PL 700 II Clair Bros Rising from the Ashes

Gepetto said:
The PL400 I measured was connected to the Acurus Preamp which has its DC ground connected internally to its safety ground via a 3 prong plug. That is my system ground which is established at the pre. It is important to have only one safety ground connection point in any system for prevention of ground loops.
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Joe,

I thought I'd point out that your Acurus pre has a 10 ohm resistor bypassed with a film cap isolating the DC and Chassis Mains ground. So resistance between RCA ground and the IEC ground pin should show 10ohms. Mondial did this in all of their pre's including the Aragon's.
 
mondialfan said:
Joe,

I thought I'd point out that your Acurus pre has a 10 ohm resistor bypassed with a film cap isolating the DC and Chassis Mains ground. So resistance between RCA ground and the IEC ground pin should show 10ohms. Mondial did this in all of their pre's including the Aragon's.
Click to expand...

Yup, agree...
 
grapplesaw said:
Have a look at the rane BB 44x

here is one one eBay for less tha $100

https://www.ccisolutions.com/StoreFront/jsp/pdf/RAN-BB44X.PDF

I have few. They are great. Total isolation with converting from balanced to unbalanced connects.

See eBay item. 143462868003
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Those are nice units. They also do a level conversion with the transformers wound 1:3.89 turns. I am just trying to create an isolation in the interconnects. But it is interesting that the Rane units are priced so much less. Less is more...
 
With input wiring disconnected from the PL14_20, and replaced with 10k resistors, there is the same hum/buzz. Scope is set to 0.5 mV per division.
The hum almost doubles when my finger gets within 1/2" of the resistor
 

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I am not an expert in transformer design, but I echo Joe's recommendation that you get your transformer tested. Here are some tests I found on the Internet:
1. Winding resistance test.
2. Transformer ratio test.
3. Measurement of insulation resistance.
4. Dielectric tests of the transformer.

The transformer winding resistances can be measured by current voltage method. In this method of measurement of winding resistance, the test current is injected into the winding and corresponding voltage drop across the winding is measured. By applying simple Ohm’s law i.e. Rx = V ⁄ I, one can easily determine the value of resistance.

The procedure of the transformer ratio test is simple. Just apply power to the primary winding, with keeping secondary winding open. We measure the induced voltages at primary and secondary terminals of the transformer to find out actual voltage ratio of the transformer.

Insulation resistance measurement steps:
  1. Disconnect all the primary and secondary terminals of the transformer
  2. Megger one of the leads to primary and one of the leads to secondary to measure insulation resistance in between the primary and secondary windings
  3. Megger leads to primary and transformer earth point to measure insulation resistance in between the primary windings and earth
  4. Megger leads to secondary and transformer earth point to measure insulation resistance in between the secondary windings and earth
Dielectric test can be done by the Separate Source Voltage Withstand Test:
  1. The terminals of the primary winding are connected together.
  2. The terminals of the secondary winding are connected together and also the frame of the transformer are connected to earth.
  3. Then a 60 cycle 120 Volt source is applied for 60 seconds across the terminals of the primary winding and earth.
  4. The test is successful if no break down in the dielectric of the insulation occurs during test.
Thoughts?
 
Hope to do some testing on the transformer of the second amplifier today. Megger testing would certainly yield some worthwhile info but both transformers are presenting issues under normal line voltage conditions.

Can these transformers be re-wound? I have called a couple of shops in Barrie Ontario, no luck. This may not be an economically logical way to go, but has anyone heard of it even being possible?
 
Peter S said:
Hope to do some testing on the transformer of the second amplifier today. Megger testing would certainly yield some worthwhile info but both transformers are presenting issues under normal line voltage conditions.

Can these transformers be re-wound? I have called a couple of shops in Barrie Ontario, no luck. This may not be an economically logical way to go, but has anyone heard of it even being possible?
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Anything is possible but you will not like the cost involved with doing so. The windings in a transformer become landlocked inside a varnish impregnated core
 
I know a guy in the UK who rewinds transformers. He's not cheap, but he is fair. Shipping costs will be high. Let me know if you want to contact him.
 
Hi Blue; I don't know if I should waste his time, but if it can be done in the UK, I should (hope to) be able to find a service like that in Toronto. As Joe said, it will likely be expensive, but shipping 40 lb's back and forth to England would be "over the top".
I have taken an "E I" transformer apart 40 years ago....but it wasn't going back together. How can they separate the laminations without it looking like a deck of cards that went through the laundry?
Also, do I need the specs for this transformer or can these places figure all this out? Would your contact be willing to answer a couple of questions regarding this project?
 
Let me see if I can get a contact. He's on a forum for the restoration of vintage radios. I lived in the UK for a long time, still participate on the forum. He rewound an output transformer for a 1945 Ekco radio I have.

Still, I think the megger is the way to go - just to see if you really have a problem. I'll wager a beer or two, you don't.
 
Would be much more cost effective to buy another dog on the bay for the transformer. One just went for low dollars. A waiting game.

I would be inclined to try putting a steel shield sheet between the transformer and remainder of chassis to see if you can cut down on the stray flux it seems to be emitting. See if that affects anything in the way of a reduction or not.
 
So here's what he had to say:

I'm still winding transformers and you can contact me directly at ed_dinning@yahoo.co.uk (note _ underscore).

If you can send me some pictures and the electrical spec/ schematic I can give you a quote. Cost could be in the region of £100-£200 for such a massive transformer and carriage will be expensive. You might like to investigate if your carriers do a "collect and return service" as that would certainly be cheaper.
Some companies operate a "groupage " service where I would dispatch to a UK address and they would forward it.
It would also be cheaper if only the windings can be sent and the transformer re-assembled in the US

Some idea of the problems would be helpful and you can also let me know if you need different primary voltages as I believe some parts of the States are now on 125/127 volts. Not good if your transformer is wound for 120v !

The current exchange rate is about $1.30 = £1.00.
 
Gepetto said:
Would be much more cost effective to buy another dog on the bay for the transformer. One just went for low dollars. A waiting game.

I would be inclined to try putting a steel shield sheet between the transformer and remainder of chassis to see if you can cut down on the stray flux it seems to be emitting. See if that affects anything in the way of a reduction or not.
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Joe,

I mentioned to Lee a while back that at some point I would test an original 700 with and without the aluminum "hum shield". (I haven't had the time to do that yet.) Dean didn't seem to think the hum shield had any effect. However he didn't back it up with any data. Bob Carver must have thought it made a difference. Otherwise he probably wouldn't have included it.

How much of a difference would say steel have over aluminum? Is there some form of mu metal that can be used?

Ed
 
oldphaser said:
Joe,

I mentioned to Lee a while back that at some point I would test an original 700 with and without the aluminum "hum shield". (I haven't had the time to do that yet.) Dean didn't seem to think the hum shield had any effect. However he didn't back it up with any data. Bob Carver must have thought it made a difference. Otherwise he probably wouldn't have included it.

How much of a difference would say steel have over aluminum? Is there some form of mu metal that can be used?

Ed
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Hi Ed
Aluminum will not do much of anything for leakage magnetic flux, it will handily contain electrostatic fields however. In order to have aluminum be effective it has to be a canceling loop around the transformer leakage flux in which case it will generate an eddy current to cancel out the impinging magnetic field. But as an open plate, that circuit is not completed. Steel or MuMetal will contain magnetic flux fields by shunting those stray fields through the ferromagnetic material and then back to the source to complete the magnetic flux line.

Aluminum has a relative permeability of slightly over 1 (nearly the same as air), steel up to ~4000 and MuMetal in the ~18K range, Permalloy even higher. The more permeable, the better magnetic shield it is.

Would be interesting to see if the original PL700 hum shield makes any measurable difference, it would show how much electrostatic field is present. I often wondered if that aluminum plate was intended to be a ground bus or shield in the original 700.

Hope this makes sense.

Joe
 
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