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Vintage Quad 33 Preamp & 303 Power Amp - Service & Bench Testing

Novalux Stereophonic

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In this video, I perform a complete inspection, diagnosis, and restoration of one of the most iconic British hi-fi combinations ever produced. What begins as a routine service quickly turns into a troubleshooting session when a fault is discovered in one channel of the Quad 303. Using oscilloscopes, ESR testing, signal tracing, and performance measurements, I track down the issue, rebuild critical sections of the amplifier, replace aging electrolytic capacitors, install new adjustment trimmers, and optimize the amplifier for maximum performance.

Along the way, we take a close look at the unique engineering behind the Quad 303's capacitor-coupled output stage, regulated power supply, and highly serviceable modular construction. We also inspect the Quad 33 preamp, evaluate previous restoration work, test its phono stage performance, and uncover an interesting volume control tracking issue that affects channel balance.

Chapters:
00:00 Introduction
01:06 Initial Inspection of the 303 Amplifier
06:17 120/240 Input Voltage Verification on the 303
12:59 Initial Bench Testing of the 303
15:40 Diagnosing the Ailing Left Channel
21:38 Preventative Maintenance
30:56 Initial Inspection of the 33 Preamp
40:41 Final Adjustments on the 303
48:49 Bench Testing the 303 After Repairs & Adjustments
50:42 DC Balance Final Tweaks
53:21 Balance Issues in the 33 Volume Control
57:52 Conclusion

Quad 303, Quad 33, Quad 303 repair, Quad 33 restoration, Quad amplifier repair, Quad preamp repair, vintage audio restoration, vintage amplifier repair, hi-fi restoration, British hi-fi, audio repair, capacitor replacement, ESR testing, amplifier troubleshooting, audio technician, stereo restoration, phono stage testing, Quad ESL57, vintage stereo repair

YTL074

20 Comments

@NovaluxStereophonic Creator 1 year ago Pinned

Thanks to a keen eyed viewer I was able to identify a math error in my distortion analyzer. The scope screen reads 19.5Vrms and 19.1Vmrs at clipping into 8 ohms which equates to approx. 45W per channel both channels driven which matches the specification of the original Quad 303 amplifier.

@Zalgol 1 year ago

Thank you for producing such a clear and informative video of the Quad 303 and 33. Your descriptions were clear, and accurately related to the images. (unlike some others). I was particularly pleased to see the recalibration process for the power amp.
QUAD audio was the brain child of Peter Walker He was the inspirational designer of the early “Quad” equipment, and he devised the 303 circuitry which incorporated "symmetrical triples" output stage which produced low distortion, and high reliability.
His designs were conservatively rated, and as such many continue to survive and work more than 60 years after their introduction.
The things that tend to fail are the components due to age, rather than the design or use. Unfortunately some original semiconductors are now obsolete and virtually impossible to obtain, but there are some good modern work arounds which can keep these venerable products functioning for many more years.

@DurokSubaka 3 months ago (edited)

I knew that Lafayette Four channel decoder I still have in the basement would become fashionable again

@adeaston6553 1 year ago

Hi from the UK. I also watch a UK YouTube cannel called "Mend It Mark". Back in December last year he was sent from the manufacturer a "Tom Evans" pre-amp worth £25.000 which they said he couldn't fix, all the components had all the values filed off so he had to reverse engineer it which he did and repaired the unit. he was then hit with a copyright strike from the Tom Evans electronics. You won't find this episode on his own channel but is freely available on other channels. easy to find. Well worth a watch.

@FaintSignals 1 year ago

As it's been repainted, plug up that hole and paint it again 🙂

@ECStrat 4 months ago

Good ole discrete circuitry is what makes them sound so good. Love my Quad33

@LawsonsStudio 6 days ago

Very informative. A client is sending me two 33/303 sets for service next week. I've worked on their younger brothers the 44/405 pair. They were a joy. So easy to work on.

@edherdman9973 4 months ago

On top of everything else, these units have a beautiful industrial design, and a bit cheerful too. "Nothing new under the sun" but it's new to me.

@bobfrankish8883 1 year ago

Hi from the UK Ben! I'm a 73 year old audio engineer so was brought up on these, it's great to see them again. I used to work for a company in the UK called Stereosound Productions, we also made domestic audio equipment. We used those Callins capacitors, they were made in Ireland and were rubbish, many were faulty from new. Because they are sealed and can't vent they used to explode dramatically! Quad also made a mono-block amplifier called the 50E. It used the same case as the 303 but had an output transformer so could match any load right up to 100 volt line without power loss. It's true that the 303 just didn't have the punch and dynamics of the 405, but in its' time it was still a nice sounding amplifier.

@favorit601 3 months ago (edited)

The power amp is still a beast in sound quality. The way to PLAY MUSIC. Forget what you learned from 50 years of marketing of other companies.
And to be honest: Wasn’t it a pleasure to get this amp done. How serviceable.

@gratmatassa5432 1 year ago (edited)

Hi from the UK great video. Quad are now manufactured in China under IAG who own several other British companies, that being said the Quad service centre in the UK will still repair & service legacy products so classic gear is going to be around for quite a while. fair play to the Chinese owners.

@ronniepirtlejr2606 3 months ago (edited)

As a electronics hobbyist & not having a whole lot of hands-on experience. I would feel l comfortable working on this. This is a beautiful amplifier without all the BS.👍

@flowntn1989 1 year ago

1973, my first foray into real hifi. All second hand, 33/303, single ESL57 (serial number round 650) and a Thornes td150 with some nondescript arm and cartridge. Numerous anecdotes about Quad, visits to the factory in Huntingdon, how the phono stage was adjusted etc etc. won’t bore you with this. Look forward to the full test video.

@johnbirchall7077 9 months ago

Excellent. The Quad gear, especially that made during the lifetime of Peter Walker, are "cultural icons" for many British enthusiasts, were heavily used by BBC who were pioneers in audio engineering, and bring up a lot of memories for older guys. The marketing for the company's recent 33/303 homage claims that one of the original 33 is in MoMA, New York.

@sureshlala6462 1 month ago

This is very nicely documented, thank you for spending the time showing the testing and commentary.

@matthiasmartin1975 1 year ago

That was great, thanks for caring enough and shooting it all in pristine 4k resolution.

@derekheeps8012 1 year ago

The Bulgin power connector is not some 'weird' or proprietary thing ! Bulgin power connectors were widely used up to the 1960s and beyond , only being replaced by the 10A IEC power connectors when they started to come out around the 1970s . Even now , Bulgin connectors are still quite widely used .

@LawsonsStudio 3 months ago

A timely video for me. I'm being sent two 303 power amps next week. I've just finished a Quad 405 and was amazed how well designed it was for the tech. 12 screws and it was in bits.

@TheEmbeddedHobbyist 1 year ago

Nice to see one in bits as I have one sitting behind me. Not been powered up in about 30years so well over due a service and maybe a cap replacement.
Having watched your video it needs to be put on this year's todo list.

@MrMersh-ts7jl 1 year ago (edited)

I have a quad 405 I found in the garbage. Works great. Very solid chassis. Sounds good, and an interesting current dump system. Lots of quality components, including polystyrene caps. Glad to see your doing these. You don't see them all the time.