Quad II - PT autopsy begins...

[trobbins]

That's a pretty clear distinction.

Perhaps you just have the knack of restoring poor PT's by un-potting them!

[dougietamson]

Originally Posted by trobbins

That's a pretty clear distinction.

Perhaps you just have the knack of restoring poor PT's by un-potting them!

I'll still check the winds for a short and hope it's just on the outer filament ones for an easy rewind.

[kalee20]

Originally Posted by trobbins

If so, then perhaps if you had time, you could measure the mains primary current with increasing voltage. To me, that could be the better characteristic to use for comparison with a 'good' original PT, and also for comparison with the unpotted PT. For an unloaded PT, the primary current waveform would likely be quite distorted, but an rms value could still be a good comparison metric.

The other valid metric would be 'joule heating' as previously mentioned.

It's worth doing, if you have a Quad transformer that's good, or at least comes intermittently good and you catch it on a good day.

For what it's worth this is the type of characteristic you get (this is actually on a 500W toroidal transformer, 240V nominal primary). I measured RMS magnetising current, and true power using a dynamometer-type wattmeter. You can see that the off-load current is a couple of tens of milliamps, though it starts to take off at 270V.

[Richardgr]

I suppose a defective transformer could just be a symptom and not the root cause. Are there other signs of distress in the amplifier?

[G6Tanuki]

Originally Posted by Richardgr

I suppose a defective transformer could just be a symptom and not the root cause. Are there other signs of distress in the amplifier?

Either way, I would not be trusting this failed transformer even if the apparent shorted turns are found and fixed.

It's been cooked in service, meaning the condition of the other windings must be suspect.

A full rewind is the only real way to guarantee any sort of future trustworthiness. You wouldn't want to go away for the weekend without remembering to turn off the amplifier and come back to a fire scene.

[Ted Kendall]

Agree with G6 and RW. They're mullered. Full rewinds - you know it makes sense.

[dougietamson]

It was serviced by Quad in 1988, some components have been replaced.

Photos of inside attached

[Craig Sawyers]

The mains transformer was always somewhat underrated.

Apart from the power amp requirement, also intended to supply power to the preamp, FM tuner and AM tuner. HT and heater supply to all of them. With two power amps, one or the other would have to supply power to a total of 13 valves in the ancillary units.

Craig

[dougietamson]

Originally Posted by Craig Sawyers

The mains transformer was always somewhat underrated.

Apart from the power amp requirement, also intended to supply power to the preamp, FM tuner and AM tuner. HT and heater supply to all of them. With two power amps, one or the other would have to supply power to a total of 13 valves in the ancillary units.

Craig

The 2 sickly Quad IIs also came with the 22 preamp and AM tuner.

[retailer]

Not that long ago I rewound a LUX opt that had been potted in pitch - a third person had started the job and given up so even though a good deal of the pitch had been removed, cleaning it up what remained prior to the rewind was not a pleasant job so I can understand a reluctance, however if you do decide to go ahead Xylene from the local hardware store will dissolve the pitch from the laminations, you'll need to get all of the pitch off to stand any chance of getting them all back in. I left the Lux laminations soaking for a few days and 95% of the pitch just washed off leaving very little to scrub off with a coarse cloth.

[GrimJosef]

I know someone who recommended petrol for cleaning bitumen off lams. It does work, but with the obvious risks of fire and benzene (and other hydrocarbons) toxicity.

I've used paraffin (kerosene) successfully. I even once tried it in an ultrasonic bath, with the lams suspended along a wire. I did that outdoors though and spent most of the time some distance away. The ultrasonics heat the paraffin.

Cheers,

GJ

[kalee20]

I've used paraffin, too - especially if the lams are warm it does a good job. You don't need a lot, use half a cupful, let it soften and remove most of the bitumen. Then another half cupful as a second wash, and a third half-cupful to rinse.


With bonfire night coming up, you've got a ready source of flammable fuel for lighting-up!

[dougietamson]

I found a tin of cellulose thinner in the shed, I've tested and it disolves the bitumen nicely.

It's warm today for the season so I can do it out doors for ventilation.

I'm also off down the the mens shed, there might be some 'stuff' donated from items found in memebers sheds that probably fall foul of modern rules

[toprepairman]

Personally I wouldn't bother with a rewind.
Steve of Valvepower Stereo 20 fame has replacement transformers for a very reasonable price.

[Richardgr]

Did you see this thread?
https://www.vintage-radio.net/forum/showthread.php?t=210698

[Radio Wrangler]

Be careful with cellulose thinners. It's an effective solvent but attacks a number of plastics. However, it is very inflammable and like petrol can fill an enclosed space forming an explosive mixture with air.

The value attached to these amplifiers is a function of perceived originality. So you'd want any replacement to be in something indistinguishable from an original can.

David

[dougietamson]

The laminations are out, gentle warming of the core and the wax impregnation between them allowed for an easy disassembly with a thin plastic guitar pick.

The 5v wind was 1st off the bobbin, 14 turns of 1mm OD wire, looks a bit rough in places.

Then the 6.3v, actually a 3.15-0-3.15v with a CT (thr 'E') turret) 9+9turns of 1.3mm OD wire, looks ok, the last turn wandered off needlessly close to the laminations.

The HT next, one of the leads fell off, looks like a bad solder joint to the strip of thin corroded copper. may have been arcing too.

Will resume the process during daylight.

photos attached.

[dougietamson]

Originally Posted by Radio Wrangler

The value attached to these amplifiers is a function of perceived originality. So you'd want any replacement to be in something indistinguishable from an original can.
David

Quite so, I did buy a suitable modern replacement PT at a very reasonable £61 inc p&p from China with the idea of dropping in in the can, soldering to the turret board and potting it with the original bitumen.

The only way to tell it wasn't original would be to un-pot it or measure the specc to find it has better insulation due to the modern wire coating and runs cooler.

[kalee20]

Super photos!

That actually looks not a very good winding design - there's a lot of wasted space, especially with the 5V winding. Two strands (bifilar), 0.7mm diameter, look as though they would nearly fill the winding width (while still leaving some margin at the edges) if close-wound without the huge gaps between the turns - and would occupy less winding height. And the resistance would be the same.

[dougietamson]

Originally Posted by kalee20

Super photos!

That actually looks not a very good winding design - there's a lot of wasted space, especially with the 5V winding. Two strands (bifilar), 0.7mm diameter, look as though they would nearly fill the winding width (while still leaving some margin at the edges) if close-wound without the huge gaps between the turns - and would occupy less winding height. And the resistance would be the same.

Spot on, and bifilar is so much easier to wind.