DYNATRON HFC77 schematic / transformer

[radio88]

Hello,

I have a DYNATRON HFC77 radio/turntable with no power at all.

I just checked the output from the transformer and there is nothing at all and no resistance on the primary winding. Transformer type 18897

I could replace the transformer but I do not know the output voltages.

Does anyone have a schematic? if so could you have a look to see if it says the voltages.

there are 4 wires on the output
I have included links to photos below.

thanks

[Vicboduk]

I wouldn't write-off your transformer yet. Looking at your pictures there appears to be a lump under the label. This is probably a thermal fuse which has gone open circuit.
If you're lucky the terminals will be brought out to the TX connector. If not, you can use a scalpel to gently cut away the label until you find the device - usually a small square of plastic with two leads. You have nothing to lose!

See post 3 of this thread https://vintage-radio.net/forum/showthread.php?t=154145


Vic

[Nuvistor]

The part circuit I have of the SRX30 show a thermal fuse fitted to the transformer so definitely worth checking.

[radio88]

Thanks a lot for the info,
I removed the paper,
The bulges on the sides were just the wires, there does not appear to be a fuse, although I dont know what that type of fuse would look like. how big would the fuse be? would it be embedded deeper somewhere? the other sides are dead flat so no space for much really.

thanks

[Nuvistor]

Where the brown and green wires connect on both sides, lift that black paper, may under there.
The ones I saw in the 70’s were about a cm square a couple of mm thick in translucent plastic and the fuse wires could be seen through the plastic, easy to see if o/c but of course a meter check is best.

[G6ONEDave]

I have the Dynatron HFC77 cross referenced to the SRX29 and SA29 Dynatron models. The HFC77 is also listed for the RTS books 1974-75 page 461 and 1975-76 page 483.
Dave

[radio88]

Thanks to all, Vic, Frank and Dave for the help.

I found the fuse, not under the paper but wedged in a plastic housing under the green and brown wires.
I shorted the fuse out and low and behold it fired up. but the transformer quickly got hot and the large capacitor near the bridge rectifier went BANG!

it looks like someone could have been in there messing around before me and could have put the mains cable on the wrong connectors. I did photograph all the cables to make sure it went back correctly so not my error.

I will keep digging and test the ohm values at the primary and secondaries so I am sure of the primary 240v input.

[Nuvistor]

Has noted in the link in post #2 there is usually a reason for the fuse blowing, if that large cap went bang it could have been faulty or very likely the rectifier is S//C.

Edit. Yes that’s the type of fuse I remember.

[agardiner]

The bridge rectifier has almost certainly shorted out. Check/replace that and the cap and fingers crossed you will be good to go. RS sell replacement thermal fuses which would allow you to continue to protect the transformer.

Otherwise I would add a 0.47R fusible resistor in series with the secondary of the transformer to protect against future failures.

[radio88]

Thanks again all.
I will have another look at the transformer and bridge/cap and post my results.

[radio88]

It looks like the transformer is "past it" as it immediately gets very hot and is outputting 61 volts and 47 volts so the insulation on the windings must have broken down. "hence the thermal fuse blowing"

Does anyone know where to get a replacement? It's transformer type 18897

According to the schematic it should output 31 volts and 5.8 volts

I have not had any luck finding anything similar on google so maybe I will have to use 2 different transformers, one for the 31v and one for the 5.8v.

Thanks

[Nuvistor]

Does the transformer get hot with the secondaries disconnected, if so it’s is faulty, if not then there is a problem in the rest of the set, possible the rectifier or reservoir caps.
Of course even if it get hot with the secondaries disconnected it is faulty but probably was damaged by a fault in the rest of the set.

[radio88]

Originally Posted by Nuvistor

Does the transformer get hot with the secondaries disconnected, if so it’s is faulty, if not then there is a problem in the rest of the set, possible the rectifier or reservoir caps.
Of course even if it get hot with the secondaries disconnected it is faulty but probably was damaged by a fault in the rest of the set.

yes with the secondaries disconnected it gets very hot and the cap near the bridge rectifier popped with lots of "magic blue smoke"
once I change the cap and check the bridge ill try it with 2 different transformers, one for the 31v and one for the 5.8v as it will be near impossible to get an exact replacement.

[Nuvistor]

Originally Posted by radio88

yes with the secondaries disconnected it gets very hot and the cap near the bridge rectifier popped with lots of "magic blue smoke"

If the cap popped you still had the secondaries connected.

[radio88]

Originally Posted by Nuvistor

If the cap popped you still had the secondaries connected.

yes when I discovered the thermal fuse had blown I shorted it out to test the radio and the magic blue smoke came from the cap.

after disconnecting the secondaries the transformer still gets very hot so im guessing the transformer insulation was breaking down probably the reason the thermal fuse had blown. its a pity I didnt test the transformer and see the high voltage before I plugged in the secondaries. oh well hopefully its just the cap and bridge to be replaced.

[Ed_Dinning]

Hi If you can post a picture of the transformer we can advise if it would be possible to rewind it

Ed

[radio88]

Originally Posted by Ed_Dinning

Hi If you can post a picture of the transformer we can advise if it would be possible to rewind it

Ed

Thanks Ed

https://drive.google.com/file/d/1-rC...WioDu0L36/view

https://drive.google.com/file/d/147M...K3_LW4nRr/view

https://drive.google.com/file/d/12lt...lw8D6DZQM/view

[Chris55000]

Hi!

Will this meet your needs?

https://www.ebay.co.uk/itm/174260979116

If the 5.8V red and white leads to SKT104 and SKT105 are only used to feed scale lighting lamps, then all you need to do is wire this transformer as follows:–

SKT103 yellow goes to tag "0",
SKT104 white goes to tag "31.5"
SKT105 red goes to tag "26", but add a 1 ohm to 2.2 ohm 2W resistor in series with the red lead to tag "26" to reduce the voltage supplied to the bulbs in order to prolong their life – choose the resistor to give about 5.5V across the bulbs;

. . .and connect the mains input to tags "0" and "240" on the top primary side!

Chris Williams

[Ed_Dinning]

Quite a big beast, certainly rewindable, but cheaper if you can get something close to your spec; no shortage of spave to fit things either

Ed

[radio88]

Originally Posted by Chris55000

Hi!

Will this meet your needs?

https

If the 5.8V red and white leads to SKT104 and SKT105......

Chris Williams

Thanks for the link and the info! You have given me an idea. if the 5.8v only is for the lights I might be able to get it working as I just found a 32v transformer in my shed, only 1.5 amps though so not sure if its quite enough. I will give it a try and post results if it worked, of-course I will have to fix the smoky cap etc first...