Quad 405 overheating

[KeithsTV]

I can vouch for the fact that the output crowbar does work as R7 in the left channel of my 405 went open circuit. The output tried to go to -50v but the crowbar caught it and protected the speaker. The heatsink got extremely hot but the fuse didn't blow. Fortunately It didn't stay in that condition for long and replacing R7, the +ve feed to the op amp, brought it back to life.

Keith

[Leon Crampin]

Keith, you've just reminded me why I don't especially like my 405. I too had a failure of R7 and before replacing it did a tolerancing exercise on dissipation (worst case mains voltage, resistor at minimum value etc.) Mr. Walker got it wrong - the specified component is unsuitable for the job. I replaced it with a 1W tin oxide resistor.

The 405 is a clever design but a terrible execution. I've had mine since new and have had many component failures including the mains transformer (failed by becoming intolerably noisy due to lamination buzz) and some other important parts including of course R7. Apart from bad design, many components are of very poor quality compared with contemporary Japanese offerings and its reliability has been the worst of any electronic equipment I own - with the 33 coming second, although better due to there being no appreciable power dissipation. Why do the blasted buttons come loose and why is there slop in the ganged volume control which gives a balance shift as it is adjusted in each direction?

Rant mode now off.

Leon.

[Ed_Dinning]

Hi Jeffery, I did some work with the transistor supplier to Quad many years back. They were selling 2N3773 die, but specially selected and matched devices on both Ft and gain. It will probably work Ok without this, but there may be a slight increase in distortion.

Ed

[mastermanx2001]

Shouldn't the circuit correct for that, thought that the Quad 405 system cancels out distortion in the dumpers. I never did fully understand how this system works. I wonder if someone can explain it simply.

[mastermanx2001]

Originally Posted by Leon Crampin

The 405 is a clever design but a terrible execution. I've had mine since new and have had many component failures including the mains transformer (failed by becoming intolerably noisy due to lamination buzz) and some other important parts including of course R7. Apart from bad design, many components are of very poor quality compared with contemporary Japanese offerings and its reliability has been the worst of any electronic equipment I own - with the 33 coming second, although better due to there being no appreciable power dissipation. Why do the blasted buttons come loose and why is there slop in the ganged volume control which gives a balance shift as it is adjusted in each direction?

Rant mode now off.

Leon.

I have to agree with certain points that Leon makes, yes, the buttons on the 33 can crack and fall off (mine haven't in about 30 years) and yes why did they consistantly use a ganged volume control with slop in it giving imbalance at low volumes? Incidentally, if the 33 is fed with the correct voltage inputs, its not too bad. Oh, yes and my 405 had the transformer buzz from new. It was only mild and has not got any worse over the years. I could add to Leon's list the poor durability of the paint on some Quads, the lack of an on off switch on early 405s and 303s and in my view the standard 405 is about the worst sounding amp that Quad made.

But, I have had no failure in mine (it was made into a 405-2) in 1987 and 405s/405-2s that I have upgraded for other people have never given any trouble in years (holding breath). Jeffrey says his has given 30 odd years service, so its not all bad. A good bit is that the second hand value of Quad gear is not bad (if one is selling!) and Quads are easy/cheap to service/ repair.

[KeithsTV]

Like Leon the mains transformer in my 405 started humming and after getting a quote for a replacement I put up with it until it finally got worse and I had to replace it. I have a 44 and FM4, all from new and both have had faults which fortunately have been easy to fix being due to poor soldering. The through hole links on the 44 were particualy bad. Those have been the only problems in the 30 years I've had the 405 and 20 years I've had the 44 & FM4.

Keith

[ppppenguin]

Now I remember the mains transformer hum problem. At the time (1978/79 in my case), Quad were issuing replacement transformers FOC to anyone who complained. These were normally fitted by dealers or by Quad themselves but I actually did mine myself. I've probably still got the old transformer somewhere.

I can only speak from my own experience but this is the first failure I've had on my Quad 33/FM3/405 system in 30 years. The only other problem was the balance control going a bit scratchy which was sorted by a shot of lubricant. I have a spare volume control assembly ordered from Quad years ago as a contingency.

[ppppenguin]

Fitted the new 2SD424, found a 3.15A fuse to replece the blown 4A one and about to refit with current limiting resistors in the +/-50V. Thought I'd better check the crowbar first. The TIC226 triac in the faulty channel is open circuit. I've ordered a couple from RS. I just hope the crowbar in the ESL63 did it's job properly.

Getting the crowbar board out of the case is an awkward job.

[mastermanx2001]

Hi Jeffrey, so progress, current limiting resistors in supply, good idea. I use some lamps in series sometimes for the current limiting, also gives visual indication of whats going on! Several Christmas tree lamps in series as I have a lot of those!

Somewhere I have a video of a repaired NAD 3130 with the lamps in series and playing music. Entertaining!

[jamesperrett]

Originally Posted by KeithsTV

Like Leon the mains transformer in my 405 started humming and after getting a quote for a replacement I put up with it until it finally got worse and I had to replace it.

I have 2 405-2's. One has a noisy mains transformer while the other has started to crackle a bit on one channel. I had to replace a couple of transistors in the crackling amp a while ago but it worked OK for a few years after that before showing the current problem.

I'd be interested to know how much a new mains transformer would cost - how much was your quote?

I'm tempted to replace the amplifier modules in the crackling amp with a couple of the Hypex modules which seem to be gaining a good reputation.

Cheers

James.

[Leon Crampin]

James, When my 405 developed transformer buzz I was earning a lot of money and simply ordered a new one from Quad. I remember at the time thinking how expensive it was for a pretty shoddy looking C-core transformer potted into a tin.

If it happened again I would unpot the transformer and place a large worm drive hose clip around the core and tighten it. I've done this on other C-core transformers and it's very effective if you clamp it up tightly in the right spot. To re-pot the transformer, you might have to cut a slot in the shield to accomodate the hose clip adjuster.

If you have a crackly amplifier check R7 and R8 as detailed. The specified component is an Iskra UPM050 which is a cheapskate 0.5W carbon film resistor. Replace with 1W metal film or tin oxide. Other problems I have had are solder fatigue on the joints of R30/31 and failure of the bootstrap electrolytic C10 and also C5 which causes a massive 100 Hz hum.

Shoddy stuff, I'm afraid - despite a clever design. Until I looked in the file I'd forgotten how many times I've had to fix it.

Leon.

My bottomless filing system has just produced the transformer invoice - £42 in Oct '87.

[KeithsTV]

The price of the transformer had risen to £98.62 by 1994 when I replaced mine. I'd hate to think what it is now. Make sure you're sitting down when you find out

Keith

[mastermanx2001]

I think its quite common to replace this with a toriodal transformer, think there is plenty info on the internet about it. But if one wants originality, suppose that will cost more.

[jamesperrett]

Thanks for the info Leon and the cost of the transformer Keith. I'll check those resistors when I get the amp out of storage but I might be tempted to make one good amp out of the two and just use the case of the other one for a DIY Hypex based amp.

Cheers

James.

[ppppenguin]

Ran up the repaired channel on the bench and it seemed happy. Repaired the crowbar with a new triac. Put everything back together and one channel didn't work. Only a speaker lead fallen off. Phew!

One channel, the repaired one, sounds horrible with obvious fizzy distortion. Not the speaker, I swapped them over. Might not be the 405 at all, could be the 33. Seems to be getting better when it's left on for a while. I'm going to mod the 405 to fit phono inputs (I'll be able to swap the channels easily then) and something better than those wretched speaker connectors that won't take anything more than very modest size cables.

I'm not buying an expensive kit of parts. The phonos will be out of the junk box and I'll try to find some ordinary binding post terminals or maybe 4mm wander sockets. What I ought to use are the professional XLR speaker connectors but I don't have any and can't see a way to make them fit.

[mastermanx2001]

Hi Jeffrey, you had me sitting on the edge of my seat here, thank goodness speakers are OK. I expect you saw my 405 in the photo earlier in this thread. I just used cheap phono sockets wired in parallel to the existing Din socket. I expect your 405 has the protection circuits mounted on the speaker spring connectors. This makes thing difficult to change speaker terminals.

The 405 in the pic has got 405-2 boards with the protection circuitry on them, so it is easy to fit 4mm sockets. What I did before this (not elegant but works) is to solder some wire on some 4mm sockets, insulate the joint and have these behind the amp.

I also fitted the on off switch.

[retailer]

Back in the early eighties I worked part time for a local music/pa hire company and I built dozens of 405 clones for pa use, I did the whole lot from winding power trannies to pcb etching to chassis making, I only ever used 2N3773's , although I did hear of 2N3442's being used. In 2.5 yrs that I was there apart from the odd one that was dropped, not one ever came back for repairs.

[Leon Crampin]

As a result of one of its many faults my 405 once consumed a pair of output transistors. As I worked for a firm which bought huge numbers of best quality TO-3 power devices, I pressed a pair of their selected 2N3442s into use. These were very high quality hometaxial devices in steel cans made by RCA in Belgium (the letter "L" stamped on the flange of some RCA devices signifies packaging at their plant in Liege). They were checked and matched on a curve tracer before fitting.

These have given no problems whatever but I have to stress that not all 2N3442s are the same - I used to spend quite a lot of time dissecting them and even unsoldering the chips from the headers. If you can find old steel can RCA devices, these are the best for this job.

Leon.

[ppppenguin]

I've done the 33. Replaced all the signal electrolytics (probably should have done the PSU ones too) and a shot of switch cleaner in all the switches and pots.

Things sound rather better now with much less difference between the 2 channels. Previously one channel was sounding rather muffled. But there's still a tendency for fizzy distortion on one channel. Definitely not the speakers, could be the 33 or 405. Now I've found my 4 pin DIN to phonos lead I can do some careful channel swapping experiments.

I suppose I shouldn't complain about having to do a little work on 30 year old equipment.

[mastermanx2001]

Hi Jeffrey

I'm fairly sure you don't really need tips from me, but here goes anyway. You've got you special 4 pin Din to phono, perhaps feed the 405 (both channels) from a mono source.

Monitor across the two live outputs with and old speaker in series with a suitable wirewound resistor, perhaps 25 to 100 Ohms.

Of course, if the two channels are the producing the exactly same there will be no output at the speaker.

Used this many times to check amps.

I found quite a difference between channels on one of my Quad 303s this way. I then found the culprit, (resistor) replaced it and did the test again, virtually no output, so channels were close to each other.