Transformer repair

Was this the PSU?

Both PSU's for both Mics in this picture are identical internally, apart from one has different value Zeners to produce the HT for the different mics.

In this case the PCB's are both marked at 9v5AC and 200v AC at the TX inputs. The S.E. Electronics one has a shorted primary (One Zener dead also).

Just wondering if all these cheapo tube mics are all made in the same factory.

Cheers,

Gareth.

"Gareth Magennis"

The AC tranny is smaller and mounted off the PCB.

I expect that most like it are coming from the same Chinese maker.

BTW:

The MA-200 uses a type 5840W sub-miniature pentode with wire leads.

Connected as a cathode follower and with a DC heater supply - it's as QUIET as a mouse.

.... Phil

OK, cheers.

Both mine use ECC83. The HST had a cheap chinese one that sounded like a mouse caught in a sandstorm.

Gareth.

nice save. I'd never have sat there counting thousands of turns.

"Cydrome Leader"

The 2000 plus turns figure was an estimate based on the core cross section ( 0.4 sq inch ) and a rule of thumb about turns per volt. The wire was

My working assumption was that for identical cores, 230V @ 50Hz primaries ought to be all the same - especially so when the Chinese mass produced both examples I had.

Turned out to be a very good assumption.

.... Phil

This bodes well for the future, perhaps the Chinese have indeed unwittingly done us all a favour.

The transformer I have here in the SE Electronics PSU appears to be of similar construction to yours. Likewise a pair of Tannoy powered monitors also appears to follow the same design but obviously scaled up a lot.

I guess it would make perfect sense if, say, one factory was making all the transformers, that they would have a modular system in place. i.e. a few different VA sizes of cores with standard plug-in primaries, and producing secondaries to order.

Gareth.

This is how it works for the most part. There are standard sizes of cores/laminations and bobbins.

For high volume stuff, you might start to see customized parts like bobbins and other mounting stuff.

I asked a transformer designer how he designs custom transformers. The short answer was grab a previous design from the filing cabinet because anything anybody wants has already been designed. Sometimes people want different lead colors, and everybody gets a new part number.

Hmm, I'm not sure that in the real world you could easily take a transformer from a random piece of equipment and replace the primary.

Gareth.

"Phil Allison"

Just gotta find a tranny that uses the same size SPLIT bobbin.

.... Phil

be

nto

it

n.

alf

ic.

This may sound like a stupid question, and at the risk of insult I had to ask. You never mentioned it but I'm just curious Phil did you check the mic out using separate power supplies before going to all that trouble, just in case the unit supplies as well as the amp was fried? Usually what you described generally, (as you found) just opens a primary and/or a fuse but you never know. Around here transformers get blown up by lightning all the time. I've subbed transformers in microwave ovens and othr equipment with some degree of success but I usually try to check out the boards on the bench first.

By the way, while we're having this discussion, it seems like I recall some time ago seeing a small article somewhere about how to determine the secondary voltage or voltages of a transformer with an open primary. Of course you can come close by looking at the voltage ratings on the electrolytics that are closest to the secondaries but this method didn't involve that or dis assembly either. In any event Phil I applaud you you all your work in repairing that transformer. Lenny

"klem kedidelhopper"

This may sound like a stupid question, and at the risk of insult I had to ask. You never mentioned it but I'm just curious Phil did you check the mic out using separate power supplies before going to all that trouble, just in case the unit supplies as well as the amp was fried?

1. The mic and its PSU looked brand new.

2. The primary of the AC tranny had visible damage and smelt burnt.

1. The secondary winding look fine.

2. There was no fuse in the clip associated with the IEC inlet and a new one blew at a low AC voltage.

1. The primary read 26 ohms when it should have read 700 ohms or so.

2. The secondary ohmage readings looked good at 3.5 and 450 ohms.

1. After I pulled the tranny apart and cut away the primary winding, the secondary tested the correct 9:120 ratio.

2. The PSU consisted only of rectifiers, resistors and zeners - all of which looked fine.

1. There was an IEC lead in the case with the mic fitted with a US style 3 pin plug.

The fact that over heating occurred ONLY in the primary and NOT in the secondary windings proved that there was no overload applied to secondary of the tranny.

It had to be an AC supply over voltage that caused the damage.

.... Phil

one

=A0all of

le 3

of

I'm wondering though if a secondary is supposed to deliver say 20 volts with a 120V primary and it's hit with 240 then that secondary will produce 40 volts out. If that 40 volts happens to be the input to a regulator then perhaps the amplifier can survive until the transformer primary opens, but maybe that 40 volts might be rectified, and unregulated and is then applied to the first stage. And then that's all she wrote. It sounds like luckily you didn't have to deal with any of that. Lenny

"klem kedidelhopper"

I'm wondering though if a secondary is supposed to deliver say 20 volts with a 120V primary and it's hit with 240 then that secondary will produce 40 volts out.

Try it yourself and see what REALLY happens.

... Phil

I'm not criticizing your deductions and how you arrived at them. Your theories do make a lot of sense. I've never tried this, (over voltage to a primary) knowing that it would be a destructive test. I guess that I didn't explain this properly. I just thought that perhaps the momentary 240V surge into the 120V primary might cause the 20V secondary voltage to approach 40V before the primary opened, (unless the transformer saturated first)? Even so I think there would still exist the possibility of a significant over voltage before all this happened. And if this AC supply was merely being rectified and filtered but not regulated, I would suspect some possible damage from over voltage to the electronics further down the line. Lenny

"klem kedidelhopper"

I'm confused. I would have thought that, at least for the first instant, the turns ratio means there would be 40 VAC on the output, until the primary's turns start shorting together as the enamel combusts because of heating caused by the excessive input current.

If this is a correct analysis, then the question becomes, does an instantaneous overvoltage on the secondary damage any downstream circuitry here?

I have to say Phil I can always count on you to liven up the discussion if anyone ever has an opinion or even a thought other than yours. It really is rather refreshing when it's all said and done. We're all ignorant imbeciles, morons and fuckwits whom you've tolerated. I truly don't know what we would do without you there to continuously correct us. I suppose we should all be grateful for the consideration and latitude you've so graciously afforded us. Lenny

"spamtrap1888"

I'm confused. I would have thought that, at least for the first instant, the turns ratio means there would be 40 VAC on the output, until the primary's turns start shorting together as the enamel combusts because of heating caused by the excessive input current.

Magnetic saturation is an instantaneous phenomenon.

The cores of most small transformers operate well into saturation, even at rated voltage - so doubling the incoming primary voltage causes complete core saturation and the primary side current flow is limited only by the resistance. The RMS current goes up by 5 or 10 times the normal level.

Crucially, the secondary voltage rises only slightly due to a large voltage drop being caused by the primary's resistance.

A correctly rated fuse will blow immediately and the event is all over.

With the PSU in question, the AC fuse was missing - but very likely had been replaced with a larger one at some stage.

... Phil

"klem kedidelhopper"