Quad 303 thundering (in a bad way) - Page 3 - UK Vintage Radio Repair and Restoration Discussion Forum

SteveP02 · 15th Oct 2020, 5:58 pm

Correction: it's both channels. I was standing next the amp when it just did it and was able to pull the source and speaker connections in sequence before it stopped (it usually lasts 5-10 seconds). It's definitely both channels and sounds like the wind blowing over an open mic with the volume turned up high.

Radio Wrangler · 15th Oct 2020, 6:28 pm

Did the two channels produce the same noise, in synchronism?

I'm wondering about the PSU regulator. The two amps being AC coupled outputs will couple PSU fluctuations to the speakers.

Alternatively a prob in one amp could pull the PSU down a bit and affect a good channel.

David

SteveP02 · 15th Oct 2020, 9:22 pm

I couldn't say for sure about synch, but I think so. I have one 303 per speaker, the one channel drives the low and the other the high range. The noise from the low range drowned out the high until I disconnected that channel. The remaining high range rumble was much quieter, more a slight rustling and hissing, but of course that may simply be because it's higher range.

Radio Wrangler · 15th Oct 2020, 10:09 pm

It'll be easier to diagnose set up simply, one 303 only with each channel driving one full range speaker.

Otherwise you won't be able to compare channels, and you'll miss some valuable clues.

David

SteveP02 · 16th Oct 2020, 6:09 pm

Yes, makes sense. It's one of those maddeningly intermittent faults that means I can run the amp for weeks without problem then when it happens I'm still running towards it as it all settles down again. Perseverance.

SteveP02 · 7th Nov 2020, 11:45 am

Well, the original problem was getting worse, coming every few days rather than weeks and somehow the whole thing just not sounding quite right, so I went for the obvious problem and changed out two transistors (TR105 & TR106) on the driver board.

I followed the tuning steps below again and now the final step has a steady reading without the wandering and spiking that was present before:

1.Adjust PSU board (not shown) for 67v
2.Adjust RV100 to set 33.5v between pins 5 and 9 on the line of connections coming onto the board
3. Adjust RV101 to measure ~10mV between pins 4 and 6

It's too early to say if it's had any impact on the "growl" but to my ears the sound is improved, back in focus.

Thanks to all for the information and encouragement.

SteveP02 · 8th Dec 2020, 12:49 pm

All has been quiet for a month now...... I'm not going to tempt fate but.......

Radio Wrangler · 8th Dec 2020, 4:34 pm

Maybe it was a dirty pot on the PSU board and that tweak cleaned it up?

David

SteveP02 · 10th Dec 2020, 6:20 pm

FWIW I think I can rule out the PSU board as that was the one I didn't adjust as the driver boards were steady at 67V. That said, I take the point that the amp may have been just craving my attention!
I'm happy to settle for everything sounding great again. Lot's of listening time has been one of the few upsides of the Covid months, so having a my hifi back together has been great.

15th Oct 2020, 5:58 pm	#41
SteveP02 Triode Join Date: Aug 2019 Location: Basel, Switzerland Posts: 32	Re: Quad 303 thundering (in a bad way) Correction: it's both channels. I was standing next the amp when it just did it and was able to pull the source and speaker connections in sequence before it stopped (it usually lasts 5-10 seconds). It's definitely both channels and sounds like the wind blowing over an open mic with the volume turned up high.

15th Oct 2020, 6:28 pm	#42
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,866	Re: Quad 303 thundering (in a bad way) Did the two channels produce the same noise, in synchronism? I'm wondering about the PSU regulator. The two amps being AC coupled outputs will couple PSU fluctuations to the speakers. Alternatively a prob in one amp could pull the PSU down a bit and affect a good channel. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

15th Oct 2020, 9:22 pm	#43
SteveP02 Triode Join Date: Aug 2019 Location: Basel, Switzerland Posts: 32	Re: Quad 303 thundering (in a bad way) I couldn't say for sure about synch, but I think so. I have one 303 per speaker, the one channel drives the low and the other the high range. The noise from the low range drowned out the high until I disconnected that channel. The remaining high range rumble was much quieter, more a slight rustling and hissing, but of course that may simply be because it's higher range.

15th Oct 2020, 10:09 pm	#44
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,866	Re: Quad 303 thundering (in a bad way) It'll be easier to diagnose set up simply, one 303 only with each channel driving one full range speaker. Otherwise you won't be able to compare channels, and you'll miss some valuable clues. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

16th Oct 2020, 6:09 pm	#45
SteveP02 Triode Join Date: Aug 2019 Location: Basel, Switzerland Posts: 32	Re: Quad 303 thundering (in a bad way) Yes, makes sense. It's one of those maddeningly intermittent faults that means I can run the amp for weeks without problem then when it happens I'm still running towards it as it all settles down again. Perseverance.

7th Nov 2020, 11:45 am	#46
SteveP02 Triode Join Date: Aug 2019 Location: Basel, Switzerland Posts: 32	Re: Quad 303 thundering (in a bad way) Well, the original problem was getting worse, coming every few days rather than weeks and somehow the whole thing just not sounding quite right, so I went for the obvious problem and changed out two transistors (TR105 & TR106) on the driver board. I followed the tuning steps below again and now the final step has a steady reading without the wandering and spiking that was present before: 1.Adjust PSU board (not shown) for 67v 2.Adjust RV100 to set 33.5v between pins 5 and 9 on the line of connections coming onto the board 3. Adjust RV101 to measure ~10mV between pins 4 and 6 It's too early to say if it's had any impact on the "growl" but to my ears the sound is improved, back in focus. Thanks to all for the information and encouragement. Last edited by SteveP02; 7th Nov 2020 at 11:55 am.

8th Dec 2020, 12:49 pm	#47
SteveP02 Triode Join Date: Aug 2019 Location: Basel, Switzerland Posts: 32	Re: Quad 303 thundering (in a bad way) All has been quiet for a month now...... I'm not going to tempt fate but.......

8th Dec 2020, 4:34 pm	#48
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,866	Re: Quad 303 thundering (in a bad way) Maybe it was a dirty pot on the PSU board and that tweak cleaned it up? David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

10th Dec 2020, 6:20 pm	#49
SteveP02 Triode Join Date: Aug 2019 Location: Basel, Switzerland Posts: 32	Re: Quad 303 thundering (in a bad way) FWIW I think I can rule out the PSU board as that was the one I didn't adjust as the driver boards were steady at 67V. That said, I take the point that the amp may have been just craving my attention! I'm happy to settle for everything sounding great again. Lot's of listening time has been one of the few upsides of the Covid months, so having a my hifi back together has been great.