Quad 34 question - UK Vintage Radio Repair and Restoration Discussion Forum

smalldr0 · 23rd Mar 2009, 5:31 pm

I have a quad 34 control unit (S/N 4113) that has distortion on its output.
My first thought was the power supply and the usual capacitor problems, and I therefore changed all the PSU cap's and the signal path tants without any further consideration.
However the distortion is still present.

I have therefore had a quick poke around with a scope and there still seems to be more noise on the supply rails than I would expect. However, I can't for certain say if this is a supply issue or noise being caused by something connected to it. The voltages themselves are within a few milli volts of the values stated on Keith Snooks excellent circuit diagram.
I should also confirm that the distortion is present on all the line level inputs.
With 1k tone at 200mV on the input of say the Aux input. It's distorted as soon as it passes through the first active component (IC2). The distortion looks like noise on the peaks of the sine wave.

Has anyone out there had a similar experience, before I really have to spend a significant amount of time looking at this thing.

Thanks in advance....

dave cox · 24th Mar 2009, 12:05 am

I'm not familiar with the quad, but I'll have a stab anyway

Does the same happen on the tape input ?
IC2 switches the L/R input from radio and aux but,
IC1 is used to control the tape input ...

What about the disk inputs ?

What does the 'distortion/noise' look like ?
Is it sharply delimited 'blurry patch' at the signal peaks and troughs ?
if so, it could be a burst of HF oscillation.

Did you measure the noise on the supply rails ?
I would expect some tens of mV - at a guess.

I would check the control voltages on IC1 , IC2 & IC22 (pins 5/6 & 12/13)
maybe more than one output is trying to drive the signal bus ...
datasheet

good luck

DC

smalldr0 · 24th Mar 2009, 4:02 pm

Thanks very much for the response.

Yes the probelm is also present on the tape input. Unfortunately I have not tried the phono input.

It is also as you suggest, like a bust of noise on signal peaks, at say pin 9 and 3 of IC2. This However looks more like the 'squaring off' of the peaks once it gets through to the output DIN.

I have some 4066's but I still suspect the problem is the supply. The rails have at least 1.5v of noise (I wouldn't describe it as ripple) on them. As previously stated I have changed all the electrolytics all ready.

I guess I'm going to have to start lifting components.....
Best regards

dave cox · 31st Mar 2009, 2:13 pm

Quote:

It is also as you suggest, like a bust of noise on signal peaks, at say pin 9 and 3 of IC2. This However looks more like the 'squaring off' of the peaks once it gets through to the output DIN.

Yup, the O/P opamp won't have much gain at high frequency ...
BTW, do you see the same signal at the input socket being driven ?
Is anything else connected to the pre-amp ?

Quote:

I have some 4066's but I still suspect the problem is the supply. The rails have at least 1.5v of noise (I wouldn't describe it as ripple) on them. As previously stated I have changed all the electrolytics all ready.

Opamps have a pretty good PSRR, even a volt of supply line trash won't show much on the output (not with a scope, anyway). IME, the supply line garbage is more likely an effect rather than a cause. The opamp in the PSU forming an active ground reference is a likely candidate - if the supply is asked to supply 'too much' current from one rail, this opamp won't be able to dynamically balance the supply. If you have a differential probe you could look at the voltage/noise between the + / - rails rather than from each rail to ground.

DC

smalldr0 · 3rd Apr 2009, 12:56 pm

Quick update for all you kind souls who have taken an interest in my woes.

After some poking about the main culprit was IC17.
It ws not only slightly distorting the audio passing through it, it was also the cause of a large amount of noise on the supply rails.
The give away for future reference, was that operating the filter selection switches (i.e. switching off pin5 on IC17) changed the distortion shape.

Anyway after changing this IC the pre-amp ran for three hours sounding as good as it should.

However, I then put the case back on.....
It then worked ok for about 30mins, at which point it disolved into distortion with again, both L&R channels effected.
I have just purchased a new can of freezer spay, and will be reving my girlfriends hair dryer up at some point over the weekend.

Best regards...

23rd Mar 2009, 5:31 pm	#1
smalldr0 Retired Dormant Member Join Date: Mar 2009 Location: Essex, UK. Posts: 4	Quad 34 question I have a quad 34 control unit (S/N 4113) that has distortion on its output. My first thought was the power supply and the usual capacitor problems, and I therefore changed all the PSU cap's and the signal path tants without any further consideration. However the distortion is still present. I have therefore had a quick poke around with a scope and there still seems to be more noise on the supply rails than I would expect. However, I can't for certain say if this is a supply issue or noise being caused by something connected to it. The voltages themselves are within a few milli volts of the values stated on Keith Snooks excellent circuit diagram. I should also confirm that the distortion is present on all the line level inputs. With 1k tone at 200mV on the input of say the Aux input. It's distorted as soon as it passes through the first active component (IC2). The distortion looks like noise on the peaks of the sine wave. Has anyone out there had a similar experience, before I really have to spend a significant amount of time looking at this thing. Thanks in advance....

24th Mar 2009, 12:05 am	#2
dave cox Nonode Join Date: Jan 2009 Location: Bristol, UK. Posts: 2,061	Re: Quad 34 question I'm not familiar with the quad, but I'll have a stab anyway Does the same happen on the tape input ? IC2 switches the L/R input from radio and aux but, IC1 is used to control the tape input ... What about the disk inputs ? What does the 'distortion/noise' look like ? Is it sharply delimited 'blurry patch' at the signal peaks and troughs ? if so, it could be a burst of HF oscillation. Did you measure the noise on the supply rails ? I would expect some tens of mV - at a guess. I would check the control voltages on IC1 , IC2 & IC22 (pins 5/6 & 12/13) maybe more than one output is trying to drive the signal bus ... datasheet good luck DC

24th Mar 2009, 4:02 pm	#3
smalldr0 Retired Dormant Member Join Date: Mar 2009 Location: Essex, UK. Posts: 4	Re: Quad 34 question Thanks very much for the response. Yes the probelm is also present on the tape input. Unfortunately I have not tried the phono input. It is also as you suggest, like a bust of noise on signal peaks, at say pin 9 and 3 of IC2. This However looks more like the 'squaring off' of the peaks once it gets through to the output DIN. I have some 4066's but I still suspect the problem is the supply. The rails have at least 1.5v of noise (I wouldn't describe it as ripple) on them. As previously stated I have changed all the electrolytics all ready. I guess I'm going to have to start lifting components..... Best regards

3rd Apr 2009, 12:56 pm	#5
smalldr0 Retired Dormant Member Join Date: Mar 2009 Location: Essex, UK. Posts: 4	Re: Quad 34 question Quick update for all you kind souls who have taken an interest in my woes. After some poking about the main culprit was IC17. It ws not only slightly distorting the audio passing through it, it was also the cause of a large amount of noise on the supply rails. The give away for future reference, was that operating the filter selection switches (i.e. switching off pin5 on IC17) changed the distortion shape. Anyway after changing this IC the pre-amp ran for three hours sounding as good as it should. However, I then put the case back on..... It then worked ok for about 30mins, at which point it disolved into distortion with again, both L&R channels effected. I have just purchased a new can of freezer spay, and will be reving my girlfriends hair dryer up at some point over the weekend. Best regards...