Quad 33 desoldering PSU

[John_73]

Is the voltage on C101 stable?
I would suspect C100, C101, C104 and C106 before changing any transistors if you get music

I will measure them this evening David, though all caps are new, bar the C100 input cap.

All information to date collated below (getting a bit confusing to follow between two threads & forums):


Q303 33.5V adjustment summary:

I have replaced all electrolytic caps except C100 on the driver boards, and all adjustment pots. R130 was also replaced on both boards as that had gone high value on the left side. All four chassis mount 2000uF caps are original, as there's no sign of any bulging, leaking, and the unit has had a very easy life. I have played music through it and it sounds fabulous - zero complaints. No obvious distortions or noises. No transistors including outputs are obviously overheating.

The 33.5V setting varies constantly on the left board, and is very stable on the right.

I have already checked the following components:

R110 - fine.
R111 - fine.
R112 - fine.
R130 (replaced in both channels).

Circuit diagram for my earlier model is below. Voltages listed under R104 in the diagram measure 8.76V on my 303, and above R111 8.51V, so all OK there. PSU output voltage is a stable 67V:

Referring to the circuit diagram, if I measure 'below' R113 (82K) I get a stable reading of 8.48 volts. Moving to the 'upper' side of R113, I get the wandering voltages, around the desired 33.5V (dipping to 31V, upto 33.85V).

UPDATE: PCBs thoroughly cleaned of old flux residue. Two very slightly suspect looking solder joints resoldered.

Disconnecting the collectors of the left channel output transistors TL1L & TL2R the voltage between pins 1 and 5 is a constantly fluctuating 33.35 - 33.70V. An improvement on the readings earlier which seems a bit weird, but there you go. Reconnecting the collectors results in the same readings.

Can set the quiescent current between pins 4 and 6 to approx. 10mV. Music sounds fine & undistorted.

 

[Arkless Electronics]

The problem you will find is that in a DC feedback loop such as the one which sets the output midpoint voltage, the majority of the components can influence the output and as the loop is always trying to correct itself it can be close to impossible to find the faulty component. When you use freezer spray (and heat) you will find that huge variations can be caused by heating or cooling a wide variety of components... My best guess would be TR100, 101 or 102 as the culprit but it could even be one of the nasty carbon resistors. The temp coefficient of these resistors is bad and you will probably find heating/cooling many of the resistors will have quite an effect.
Chances are that the unit will work just fine as it is and without reliability issues.
It could well be just higher 1/f noise ("flicker noise") in one of the above mentioned transistors, especially TR100, most of which will be infrasonic anyway. c100 could cause it but I would expect audible noise as well. Do you have an oscilloscope?

 

[Dowser]

I stand - or sit - corrected

Can of freeze spray ordered and will be collected tomorrow. If it doesn't work I can always use it on my cans of cider instead...

Freezer will drop temps below normal operating point - it generally highlights the suspect component in these sorts of issues, but as above note that spraying any component is likely to have an effect on bias, you are looking for a big one or a temoraryp return to stability.

 

[John_73]

Freezer will drop temps below normal operating point - it generally highlights the suspect component in these sorts of issues, but as above note that spraying any component is likely to have an effect on bias, you are looking for a big one or a temoraryp return to stability.

Thanks for that; I'll have a better chance of knowing what to expect tomorrow now then. Still hoping it turns out to be something relatively simple.

 

[John_73]

The problem you will find is that in a DC feedback loop such as the one which sets the output midpoint voltage, the majority of the components can influence the output and as the loop is always trying to correct itself it can be close to impossible to find the faulty component. When you use freezer spray (and heat) you will find that huge variations can be caused by heating or cooling a wide variety of components... My best guess would be TR100, 101 or 102 as the culprit but it could even be one of the nasty carbon resistors. The temp coefficient of these resistors is bad and you will probably find heating/cooling many of the resistors will have quite an effect.
Chances are that the unit will work just fine as it is and without reliability issues.
It could well be just higher 1/f noise ("flicker noise") in one of the above mentioned transistors, especially TR100, most of which will be infrasonic anyway. c100 could cause it but I would expect audible noise as well. Do you have an oscilloscope?

Thanks for that Jez. I'm wondering if it might indeed have something to do with C100. I'm using a Marantz HD-DAC1s variable output directly into the 303. I've just noticed whenever it stops playing, or changes sample rates - everytime a relay clicks internally in fact - I get a 'plop' noise from the left speaker... I would have thought the Marantz would have DC blocking caps on its outputs, but if not perhaps that might indicate the left input cap is 'iffy', though music still sounds fine for the time being. Hoping my Marantz isn't faulty too :/
I've got some 1uF polyster caps arriving tomorrow for another project, so will solder a pair in place of the existing C100 electrolytics to see if that has any effect.
Was wondering if 46+ year old output transistor heatsink mounting compound might have all dried by now and could cause slightly erratic voltages here too? Would it be worth changing to sil-pads?
Or it could just be the flicker noise you mention, and it's always been that way in this particular amp.

Ta, John.

 

[Arkless Electronics]

Thanks for that Jez. I'm wondering if it might indeed have something to do with C100. I'm using a Marantz HD-DAC1s variable output directly into the 303. I've just noticed whenever it stops playing, or changes sample rates - everytime a relay clicks internally in fact - I get a 'plop' noise from the left speaker... I would have thought the Marantz would have DC blocking caps on its outputs, but if not perhaps that might indicate the left input cap is 'iffy', though music still sounds fine for the time being. Hoping my Marantz isn't faulty too :/
I've got some 1uF polyster caps arriving tomorrow for another project, so will solder a pair in place of the existing C100 electrolytics to see if that has any effect.
Was wondering if 46+ year old output transistor heatsink mounting compound might have all dried by now and could cause slightly erratic voltages here too? Would it be worth changing to sil-pads?
Or it could just be the flicker noise you mention, and it's always been that way in this particular amp.

Ta, John.

No that's definitely nothing to do with it.

 

[davidsrsb]

It is worth double checking the voltage across those four capacitors is the right way round. Sometimes board silkscreen markings can be wrong

 

[John_73]

There are no markings on the Quad 303s boards, but all caps are most definitely installed correctly with plenty of photos of the board fitted with original caps prior to new ones taken as a guide, as well as the DaDa and Quads upgrade & service manuals. Plus the right-hand channel is fine.

 

[John_73]

Update on this little project. Managed to get the Quad 303 working properly - I replaced all the chassis mount caps with 2200uF 200V BHC caps (might try out 4700uF caps on the outputs in the future, but want to get a handle on the 'near stock' sound right now). I also replaced the original axial electrolytic 0.68uF input caps (C100) with 1uF axial tants (which look exactly the same as the originals, which was a nice bit of luck in terms of keeping some of the stock look). Not sure which capacitor change was responsible, but all voltages are now stable, just subject to the usual slight drift as the components warm up, especially bias (which I'm led to believe is quite normal). Set it all up and let it play for a couple hours, then reset all voltages & bias once again, then put the lid and base back on. Ready for business.

It all sounds very nice indeed, definitely not 'vintage' or 'pipe & slippers' in the stereotypical sense - surprisingly dynamic in fact - though a slight amount of hiss and a tiny amount of hum with ears close to the speakers. I'm using the original (1969) cruddy old DIN to DIN cable between 33 & 303 so hopefully that's responsible - will get a new one shortly now I know it all works. To put it into perspective, the hiss is less than that I've experienced with two different Naim 72/140 combos I've owned in the past, so not too worried about that, probably those older transistors mixed with lots of carbon film resistors & the gain, perhaps. I can honestly say I'm not hearing anything sounding 'rolled off' or soft sounding, so not quite sure where that stereotype came about, unless it was via people listening to unserviced units with tired caps, against more 'in your face' modern gear...

Very happy with how it's looking and sounding indeed. No problems using a DAC either via the Tape input, with the input sensitivity screws set to 1V - not quite sure why many seem to think the 33 isn't suitable for modern components to be honest. Phono stage sounding great too, surprisingly so, but need to tailor the load capacitors a little still.

Still haven't tried out the FMII yet, that's on the seemingly never-ending to-do/to-tweak list
Photo below - It's morphed into a vintage system over the past few years, and I absolutely love it!

- John

 

[John_73]

 

[Radfordman]

Looks a very neat job, much better than I could do these days.

 

[Dowser]

Great work...you need a clear case

 

[Tony L]

How do you feel the 303 shapes up to your IIs? I've never spent any real time with IIs so don't really have any handle as to their strengths/weaknesses.

 

[John_73]

Thanks guys. My aim is to always try and make it look 'factory perfect', which can drive my ocd side crazy sometimes Fortunately both the 33 & 303 are a joy to work with and were obviously designed with easy servicing in mind.

Tony: I haven't heard my IIs since Jan/Feb time. But allowing for the unreliability of audio memory, I'd say the 303 was very similar, with perhaps a little more 'heft' in the lower mids and bass region; more or less what you'd expect really. I wouldn't call the IIs bass light at all though if serviced properly, but the 303 has a slightly better grip on more modern 6-8 ohm speakers. The II counters this with an addictive fluid midband and open and airy treble..... I can hear the objectivists groaning at my descriptions already heheh
In all honesty, there's not a day and night difference between them - both have the same sonic dna as it were; the diffferences are in real-world speaker driving ability.

 

[stevec67]

That's entirely what you would expect. Quad launched the 303 with the then all-new transistor technology, it had to be at least as good as a valved Quad 2.

 

[Tony L]

That's entirely what you would expect. Quad launched the 303 with the then all-new transistor technology, it had to be at least as good as a valved Quad 2.

Though many tried and failed, e.g. the transistor Leak amps are considered pretty terrible compared to the classic tube amps.

 

[Barrymagrec]

If I remember correctly It was a time of fast moving development in power transistors and Quad waited longer than many other manufacturers to produce their first transistor amplifier.
The Leak Stereo 30, like some other products that emerged at the time, was based on the Dinsdale circuit from the Wireless World and used Germanium output transistors, again if I remember correctly.

 

[toprepairman]

You do remember correctly.
Those early amps were considered badly by a few, and the trend continues, perpetrated by dimwits like 'select45'. The reality is that they were not that bad, the early Germanuim St. 30 particularly panned by said twit, is really quite good. I'd prefer that one to the sucessor Silicon one.
Probably thr worst of that early bunch was the Rogers silicon job, but that used rather crude driver transformers and vague biassing so had quite a handicap.
Henry