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Old 16th Sep 2020, 5:08 pm   #1
Bufo Bill
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Default Clumsy Quad 2 error

Hi, I decided to give the hifi a dust, and upon return to service of the equipment I switched on and there was a spark and burning smell from the power connector into the quad 2. The one wire had broken from the contact and shorted. Removed the QCII to the bench and re-soldered the wires (both joints were cruddy). Returning the set up, I found the valve heaters light up, the amp's fuse had not blown, but the sound level was low. Allowing the caps to discharge, I then opened up the amp the danger sign visible was a little crud around one choke turret (see pic). Is this the problem? Is it solvable? Or has that disaster of a short done terminal damage? It occurs to me that there could be some problem in the preamp also, as it does not sit in its case on the workbench as it normally sits in a stand.
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Old 16th Sep 2020, 5:21 pm   #2
GrimJosef
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Default Re: Clumsy Quad 2 error

The crud is unlikely to be the problem. The choke is potted in bitumen and the lighter components of this can seep out around the turret base. Solder flux can also look dark like that.

Normally the power connector (I take it you mean the 2-pin Bulgin mains connector) has screw connections to the terminals, not solder. Is that the connector you've been working on ?

Cheers,

GJ
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Old 16th Sep 2020, 6:20 pm   #3
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Default Re: Clumsy Quad 2 error

Hi GJ, the item in question is 2 pin connector that emerges from the QCII, alongside the Jones connector umbilical, I can provide a picture if required. No screws were inside the bulgin, although I see the holes for them in the brass contacts inside.
Cheers from Bill.
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Old 16th Sep 2020, 6:56 pm   #4
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Default Re: Clumsy Quad 2 error

I'm afraid that does happen. When the screws get lost, or the threads stripped, or the heads broken, rather than replace them some folks just get the soldering iron out.

Disconnecting the mains from the power amp, which is what the fault you describe would have done, shouldn't affect the audio circuitry at all. After all, we disconnect the mains every time we turn the kit off and that doesn't make it go quiet next time we turn it back on. When a fault inexplicably appears the first question I ask myself is "What have I just fiddled with ?". Since you'll have pulled the Jones connector out of the power amp I think I would start by wiggling the cable that goes into that and seeing whether the volume suddenly changes.

Cheers,

GJ
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Old 16th Sep 2020, 7:06 pm   #5
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Default Re: Clumsy Quad 2 error

Cheers GJ, I had visions of chokes blowing and all sorts of horrors. Will check out the Jones plug. The amp has old crusty flux on a lot of the solder joints, should I clean this off?
Thanks from Bill.
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Old 16th Sep 2020, 10:11 pm   #6
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Default Re: Clumsy Quad 2 error

Is the lead on C2 touching the capacitor clamp and shorting to ground?
Glyn
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Old 17th Sep 2020, 5:33 am   #7
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Default Re: Clumsy Quad 2 error

Is that resistor and cap in parallel with the choke? Surely that ain't right? Also noticed some solder balls top right, potential SC waiting to happen and the radial cap wants anchoring properly. Apart from that and some dodgy soldering around the Russian PIO where the connecting wire insulation got melted and excess flux can't see owt wrong. Excess flux can be an issue, it can go conductive not to mention corrosive so a good tech always carefully removes flux.

If you have heaters but no sound, remember "thoult shalt check voltages" - do you have full HT? Do you have AC sig in and out V1?

Andy.
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Old 17th Sep 2020, 8:08 am   #8
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Default Re: Clumsy Quad 2 error

Don't worry about the choke wiring Andy. There are four terminals on the choke plate. The two on the left (mostly hidden under the resistor) connect internally to the choke and externally to the right place in the HT smoothing circuit. The two that you can see aren't connected to anything at all inside the choke can*. They are put there for exactly the job they are doing i.e. mounting the KT66 cathode circuit R and C.

Cheers,

GJ

*Except that the choke itself is potted in bitumen and is held in place by nothing else. Over decades of warm running the very slightly fluid bitumen can allow the choke gradually to sink under gravity, and one of its connecting wires can come into contact with the back of one of those cathode component terminals. Bang !
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Old 17th Sep 2020, 8:53 am   #9
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Default Re: Clumsy Quad 2 error

It's likely that that the arcing mains connector will have caused a sudden surge following the undesirable removal then immediate restoration of mains power to the Quad II. With valve equipment that runs at relatively high HT current that surge can sometimes cause a marginally serviceable component to fail. I agree that voltage checks are the way to go next. I've also found that oxidation of the Jones plug pins introduces high resistance that can reduce HT or heater supply to the QC2 or Quad 22 control unit so worth cleaning them and the Jones socket holes first. Also the photo looks like the Russian PIO decoupling capacitor has been pushed left such that the KT66 grid lead is shorting to the clamp. That would certainly explain it! Cheers, Jerry

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Old 17th Sep 2020, 11:17 am   #10
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Default Re: Clumsy Quad 2 error

I have seen burned resistors caused by intermittent power.
If the power is interrupted briefly the grids of the valves act like a rectifier charging the coupling capacitors. This turns the valve hard on for a second or so and can burn anode resistors or even worse output transformers.
I was once asked to fix an AR88 that had the mains voltage selector all full up with charcoal. It had taken out the power feed filter resistors in the IF stages. The reported fault was intermittent heaters...
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Old 19th Sep 2020, 5:47 pm   #11
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Default Re: Clumsy Quad 2 error

Hi, i guess I should fill you in on my silence. Dad got taken to hospital with an infection and blood clot. While he was waiting for treatment he fell and broke his hip. Spent the day moving furniture to make his life easier when he gets home. I will be getting back to electrical issues soon though, i need to keep busy.
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Old 19th Sep 2020, 7:09 pm   #12
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Default Re: Clumsy Quad 2 error

Wishing your Dad (and your amp!) well, hope both are up and running soon
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Old 19th Sep 2020, 7:32 pm   #13
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Default Re: Clumsy Quad 2 error

Thank you so much Colin, kind wishes really mean a lot just now.
Bill.
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Old 19th Sep 2020, 7:55 pm   #14
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Default Re: Clumsy Quad 2 error

I'll send mine too Bill. There's no rush with the electronics. Our families come first.

All the best,

Graeme
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Old 19th Sep 2020, 8:12 pm   #15
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Default Re: Clumsy Quad 2 error

Thanks Graeme, it's good to know I have good friends here, i will hopefully be able to have a session tomorrow. Currently Covid rules prevent me from visiting, so distractions are most welcome and actively sought out just now.
Cheers from Bill.
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Old 20th Sep 2020, 9:55 am   #16
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Default Re: Clumsy Quad 2 error

All the best, Bill. Cheers, Jerry
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Old 20th Sep 2020, 12:04 pm   #17
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Default Re: Clumsy Quad 2 error

Sympathies - similar sort of situation here, though at least the last fall didn`t break anything....
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Old 20th Sep 2020, 1:17 pm   #18
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Default Re: Clumsy Quad 2 error

Thanks guys, you guys are helping me through this.
Bill.
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Old 20th Sep 2020, 5:34 pm   #19
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Default Re: Clumsy Quad 2 error

Got in a little action on the quad front. The EF86 works well, nothing unusual in the meter readings.
Anode:70V
Cathode:1.45V
Grid 2: 69V
Will check Jones plug for bad connections as in post#4 by GJ.
DAD UPDATE: Hopefully going to theatre late this afternoon (for surgery not Les Mis).
Cheers from Bill.
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Old 20th Sep 2020, 8:01 pm   #20
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Default Re: Clumsy Quad 2 error

We have a little progress. Jones Plug pin 2 had frayed wire, and a single strand was fouling pin 1. Resoldered and applied heat shrink. But no real change. I then tested the radio, which I had neglected to try previously, and the volume was just the same as always (no.2 on the volume dial).

So can we say for certain that the fault lies in the Phono preamp circuit? Will have a look at the schematic, they are not yet second nature to me so would appreciate suggestions.

Re the clamp on C2: The angle of the photo is deceiving, the lead and clamp are not in contact. It does look like it but I think that is because of the limitations of the phone camera. I have checked and there is no contact.
DAD UPDATE: No surgery today, but they are saying he is on the list for tomorrow.
Cheers from Bill.
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