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Old 17th Aug 2019, 8:59 pm   #1
Norman Raeburn
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Default Quad 33 one channel brighter

Hi Folks, Looking for some help on this, this was my old set up and a friend bought it from me a few months ago.
He complains that one side is brighter sounding than the other. I have had a listen and it is brighter on one channel but only on phono input. I assume the "dull" channel is the faulty one.
The difference on my speakers isn't huge but he says it is much worse on his speakers.
I swapped the tone boards over but didn't make any difference. I have checked everything on the phono stage board and can't find anything amiss.
I should mention that all the electrolytics were changed a few months back and I have been over them in case I had fitted an incorrect value somewhere.
There is very little left and I am at a loss as to where to go next. Any suggestions gratefully received. Norman
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Old 17th Aug 2019, 10:25 pm   #2
vidjoman
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Default Re: Quad 33 one channel brighter

Swap the input connections from the cartridge as it may be that at fault.
Have you tried it on headphones? If one speaker is close to heavy curtains that can make it sound dull compared to one against hard surfaces.
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Old 17th Aug 2019, 10:27 pm   #3
teetoon
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Default Re: Quad 33 one channel brighter

Hi Norman. Is it the same with the cancel button pressed ? That would rule out the tone circuit, but you say it's only on the phono input I see.
Try the 2 input sensitivities on the phono board by rotating the board to select the other MM input.
I would have thought it would be around that module if it's not affecting the tuner or tape inputs.

Edit: a dirty switch might cause a problem, even the stereo mono buttons can do weird things.

David.

Last edited by teetoon; 17th Aug 2019 at 10:33 pm. Reason: more info.
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Old 17th Aug 2019, 10:29 pm   #4
Radio Wrangler
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Default Re: Quad 33 one channel brighter

Tried swapping the phono connections from the turntable around?

Check load impedance presented to the cartridge?

If it's phono only, then that rules out all stages common to other inputs (if they are OK)

It may be a pre-existing condition with say the cartridge, and it's only been noticed after the amp got changed.

David
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Old 18th Aug 2019, 12:00 am   #5
Synchrodyne
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Default Re: Quad 33 one channel brighter

If the fault, affecting the disc input only, is in the amplifier and not in the cartridge or its connections, then it would appear to be in either the disc adaptor board, the preamplifier board or perhaps the input switching. Most of the equalization components are on the disc adaptor board, and are shared between the M1 and M2 positions. My understanding – from the board diagram - is that they swap channels between the M1 and M2 positions of the board. So a fault in the M1/M2-specific circuitry on this board that appears in one channel only at the M1 position should appear in the other channel at the M2 position.

Click image for larger version

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If it the fault doesn’t move with the M1-to-M2 swap, then it probably lies in the preamplifier board, or in the input switching just downstream of that board. As an additional check, perhaps fed a “line” source into the disc input with the board at the C1 position. It will sound odd (bass boost) but should show up any channel differences whose cause lies not in the M1/M2 equalizing circuitry.

Here is a simplified version of the Quad 33 disc input amplifier, positions M1 and M2, taken from P.J. Baxandall’s writings, showing commonality of most components, although not showing the channel swap:

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And for completeness, derivatives of that for the C1 and S positions of the disc adaptor board:

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ID:	188505 Click image for larger version

Name:	Quad 33 Disc Input Amplifier Position S1.jpg
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ID:	188506


Cheers,
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Old 18th Aug 2019, 12:08 am   #6
Norman Raeburn
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Default Re: Quad 33 one channel brighter

Hi Folks, Thanks for the suggestions, we have tried swinging the phono input board around, three different turntables and it is exhibiting the fault in my friends house and my workshop. The other inputs are ok according to my friend, he has much better hearing than I do. I will try it tomorrow with the tone cancel button pressed although I think it's the same. I have previously cleaned all the switches but will clean them again. Another thing that was tried was a phono amp connected to the tape input which was fine. I will report back tomorrow on the latest finds, Norman
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Old 18th Aug 2019, 6:50 am   #7
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Default Re: Quad 33 one channel brighter

If you get stuck, Norman, I can lend you an audio spectrum analyser which has a tracking generator and produces on-screen frequency response plots. It'll measure the RIAA responses.

David
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Old 18th Aug 2019, 7:43 am   #8
Hartley118
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Default Re: Quad 33 one channel brighter

On my Quad 33, I’ve found the plugin adapter board connectors a source of unreliability. Treatment with Servisol is sometimes necessary.

Martin
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Old 18th Aug 2019, 8:56 am   #9
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Default Re: Quad 33 one channel brighter

Quote:
Originally Posted by Hartley118 View Post
On my Quad 33, I’ve found the plugin adapter board connectors a source of unreliability. Treatment with Servisol is sometimes necessary.

Martin
I agree I have the same issue with 1 of my 33 and Servisol cured it

Dave
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Old 18th Aug 2019, 8:41 pm   #10
Norman Raeburn
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Default Re: Quad 33 one channel brighter

I have had another look at this today, I am now getting confused with the results.
It has the fault with the cancel button in or out which eliminates the tone circuitry. I have tried cleaning the connectors on the boards, wiggling the switches to see if the fault changes, it doesn't. I tried the turntable on the M1 and M2 inputs and it doesn't reverse the channel the fault is on. I have checked every component on the phono amp board, also swapping the tone circuit boards makes no difference. I have tried metering the input switches for high resistance but that seems ok.
Another thing is, when you swing the balance control from side to side the volume comes up considerably on the left channel (the bright side)and by quite a bit less on the right. Looking at the circuit for the balance I can't get my head around what's going on there.
I will get back to this tomorrow and note my findings. There isn't much left to check so I am thinking I have missed something. Sorry for the long post and thanks for the suggestions. Norman
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Old 18th Aug 2019, 9:48 pm   #11
teetoon
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Default Re: Quad 33 one channel brighter

Re the balance control, the slider doesn't cover the full travel of the pot and you can adjust the area of movement by loosening the screw and re setting position of the slider until you get the same adjustment to the left and right.
Try the pot at each end of travel to see if there is a gross difference in levels.

Back to the original problem. Have you tried swapping the transistors over in the phono pre ? They are in the phono eq path as it were.

David.
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Old 19th Aug 2019, 7:51 am   #12
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Default Re: Quad 33 one channel brighter

OK so you've narrowed down the fault to the Disc Input Board. I notice you've replaced all electrolytics in the set? but apart from re-checking that the correct values were used, have you double checked one isn't back to front?
In the service manual, Quad recommend...."If Radio & Tape replay are satisfactory, but not Disc, try another position of the Disc Adaptor Board. If this has no effect other than that expected and another Pickup is not available, plug a radio Tuner into the Disc Input with the adaptor board in the C1 position, to prove whether the fault lies in the pickup or the Control Unit.If the latter, the Disc Pre-amplifier Board (M12019) is suspect".......
That should narrow down where the fault is likely to be.
Actually a sig. gen and a 'scope would be really handy in a case like this as you can actually see what is going on.
When setting up the balance control for equal output at usual listening levels, I use a sine wave at say 1khz into both channels(use Radio or Tape input) & monitor the output on the 'scope. Then I invert one scope channel and sum the 2 channels together. Result will be a straight line if both channels are indentical - invariably this generally doesn't happen but adjusting the balance control will make it so. Then you loosen the screws holding the connecting link to the shaft of the balance control and set the slider in the centre (without moving the actual control), make any minor adjustments to retain the null output and retighten the screws. Note that the dual gang vol. control is unlikely to have matched sections so the above adjustment will, in practice, be correct in only one spot.
Mike
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Old 19th Aug 2019, 10:33 am   #13
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Default Re: Quad 33 one channel brighter

Quote:
Originally Posted by Norman Raeburn View Post
Another thing is, when you swing the balance control from side to side the volume comes up considerably on the left channel (the bright side)and by quite a bit less on the right. Looking at the circuit for the balance I can't get my head around what's going on there.
It might be worth double-checking (or even swapping) the electrolytics either end of the balance pot.

Cheers
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Old 19th Aug 2019, 12:40 pm   #14
Norman Raeburn
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Default Re: Quad 33 one channel brighter

Hi Folks, I have finally managed to get to the bottom of this. Two faults? There was an incorrect value capacitor on the disc adaptor board and I would say it's never been touched so must have been there all the time although I think it was on the ceramic input. The other fault I found after using a signal generator and scope was the levels after the disc preamp board were approx. the same but at the volume control top end there was a huge difference in signal level. This left the switching and the tape adapter board, the signal level going into the tape adapter were approx. the same but coming out were drastically different. So the second fault is on the tape adapter board, why this only showed on disc I have no idea. Perhaps the lower levels involved made a difference.
Now I found a broken piece of track on one of the transistor collectors so I assume it was bleed through that was making it work.

I would like to say thanks to all for there input and keeping me on the trail. I was beginning to loose sight of what I was doing.

Very many thanks again for the help it is much appreciated. Norman

Last edited by Norman Raeburn; 19th Aug 2019 at 12:55 pm.
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Old 20th Aug 2019, 11:51 pm   #15
Synchrodyne
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Default Re: Quad 33 one channel brighter

It’s great that you have found and fixed the fault!

Reconciling the cause and symptoms is not so easy though. I have pondered it somewhat, and have come up with a possible explanation for why the disc input was apparently differently affected.

The tape adaptor board has a single emitter follower in each channel. This is configured to provide a 100k input impedance. If a collector connection is lost, then the incoming signal would see what is effectively a P-N (base-emitter) junction diode with a positive bias (around 6 volts) on the base. This would present only a small impedance to the incoming signal. And that diode would be in series with the emitter resistance, which totals 4k9. In turn that would be in parallel with the volume control (50k), and from the source viewpoint, also in parallel with the 100k emitter follower input resistance. (Two 220k bias resistors effectively in parallel as far as the signal is concerned, and a 1M bleed resistor ahead of the input capacitor). So the source would be looking at 4k3 give or take whatever adjustment is needed for actual diode resistance.

Click image for larger version

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A line source with a buffered, low impedance output might not be too bothered by such a low impedance load. If the source impedance is 1k, then the loss as compared with a 100k load is 1.8 dB (neglecting any additional bass loss due to coupling capacitors sized for higher load impedances), probably only just noticeable.

In the case of the disc preamplifier though, the situation is different. Whilst the output impedance of the feedback pair is probably quite low in and of itself, it is looking into an R-C network that is designed to correct the HF error that is inherent with series feedback RIAA equalizing amplifiers. In this case there is a 3k3 resistor and a 1500 pF parallel capacitor, for a time constant of around 5 microseconds. The last-mentioned is situation-specific, and I should guess is calculated for the M2 disc position, where the unwanted HF turnover would occur at a lower frequency than with M1 position because of its lower gain, 25 dB as compared with 34 dB.

Anyway, looking back from the emitter follower, the disc preamplifier appears to have a 3k3 source impedance. When loaded by 4k3 as in the broken collector connection case, the signal attenuation will be around 5 dB, which would be quite noticeable. Not only that, but the time constant would be modified – I haven’t done the calculations – and it might be that there is some additional treble rolloff.


Cheers,
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Old 21st Aug 2019, 9:13 am   #16
Norman Raeburn
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Default Re: Quad 33 one channel brighter

Thanks for the explanation, it makes more sense to me now. I was initially at a loss as to how it was only effecting the phono input. I think it was effecting the other inputs but to a much lesser degree. The tape input of course was fine as it is after the emitter followers. I am just glad that it's working now. Norman
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