Quad 33/303 shutdown noise question

[mikmas]

If this is in the wrong place please do move it :)

Firstly need to say up front that this is not a new issue but has been bugging me both before and after carrying out a complete re-cap of both units and also further work. It was one of many problems that started a couple of years ago, all of which are now resolved bar this one. Apart from this problem, the setup has never sounded or behaved better in the 20 years I have owned it.

Basically on some occasions (but not all) when switching off the amps I get the sound of rustling paper through the speakers that slowly dies out. This has also occurred at start-up and also fades within a second or two. I believe the problem lies with the 33 because it never occurs when using the 303 with my passive-pre – although I am not ruling out either unit as possible cause.

I can provide all details of what has already been done but would welcome any and all suggestions of possible causes and (hopefully) a cure.

[DSJR]

Well worth a look on the Dada website for ideas and their webshop often has a small stock of replacement 33 cards. My first advice would be to remove the disc boards as well as the tape board, turn the volume to minimum and see if the rustling on/off is still there. if so, it could be a transistor going off on the output cards, or possibly the two caps adjacent to the tone/filter inductors on the mother board. My expertise is already severely stretched here and you may have to look to Quad for a service (they're not expensive and obviously know this unit backwards).

Using a 303 with a passive preamp is a sonically perfect way to go, if not always as attractive as the 33 to the eye ;) Even fully restored and updated (the net-Audio mods change the 33 into something else IMO), I've found a 33 used with modernised original circuits will be a bit of a bottleneck on the sound in all honesty, the perceived sound-field shrinking a little in comparison with a passive. Just my experiences obviously and for whatever reason, some 33's sounded better than others on rock music program - I don't know why and I don't 'think' I was imagining it either...

[The Barbarian]

If you pinned it down to the '33' & really have no clue as to what it could be id work on the PSU first then try the Output buffer boards. Good thing about this amp is you can rebuild a card as you can afford to.. or just rebuild using Net Audio jobbies..

[mikmas]

Thanks both for the rapid responses and suggestions - kind of confirms my suspicion that the noise might be imminent transistor death.

I have a bunch of BC549 that I intend as upgrades for all in the 33; already did the tape board and that appeared to improve the sound already - could be in my mind though :lol:
The recap I did was using the DADA 33/303 kits pretty much to the letter - with the exception of changing the 33 PSU to 15v (which I may yet do) I also renewed the switch suppressor at the same time to the recommended RIFA model. I have trawled their blog many times but couldn't find much on the 'rustling paper' syndrome though.

Dave - completely agree that passive-pre/303 provides a juicy difference - here's my earlier post on this:

http://theartofsound.net/forum/showthread.php?36140-My-new-cheapie-amp
Only downside here is the need to add a phono stage - although I could still use the 33 for playing vinyl I suppose and feed it to the passive.

Will do the transistor replacement in either event but if anyone has any other suggestions while I have the box open I would be most grateful ;)

[mikmas]

Well .... I thought I'd cracked it but seems not :scratch:

Monday I replaced all 6 transistors on the two amp boards - and all went really sweet for a couple of days; a noticeable sonic improvement in all respects and silent as the proverbial lamb at startup and shut down....

Tonight I switched it on and the noise beast was back, just as annoying as before !!!!! Turned it off (more noise) and left it for a while .... after half an hour wait -back on and still noisy !!!!
I am now tearing what's left of my hair out and close to binning the bloody thing :steam:

The only ideas I can come up with is that either the transformer or volume pot power switch are going bad - If anyone has any ideas at all I would really really really appreciate it :)

[struth]

the fact that by replacing the transistors fixed it for a short time suggests something is taking them down. could be some ascociated parts in the circuit that are out of spec.

[The Barbarian]

I keep telling people these old amp will only be 100% reliable if totally rebuilt let alone sound a hundred times better.

btw: The original 50K Volume pots with are Spanish if i remember. Bit of advice that Switch noise Supressor you see wired across the switch need replacing.

Here's a replacement pot.

http://www.dadaelectronics.eu/shop/components/potentiometers/carbon-potentiometers/47k-stereo-log-potentiometer-with-switch

My next job on the '33' is outboard PSU which will carry a power switch, I'll fit a high quality Pot with no switch ofc to the '33'

[mikmas]

... thanks for the advice, already did the suppressor when I swapped out the el-caps

bit loath to chuck more good money at bad until I have an inkling what the cause might be - the 33 sound isn't that holy

guess I'll just root around the boards again and look for dodginess .....

[mikmas]

.. just to close this off with a happy face

I finally narrowed down the problem to one of the amp boards; close inspection with a loupe revealed some very dodgy tracks and stress from repairs carried out in its previous life. Rather than attempting to bodge it any further I ordered a working board from the DADA site (very cheap really...) and sure enough this sorted the problem and the 33 is now singing pure and sweet :)

In the meantime I have also acquired a recent model Quad 34 (RCA type) and have been alternating them into the 303 - in all honesty I have to say the 33 has a definite edge in all respects bar compatibility of interconnects and will probably end up being the queen of the stack.

I really don't understand why the 33 gets such a harsh press - definitely not the weakest link once age-related problems are sorted :lol:

[The Barbarian]

I really don't understand why the 33 gets such a harsh press - definitely not the weakest link once age-related problems are sorted :lol:

:respect:

[mikmas]

... well - the beast came back !! - - and this time with a vengeance :steam:

At least now I'm pretty sure why. Suspecting dodgieness in the PSU dept I decided to complete my DADA upgrade with the 16v conversion to see if this would finally cure it. This requires lifting the board to change diodes, etc and when I did I made a discovery ... moving one of the trafo feeder wires out the way to lift out the board it literally fell off !! it's the red/black wire resting on the power sockets which is normally connected to the lower rail:

https://farm8.staticflickr.com/7657/16668388264_4773a9a1ce_z.jpg

How long this has had been hanging on there is anyone's guess but it took little more than a touch to fall off ....