge. I struggled with one of my Quad 405s because I prefer the 'one component at a time' replacement policy when faultfinding, but the practice failed me with the 405. What David says in post 6 is so true where directly coupled audio amps are concerned - a single component failure tends to lead to a cascade of faults. Then when one faulty transistor is identified and replaced, you find that the amplifier has transformed itself into an evil transistor destroyer. The only solution is to root out all the faulty semiconductors before powering up again.

An approach which I've found useful is, with power off, to test transistors individually in circuit simply using the 'diode test' facility on a digital multimeter to establish that each one still contains two junctions and no shorts or open circuits. A silicon junction will normally read between 0.6V and 0.8V on diode test in its forward biased direction. In the other direction, it may read anything from a few Kohms to infinity depending on the surrounding circuitry.

Incidentally, Murphy's law ensured that I had to go through this twice with my faulty 405: having got it working again very nicely, one careless slip of a test prod, a flash bang, and I had a whole new fault to find!

By the way, I too can recommend Dada as a fast and reliable supplier of genuine parts for the 405.

Martin
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Old 11th Jan 2019, 12:53 pm   #10
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Default Re: Quad 405-2 help

Yes, that's a good poinr, Martin. A trustworthy source of components is most important. One sonky fake transistor and flash bang all over again and you're back to square one.

Sometimes transistors are so intimately interlinked that junction tests leave you unsure about which junction of which device you're reading. Be prepared to unsolder and test some off the board.

Once you accept the inevitability of the long way round fixing transistor amps is merely tedious. Without that acceptance it is often impossible.

The 405 'current dumper' circuit is unique, weird, and probably not the best amplifier to start on, if you had any choice! But it seems to have chosen you!

David
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Old 12th Jan 2019, 3:35 pm   #11
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Default Re: Quad 405-2 help

Many thanks to all for your advice and contributions.

Radio Wrangler

Thanks for your useful summary. Iíve obviously been lucky with my dabbling and quick fixes so far but Iím learning as I go. The only test gear I have is a DMM and I will go through testing and measuring what I can with that. A long slow process I think.

Diabolical Artificer

As you say, thereís a lot more to these things than I know or understand yet. I donít have a scope (and currently wouldnít know how to use one anyway) or any other test gear so I have been Ďgetting away withí it so far. Iím sure the friendship will survive, the amp was inherited so cost nothing and Iím happy to pick up the parts bill if I canít make it work.

nutteronthebus

I have searched online and, as you say, there is plenty of info out there. The biggest problem for me at the moment is understanding some of the technical terms and references. I go off and look them up but they often take me down another winding rabbit hole until I get lost! Iíve been having fun so far but this one is a bit trying.

Hartley118

Thanks for your sympathy. I have tested some transistors with the DMM but, when in circuit, have had some odd readings that Iím not sure are due to a fault or the effect of other components in the circuit. I do find taking them all out to test, whilst obviously preferable, somewhat tedious, as David mentions, but thatís probably me wanting instant results. I buy my components from Farnell so I think quality shouldnít be a problem. I have had a good look at the Dada site now and may well use their kits in the future. Though it seems my current problem is beyond a kit of parts so Iíll keep checking.

Thanks again to you all for the insight, advice and encouragement. Iíll keep trying and, hopefully learning. I guess if all else fails I can always send it to Quad/IAG to be done 'properly'!
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Old 12th Jan 2019, 9:02 pm   #12
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Default Re: Quad 405-2 help

Quote:
Originally Posted by soundray View Post
I donít have a scope (and currently wouldnít know how to use one anyway) or any other test gear so I have been Ďgetting away withí it so far.
I wouldn't be without one - the great thing is that it actually allows you to see what is going on, and the initial learning curve is not very steep. A decent CRT 'scope, such as a Hameg, can be had for tens of pounds and will pay for itself very quickly in terms of clarity of diagnosis.
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Old 12th Jan 2019, 10:58 pm   #13
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Default Re: Quad 405-2 help

An oscilloscope is a scary machine if you don't know how to use one. This view is reinforced by a small horde of people ready to chip in that they fix video recorders/colour TVs/spacecraft and never needed anything more than a basic multimeter. Their statements are quite likely true.

What they miss is the fact that oscilloscopes, once you learn to use them at a basic level, make life a lot easier.

If you approach one with the thought that "Here's a tool which can really help me" it will.
It turns electrical signals into patterns and the human mind is a pattern-handling specialist.

A voltmeter is like an altimeter and gives you a spot height

An oscilloscope gives you the whole damned contour map and you can see the shape of the land. Teaching and learning becomes easier too.

David
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Old 13th Jan 2019, 12:28 am   #14
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Default Re: Quad 405-2 help

I have to agree with you David, scopes have always seemed scary to me.

Iím sure youíre right about their usefulness Ted. Not so sure about the learning curve though for someone like me with little experience or knowledge.

When I was working in professional situations the maintenance guys used them all the time of course, but Iíve always thought them out of my league without a more in depth knowledge of circuits and electronics, and probably a lot of instruction. I can handle a mixing desk with more knobs, buttons, routings and options than you can shake the proverbial at, but the few on the front of a scope always appeared beyond my understanding.

Perhaps though, as you say David, thinking of them with a different mindset may be all I need to try one, though Iím not sure the little bit of hobby work I do would justify it. Iíll give it some thought and see if I can persuade myself to get a basic one, and a good instruction manual!

Ray
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Old 13th Jan 2019, 1:45 am   #15
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Default Re: Quad 405-2 help

If you can drive an SSL then learning how to get useful information out of a Hameg 'scope or similar is well within your compass and in many ways simpler - I've done both. It's more a question of familiarity than anything, and as I've said, decent 'scopes are not expensive these days.
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Old 13th Jan 2019, 2:14 am   #16
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Default Re: Quad 405-2 help

You are very welcome here soundray [like everyone else, which seems to be a bit unusual on the net-as far as I can judge!] There used to be an accepted ethos of general slow development with support but this has all changed with the advent of You Tube Demos [some of which can be very helpful] a general emphasis on retro items-especially record players believe it or not, plus changing all the capacitors and hoping to get lucky! I am very ignorant of Oscilloscope Techniques myself but I would take it up if attempting high end solid state audio repairs. There is little else to tell you what is really happening as far as I can see By the way, you are not a subscriber. Many of us have offered to pay during difficult times in the past but Paul Stenning is our benefactor.

Cheers,

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Old 13th Jan 2019, 8:48 am   #17
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Default Re: Quad 405-2 help

From time to time suitable scopes crop up on here, modern-ish basic ones being cleared out by educational establishments. Pocket money prices are usual, a small fraction of the going price of a second-hand 405.

You could try a post in the wanted section of this site. People grow out of their first scope and go looking for something a bit higher performance and feature rich, so there may be basic ones sitting around unused.

If a fancy one turns up, you can use it, but you'll need someone to tell you where to park all the controls for the features you aren't going to use. There's plenty of help on here. It's analogous to you showing a beginner how to do something basic on a big desk. These are the controls you need, park those like this, and all those other channels don't have mikes on them... A friend had the job of running Soundcraft for a while until H-K pulled the plug on them.

1) You'll need a couple of 'times ten' scope probes to go with it. They allow you to probe around in whatever you're testing in a non-intrusive way, and get the signal into the scope.

2) Couriers don't handle packages well enough to move scopes safely. I think they score 10 points for a rugby conversion, 3 points for a touchdown. The people on the forum move stuff around gently for each other in an informal way. Look up FCS - forum courier service and you'll see what happens.

3) High end or low end, the trip-ups for the unwary in fixing solid state amplifiers are the same. The posh amp with discrete transistors may be easier to get bits for than the power modules that infested the cheapies and the mid range stuff of the last few decades. The 405 is a well trampled path, all of its weaknesses are well documented. Much better to work on than a lot of the esoteric stuff where manufacturers guarded service info like the crown jewels, and sanded the type numbers off of transistors and ICs. Quad were open about their circuitry, they even (Walker and Albinson) wrote an article explaining it for Wireless World. Quad still exist and will fix your amp if you want them to.

You might not need a scope often, but you'll find a lot more things you can do, not only because of having a scope, but also because of the things you'll learn from one.

David
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Old 14th Jan 2019, 1:24 am   #18
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Default Re: Quad 405-2 help

Hi Dave W, thanks for your welcome. I have been encouraged by the response I’ve had here. I realised that ‘subscribe’ wasn’t perhaps the right term but new member or similar seemed a bit presumptuous for my first post ��.

David and Ted, I’m trying to resist being drawn to the ‘dark side’ with your suggestions of getting a scope, however, I’ve seen a Hameg HM 312 dual trace scope advertised. It says it’s fully working and comes with a new pair of 100 MHz probes, associated hardware and the manual. Quoted price is £55 and, as a bonus, it’s reasonably near me so I can avoid the perils of a courier. Any thoughts?

Ray
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Old 14th Jan 2019, 9:00 am   #19
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Default Re: Quad 405-2 help

Well, used Hamegs are generally a good bet, and that sounds like a nice package to get you going. They use off-the-shelf components for the most part, too, so are straightforward to fix if needs be - but I'd put money on it working "out of the box". My nearly-thirty-year-old Hameg has nothing wrong with it, and it's been well used, believe me...
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Old 14th Jan 2019, 9:58 am   #20
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Default Re: Quad 405-2 help

Hameg scopes are very reputable. It's the brand name used by Rohde & Schwarz of Munich, a very well respected company and the big competitor of HP/Agilent/Keysight. As Ted says, they are well designed and made out of mostly standard parts.

The HM312 is a good basic scope, minimal fear-factor and will do an awful lot of work for you. Mention of a pair of probes suggests it's the HM312-8 model which has twin input channels which is a very useful advance on the basic model. If it's in good order, it's ideal for you.

You'll find on here that for a lot of people, a multimeter is their primary bit of test equipment. It's what they go probing with first. It's what they feel most comfortable with. They're cheaper in new prices, they look simpler, and if you worked in a service centre, you were largely limited to them, with one shared scope for special jobs. Cost was the real reason.

You'll also find on here a smaller number of people whose background was in research and development, where they were working on prototype equipment and problems that had never been seen before. You'll find these people use scopes as their 'eyes' and only switch to a meter when they need to know a voltage particularly accurately, or measure resistance.

Part of the difference came from the funding available, less from fear factor.

If you probe with a voltmeter, you get to see the mean DC voltage quite accurately, but don't see any other clues.

If you probe with a scope, you don't see voltages with anything like the accuracy of a meter, but you get enough for most things. hat you do get to see are any super-imposed signals, wanted ones and unwanted ones. You can see ripple on power supplies to hunt and kill hum problems, you can see spurious oscillations (the bane of Quad 303s) You can check TV waveforms, logic waveforms etc.

All told, YOU become a lot more versatile with a scope probe in your hand. Because you can see what's going on and not have to deduce it from spot voltage readings, it becomes easier for us to explain and for you to understand.

First you learn wow to set it up for a basic measurement, and from there on it's just a matter of acquiring tricks for special circumstances.

Together a multimeter and a scope are very complementary.

David
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