Hotrod Deville PT intermittent.

[Luthin You]

Hi guys , Im a tech . Im working on a HR Deville where the PT comes and goes , it seems to have primary connections but the secondaries all die now and then . A whack with a little mallet on the chassis brings it back . Its a 240V country here in australia so the primary is 2x 120 in series , I can measure 240 going to one side and 120 at the centre connection which tells me there is current through pri. I have wiggled and jiggled all wires , now Im wondering if there is a faulty thermal fuse in the PT ? I can't see all the primaries being at fault ?Thanks for your interest .

[Mick Bailey]

An intermittent thermal fuse would be way down on my list of possibilities but is easy to monitor by measuring the voltage across it (CP3, CP5) when the failure occurs. If the secondaries all die then the fault would most likely be on the primary side as the secondary windings are each isolated. You could have an internal connection issue where one of the flying leads is soldered onto the actual winding. My first thought though is to suspect the supply side - in particular the terminals and their joints on the PCB.

Are seeing 120v at the centre when the transformer fails?

[Luthin You]

Thanks Mick , yes I see 120 at centre of primary when the secondaries are dead , strange . I can't see a logical scenario , think I'll just pull the chassis and poke around . Also I can hear the feint hum of a transformer . In my 1st post above i meant to say "I can't see all the secondaries being at fault"

[glebert]

On my HR Deluxe that had a failed PT due to thermal fuse the primary measured as open. Which kind of makes sense to put the thermal fuse on the primary side.

[Mick Bailey]

The 120v at the CT does suggest the primary is OK. I'm trying to think of a scenario where the primary is good but all the secondaries fail and I can't come up with anything. A collapse in the magnetic field would do this (say from a shorted turn in just one secondary section) but this would cause excessive current draw and usually blows the fuse. I did have a transformer which had very low output across all secondaries and it didn't blow the fuse, but when I measured the current on the mains side it was pulling way over what it should and a post-mortem showed one of the secondaries was blackened through many layers. That wasn't intermittent, though.

Maybe it's worth checking the current draw when the failure occurs just to see if the quiescent current rises or falls.

[glebert]

The 120v at the CT does suggest the primary is OK. I'm trying to think of a scenario where the primary is good but all the secondaries fail and I can't come up with anything. A collapse in the magnetic field would do this (say from a shorted turn in just one secondary section) but this would cause excessive current draw and usually blows the fuse. I did have a transformer which had very low output across all secondaries and it didn't blow the fuse, but when I measured the current on the mains side it was pulling way over what it should and a post-mortem showed one of the secondaries was blackened through many layers. That wasn't intermittent, though.

Maybe it's worth checking the current draw when the failure occurs just to see if the quiescent current rises or falls.

Transformer should get hot when this happen, and start to smell at some point, right?

[Mick Bailey]

It will get hot (or at least hotter) than normal and if severe enough will start to cook. If you think the amp has a 1.6A fuse on 240v the amp can sustain a fault condition of up to 384W before the mains fuse blows. That's a fair bit of heat if confined to the PT, though if that were the case at some stage the internal thermal fuse should operate if fitted.