Philips EL3541/15k transformer

[DarthGrapes]

Hi all,

Recently came to acquire an EL3541 and I'm having some trouble getting the thing powered up.

I'm no electronics wiz but I've taken some basic measurements, when the machine is plugged into the mains I'm getting a nice 240V across what I think is L4 (The blue and white wires on one side of the transformer).

I'm not reading any voltage across anything else however. The motor isn't running (no voltage measured across either coil) and none of the valves are lighting (also no voltage measured across pins 1 and 7 of V5).

I'm pretty stumped as to what the issue could be, is the thing toast?

Any advice would be greatly appreciated.

[Sideband]

I don't have any information on these but I seem to recall from the dim and distant past that the mains transformers have a thermal fuse. If this goes open circuit than everything will be dead. Unplug the recorder from the mains. Switch your test meter to read low ohms (choose the lowest range to comfortably measure about 500 ohms). Connect the test leads of the meter across the mains lead of the recorder live and neutral....for this test it doesn't matter which way round you connect the test leads. Don't hold the test prods, use something to hold then securely. Switch the recorder on. The meter should indicate a resistance of around 50 to 200 ohms or so.....it may take a few seconds to settle.

If you read somewhere around the stated figure (it may be slightly higher or lower) then the thermal fuse is probably OK. If you read nothing, then it's likely to have failed open circuit for some reason.

The purpose of the thermal fuse is to protect the transformer from overheating. Sometimes they failed through age. the transformer does look like it has got damp at some point.

It might be possible to resolder the fuse. Others here may be able to advise.

[DarthGrapes]

Thanks for the quick reply, I've got a reading of about 60 ohms so I'm assuming that the fuse is fine.

The more reading I do the more I think the transformer is dead, here's to looking for a replacement!

[paulsherwin]

A 60 ohm DC resistance sounds plausible.

[DarthGrapes]

Potenitally silly followup question, might this be the thermal fuse?

It looks like it may have been soldered at some point but it's hard to tell. I dont want to re-solder the connection and plug it back in only for it to go pop.

[DMcMahon]

Looking at the schematic with a continuity reading, would expect to get some voltage reading across one or both motor windings (especially as you measure 240 volts at L4), so your results are confusing.

Not easy to do but I would recommend you need to prove conclusively that the L4 and L3 primary windings each have good continuity by identifying the 1, 2 (L4), 3 & 4 (L3) taps (by continuity checks as per schematic) and disconnect one end of each primary to measure the 2 primaries individually (can ignore taps 5 and 6 section of L3).

David

[DMcMahon]

Originally Posted by DarthGrapes

Potenitally silly followup question, might this be the thermal fuse?

It looks like it may have been soldered at some point but it's hard to tell. I dont want to re-solder the connection and plug it back in only for it to go pop.

Not silly at all. Even though I have heard of these transformer fuses I have never seen one but would say it most likely is a fuse. I would certainly resolder it, first checking as far as you can for no shorts to earth or dead shorts across the windings.

I think the transformer fuse most likely would be on the primary side of the transformer.

The schematic shows an external fuse going to tap #1, you could lower the value of this fuse to provide more protection if you are concerned of the transformer fuse going pop.

[vidjoman]

You can't use ordinary solder to repair those thermal fuses. It uses a very low temperature solder. Only way is the use the existing solder - just mend by heating with a clean iron whilst holding the bits together. They usually fail for a reason - the transformer has got very hot. You need to check out the rest of the recorder to find the reason before re-appyling the mains.

[DarthGrapes]

Originally Posted by DMcMahon

The schematic shows an external fuse going to tap #1, you could lower the value of this fuse to provide more protection if you are concerned of the transformer fuse going pop.

One plate of the fuse does join to one of the taps which is connected to the mains power, also, manually closing the thermal fuse and testing between the primary tap and the secondary tap with the red wires in one of the previous pictures now gives continuity too, it all makes sense now!

Originally Posted by vidjoman

You can't use ordinary solder to repair those thermal fuses. It uses a very low temperature solder. Only way is the use the existing solder - just mend by heating with a clean iron whilst holding the bits together. They usually fail for a reason - the transformer has got very hot. You need to check out the rest of the recorder to find the reason before re-appyling the mains.

Thanks for the heads up, I've managed to tack it back together with what was there. I've spied that the rectifier tube (EZ80) looks bad with a cloudy white cap as opposed to silver, same thing for the magic eye (EM84), so they will be replaced.

Are there any obvious things that I might be missing as to why the transformer would overheat?

[ricard]

The two most common reasons I would say would be either a leaky filter capacitor after the EZ80, or, perhaps more likely, a leaky grid capacitor on the output valve (often called that capacitor), driving the grid positive, increasing its anode current.

None of these cause instantaneous overheating, so once you get the dud valves replaced, I would power it on with a voltmeter measuring the B+ voltage after the rectifier, if it drops alarmingly low there's still something drawing excessive current.

[DMcMahon]

Before powering up I would do at least the following:-

1. Measure for any obvious dead shorts to ground/earth on primary and secondary windings. Note there will be low resistance continuity to ground on the secondary windings

2. Measure for no dead shorted windings.

3. Ensure mains voltage selection switch is correctly set to 240 volts.

4. Visually check for any ruptured/physically leaking electrolytic capacitors, particularly the reservoir and main smoothing electrolytics C28/C26/C11 (the triple 50uF/32uF/32uF).

5. Check the anode to grid coupling capacitor as Ricard references, C15 (0.1uF) on schematic I am looking at. If you have a Megger/insulation tester, disconnect C15 and check for good insulation across it, at capacitor working voltage. Replace it if any doubt.

Note - There may be other old paper capacitors that might need checking later on.

6. Visually check for any overheated resistors etc.

Then power up with the 2 suspect valves not fitted.

Check for:-

1. Good heater voltage X-X, other valves should light up.

2. Good voltage to EX80 anodes 1 and 7.

3. Motor power and control.

4. Leave running and check for Mains transformer not getting hot, use temperature probe or safe finger test.

If above OK then refit the EZ80 and check for good +HT voltages at C28/C26/C11, keep monitoring the transformer temperature. If OK refit the EM84 and retest.

[DMcMahon]

The schematic I was looking at had no DC voltages referenced. Looking at another 3541 schematic it shows +262V at EZ80 output/reservoir capacitor, +232V at first main smoothing capacitor and +196V at second main smoothing capacitor, also +242V at ECL82 output valve anode.

These are just ballpark figures for reference.

[DarthGrapes]

Thank you so much everyone for the detailed instructions, I managed to have a thorough check of the transformer and made sure that the X-X voltage was good, I don't have an insulation tester though so I'm not sure how I'd test the caps properly.

The machine is working now with the new valves and doesn't appear to be overheating, however there is quite noticeable hum at times so I will more than likely just replace the electrolytic capacitors anyway. My initial thought when I bought the machine was that the caps would have gone bad so I'm quite surprised that it works as well as it does.

[Station X]

Check that there's no positive voltage on the grid end of C15.

[DMcMahon]

That is good progress.

C15 connects to the control grid (via 1k resistor R38) of the output pentode (pin 3), which is probably the ECL82. Note - that some of the component number designations differ depending upon which EL3541 schematic is viewed.

David

[DarthGrapes]

Originally Posted by Station X

Check that there's no positive voltage on the grid end of C15.

That'll be my next thing to do. Where am I actually placing the probes, is it at the grid end of the cap and ground?

[Station X]

Yes. That's correct.

[Sideband]

Originally Posted by DarthGrapes

Thanks for the quick reply, I've got a reading of about 60 ohms so I'm assuming that the fuse is fine.

Just trying to work out why you had a resistance across the mains lead with the thermal fuse open....of course there is a motor involved so the 60 ohms was probably the motor winding.

The good thing is that you got it sorted. I can't remember what solder we used at Philips to resolder these fuses but it was low melting point stuff in tape form...it would melt with a match flame.

[DMcMahon]

I wondered the same at the time, assuming that the transformer fuse is on the primary side then (depending upon exactly where the the fuse is connected) I think there is a partial circuit though 1 motor winding in series with one transformer primary winding.

David

[DarthGrapes]

Originally Posted by Sideband

Just trying to work out why you had a resistance across the mains lead with the thermal fuse open....of course there is a motor involved so the 60 ohms was probably the motor winding.

The good thing is that you got it sorted. I can't remember what solder we used at Philips to resolder these fuses but it was low melting point stuff in tape form...it would melt with a match flame.

I think I was taking a measurement at the wrong place, and I hadn't removed the transformer at that point, so it was probably completing a circuit somewhere along the line.