Quad 34 no output

[aeromech]

My Quad 34+306 set up suddenly stopped working; I separated the units and found that both channels of the pre-amp (34) had almost no output V at full volume, regardless of input source or input level; common to both channels would be the power supply, I have now checked this and get the various voltages shown on the circuit diagram.The 306 works fine. Before I send the 34 off to Quad, any ideas on other simple checks or what might cause this Thanks

[Mooly]

First checks now after establishing supplies are OK would be to see if the output is muted.

T14 and 15 short the audio to ground when turned on so check the voltage on the base of those transistors. It should be around zero. Anything over 0.6 or so and the transistors will be conducting. See what that reveals first...

[aeromech]

Hi Mooly-thanks,
With audio inputs to ground, both T14 & T15 match,
base to ground 0.719v,
collector to ground 0.004v,
output 0.001v regardless of volume position!
Appreciate next step/interpretation?

[Mooly]

Its muted with 0.7 on the bases... interesting.

We have to piece the evidence together now with a few voltage measurements.

For 0.7 volts to be on the base there is a problem around IC21 and T13. Can you record and report back the following.

With the BLACK lead of your meter on A what is the voltage at point B. it should be -18 volts DC.
With the black lead of your meter on A what is the voltage at point C. it should be around -22 volts or higher.

Now read the DC voltage ACROSS C69. It should be around 0.7 + the volt drop of D33 (LED) so I would guess around 2.4 volts total if the LED is red and a bit higher if its green.

Is the LED lit by the way ? Is it the power light on the front ? If it were open circuit it would give this fault so if in doubt measure volt drop across it.

Is D32 the zener leaky. If in doubt remove it and see if fault clears.
A leaky D31 would also cause this fault.

[aeromech]

Mooly great directions, I will jump to the most obvious 1st.
Whilst the front panel lights for source and filters work the D33 LED inside does not light, the v across its contacts +0.003 or -0.001!!!!
IC21: A to B gives -18.02v
A to C gives -1.0v only!
C69 gives 0.023v only!
Is there an obvious solution, as I did not de-solder D32?
Thanks

['LIVEWIRE?']

Perhaps I'm missing something, but I don't see how you can have -18v at point B, yet only -1v at point C. IC 21 is a voltage regulator, and will only provide an output if it's input pin
(Point C) is supplied with around -20v(with reference to point A). As to why D33 doesn't light, it wouldn't if there is only -1v at point C, but it could obviously be open circuit, C69 could be short circuit, or T13 faulty.

[Mooly]

Originally Posted by aeromech

A to C gives -1.0v only!

Just recheck the A to C reading. As LIVEWIRE says it has to be there if you have -18v from the reg, or is your meter on a 20vdc range and its showing -1.000 as an "over-range indication". Just a thought.

From the readings you give I suspect C69 is short or very leaky. Remove it and see if it all works normally.

If it doesn't then just apply a short across C and E of T13 to test. That will (should) light the LED and unmute the audio.

See what all that shows...

[aeromech]

Gentlemen, I humble before you, this digital meter (Laser) does not give an out of range like my old Radio Shack one. Selected range from 20v to 200v and sure enough have -31.4v on A to C.
I also thought I might have a bad joint and went onto the legs of IC 21 for this reading, when I saw the volts I was so excited the probe slipped and a brief short occurred between B & C ugh! But wait, the LED is now on and I hurry around and press into service an MP3 and a small speaker set and low and behold there is music in both channels-unbelievable; take me a while to rig the 306 and Bose but what a turn up.
Many many thanks.

[Mooly]

I'm pleased its working, but will it stay like that

In your excitement did this accidental short occur on the transistor T13, as it reads as if you were on IC 21 when the probe slipped.

I wouldn't hesitate now, I would replace the transistor, LED, D31 and C69 now.
Maplins for all if needs be lol, cost should be under £2. If you do and your not sure what to get then ask.

[aeromech]

Hi Mooly, yes it was IC21 I shorted and correct, why and how will it last! Got it rigged with 306 and speakers, all seems fine.
But would appreciate a pointer, as Maplins did not list ES270, I am guessing there is a standard equivalent out there and the other components if you have reference to hand and time. Thanks.

[Mooly]

Here we go... Maplins prices are outrageous.
The cap... it will never see more than a couple of volts across it so physical size may be important as to what you choose. I've never heard of those transistors either but the BC546 will be perfect... just watch the pinouts as they are probably different to the original. The diode... I've chosen a 4001 over a smaller IN4148 type as it does discharge the 100uf cap at switch off.

I would just (calmly ) check the -18volts is still correct... regs are pretty tough but lets be sure.

The transistor can be any small signal NPN type of suitable rating such as BC546,
http://www.maplin.co.uk/low-power-lf...rdercode=N57AC

The LED... colour matters as the volt drop is different for each.
http://www.maplin.co.uk/3mm-leds-35702

Capacitor, 100uf 50volt
http://www.maplin.co.uk/radial-elect...pacitors-13472

Diode, IN4001
http://www.maplin.co.uk/rectifier-di...rdercode=QL73Q

Edit... the links don't work well, you'll have to select the value from the drop downs.

[Catchhq]

Hello there, I am new to the forum and I would like to thank Mooly01 for his excellent advice for diagnosing and repairing a muted Quad 34 preamp.

I followed the instructions and advice carefully as I had a Quad 34 with the same power supply problem causing the preamp to mute.

The following information may be helpful to someone thinking about a similar repair.

The voltage of the base of T4 and T15 and ground was 0.731v confirming the problem highlighted by Mooly01.
The voltage across A to B was -18.4v.
The voltage across A to C was - 30v.
The voltage across C69 9.4mv.
The led D33 was not on.

I replaced the following:

T13 with a BC547C (Maplin code: QQ14Q)
D33 with a 3mm green led (Maplin code: WL33L)
C69 with a 100uf 35v capacitor (Maplin code: VH38R)
D31 with a IN4001 rectifier diode (Maplin code: QL73Q)

All is working well now.
Thank you for an excellent forum.

[tri-comp]

There's really only one or two components that will cause this problem of the all components in question with the given voltages.
The problem arise because T13 is robbed of it's base-current and therefore switches off.
This extinguishes the LED as there's no current passing T13.
Collector of T13 is pulled-up by R103/R104 and the voltage at the junction of these resistors is high enough to turn-on the muting transistors T14/T15 fully.
Only a shorted C69 (Most likely) or an open R102 (Least likely) will cause this situation.
A defective (shorted) D32 will also activate the muting circuit but the LED will still be on, as T13 will conduct normally.
A defective T13 (Open, Shorted or Leaky) will NOT produce a reading of 9.4mV across C69.

rgds,

/tri-comp

[Mooly]

Originally Posted by Catchhq

All is working well now.
Thank you for an excellent forum.

Pleased it was of help and great to hear you have fixed a similar problem.

[waterssc]

Originally Posted by Mooly

Its muted with 0.7 on the bases... interesting.

Hi Mooly,
I have just joined this site as my Quad 34 has the same problem... muted/no output on both channels.

All the supply voltages are good as per your diagram, and D33 is illuminated. However there is 3.5v on the base of the muting transistorsT14/15. I tried removing D32 and the bases then measured 6v. I have changed out T13 and C58 for new but the problem remains. Could you point me to the next step please.
Thanks,
Steve

[TonyDuell]

If you have 3.5V _with respect to ground_ (that is, with the black lead of your meter on the ground rail) on the bases of those transistors then they must both be open-circuit base-emitter. That means the transistors have failed, but something else might have damaged them.

[waterssc]

Sorry my text should read -3.5v measured on the bases (with black lead to ground)

[TonyDuell]

In which case the 2 muting transistors should be cut off and have no effect on the signal, I think.

I would be inclined to start tracing the signal. Feed some input into the unit, and using a 'scope, small amplfier, or crystal earpiece see just how far the audio gets.

[Mooly]

Yes, minus 3.5 volts means the mute transistors are cut off.

Do you have any test equipment such as a scope ?

[Radio Wrangler]

So you have the same basic symptoms as the original poster, but a different cause.

Information is the key to finding out what's wrong. Somethings can be found out without test-equipment... things which narrow down the field of search.

1) Is it dead on both channels if you try signals into and selecting each input in turn?

2) Turn up the volume and do you start to get a tiny bit of increasing noise out of the.speakers?

3) does moving volume or balance control create any little scratchy sounds out of the speakers?

4) How do you know it's not the power amplifier?

David