Quad 405 overheating - Page 3 - UK Vintage Radio Repair and Restoration Discussion Forum

ppppenguin · 3rd Jun 2008, 9:07 am

Paul, thanks for reminding me of that method. I'd totally forgotten about it.

ppppenguin · 28th Jul 2008, 12:17 pm

Things come back to bite you.

When I repaired my 405 I decided to recap the 33 preamp. All done, put it back together, worked OK. Except that I didn't test the phono input at the time and now it's very dead on both channels. Checked that the phono amp and adapter cards were well seated. There's 12V getting to the amp card. It's not an easy card to probe but TR302 had 5.8V on its collector, TR303 had 6V. The latter seems a bit high compared to the nominal 5.4V.

Touching pins 3 or 14 of the adapter card gives a slight crackle in the speakers so the selectors switches should be OK.

I suppose I could have got an electrolytic backwards on both channels.

Any other ideas before I dive in and faultfind it properly?

PS: I've checked the electrolytic and they are all the right way round. I ought to check the lead from the deck to the amp but that's probably OK since I didn't get a buzz from a damp finger on the amp input.

ppppenguin · 28th Jul 2008, 1:27 pm

That'll teach me not to jump in until I've looked properly. The multiway connector from the SME arm to the cable feeding the amp had fallen off under the deck. The reason I didn't get any hum when touching the phono input was because it's hard to get at the pins on the back of the DIN connector and I was probably not making contact with the actual inputs.

Nickthedentist · 28th Jul 2008, 2:27 pm

Ha ha! I'm glad I'm not the only one that that kind of thing happens to. Glad it's all ok again.

ppppenguin · 4th Dec 2009, 5:54 pm

I still felt the 405 was making odd fizzy noises so I finally got it back on the bench today. This time I could see gross HF instability at about 1MHz on one half cycle of a sine wave. Only happened at medium to high volume and when the output was loaded. No idea why I hadn't seen it before.

Was probing around, looking for inspiration and the magic smoke escaped. A resistor had burnt out, so had the new output transistor. The fuses, of course, stayed intact. I replaced the resistor and the transistor with a 2N3773 that I had to hand. I used current limiting resistors at first which was a good thing because I had dropped a solder splat on the PCB. Cleared that and it's all been 100% fine. No trace of instability, sounds like it should. I can only conclude that there was somethhing funny about the 2SD424 that I used as a replacement first time round.

Odd quirk. I was using a big 20R open frame linear rheostat as the dummy load. It whistled along nicely with the test tones.

cmjones01 · 4th Dec 2009, 6:10 pm

Quote:

Originally Posted by ppppenguin

Odd quirk. I was using a big 20R open frame linear rheostat as the dummy load. It whistled along nicely with the test tones.

When soak-testing power amplifiers into a dummy load after repair, I find it quite handy to put a magnet near the load resistors. They'll then 'sing' quietly in time with the test tone or music, but you can run the amplifier at high outputs without the risk of deafening anybody! I've found it quite handy to be able to get on with something else and still hear in the background whether all is well with the soak test.

Chris

Herald1360 · 4th Dec 2009, 7:35 pm

I expect the winding on the rheostat was carrying enough current for its tendency to fly apart under the influence of its own field to move the winding enough to rattle against its former.

Sort of a small scale AC version of the tonnes of hoop stress that MRI magnets are subjected to when run up to field

Chris

VHFHerald · 5th Dec 2009, 1:07 pm

The DC-Daylight website has drawings for all incarnations of the 405.

http://www.dc-daylight.ltd.uk/Valve-...chematics.html

Kevin G8FYK · 6th Dec 2009, 1:00 am

Just something of general interest to add here.
I used a Quad 405-2 many years ago as a mobile disco amplifier, for a good ten years.

In the earlier years it ran into a pair of HH 2x12 dual concentric speaker cabs. Even pushed hard (and getting very hot) it still put out very loud high quality distortion free music.

In the later years it was used within a three way active xover system as the amp to drive the treble horns. Again it did it's job without complaining, and at high volume levels was still amazingly free from distortion.

More to the point, it never ever let me down. It never even cut out once. It just went on and on (and with it's 100w per channel output it was often louder and certainly much clearer than any other 100+100w disco amps I heard).

The only mod ever I did was to change the input to phono sockets, and rewire the output to 1/4" jack sockets mounted in a little diecast box fastened to the rear using the existing case fastening screws.

It ended it's days as a home Hi-Fi amp, but due to it being too powerful for that so much more sedate environment I eventually sold it. For £50! And I now regret it!

dave cox · 8th Dec 2009, 5:09 pm

Quote:

I can only conclude that there was somethhing funny about the 2SD424 that I used as a replacement first time round.

Fake ?
Were they marked TOSHIBA ?
They don't seem to be a current production item ...

dc

ppppenguin · 8th Dec 2009, 5:24 pm

One other very remote possibility. I had to clean a solder splat off the PCB to get it to work. I suppose it's possible that there was another solder splat from the first time I replaced the transistor and this just disappeared when I was doing the 2nd repair.

The fake transistor argument has some merit. The problem only showed up when the dumpers were actually having to do something.

orbanp1 · 14th Dec 2009, 3:22 pm

Hi Jeffrey,

You wrote:

> A helpful fellow, not a forum member, emailed me and commented on some
> aspects of the crowbar design which could give trouble. He also suggested
> that the cooked choke could have shorted turns and I will check this on an
> LCR meter.

Would you care to share that information on the crowbar design?

Thanks, Peter

ppppenguin · 14th Dec 2009, 3:27 pm

I've had a look and can't find the message about the crowbar. I did find a faulty triac which I replaced. I did some tests to prove that the crowbar does indeed operate. I suspect the crowbar didn't operate when I had the original failure - the fuse should have blown pretty much immediately if it had, without cooking a choke first.

Station X · 29th Dec 2009, 10:17 pm

Three posts moved to a new thread here:-

https://www.vintage-radio.net/forum/...ad.php?t=49014

3rd Jun 2008, 9:07 am	#41
ppppenguin Retired Dormant Member Join Date: Dec 2003 Location: North London, UK. Posts: 6,168	Re: Quad 405 overheating Paul, thanks for reminding me of that method. I'd totally forgotten about it.

28th Jul 2008, 12:17 pm	#42
ppppenguin Retired Dormant Member Join Date: Dec 2003 Location: North London, UK. Posts: 6,168	Re: Quad 405 overheating Things come back to bite you. When I repaired my 405 I decided to recap the 33 preamp. All done, put it back together, worked OK. Except that I didn't test the phono input at the time and now it's very dead on both channels. Checked that the phono amp and adapter cards were well seated. There's 12V getting to the amp card. It's not an easy card to probe but TR302 had 5.8V on its collector, TR303 had 6V. The latter seems a bit high compared to the nominal 5.4V. Touching pins 3 or 14 of the adapter card gives a slight crackle in the speakers so the selectors switches should be OK. I suppose I could have got an electrolytic backwards on both channels. Any other ideas before I dive in and faultfind it properly? PS: I've checked the electrolytic and they are all the right way round. I ought to check the lead from the deck to the amp but that's probably OK since I didn't get a buzz from a damp finger on the amp input. Last edited by ppppenguin; 28th Jul 2008 at 12:32 pm.

28th Jul 2008, 1:27 pm	#43
ppppenguin Retired Dormant Member Join Date: Dec 2003 Location: North London, UK. Posts: 6,168	Re: Quad 405 overheating That'll teach me not to jump in until I've looked properly. The multiway connector from the SME arm to the cable feeding the amp had fallen off under the deck. The reason I didn't get any hum when touching the phono input was because it's hard to get at the pins on the back of the DIN connector and I was probably not making contact with the actual inputs.

28th Jul 2008, 2:27 pm	#44
Nickthedentist Dekatron Join Date: Jan 2005 Location: Oxford, UK. Posts: 17,846	Re: Quad 405 overheating Ha ha! I'm glad I'm not the only one that that kind of thing happens to. Glad it's all ok again.

4th Dec 2009, 5:54 pm	#45
ppppenguin Retired Dormant Member Join Date: Dec 2003 Location: North London, UK. Posts: 6,168	Re: Quad 405 overheating I still felt the 405 was making odd fizzy noises so I finally got it back on the bench today. This time I could see gross HF instability at about 1MHz on one half cycle of a sine wave. Only happened at medium to high volume and when the output was loaded. No idea why I hadn't seen it before. Was probing around, looking for inspiration and the magic smoke escaped. A resistor had burnt out, so had the new output transistor. The fuses, of course, stayed intact. I replaced the resistor and the transistor with a 2N3773 that I had to hand. I used current limiting resistors at first which was a good thing because I had dropped a solder splat on the PCB. Cleared that and it's all been 100% fine. No trace of instability, sounds like it should. I can only conclude that there was somethhing funny about the 2SD424 that I used as a replacement first time round. Odd quirk. I was using a big 20R open frame linear rheostat as the dummy load. It whistled along nicely with the test tones.

4th Dec 2009, 7:35 pm	#47
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: Quad 405 overheating I expect the winding on the rheostat was carrying enough current for its tendency to fly apart under the influence of its own field to move the winding enough to rattle against its former. Sort of a small scale AC version of the tonnes of hoop stress that MRI magnets are subjected to when run up to field Chris

5th Dec 2009, 1:07 pm	#48
VHFHerald Retired Dormant Member Join Date: Nov 2009 Location: Brighton Posts: 20	Re: Quad 405 overheating The DC-Daylight website has drawings for all incarnations of the 405. http://www.dc-daylight.ltd.uk/Valve-...chematics.html

6th Dec 2009, 1:00 am	#49
Kevin G8FYK Retired Dormant Member Join Date: Jan 2009 Location: Manchester Posts: 17	Re: Quad 405 overheating Just something of general interest to add here. I used a Quad 405-2 many years ago as a mobile disco amplifier, for a good ten years. In the earlier years it ran into a pair of HH 2x12 dual concentric speaker cabs. Even pushed hard (and getting very hot) it still put out very loud high quality distortion free music. In the later years it was used within a three way active xover system as the amp to drive the treble horns. Again it did it's job without complaining, and at high volume levels was still amazingly free from distortion. More to the point, it never ever let me down. It never even cut out once. It just went on and on (and with it's 100w per channel output it was often louder and certainly much clearer than any other 100+100w disco amps I heard). The only mod ever I did was to change the input to phono sockets, and rewire the output to 1/4" jack sockets mounted in a little diecast box fastened to the rear using the existing case fastening screws. It ended it's days as a home Hi-Fi amp, but due to it being too powerful for that so much more sedate environment I eventually sold it. For £50! And I now regret it!

8th Dec 2009, 5:24 pm	#51
ppppenguin Retired Dormant Member Join Date: Dec 2003 Location: North London, UK. Posts: 6,168	Re: Quad 405 overheating One other very remote possibility. I had to clean a solder splat off the PCB to get it to work. I suppose it's possible that there was another solder splat from the first time I replaced the transistor and this just disappeared when I was doing the 2nd repair. The fake transistor argument has some merit. The problem only showed up when the dumpers were actually having to do something.

14th Dec 2009, 3:22 pm	#52
orbanp1 Heptode Join Date: Nov 2009 Location: London, Ontario, Canada Posts: 675	Re: Quad 405 overheating Hi Jeffrey, You wrote: > A helpful fellow, not a forum member, emailed me and commented on some > aspects of the crowbar design which could give trouble. He also suggested > that the cooked choke could have shorted turns and I will check this on an > LCR meter. Would you care to share that information on the crowbar design? Thanks, Peter

14th Dec 2009, 3:27 pm	#53
ppppenguin Retired Dormant Member Join Date: Dec 2003 Location: North London, UK. Posts: 6,168	Re: Quad 405 overheating I've had a look and can't find the message about the crowbar. I did find a faulty triac which I replaced. I did some tests to prove that the crowbar does indeed operate. I suspect the crowbar didn't operate when I had the original failure - the fuse should have blown pretty much immediately if it had, without cooking a choke first.

29th Dec 2009, 10:17 pm	#54
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,288	Re: Quad 405 overheating Three posts moved to a new thread here:- https://www.vintage-radio.net/forum/...ad.php?t=49014 __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.