At my local repair café recently we were asked to explore a problem on a vintage amplifier. Apparently one channel is quiet. So far I’ve not had time to prove that fault to be correct, but I thought I’d share a couple of photos.
The item is a Quad 303 power amp, and it’s accompanying Quad 33 preamp. These are quite collectable so it is unusual that one is handed over to us “amateurs”.
It is a very distinctive design and weighs a tonne, having an enormous linear power supply in the back. What most impressed me though was that the owner brought it in together with the original owner’s manual. In the back of the owners manual is the full schematic. A real delight to see. This device was clearly built with longevity in mind and designed for field servicing. Compare that to a more modern “Audiophile” manufacturer, such as Linn and you see the stark difference. I had a Linn Classik (a relatively low-end product in their range) and it developed a fault. Unsurprisingly it was the caps, but I approached Linn to request a schematic for the (now discontinued) product and was told, in no uncertain terms, that they would not provide anything of the sort and that I would have to go to one of their approved repairers. I re-capped it myself, of course, and it as good as it ever was (arguably better as I was a little disappointed in Linn’s choice of caps). Needless to say, attitudes like this lead to many devices being scrapped when all they need is a little love and a few quids worth of components. The flip side to that is that the Quad 300/33 will be living on as a useful and probably in-demand item long after the last Linn products have been have been recycled and forgotten.
Here’s a shot of part of the schematic. I’ll be making proper copies on Wednesday when I am back in the workshop.
Very nice bit of vintage history from the audio field. You don’t see them too often in this visual condition
Woodb180
An update.
I had time to make up the cables I needed today to prove the fault was as reported (I didn’t have any 5 pin DIN cables last week, though there were, of course, a number in the “cable cache/graveyard” in the garage at home). As reported by the user, the left channel is very weak compared to the right, so it was time to remove the case. There is a small amount of crackle on the balance pot (another lovely little mechanism with the user-friendly left/right slider, operating a mechanical linkage to the rotary pot, that you can see in the centre top.)
This is a wiring work of art. Just look at the routing, the neatly bent elbows, even the knotting on the top left. The attention to detail is incredible.
The label shows that it was serviced in 2004, though it is unclear whether that was by Quad. The modular design made it very easy to narrow down the fault. The two identical boards on the left are the left and right amplifier stages of the Quad 33. The top is the left channel the lower is the right and it was a very simple task to swap them around and prove that it was the left channel amplifier board and not anything else. I ran out of time to complete the fault finding. The film caps all tested ok (the board design means that they are all connected to the edge and effectively open circuit making in-circuit testing more reliable than it would typically be. The voltages on one of the transistors look to be decidedly dubious, and the most likely cause of the issues, but the voltage at the base pin seems higher than I’d expect, suggesting that it might be one of the electrolytic caps. I’ll continue next Wednesday.
In the meantime, here is another photo, this time of the underside of the simple amplifier board. With this neat and simple design, you could make your own replacement using toner transfer. The curious will note the flux marks on the board where the transistors are, suggesting that they’ve been replaced before at some point. That does not rule them out, of course, and it may simply be that they were desoldered to be tested and not actually changed.
This device was clearly built with longevity in mind and designed for field servicing. Compare that to a more modern “Audiophile” manufacturer, such as Linn and you see the stark difference. I had a Linn Classik (a relatively low-end product in their range) and it developed a fault. Unsurprisingly it was the caps, but I approached Linn to request a schematic for the (now discontinued) product and was told, in no uncertain terms, that they would not provide anything of the sort and that I would have to go to one of their approved repairers.
It’s easy to see what Linn’s main goal is and that doesn’t seem to be either customer service or designing things to last.
In the old days this was known as Planned Obsolescence which is a nice way of forcing people to spend additional monies to keep the current item of to buy again (and again).
I miss the old days of ‘built to last’.
The neat thing about that Quad 33 preamp is in it’s design, looks more like military style of layout and wire routing which was more common back in the 40’s thru the 80’s.
Just from having a peak under the hood (via your pictures), gives me great respect for the Quad.
I’m wondering what vintage this unit is as it appears to be 80’s or 90’s vintage?
In the meantime, here is another photo, this time of the underside of the simple amplifier board. With this neat and simple design, you could make your own replacement using toner transfer. The curious will note the flux marks on the board where the transistors are, suggesting that they’ve been replaced before at some point. That does not rule them out, of course, and it may simply be that they were desoldered to be tested and not actually changed.
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Doesn’t appear to be wave soldered either?
Looks more like a labour of love in both design, assembly and function
I’m not sure where HiFi Engine got that data, but it is definitely wrong on that one. The 33/303 was Quad’s very first solid-state product, appearing in 1967.
The 33 was in production until the end of 1982; the 303 carried on to ’84/5.
The best source for that was straight from Quad themselves. https://www.quad-hifi.co.uk/ourstory/
This unit had a serial number of around 23000, dating it to 1972 (if we believe the records in this thread https://lencoheaven.net/forum/index.php?topic=15705.0 )
Final update on this project.
I fixed the unit last week, and the owner was meant to collect it today but couldn’t get over to us in time.
Having isolated the issue to the left channel amp stage in the 33 “pre-amp” and noted that the transistors’ voltages were way off where they should have been, I removed the electrolytic caps that tied the emitters to the ground. These tested poorly and would have acted like resistors rather than caps, thus dragging down the bias voltage. I carefully replaced just the one cap first. I confirmed that it had restored about 80% of the performance of that channel (measured pseudo-scientifically, using a simple sound pressure app on my phone to compare the two channels). I then replaced the remaining electrolytic on the card and that restored it to about 110% of the right channel. At this point, I decided to replace the electrolytic on the right channel proactively to head off a future failure and to ensure that balance was restored. After the two channels were both re-capped, they measured very close (which, in the far-from-ideal setting of our workshop, was more than adequate).
Balance and sound pressures aside, it sounded really nice, and for my money, that is the most important part.
Today’s repair café forays included finishing up a Marshal guitar amp (mostly a cleanup of the pots, with two needing to be replaced) and a peculiar wild goose chase for three Robert’s radios (all small transistor models, likely ’80s-’00s) that the owner had brought in together announcing that they were all “not working”. One was a broken SM power supply (wall wart), one just needed new batteries, and one had no fault at all that I could identify. So they’ll go back as “need more information”). I have a niggling suspicion that the “not working” might be that their favourite frequencies have been abandoned and that they’d not realised this. We shall see no doubt.
I doubt I’ll get to work on another Quad for a while, but I can only say it was bliss. It is such a work of art, and only those of us who look under the covers would ever really appreciate it.