I agree with AlbertHall that this is typically a place where a time delay fuse would be used. I would take the "T250 mA" to mean time-delay 250 mA. The transformer may be OK. (at turn-off, a transformer can be left with residual magnetism in the core; if the AC phase is "wrong" at turn-on, the core can magnetically "saturate" for half a cycle resulting in very high transient current which can blow a non-time-delay fuse; there is also big transient due to the filter capacitor, but testing with the bridge removed eliminates that)
Can you show us a good photo of a blown fuse? Sometimes you can estimate magnitude of overcurrent by appearance. Fuses that have been in use for a long time, particularly if they run close to rating, will sometimes just get tired of life and die. Replacing with the wrong type leads you in the wrong direction. But even looking at the wrong fuse carefully may provide a clue.

Do you have a source of lower AC voltage? If there is a shorted turn, the primary current would probably be quite high even with say 24 VAC applied.

A low-voltage secondary winding usually has quite low DC resistance, so the continuity test result is expected. In general, resistance checks on transformers are good only for detecting open windings but almost completely useless for detecting shorted turns. A single shorted turn will make a transformer useless. It may be detectable on a low turns count secondary if another identical transformer is available for comparison. A shorted turn in the primary is much more difficult to find with resistance measurement. You would definitely need an identical transformer and either a differential resistance measuring set up or a high-resolution (prob 6 digits, at least) ohmmeter with Kelvin connection.

The good news is that because there is only one output, there is some hope of finding a suitable replacement for the transformer. But it will be necessary to determine what the output voltage should be. If the output voltage is known, you can "match" the current rating by selecting a transformer that weighs about the same. I agree with Max that the regulator is zener-based, so if you can identify the zener, you can do a good estimate of the voltage required. If you have an adjustable DC power supply, you could test the regulator circuit with that - connected it across the big capacitor, set it at zero to start, then slowly raise the voltage until the output from the regulator stops rising, but don't go over the cap voltage rating in any case. What is the voltage rating of the electrolytic capacitors?

 

2SC1740 NPN transistor.
Is it possible to trace the circuit, may help to identify the secondary needed.
Max.

Ill give it a go when I got some time, thanks.

 

I would take the "T250 mA" to mean time-delay 250 mA.

Yes, agreed.
These T250mA fuses look like fast blow fuses at first sight but they are not.

 

Oh ok. Ill give those a go. Will these ones do as the store is just round the corner? They look different which is why I assumed the original was quickblow.

https://www.maplin.co.uk/p/250ma-time-delay-glass-20mm-fuse-10-pack-gl53h

 

Also a possibility if you have a couple of wall-warts or the odd dc power supply, is the feed the trace where the bridge outputs and see what the regulated output is, start with around 9v-12v DC.
Different ways to find out what is needed.
A Variac would be the ideal.
Max.

 

The 25 volt rating on the big cap sets an absolute upper limit for the transformer winding of about 17 volts, more likely about 16. Typically the cap rating will be selected to handle the no-load condition with line voltage at about 10% above nominal, so 25 * 0.7 * 0.9 = 15.8, ignoring the diode drops.

Testing the regulator with a bench supply is quicker than tracing, but it is a very simple circuit. It looks like the zener, ZD1, is 7.5 volts (violet, green, something) - color banded zeners are very rare in my experience! The output of the supply is probably about 7 volts, which would match well with further regulation to 5 volts in other circuitry.

At this point, I'd guess a 10 to 12 VAC secondary. In North America, 12.6 VAC is a common rating (the oddity dating from the days of transformes for the heaters of vacuum tubes). Given that the regulator is probably a compound transistor with the power PNP & driver NPN as current buffer on a zener, the valley voltage on the big cap would need to be at about 1-2 volts [edit - that is, 1-2 volts greater than the regulator output voltage] at low line and full load. Valley voltage is tough to estimate without knowing load current.

 

Oh ok. Ill give those a go. Will these ones do as the store is just round the corner? They look different which is why I assumed the original was quickblow.

https://www.maplin.co.uk/p/250ma-time-delay-glass-20mm-fuse-10-pack-gl53h

Yup, they'll do fine.

 

Also the physical size of the existing transformer will give you an idea of the Va rating needed.
Max.

 

Also the physical size of the existing transformer will give you an idea of the Va rating needed.
Max.

Dimensions of the transformer:

Width 56mm, Depth 47mm and height 50mm.

 

What is the width of the laminated core?
Max.

 

Laminated core is W 33mm, D 28mm.

 

I would put that at around 35va-40va tops.
For anything regarding the secondary V. if the circuit and the voltage test could be done, it should wrap it up.
Max.

 

I think it's a 12V transformer, here is what i think is the circuit diagram, the output is probably 9 to 12V.

 

the output is probably 9 to 12V.

It better not be as you have a 10V capacitor across the output.

 

It better not be as you have a 10V capacitor across the output.

It's a 16V cap on the output and a 25V across the rectifier. As per post#17

 

It's a 16V cap on the output and a 25V across the rectifier. As per post#17

OK but it sure looks like 10V on your diagram.

 

The 330 ohm resistor will have 7.5 volts across it (7.5Vz + Vf1N4148 - VbeNPN), so the output will be a diode drop above that.

Applying 12 to 15 volts DC from an external source to allow measuring the supply current should be safe. I would first test with no load to make sure the output voltage doesn't go too high. While running, be sure the heatsink isn't getting too hot. If you can keep your finger on it for a few seconds, it should be OK.

In case anyone wandering by wonders:
This is a very simple closed-loop regulator i.e. there is feedback to improve regulation. The zener and the 1N4148 set the base voltage of the NPN. The base voltage, minus the base-emitter junction voltage of the NPN appears across the 330 ohm emitter resistor. Current through the NPN provides base drive for the PNP. When the output voltage rises sufficiently, the diode from the output to the NPN emitter will begin to conduct. The voltage across the 330 ohm resistor will be maintained, but the current through it now comes partly via that diode, so the collector current drops, the base drive to the pass PNP is limited, the output ceases to rise and equilibrium is established. The regulation performance is relatively poor, but the circuit is simple. I assume that the output of this reg is more precisely regulated to (probably) 5 V by other regulators for critical parts of the circuitry.

 

Yup, they'll do fine.

I just tried a T250mA fuse and it still blew. Bright flash and dead just like what happened to the quickblows.

So I guess can rule that out.

Attached pic of what it looks like (blackened at one end):

 

I just tried a T250mA fuse and it still blew. Bright flash and dead just like what happened to the quickblows.

So I guess can rule that out.

Attached pic of what it looks like (blackened at one end):

Ok, the transformer is faulty.
If/when you get a replacement transformer, you will still need the slow blow fuse.

 

Ok, the transformer is faulty.
If/when you get a replacement transformer, you will still need the slow blow fuse.

Yep, I got a box of 10 just in case!