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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment. |
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10th Aug 2010, 7:46 am | #21 | |
Dekatron
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Re: Quad II amplifier problem
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Cheers, GJ |
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10th Aug 2010, 10:08 am | #22 |
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Re: Quad II amplifier problem
There's an easy way to prove that the mains transformer has a short circuit secondary.
Meter on ohms range, one lead to chassis. Measure the resistance to each of the HT terminals on the mains transformer in turn. If the readings are widely different and not both low I'm afraid the transformer has had it. I didn't suggest this earlier because you said the the mains transformers in both amps measured the same.
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10th Aug 2010, 10:52 am | #23 |
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Re: Quad II amplifier problem
When I said the mains transformer in both amps measured the same, I was talking about the primary side - sorry for confusion.
The resistance from HT terminals to chassis are 70 and 0 ohms so this I assume is conclusive? Many thanks to everyone for your contributions. |
10th Aug 2010, 10:59 am | #24 |
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Re: Quad II amplifier problem
I'm afraid that's pretty conclusive. Half the secondary HT winding is short circuit. To be absolutely sure though remove the wires from the HT terminals and repeat the test. There's a slight chance that the fault could be in the wiring external to the transformer.
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10th Aug 2010, 11:40 am | #25 |
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Re: Quad II amplifier problem
Still get short from one of the terminals after disconnecting.
Thanks again |
10th Aug 2010, 12:39 pm | #26 |
Dekatron
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Re: Quad II amplifier problem
Hi
Im sure there's someone on the forum who has transformers for these at considerably less cost than Quad. As you've checked the valves, it's very likely that the transformer has died due to the main smoothing block being leaky or short. However, the KTs can go out of balance when warm causing this, so I'd run the pair in your other amp for a good while (don't leave the room!) and check one isn't running away. Also a GZ32 can short intermittently to the annoyance of the transformer. If a KT is the cause then it might have damaged the output transformer as well, but I hope not. Glyn |
18th Aug 2010, 8:29 am | #27 |
Tetrode
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Re: Quad II amplifier problem
What generally causes the main transformers to fail in these units? I have a pair. They're not used often but I'd like to take precautions.
Regards, Bruce |
18th Aug 2010, 9:07 am | #28 |
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Re: Quad II amplifier problem
The basic problem is that the mains transformer, especially the HT winding, seems to be the weakest link in the whole amp so it acts as the fuse when other things fail (sadly, in some senses at least, it's weaker than the actual fuse !).
First and biggest problem is that there are several faults which can cause the KT66s to pass too much current. Most common fault is failure (leakiness) of the coupling capacitors which feed the grids of the '66s. These are C2 and C3 in the standard Quad II circuit (e.g. http://www.dc-daylight.ltd.uk/Valve-...cs/QUAD-II.pdf). Even if they seem OK now I would always change these for modern ones. Second cause of too much current is a failure (gross leak, or dead short) of the cathode capacitor (C5). Again I'd recommend changing it as a matter of course. It is also possible that one of the valves could be gassy and go into thermal runaway. If it's only one valve the bias circuit would tend to switch the other one off to keep the total current more-or-less constant. But this doesn't always work well enough to protect the transformer. The trick here is to keep an eye on the valves - look out for red hot patches on the anodes and, if you can, check the grid voltages when they've been running for a while. They should be close to ground (less than half a volt ideally). Something which can occasionally cause trouble is substituting non-standard valves for the KT66s (so-called "tube rolling"). This can be a harmless pastime if you know what you're doing. But it can be risky if you don't. Other things that can see off the transformer are failure of the main reservoir and/or smoothing capacitors (C4 and C6) or, possibly, the GZ32 rectifier. It's also possible that the internal wiring of the choke (L1) can short to ground. These faults do tend to blow the fuse though. Unless of course someone has replaced it with a higher-current one or, worst of all, wrapped tinfoil round it. Never, never, never do this. In past times transformers would fail because the amps were being used to power too many peripherals (pre-amp, various tuners etc) and were also housed out of sight in badly ventilated cabinets. This sort of practice is much rarer these days. The problem with transformer failure has been around for sufficiently long that Quad themselves addressed it. For the 100V line version of the amp which they produced for Rediffusion they added an additional internal fuse in the HT line before the connection to the output transformer primary (I think it was 300mA). Finally, you say your amps are not used often. Actually, depending on what you mean by this, that can be A Bad Thing too. Unused components (particularly electrolytic capacitors, but also KT66s) can develop faults. I would strongly recommend powering the amps up for a couple of hours, say 3-4 times per year, just to keep things in good shape. If they haven't been switched on for more than, say, 2 years I would seriously consider not just plugging them straight in, but bringing them up more gently (there are various ways of doing this). Cheers, GJ |
18th Aug 2010, 9:15 am | #29 |
Octode
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Re: Quad II amplifier problem
Not a Quad fan here, so I'll take cover.
Quad were pretty clueless in terms of designing for reliability and made no attempt to evaluate the reliability of the components they specified or even design properly for worst case. In the 405 amplifier, the upper feed resistor for the op amp is used beyond its power rating - very much so if all worst case conditions apply. Damage to other equipment can result from a failure here. In the 33 preamp, the Rifa noise suppressor on the power switch suffers thermal fatigue and fails catastrophically short circuit. When I was doing component evaluation 40 years ago, I rejected these capacitors on these grounds. In the case of the Quad II, the mains transformer is not up to its job and certainly not up to powering a preamp and an FM tuner. The temperature rise is completely unacceptable for the insulation materials used and there is no proper protection in the form of a thermal fuse in the transformer (Philips and Murphy did that ) or even usefully rated external fuses. So, don't power external units from a Quad II and consider fitting a bimetal disk type overheat cutout under the transformer. Don't leave these amplifiers unattended. Bottom of the class Mr. Walker - good theoretical design, lousy detailing. Leon. |
18th Aug 2010, 9:41 am | #30 |
Tetrode
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Re: Quad II amplifier problem
Thanks GJ for such a comprehensive answer. I haven't powered mine in over a year and they're sadly used as bookstops. I'll get them up to scratch and watch the KT66s for red patches.
Regards, Bruce |
18th Aug 2010, 10:01 am | #31 |
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Re: Quad II amplifier problem
Well Leon, of course it's always possible to do a better job. But it costs time and money and Peter Walker was running a business. Given the innovative features present in a lot of Quad products and the grumpy letters that Peter Walker wrote to component suppliers, I'd be interested to see your evidence that they were "clueless", rather than making simple "let's get on with it" decisions. And they weren't the only ones who suffered from component failure - there are a few dead Leak and Rogers and Armstrong and RCA amps out there too (these folks weren't above running their transformers warm and/or using Hunts capacitors) !
I doubt Quad were designing for a 50-60 year lifetime so personally I'm surprised that there are so many Quad IIs (literally tens of thousands) still working. Many of these have spent quite a while in the bottom of cabinets with little or no attention. And if you take the trouble to have them serviced, which Quad/IAG are still prepared to do for you, they can go on for years and years . Cheers, GJ |
18th Aug 2010, 11:38 am | #32 |
Octode
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Re: Quad II amplifier problem
Whilst I would always prefer to keep my comments useful as a help to others, I think "clueless" was an absolutely fair description of some of Mr. Walker's activities.
Rating a resistor properly simply requires a knowledge of Ohms Law. I have loads of vintage radio equipment from the likes of Bush, Philips and Murphy which are more complex than a Quad II - and, importantly, older. I've very seldom had to replace mains transformers. This is because they were properly designed for their job and were used within the limitations of the materials specified. My Tektronix 'scope has a transformer warranted "for life". Any decently constructed and designed transformer will last the lifetime of the equipment to which it is fitted. Quad equipment is "very nice" and is in some respects elegant but it's not complicated stuff - not as complex as a decent radio. A failure to design properly for worst case operation earns them zero respect from me - so I rest my case. Leon. |
18th Aug 2010, 2:49 pm | #33 | |||||
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Re: Quad II amplifier problem
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Cheers, GJ |
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18th Aug 2010, 3:08 pm | #34 |
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Re: Quad II amplifier problem
Whilst Leon and GJ both have points about the design of this equipment, it doesn't help the OP who now has transformer failure and has to deal with it.
Unforunately none of us are able to travel back in time to tell Mr Walker that someone thinks his designs are flawed. What I will is that a manufacturer, any manufacturer of any product will find that problems can and do arise because of defects in the design which weren't know at the time of development. Now can we please stay on topic please? |
31st Aug 2010, 3:51 pm | #35 |
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Re: Quad II amplifier problem
Just to close this thread, I have now acquired and fitted a second-hand transformer and all is well - I guess the problem arose because I accidentally left the amp on for several days.
Thanks to everyone for helping. Gavin |
31st Aug 2010, 4:10 pm | #36 |
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Re: Quad II amplifier problem
Hi,
The Quad II was used by broadcasters and for public address and industrial applications. As such, it should be capable of being left on 24/7 indefinitely without causing mains transformer failure. I'd strongly suggest making sure the transformer failure wasn't caused by another fault elsewhere, otherwise the replacement may not last long. Regards, Kat |
31st Aug 2010, 4:20 pm | #37 |
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Re: Quad II amplifier problem
point taken
Gavin |
31st Aug 2010, 11:55 pm | #38 |
Octode
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Re: Quad II amplifier problem
The Quad II was indeed used professionally. It was used by one of my former employers - Rediffusion.
Although inevitably doomed as a "one trick pony" organisation, Rediffusion employed competent engineers (I worked with them) who obviously saw fit to modify this amplifier with extra fuse protection. This speaks volumes in terms of the adequacy of Mr. Walker's original design. I think our poster, who wishes to enjoy the performance of this unit without incurring the cost of all too frequent transformer replacement could do worse than to copy the Rediffusion mod. I don't have the precise details of this as it was done before my time. A thermal cutout bolted to the underside of the chassis in close proximity to this dubious transformer would be my first line of attack. Leon. |
2nd Sep 2010, 7:09 pm | #39 | |
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Re: Quad II amplifier problem
Quote:
I'm interested to hear that it was Rediffusion who came up with this mod. The actual wiring was, of course, done by Quad themselves during production and I'd always assumed that the fuse was simply an option they offered to those industrial customers, like Rediffusion, who were running the amps unattended for many thousands of hours per year. You live and learn ... Of course the purpose of this fuse was to protect not from internal faults in the transformer itself (it works fine under normal conditions) but from faults which occur downstream. I'd strongly support Kat's "strong" recommendation that you check out the rest of the circuit. You could choose to add additional protection to your amps if you like - it will do no harm if you're careful about the way you do it. But whatever you do don't drill the chassis, as Quad did, or cut any of the wiring short. Irreversible changes will seriously lower the amps' resale value. Cheers, GJ |
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2nd Sep 2010, 11:44 pm | #40 |
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Re: Quad II amplifier problem
I used to have two of these 100 volt line ones. I sold them about ten years ago and managed to pay for our cavity wall insulation! One didn't even have any valves in.
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