Weird circuit in Fidelity record player.

[vidjoman]

Often the sellers of items don't have an engineer's knowledge of these matters. If it has two transformers then they think it's a full mains transformer, but it's not always so.

Descriptions are often wrong on auction sites.

[vampyretim]

Originally Posted by John10b

Further to Nickthedentist post #3, and the link he provided, the seller states the following “The amplifier has a proper mains ransformer (sic) which allows for an isolated, earthed chassis “.
Cheers
John

The one that I'm working on definitely doesn't have a mains transformer but it does have two transformers which at first led me to believe that it did have a mains transformer.

[John10b]

I completely agree, any chance of a picture ?
Cheers
John

[vampyretim]

Originally Posted by John10b

I completely agree, any chance of a picture ?
Cheers
John

Hi John,

I'm away from Bristol for a week but will upload some pictures of the amp when I get back.

Many thanks.

[John10b]

Ok ta, just thought it would be interesting to see amp.
Cheers
John

[Edward Huggins]

Well it does have a Mains transformer now if there are 2 transformers there....
With photos, we can tell if it is for the filaments only or if it is double-wound and therefore offers an isolated chassis.

[Michael Maurice]

Originally Posted by Nickthedentist

Someone here claims it's got an isolated chassis: https://www.worthpoint.com/worthoped...tro-1694440334

I do hope it was an isolated chassis, otherwise God help the buyer.

Originally Posted by vidjoman

Often the sellers of items don't have an engineers knowledge of these matters. If it has 2 transformers then they think it's a full mains transformer but it's not always so. Descriptions are often wrong on that auction site.

Originally Posted by Edward Huggins

Well it does have a Mains transformer now if there are 2 transformers there....
With photos, we can tell if it is for the filaments only or if it is double-wound and therefore offers an isolated chassis.

How can you tell from a picture Edward? If the transformer acts as an autotransformer with just the heaters isolated, then at first glance you'd never know.



Especially if its like the circuit described in post #10

Originally Posted by Herald1360

So did the (cheaper?) tapping on the motor winding approach.

But they weren't available till around 1960, starting with I think the UA12.

[Edward Huggins]

My point in Post #26 is that if it were just a filament transformer (autotransformer) one would expect to see just 4 winding connections, and if isolated, then at least 5.

[vidjoman]

Should be able to tell from a photo as an isolated type will have 3 windings and heater only just 2. There might be a snag if there are extra tags for a primary tap for 200 and 240 volts but that would be rare in this type of player.

[vampyretim]

Hello guys,

I'm away from Bristol for a few days but as far as I can recall the transformer has 4 taps.

Many thanks

[julie_m]

It would need to have 5 or 6 connections to provide isolation: two for mains in, two for HT out and two for LT out, but one end of the LT and one end of the HT secondary windings could be joined giving just 5 connections. Four connections suggests just mains in and LT out, with the mains being used directly to supply the HT.

[vampyretim]

Hi,

These are the photos of the amplifier the 6x4 is hooked up directly to the mains and for some reason the 6x4's keep dying.

I will be changing the plessey before anyone mentions it.

Many thanks.

[ms660]

According to the rectifier's valve data Rs should be around 180 ohms minimum per anode so far as I can make out for 240 volt mains.

Lawrence.

[John10b]

Looking at your pictures it looks like there is a wire bridging across the Mains terminal on the transformer, or inadvertently shorting, I could be completely wrong of course! A few more pictures would be useful.
Cheers
John

[FIXITNOW]

Just had one off these come my way to sort. Thread here:-

https://www.vintage-radio.net/forum/...d.php?t=164635

[crackle]

Originally Posted by vampyretim

Hi all,

Thanks for the replies. I'm actually away for a week or so and don't have the thing to hand. From memory though the amplifier seemed untouched so I don't think that it ever had a dropper. When I said that it was taking out the 6x4's when I got the thing it had a milky 6x4 with a cracked base so I replaced with a nos one and the amp was working for 20 minutes or so as I was working on the mechanics and I happened to see a flash and the fuse was taken out in my lamp limiter. Looking at the valve it has completely disconnected an internal connection (I think cathode but can't quite remember) of one of the pins. A huge current has been drawn and I was suspecting it to be the power supply caps.

Many thanks.

Your lamp limiter appears to not be wired correctly as any short in the equipment connected to it SHOULD NOT cause the fuse to blow.

The first valve was obviously no good, the second valve may have had a fault in it.

Test the smoothing caps for electrical leakage.

With the EL84 valve removed there should be next to no current on the path from the rectifier cathode to the first smoothing capacitor. (once the caps are charged up)
To measure this connect your ammeter via a 10k resistor in series with the HT circuit before the rectifier. There will be a high initial current, (25mA) but if after 60 minutes there is still more than 1mA flowing, replace the capacitor with a new one.
Be careful as you are working with live mains.

Mike

[Station X]

[QUOTE=crackle;1224082]

Originally Posted by vampyretim

Your lamp limiter appears to not be wired correctly as any short in the equipment connected to it SHOULD NOT cause the fuse to blow.

Mike

If like my own lamp limiter, the OP's incorporates a switch to short-circuit the lamp, then it's possible the lamp limiter's fuse could blow under fault conditions.

There'll be two fuses in series. One in the lamp limiter's plug and one in the player's plug. Either or both could blow.