Quad 405-2 advice

[Jez1234]

Spot on... other than Tr2 is a common emitter amplifier not an emitter follower, to be pedantic.

Not only is the change in current due to driving the load insignificant but the op-amp has very considerable PSRR so the matter is a none issue. If willing to risk further switch on thump issues then electrolytics of say 100uF 16V could be put across the Zeners to bring the impedance down with increasing frequency, hence counteracting the falling PSRR of the op amp with frequency. I did this many years ago on a 405 original version and it worked fine.

[vintageampguy]

Great discussion! Ok, i get it regarding replacing the zeners and opamp.
However, given the age, and heat stress on that corner of the board, does that not indicate a preemptive replacement?

[Jez1234]

I'd replace the Zeners with something a little larger just for a bit better heat dissipation. 1 or 3W types. leaving the leads long on the solder side and bending them to follow the PCB tracks can help a little and improve the reliability of hot solder joints here also. For some extra peace of mind you could replace the dropper resistors with 3W wirewounds.

[fetteler]

Originally Posted by Craig Sawyers

...The crowbar is a device that simply burns out the unobtanium power transistor dumpers and class A drivers in the event of a component failure elsewhere in the amp...
Craig

This does indeed seem to be what happens but I sort of remember reading (years ago) from Quad that the idea was the output devices were rugged enough to withstand the protective short circuit across the speaker terminals and the resulting current would simply blow the fuses in the power supply lines.

Have I miss remembered?

Steve.

[Jez1234]

I can't recall which of the ten commandments of electronics it is but I think it goes something like "An expensive semiconductor will always blow before the fuse that was meant to protect it"

[Craig Sawyers]

I've been running some Zeners through a curve tracer (Peak DCA75). I've run a selection from 500mW, 1.3W and 5W. The 500mW ones have a very sharp knee, and this becomes softer the higher the nominal power. So no surprise there.

However, at the sort of current they are running at in the 405 (8-ish mA) it really does not matter too much. All of them are well away from the knee.

But since they are dissipating so little power - 100-ish mW - a 500mW Zener should be just fine. The only precaution I'd take if replacing them like with like, or using a slightly higher power, is to space it a little off the board.

A point that is often missed - and even Tektronix have made that mistake - is the rated power dissipation of components is in free air. So soldering parts tight to the board, particularly if they are running close to their rating, is asking for long term problems.

It depends on the board issue, but on early ones C10 is right in the heat plume from high dissipation R30 and R31 on one channel only. So C10 cooks on one channel only.

In later issue boards they moved R30 and R31 well out of the way of electrolytics.

Craig

[Craig Sawyers]

Originally Posted by fetteler

This does indeed seem to be what happens but I sort of remember reading (years ago) from Quad that the idea was the output devices were rugged enough to withstand the protective short circuit across the speaker terminals and the resulting current would simply blow the fuses in the power supply lines.

Have I miss remembered?

Steve.

Yup. The classic failure is when the 3k3 resistors feeding the zeners burns out. One of the power transistors hits the 50V rail, so the crowbar operates and shorts the saturated (but otherwise fully functional) transistor to ground. That transistor merrily blows - usually going into second breakdown and shorted for a short while, and before the bond wires melt it blows the crowbar up. Then the fuses blow. Usually a hell of a mess to sort out.

Been there, got the badge. Learned the hard way to always replace the 3k3 resistors with ROX 1W or 2W (or similar) spaced off the board. And mount replacement electroytics on the other side of the board so they don't cook if you have early boards (The op has later boards so should be OK).

Craig

[Jez1234]

Been there a couple of times also and yep messy to the extent of holes burnt through the PCB and tracks vaporised on one of them.... it lived after some work with a dremel and wires added to bridge gaps plus new semi's.

[GrimJosef]

I'm not very familiar with the 405 pcbs, and the devil will be in the detail, but if we 'float' a higher-rated (=heavier) 3k3 into free space using just its leads' solder joints to carry the weight is there a risk of the joints eventually cracking, or the pcb track lifting ? Might it be better to use Veropins or similar ?

Cheers,

GJ

[Jez1234]

This is one of the reasons, along with heat dissipation, that I suggested the leads should be formed to follow the PCB tracks for a little way before soldering. 1/2" will be fine.

[vintageampguy]

Yet another question. The unit is labelled 110v input, of course, it's 120v here in Canada. The service manual wiring does not match up with what's actually inside, either for 110 or 120. Not a big deal, but maybe it would operate slightly better with the correct voltage wiring, if I can trust the service manual picture?

[SteveCG]

What does the Amplifier provide in the way of mains voltage options?

Certainly 120 V into a 110 V amplifier is going to increase the heat dissipation in the unit - the question is where in the amplifier. If the amplifier is already close to the thermal design edge with the correct nominal mains voltage then overvoltage may be a recipe for a short life...

[Trigon.]

Originally Posted by vintageampguy

Yet another question. The unit is labelled 110v input, of course, it's 120v here in Canada. The service manual wiring does not match up with what's actually inside, either for 110 or 120. Not a big deal, but maybe it would operate slightly better with the correct voltage wiring, if I can trust the service manual picture?

The diagrams shown are correct, and yours is indeed wired for 110v. (Don't worry about the additional short links to the 10v windings - the other ends of the windings aren't connected.)

Running it at 120v isn't good...

(e.g. Say the rails have gone from 50v to 55v, those 3k3 resistors are now dropping 40v rather than 35v so their dissipation has increased by 30%!)

Cheers

[Craig Sawyers]

Incorrect email deleted

[vintageampguy]

Thanks, I have corrected the transformer wiring. Voltage at R7 dropped from 57 to 52.

[vintageampguy]

I installed a new bridge rectifier, and the turn on thump is substantially smaller.

[Jez1234]

Interesting! A bit of a stock fault on the 405 is failed bridge rectifiers. They used a rather odd form factor 4 in a line terminals AEI or Plessey or Marconi branded rectifier (they appear to be the same thing with alternative branding) and I've known several fail.

There was a thread about intermittent bridge rectifiers a few days ago here! I wonder if one diode was only working after a few cycles and this caused one rail to rise quicker than the other?

I had a "lucky" escape with a 405-2 I'd been working on just about a year ago. It had come in for new caps and a general service. All done no problem. It passed max power into dummy load tests etc etc on the bench and I'd used it for two evenings in my own hi fi. I was about to pack it for courier collection when I decided to leave it till tomorrow. Later that night, with it still in my system, I decided to play a few CD's and switched it on only to be greeted by a brief blink of the power LED then silence. As I'd had a few bridge rectifier failures before on these my first (and correct) guess was the bridge had just gone S/C and taken the fuse out. Imagine having to explain to the customer that it wasn't my fault and that it was quite impossible to have predicted that it would fail the first time he plugged it in!

[vintageampguy]

I spoke too early - in the shop, the thump on the scope was much smaller, but when I installed it in the stereo, the thump was bad, maybe even worse than before.

[vintageampguy]

Originally Posted by vintageampguy

I spoke too early - in the shop, the thump on the scope was much smaller, but when I installed it in the stereo, the thump was bad, maybe even worse than before.

Any ideas on next steps? I was thinking about the filter caps next, my (in circuit) ESR test seemed ok.

[Jez1234]

I really wouldn't worry about it unless it is so bad as to be very much not normal for that model. Virtually all amps make some degree of switch on thump unless there is a relay delay. My own amp makes such a huge switch on thump it could well be damaging to speakers so I don't connect the speakers until a couple of minutes afterwards. It's no big deal.

If it bothers you that much then check that the smoothing caps are fairly close to each other in value, if one's 9500uF and the other is 10600uF then that's fine. They are not close tolerance parts. I'm talking one's 3000uF and the other is 10000uF here! If OK then check resistors are within tolerance and if all OK then recap it (the small ones on the PCB) with one's of the correct value being careful to observe polarity and avoid solder splashes etc. If both channels are the same then suspect the smoothing caps having wildly different values.

I had a customer bring in a 405 mkI a few months back that had seen a LOT of use and was still on it's original smoothing caps from 1977! They were completely worn as expected and causing some mains hum. They measured around 3500uF and had excessive ESR but the customer was happy to leave it like that