It would saturate the transformer core and severely reduce its inductance leading to a much larger current flow. A blown fuse is the usual result of connecting 230V to a transformer intended for 115V.

 

It would saturate the transformer core and severely reduce its inductance leading to a much larger current flow. A blown fuse is the usual result of connecting 230V to a transformer intended for 115V.

Thanks. I've never had the occasion to try that one.

 

Sure appears to be an open winding in the transformer primary. Do you any way to bring your 240V line down to 120V, such as a variac? If so, you could test the amp using the good portion of the primary.

 

@Ylli I don't have access to a variable power supply, but I had thought about buying a 230V to 115V stepdown transformer to power the amp through the (hopefully) still good 115V input of the transformer. A stepdown transformer with a high enough power rating is about AUD $120, which is probably a bit more expensive than a replacement toroidal transformer.

I am 100% certain that I am not blowing the fuse by accidentally connecting power to the 115V input (it is unplugged), so it sounds like my best option is to buy a replacement transformer and hope that the faulty transformer is the only problem with the amplifier. It would be a shame if I put a new transformer in, and blew it up due to downstream faults on the circuit board.

Unfortunately the Hartke A70 amplifier has been discontinued, so finding replacement parts might be tricky.

The label on the transformer (example attached) says...
A70 50/60Hz
PRI:FUSE - 115V - 230V
BLK - ORG - RED
SEC:31.7V - 0 - 31.7V
BLU - BLK - BLU
TOP CHEERS

...and it is approximately 50cm high x 10cm in diameter. Power rating is not marked, but the description on the fullcompass.com website lists it as a "75 W Power Transformer For A70"...but they also list it as out of stock.

31.7 volts seems to be a pretty non-standard output voltage, so I am assuming that the transformer for the A70 is some kind of custom winding that was made for Hartke.

I am a bit nervous about buying a transformer with a 35V output and trusting the voltage regulators (JRC7815/JRC7915) to dissipate the extra heat, and I expect that a transformer with a 30V output will cause problems.

Am I being unnecessarily worried about buying a transformer with slightly different specs, or should I only buy exactly the same transformer?

 

That is one whopping tall transformer at 50cm high!

 

Look on ebay. You might find something suitable.

https://www.ebay.com.au/itm/230V-To...m43e09cdbbe:g:UcgAAOSwMmBVwJP7&frcectupt=true

 

You may be able to carefully unwrap the insulating wrap from the transformer and take a physical look. Might get lucky and find the open right where the magnet wire connects to the input wires.

MrChips, on the transformer you linked, is that the right secondary voltage? Dual 30 volt secondaries (configured as 60VCT) would probably overvolt the 7800 series regulators. I would think you would want a secondary voltage more like 36 VCT. Maybe this one: https://www.ebay.com.au/itm/230V-To...403007?hash=item463efbd83f:g:XQgAAOSwLVZVwJlL

 

This transformer has twin secondary windings which can be connected in various ways. For instance they can be connected in parallel to make a single 30V winding but at double the current of a single winding.

From the link given:

These high quality open style toroidal transformers have dual primary windings (115v + 115v) and twin secondary windings which can be connected in series, parallel, or centre tap configurations.

 

This transformer has twin secondary windings which can be connected in various ways. For instance they can be connected in parallel to make a single 30V winding but at double the current of a single winding.

But the power supply in the amplifier requires a CT secondary, so series would be the only way to get that. 30-0-30 VAC would put the inputs to the 78xx regulators over 40 VDC.

 

There is a 240Ω 5W resistor before the regulator.

 

There is a 240Ω 5W resistor before the regulator.

Guess it depends on how much current the amp is drawing from these regulators. If the secondary is 30-0-30, then whoever designed this amp is making a lot more heat than needed.

edit: Just notice the +V and -V lines. Could indeed need 30-0-30. I found a complete schematic of the unit here: https://music-electronics-forum.com...8163b2fec2147&attachmentid=39800&d=1467824172 . Still no labeling on the schematic as to the expected voltage. Need to figure out a way to get 120 VAC or less on that good primary and measure the secondary voltage.

 

OK, I have been moved over to the 30-0-30 camp. The A70 is rated at 70 W output, and 42 volt rails is not unexpected for that output power.

 

32VAC on the secondary would translate to about 45VDC after rectification.

If we assume about 30VRMS to 8Ω load, that's about 112W which makes 70W output in the right ballpark.

75mA through 240Ω would drop 18V @ 1.35W.

 

I have done some hunting for the official transformer with the 31.7-0-31.7 output, but it is not looking promising.

As I see it, my options are:

1. A toroidal transformer with a 30-0-30 output. This will mean that I won’t be able to get the full 70W output from the amplifier.
2. A toroidal transformer with a 35-0-35 output. This will mean that the voltage regulators are going to be working harder than they should, which may lead to overheating.
3. An external stepdown transformer to convert my mains voltage from 230V to 115V so I can use the (hopefully) good 115V input to the amplifier.

Is this a correct summary, or are there further risks like damaging other components on the circuit board with overvoltage, etc.?

@MrChips found what looks like the perfect link to a 120VA 30-0-30 transformer on eBay: https://www.ebay.com.au/itm/230V-Toroidal-Mains-Transformer-Dual-115V-Primary-120VA-0-30V-Twin-Secondary/291531185086 ($54 delivered)

My stepdown transformer option:
https://www.jaycar.com.au/120w-240-120v-isolated-stepdown-transformer/p/MF1080 ($119)

My local supplier option:
https://www.altronics.com.au/p/m5330a-powertran-30-30-160va-toroidal-transformer/ ($64.95)

I am inclined to go with the eBay option from @MrChips. Is there anything I have missed (or misunderstood) in my summary? Is this my best option?

 

If your supplier is nearby, one option would be to take your transformer and compare form factor, size and mounting holes with their transformer.

 

When installing transformers with dual primary and dual secondary windings, make certain that you connect the windings in correct phase.

 

@MrChips thank you for the tip about the importance of phase (and for all your other help). Having transformer taps out of phase is not a problem I had thought of, but is clearly something I need to do a little research on.

I have ordered a transformer. I will update this thread when it arrives and I wire everything up.

 

Here is an example of dual primaries and dual secondaries.

For 120VAC line input, you would connect the primaries in parallel, i.e. dot to dot (black to brown and white to orange).

For 240VAC line input, you would connect the primaries in series, i.e. white to brown. AC LINE INPUT on black and orange.

For 30V-0-30V center tap secondaries, connect in series, i.e. blue to gray.

This is an example only.

Every transformer is different. Check the manufacturer's datasheet for the correct configuration and colour code.

 

Thank you! I will be very careful when I hook up the inputs/outputs once everything arrives.