Thank you for a comprehensive description of the completed work... Very good indeed. !

Thank you M. Caldeira for a great instructional video, thanks also to the informative contributors in the comments section. All I need now is a techie in Sydney to help restore my 303/33 combo.

Fascinating video

Thank you for a good video

Thanks for the video, I've got a bid going on eBay for one of these.

Quad 303 recap

my fav stereo next to b&o

Hello, this is the best explanation I’ve seen on adjustments. I’m trying to follow your steps exactly but no matter how much I adjust the pots, nothing happens! What error can cause in adjustable voltage?

The main low end restriction is the 2.000 uF output capacitor. With a 4 Ohms load,  response will be 6dB lower at 20 Hz. The input time constant, even with the original 0,68uF, will have the -6dB at around 10Hz. Quad is known for never been a big fan of wide bandwith products... ;)

Excellent video, thank you. I did the same thing on a Leak Stereo 30+ i.e. changed all the caps on one channel only. I found as you did that the main benefit was a flatter frequency response, and also faster rise time, but I can't honestly say I could hear a difference.  Small point: I suspect you should measure voltages with an analogue meter (20k ohms/ volt) if you are to compare with the design figures; I use an old Avo 8 for this (great excuse to use a lovely piece of kit!).

May I ask what is the best option for C100 component replacement?
Thanks :)

I just picked one of these 303/33 combos up to go with the Quad ESL 57s I’ve had in storage for about 20 years. Tell me, is it dangerous to remove the cover of these amps and have a peek inside? What precautions do you have to take? Thanks.

Am taking my used pair of 57 speakers, used 303 & 33 back to their home :; Quad in Huntingdon, Cambridgeshire, tomorrow for servicing  / refurbishment/ repairs as required.
Also considering how to fabricate a mild steel box section floor stand for the speakers to lift 18 inch from the floor and to allow the units to tilt forward & backwards as my ears need !
There are absolutely no ready to use steel stands specifically to suit Quad 57’s and a few mentions of ‘spider legs ‘ made from timber or man made board . But IMO they look ugly and I am able to design and build sleeker , less clunky and better home made stands

1. All the necessary Bulgin connectors are still available and no problem at all. 2. Measuring the voltage between pins 6 and 1 is slightly invalid. 3. The difference between 3.8VRMS and 3.6VRMS is neither significant nor audible. 4. The LF response isn't affected in the slightest by the midpoint or bias adjustments, or by C101 or C106, but it is affected by C100 and C104 and the output capacitor, which should be doubled. 5. All Peter Walker's amplifiers follow a consistent theme of -3dB points at 15Hz and 55KHz, and the 303 is no exception, contrary to what is claimed in other comments here.

It seems that there is quite a lot of wind blowing around your house, is it? You can hear it clearly.

i did mine 10 years ago  or so

I am owner of a 303 and a 33 by Quad for more than 20 years.  Unfortunately here in Rio de Janeiro is very dificult to  find good technical people  for repairs and therefore we usually just have 2 options for these kind of equipment by here:  or  to Pray or go to Sao Paulo city try find some technical people, what is not also an easy option also.  Could you plese  tell me what fuse especificatrions the 303 use at its back?  I would like thank you  for the nice video and so sorry my limited English.

Did you mean to adjust the 67V supply with a dim bulb in circuit? That's what the video implied.
I don't think the original -1dB at 30Hz is bad, do you?

if anyone else ends up here because they have a 303 that runs insanely hot but sounds fine..... mine, I could smell the paint from across the room..... after a bit of research, it emerges that the original op transistors had been replaced with more modern 2N2955/3055 pairs that have a much greater power bandwidth, extending to many tens of kHz, & were thus able to sustain oscillation at supersonic frequencies. quad had not designed this out because when the amp was new, these devices didn't have this kind of bandwidth. so if you replace the op transistors, try to find N.O.S. devices, or be prepared to modify the driver circuit quite considerably!
personally, I prefer the sound, the design & the overall aesthetic of the leak stereo 30, the 30+ & the 70.
I find myself agreeing with another comment here about the 33 preamp- the switches are awful, & you will end up with a deadspot on the combined volume/power switch; good luck finding a replacement. same problem affects the leak, but it's a lot easier to add a separate power switch to that.

I have one of these amps in for repair at the moment. I would be interested to see how you input the signal to the amp and how you connected the scope to each channel. Did you use a dummy load? Great video though. I was thing of trying some audiophile caps to see if I could get any measured improvements. Thanks Graham