Finding a mains transformer for Orange CR60

[stratfordade]

I've got an Orange CR60 where one of the two 120V primaries has gone open circuit. The amp runs great if I switch it to run on 115V, and power it via my variac, but can't use it on UK 240V wall voltage directly with mains switch set to 230V, as the open primary is then in series with input..

I could have the open transformer primary rewound but would be ideal to find a replacement transformer. The original has the two 120V primaries which is standard, but has two identical secondaries 0-18V-28V wired so that the preamp/switching is fed by 18-0-18 (0.8A) and the power amp from 28-0-28 (1.5A).

I imagine the transformer was custom made for the amp, but can anyone suggest a source for such a transformer in case it's available off the shelf. It's a chassis mount.

[Loudthud]

Have you tried Orange. They may only sell to an authorized service center.

[J M Fahey]

Custom rewinding is more expensive than anything available off the shelf.

You may find easier a 28+28V secondary PT to feed the power amp, and derive +/-15V rails out of it using regulators or Zeners.

Try to find the schematic and post it here to calculate actual current demands.

[stratfordade]

Thanks that's a good suggestion on deriving the +/- 15V from a more readily available 28-0-28.

Schematic attached (home-brew by someone but it aligns well as far as I've checked with the actual amp). As far as current goes the original transformer is marked 1.5A on the 28V tap which sounds about right for a 60W output? The requirement for the preamp/switching/digital reverb powered off the +/-15 will be lower.

I have sent request to Orange, but I'm also pretty sure they will only sell handles etc to anyone outside the official service network!

[J M Fahey]

Yes, I found a .jpg version of same schematic:

You cannot view this attachment.

Main 28+28AC winding  gives you very common(for 100ish amps) +/-40V rails.
It´s a popular voltage, relatively easy to find.
A Tech might even have a "pull" from a dead Marshall/Laney/VOX/Torque/etc. he has in his amp graveyard. for low cost.

56VAC * 1.5A= about 90VA, fine for a 60W amp.
Of course, any 100-120 even 150VA transformer is fine, as long as it fits in available space.

As of the preamp supply, secondary winding 17+17 VAC gives you raw +/-24 VDC, which are then down regulated to +/-15V plus +5V DC.

Simplest solution is to add a couple LM7824/7924 on a small perfboard, bolted to chassis for dissipation, wiring as the simplest datasheet example, to get +/- 24V DC.
The rest of the supply stays as-is.

If in doubt, ask.

[stratfordade]

Thanks for guidance. I wonder why they went to trouble of a separate secondary in first place for the preamp, since the amp has similar regulation circuitry for the 15/5V supply?

I've arranged to have the open primary rewound at reasonable cost, so given convenience if this I think it's worth it given marginal price difference with sourcing a 28-0-28 and the other regulator parts.

[J M Fahey]

Thanks for guidance. I wonder why they went to trouble of a separate secondary in first place for the preamp, since the amp has similar regulation circuitry for the 15/5V supply?

In theory everything should have its own supply, windings, etc.
In practice, that adds cost.

Repeat with me:
* Money RULES.
* every cent saved is an advantage, either as profit or by offering lower price to Customer.

Adding an extra winding (actually two in series plus an extra bridge rectifier plus 2 filter caps adds significant cost and build time.

Now, in Asia parts cost is lower, salaries too, so in Japan/Korea/China made amps you will usually see the extra supply; in UK/US/Argentina amps you will usually not.

In theory LM78/79xx regulators "could" down regulate from main (+/-40V) rails BUT they have a 35V limit 
So they can´t handle 40V .
So Orange added a special +/-24V supply.

I suggest a two stage regulation: 40 to 24 to 15 or 5 V which is not practical in a mass produced amp but acceptable in a servicing job.

There are other options but I go for the simpler one if possible.

I've arranged to have the open primary rewound at reasonable cost, so given convenience if this I think it's worth it given marginal price difference with sourcing a 28-0-28 and the other regulator parts.

Good.

Restoring original configuration is worth the extra cost/effort so go for it.

Note to rewinder: I *suspect* the transformer has an internal thermal fuse which blew (winding overheated)

 , *maybe* he does not need to do a full rewinding (which is a mess) but just replace fuse, but ONLY if after inspection he finds windings undamaged.


Why do I mention "Full rewinding" if only one primary is open?

It depends on how transformer was made.

Typical is to wind everything around a single plastic bobbin.
First primaries, then secondaries, so to reach damaged primary you must remove everything around it. Ugh! 

Sometimes a split bobbin is used, so primaries are on one half, secondaries on the other, side by side, so they are more independent.
Again, only inspection can tell.

Problem with rewinding is that transformers are dunked min isolating varnish, often hard epoxy or polyester, which complicates everything.

So let winder give you his expert opinion after inspection.

Classic winding. Notice primaries are *buried* under that thick wire secondary:


Split bobbin winding.
Pray you have this 

[stratfordade]

That is an extremely astute post, thank you.

I wondered about a thermal fuse as it's explicitly referred to on the transformer. I think certain Peavey amps also had a fuse buried in the transformer.

Will be interesting to hear what the guy rewinding it finds. I think from the layout of the primary and secondary connecting wires it is laid out in the more convenient split way, but I'll update this thread once I know for certain.

[gbono]

Hello very interested in your source for rewinding the xformer. Is that something you can share?