Quad 405-1 Woes

[sennsai]

I recently bought a very early 405 from a friend for my Quad 63 speakers and was pleased with the sound, but after a few days one channel went quiet. Upon examination both 4A fuses had blown, the MJ 15003s were shorted along with the 22 ohm resistor. The board is the earliest type and has the crowbar board attached to the speaker sockets, the IC is a TL071, and a Quad service in 1993.

Now I suspect, although not 100% certain, that one of the (temporary) speaker lead connections had contacted, despite my confidence in the insulation! The speakers are ok. I found that the other board had 2N 3773 outputs fitted.

I removed the crowbar board to check the components and find that there is a noticeable difference in one of the triac resistance measurements, so I’m going to replace these. I’ve renewed the small electrolytic.

Any comments welcome.

[nutteronthebus]

have a look at R7/8 as they are under rated and take a lot of silicon out when they fail replace with 3K3 1w type off set from the PCB

[KeithsTV]

I've had R7 & R8 fail on my 405. On one channel I was lucky and the output transistors were OK but on the other channel both output transistors were short and the 4A fuses blown. It's also worth checking the protection board as the triac was also shorted.

If they haven't been replaced it's also worth changing the electrolytic caps on the boards.

Keith

[Craig Sawyers]

Failure mode:

R7 or R8 go open being underrated for power *and* tight to the board. That causes the output goes up to rail. Crowbar fires blowing one or both output transistors and possibly the drivers too.

Then the crowbar blows up.

Then the fuses blow.

Not Quad's finest design moment.

Check the electrolytics sited above a high power resistor. That capacitor cooks for a long time and goes open. Replace them.

Craig

[sennsai]

Advice taken on board!

[alanworland]

I have the later 405-2 and this has the same problems with under rated R7 and 8. I sent my two boards back to Quad and they were returned with these changed to higher wattage types and mounted away from the board - how they should have been in the first place!
Great amp, effortless in its delivery.

Alan

[sennsai]

I've had a series of amps with these esl 63s, modded NAD 3020, a Rogers e40, a Marantz 7200 90w, and an Audiolab 8000- this one extended the treble but they all still clipped when the volume was turned up a bit. The short trial with the 405 sounded much better all round, dynamics, midrange, so I'm looking forward to finishing the mods etc.

Both diacs were found to be defective. This board will be mounted inboard separate from the 4mm sockets that I've ordered. The Velleman speaker protection kit looks a good bet too.

[Craig Sawyers]

The real way to protect the speakers in the event of a fault is to dispense with the daft silicon-destroying crowbar, and fit either a relay based protection, for example https://sound-au.com/project33.htm or http://www.signaltransfer.freeuk.com/protect.htm

Or a FET based protection circuit https://www.diyaudio.com/community/t...relays.299355/ or https://neurochrome.com/products/guardian-86

For what it is worth, I use the signal transfer relay based ones on my amps (but not on a 405). That has DC offset protection, thermal sensing on the heatsink, a delay on turn on to prevent start up transients getting to the speakers, and very fast turn off. for the same reason.

Just looked at the Vellman - and that looks exceptional value!

Craig

[vinrads]

The Vellman kit gets my vote ,easy assembled 240 volt mains , relay switching of the speakers delay start very pleased with the two that I have fitted , Mick.

[sennsai]

Which of these would be best for a mains filter cap?

1. 0.22uf 250v ac X2 MKP
2. 0.1uf 630v
3. Hunts g224 0.22uf 400v

[tony brady]

number 1. the X2 filter cap as that is rated for being across the mains

[fetteler]

Originally Posted by tony brady

number 1. the X2 filter cap as that is rated for being across the mains

Yes, absolutely.

Steve.

[sennsai]

I'm feeling my way along with the basic upgrade on the early 12368-6 circuit boards (based on Bernard Ludwig's article). This unit also has a "crow-bar" board behind the speaker connectors.

Some points that I would like help-

There are short links already fitted behind the two R36 sensing resistors?

Can the bi-directional switch T1 2N4992 be inserted either way round?

The C8 mod appears to be a bit vague in the article, is it a worthwhile upgrade?

Thank-you in advance.

In the pic, some of the components have been already changed

[KeithsTV]

As mentioned earlier replace R7 & R8 with 1W resistors. Failure of these is a known issue and can take out the output transistors and the crowbar triac.

Keith

[sennsai]

yes that definitely will be done!

[Top Cap]

Here is the tale of one that I repaired. Hope it may be useful.
http://g4cnh.com/public/A_Quad_405_repair.pdf

[sennsai]

Top Cap-thanks for the your link, although my boards are the early M 12368-5.

I've been following Keith Snooks circuit which upgrades my boards to DCD Mod-3. I've managed to get most of the general components, however there's a puzzle over C8 (120pf) and he suggests this at 630v? I have some small suflex ones but I think these are 63v rated. Perhaps the re-rating is because C8 now sits in a different part of the circuit? Are there any alternative types?

[JonSnell]

The maximum voltage across C8 will be no more than rail + 50% for safety so a 150volt ceramic, mica or poly cap will be fine.
One point though, ceramic caps can become like a microphone in high gain stages but as this is not high gain, the amplifier will be fine.
My first choice would be ; https://uk.rs-online.com/web/p/mica-capacitors/7441534/

[Radio Wrangler]

There are several different sorts of ceramic used to make ceramic capacitors, usually known by the tempco codes assigned to them.

NP0 and C0G have got low temperature coefficients (around +/- 30ppm/C) they are not microphonic and ave good for linearity. The capacitance /volume limits them to under 2200pF for normal 50v parts and package sizes.

X7R, Y5V and several newer materials can pack in much more capacitance, BUT they are poor in stability over temperature, some are distinctly non-linear with applied voltage, most are microphonic. OK for decouplers in non-critical places. The newer materials were developed as alternatives to electrolytic capacitors. But in some respects, the electrolytics out-perform them!

David

[sennsai]

I finally decided on Snooks 405 2.5 version, except that he deletes the ELS 57 link.

As I have these speakers, is the addition I've made to the circuit correct? The placement of R2 there adds some confusion (for me) somewhat. R9 is 180 ohm.