Sony Tuner - Mains Transformer Problem

[stevecole]

I have a UK spec ST-S505ES with a faulty mains transformer - it has an open circuit primary winding. The unit does not have an external voltage selector, and the data plate on the back says 240v only. I notice that the primary winding has a tap on it, selected by a soldered link on the PCB, but it isn't clear what voltage the tap is rated for. The winding still measures OK from the 'Common' end to the tap, but is O/C beyond it.

I have been able to find a service manual for a similar model, the '707, and using the dc voltages given in that, it is possible to get a rough idea of what the secondary voltages are. I am thinking of energising one of the secondaries and measuring what comes out of the tap on the pri - that way I should be able to see if the tap is for 120v or 220v. I could then run the unit temporarily at reduced voltage via the tap, to see if anything else is wrong, and to judge whether it is worth repairing.

Does anybody have any thoughts or suggestions on this?

BTW I am reasonably sure the transformer is not fitted with a thermal fuse, since I would expect it to be at the 'Common' end of the winding.

[Mooly]

If the tranny is easy to isolate from the unit then just run it up with a bulb in series and see what the secondaries read.

Very rare for a tranny to go OC thermal fuse excepted of course. Not just a fractured wire between the bobbin and where it wraps/connects to the terminals.

[huibert34]

the secondaries have a very low impedance , the resistance and inductance is low.

you can make a voltage divider from 2 mains bulb in serie from 100w and feed the voltage from 1, 100w bulb to the 110v primary

[stevecole]

Thanks for the suggestions gents. Mooly - I've done as you suggested - a 60W bulb in series with the tap, and the bulb stayed off. My mains supply measured 241v at the time, and of that 240v appeared across the transformer. From this I assume that the tap must be electrically close to the end of the winding (at 220 or 230v maybe), and that it would probably be OK in the short term to apply full mains to the tap. I measured 28mA magnetising current - no idea what to expect here, but at a guess the transformer can't be much larger than 20VA. The secondary voltages were 26-0-26v and 6.5v.

BTW, no sign of fractured wires between the winding and the bobbin pins, nor any other signs of failure.

As a next step, I'm thinking of leaving it energised for some time to see if anything gets warm, and if it still looks OK I'll put it back into the tuner and see if anything else is wrong. Does that seem reasonable?

[Mooly]

As long as the voltages seem reasonable it should be OK. From what you say it sounds like a standby transformer (20va) and these often do have a thermal fuse somewhere.

Remember that even if it were a 120 v winding you were feeding it would still appear OK using the bulb method of testing it and it would still draw little current but the secondary output would be twice what it should be. So you need to be sure. Small trannys have poor regulation and the output could well be much higher off load than you might think.

It might help if you posted the PSU part of the circuit. Again from what you describe I would look at perhaps replacing any simple rectifier/reservoir caps that this small tranny feeds. Its probably gone O/C for a reason... usually something like a duff cap or rectifier is the cause.

[Andy5112405]

Originally Posted by stevecole

Thanks for the suggestions gents. Mooly - I've done as you suggested - a 60W bulb in series with the tap, and the bulb stayed off. My mains supply measured 241v at the time, and of that 240v appeared across the transformer. From this I assume that the tap must be electrically close to the end of the winding (at 220 or 230v maybe), and that it would probably be OK in the short term to apply full mains to the tap. I measured 28mA magnetising current - no idea what to expect here, but at a guess the transformer can't be much larger than 20VA. The secondary voltages were 26-0-26v and 6.5v.

BTW, no sign of fractured wires between the winding and the bobbin pins, nor any other signs of failure.

As a next step, I'm thinking of leaving it energised for some time to see if anything gets warm, and if it still looks OK I'll put it back into the tuner and see if anything else is wrong. Does that seem reasonable?

If you put a 240V 60W lighgt bulb in series with a good 240V transformer you will get a glow of some kind from the lamp.

[Mooly]

Originally Posted by Andy5112405

If you put a 240V 60W lighgt bulb in series with a good 240V transformer you will get a glow of some kind from the lamp.

Hi Andy... it won't glow feeding a small transformer primary with unloaded secondaries... not enough current to heat the filament.

[Andy5112405]

I personally don't trust the lamp check.

[Mooly]

Originally Posted by Andy5112405

I personally don't trust the lamp check.

Its not a case of trusting it as you back everything up with measurements.

It's really for safety in this case. Suppose the windings 240 was applied to were in fact between the 220 and 240 v tappings. That would draw excess current and light the lamp with no smoke or sparks No lamp and big bang.

If the lamp is dropping no voltage and 240 vac is across the test windings then we can go on to measure the secondary outputs safely knowing that full 240 v is across what we believe to be the primary, whether or not that is the case. Any following measurements will then confirm or otherwise.

[stevecole]

The tuner is now up and running. I was 95% certain that the tap on the transformer primary was for 220/230v, and as the no-load secondary voltages seemed sensible I took a chance and refitted it. The bulb limiter is still in use, and remains un-lit, with about 6 volts across it.

The tuner seems to work perfectly, but I'm not happy with running it in this state long-term, so I plan to dream up a replacement 'transformer', using 2 or possibly three off-the-shelf units. There are acres of empty space in the case, so these can be mounted remotely from the tuner PCB and hardwired to it.

While it is in pieces it occurs to me that the output op-amp could be upgraded at the same time. It has a 4558 there at the moment - any suggestions? One of the Burr-Brown 2604's perhaps?

BTW lots of nice features on this unit - Full RDS, Wide/Narrow IF (4 filters, not sure what B/W), signal readout in dB, rotary tuning control switchable between Auto/Manual/Preset Scan.