troubleshooting M40 Echolette with loud hum

[notea]

Hi Mickey,
thanks for the precision i understand a bit more the center tap thing and the loop on pin1&7 of the EZ81, it was confusing between the center tap PT/OT,
(you already noticed how i'm not a quick understanding person!.)
so i followed your procedure step by step and this what i found:
-with HT connected+center tap(PT) AND center tap OT disconnected i've got 268VAC at pins7 and 271 at pins1 (rectifiers removed).
-With one rectifier at the time we've got 243VDC at the cathodes
-with the leads connected to the circuit via filter caps we've got 360VDC.
-Now when i hooked the OT center tap this is 0VDC on the cathodes and Hum increasing as the valves warm up!.

Thanks to your procedure we now know that the PT seems to work fine isn'it?
Cheers, Nick

[Mickey]

Hi Nick, Now when you say the troubles begin when you hook up the centre tap of the O/P transformer is that with the output valves in or out? Looks like your O/P transformer has gone to heaven!
If the problem is only there with the valves in place, remove all valves and replace them one at a time.
You can also measure from the centre tap to ground with the O/P transformer disconnected from the power supply If you measure a low resistance to ground with everything disconnected from the O/P transformer then it would verify a faulty O/P transformer.
Let me know what you find.
Cheers, Mickey

[notea]

Hi Mickey,
yes i'm stupid it was without the EL84.. Now that the output valves are in place, the DC is slowly coming as the valves warm up but the hum increases to.. I tried by putting the valves one at the time and it's the same. There was a big flashing red light on he rectifier tubes as if they had receive a large amount of current, then i shutted down.
I measured the OT center tap to ground and we've got 22 ohms which is pretty low, but did you mean that i shoud desolder all the wires on each lead of the OT and just keep the wire of the center tap in place,and measure again?
it's sounds like a bad OT anyway..
Cheers Nick

[Mickey]

Hi Nick,
Pull out all the EL84s and unsolder the centre tap of the O/P transformer so there is no connection to the primary of this transformer. Make sure there is no connection to this primary and then measure the windings to ground.
If you can measure some low resistance to ground then I would think that the primary winding has shorted to the metal core of the transformer. If you look at the schematic you will se that with the centre tap completely disconnected from everything and the valves removed there should be no path for resistance to ground UNLESS there is a short in the primary winding to ground.
If you measure a short, make a carefull note of all the wires to this transformer (take some photos too) and remove the transformer from the amplifier and measure again. Now have a good look at the connections and check that there is nothing causing the short. Sometimes shorts can be caused by protruding screws, bits of solder etc, so have a good luck and let me know.
Now if the transformer is faulty you don't have to buy the exact same transformer, but have to buy one that fits physically and has the right impedances.
Cheers, Mickey

[oc disorder]

[notea]

Hi guys,
thank you Oc for your schem very helpfull, so i've unsoldered wire of the center tap directly at the OT and i still have 22 ohms.
i dissembled the OT and putted out of the chassis. I don't know if i'm doing right but i hooked the com of the multimeter at the center tap leg and try measuring each windings and i found this:
-the opposite windings that feeds pin7 we've got 100 and 102.7 ohms.
- the 2 windings for the grids are 21 ohms each.
-in the attached pics this is a recap of the readings and how i did them
-do you still think it could be shorted?
cheers Nick

[Mickey]

Hi Nick, those measurements indicate that the windings of your O/P transformer are OK and not shorted to each other.
What you have to measure is the resistance from the centre tap to the metal core of the transformer, this would show if there is a short between the windings and the core which when the transformer is in place in the amplifier would short the DC voltage to ground.
Think for a minute. When you connected the DC from the power supply to the transformer the rectifier valves began to glow red from excessive current being drawn through them. When you disconnected the DC from the O/P transformer there was no problem, so the excessive current was/is flowing from the rectifier cathodes through the O/P transformer to somewhere. Now with the output valves (EL84s) removed you still had excessive current flow so that would indicate that from where the DC lead is connected to the O/P transformer there is a short to ground which most likely would be a shorted winding to the core. The windings (from your measurements) appear to be perfectly OK that is why I suggested that you look for an INTRODUCED short such as a drop of solder etc.
Let me know your readings.
Cheers, Mickey

[oc disorder]

"i've unsoldered wire of the center tap directly at the OT and i still have 22 ohms."
At the ? :-
1. Transformer
2. Wire

Sorry I don't speak French but what your looking for is a short to ground .. ie one of the fine wires inside the transformer
has lost it's insulation and is touching on the metal case of the transformer.
It's not a very good connection so it measures 22 ohms instead of 0 ohms.
When the transformer is bolted to the chassis the transformer frame or case is then grounded.

I was hoping the 22 ohms "short" would be on the wire.
And no you can't run the amp with the transformer isolated from the case as it has the HT on the outside
due to the short , and would be dangerous perhaps even lethal.

[Mickey]

Hi Nick, looking at the photo of your O/P transformer there appears to be some rust and paint flaking. Check that under the tag strip where your wires connect there isn't a short to ground. I just checked the power transformer on a Echolette NG51s that I'm repairing and there doesn't appear to be much insulation between the tag strip and the core of the transformer, and this may also be the case with your O/P transformer.
Cheers, Mickey

[notea]

Hi guys,
we may found something. I had the output transformer on my working table eye checked everything, took my solder pump and cleaned every tag strips, i measured the resistance from the center tap to the metal core and i had no ohms reading. i tried to mount the screws and see if they were creating a short but they were not.
So i replaced the output transformer, putted back the wires, measured again the center tap to the chassis and still no ohms reading, seemed that the 22 ohms were gone.
i connected all the wires on the tag strips and also the wire that goes from the center tap to the filter caps, I replaced all the valves, switched it on and the hum was increasing.
I switched it off, measured again and the 22 ohms were back on the center tap.
but i found something interesting cause the 22 ohms are coming from the tag strip where the wires connect to the pin9 of valves 6&8.
When i disconnected this wire there is no ohms reading on the center tap and on each tag strip of the Output transformer.
If i follow the schematic, i could try to isolate this section Rö6&8 by unsoldering R80 and C22 and measure the resistance again isn'it?
Cheers Nick

[oc disorder]

"If i follow the schematic, i could try to isolate this section Rö6&8 by unsoldering R80 and C22 and measure the resistance again isn'it?"
I don't see how changing the input to the valve/tube (presume you mean C25) will alter the fact there is some kind of short around the screen grid (pin 9)
or more likely around the wire that goes from one of the ultralinear taps to the tag strip.
The first idea? I thought was there may be arcing (lightning - flash over - electricity jumps through the air to complete circuit) at the socket .
When it arcs (good example ? https://www.youtube.com/watch?v=6GiIVze2Tac ) (make a good stage show!) the valve/tube base/socket can turn into carbon and acts like a resistor or in your case a short circuit.
Then I realised that was wrong an imperfect conclusion as when you measured between the o/p trans. C.T. & ground you found ~22ohms .
If the short was at the socket at pin 9 from the ultralinear tap, you should have also measured the 200 ohm screen grid (p9) resistor which should have read ~222 ohms .
Can you see this? (an Eureka moment ?)

Therefore the short must be on one of the wires that run from the ultralinear tap to the tag strip ( I guess the 200ohm resistor goes from the tagstrip to pin 9) as when you measure 22 ohms it is really the 21 ohms or 21.5 ohms from the C.T. to the ultralinear winding you are measuring as the wire is touching the chassis somewhere.

The wire is in a loom (bunched/gathered together) bound by string (olden quaint way) so I would as a test/experiment disconnect those wires from the o/t trans. ultralinear taps that go to the tag strip (then on to 2 x 200 ohms to p9) and try a temporary wire of yours there instead (sticking out - doesn't have to be neat for the test). Hope I'm making sense.
So new wire from o/p trans U.L. tap to 200 ohm screen grid resistor.

[Mickey]

Hi Nick,
Take out all valves except the rectifiers.
Disconnect all primary leads to the O/P transformer. Check operation of amp, should be all OK.
Connect only HT wire to centre tap on transformer. Anode and screen still not connected. Again everything should be OK.
Connect anode leads only. OK ?
If OK then disconnect anode leads and connect screen grids. Short now appears ?
If short is now present check each screen connection. There are four screen grids all individually connected to the transformer via screen resistors. If these resistors aren't burnt then the fault must be between the connections to the resistors.
There are two screen taps on the transformer. Disconnect both and again ascertain that the fault is only present when the screens are connected. If OK so far connect only one screen wire, what happens? If OK disconnect and connect the other screen wire and you should now have determined where the short is.Is it in one of the anode connections or is it in one of the screen connections?
A point to think about - when measuring with a meter there is very very small voltage applied by the meter to whatever you are testing, however when you connect the HT there is a large voltage applied to whatever you are testing. This large voltage may be causing the short.
Also your amp has a SERIOUS short, if a resistor was shorting internally you would see the resistor glowing and arcing but it would not have a serious affect on the power supply HT. If however you have a direct short to ground yes then your HT will drop and cause serious overload.
Your O/P transformer may still have an internal short which only appears when you apply the high HT voltage
What voltage do you measure on pin 2 of each output (EL84) valve?
Good luck, Mickey

[oc disorder]

What voltage do you measure on pin 2 of each output (EL84) valve?

I asked this before although without the valves/tubes plugged in...

Notea replied

i measured at the lug of the diode OA150 one side and the other i've read respectivly 16 v and -16V
- on pin 2 of each EL84 i've read 242k ohms and -11VDC

[Mickey]

Hi, I was trying to ascertain if one or more of the coupling capacitors from the phase splitter stage may be leaking. If there was a positive voltage on pin 2 of any of the output valves (with output valves removed) this could indicate a leaky capacitor

[oc disorder]

Well from those initial readings we hope not ..

Notea

I measured the resistance from the center tap to the metal core and i had no ohms reading. i tried to mount the screws and see if they were creating a short but they were not.
So I replaced the output transformer, putted back the wires, measured again the center tap to the chassis and still no ohms reading, seemed that the 22 ohms were gone.
reconnected all wires ,replaced valves, switched it on and the hum was increasing.
I switched it off, measured again and the 22 ohms were back on the center tap

Mickey

A point to think about - when measuring with a meter there is very very small voltage applied by the meter to whatever you are testing, however when you connect the HT there is a large voltage applied to whatever you are testing. This large voltage may be causing the short.

Just wanted to emphasise this point .. and I'll try and use lightning as an example.
With a power supply it's kinda like an imbalance .. the positive and negative want to get together and have current flow
In the same way a charged cloud wants to discharge to earth (the one we live on) to balance out ie discharge...

The cloud will discharge more easily through a Church spire or a TV antenna the path of least resistance.

It has to overcome the gap somehow (air) to be able to jump.

Another way takes a bit longer if there is no Church spire or TV antenna... (ie just air)

It builds up enough voltage.. difference...imbalance and jumps just through air because of the very high voltage.

Your meter has a very low voltage going through it to be able to compare/measure resistance.

There is not enough voltage between the probes to make any jumps through air even .00000001mm .

But a very high voltage would jump like in the arcing YouTube Video.

So as Mickey said the higher voltage is or could be causing the short only because somewhere the insulation is bad
or something is almost almost almost !!!! touching when there is no high voltage.

Sometimes for instance a transistor may read ok on a low voltage tester but when in circuit with a high voltage across it
it temporarily breaks down --- collapses under the strain but when the high voltage is removed it seems ok.
Unusual case and sometimes very hard to find when there are a lot of them connected together.

The above is not accurate but just trying to convey the situation .. as sometimes we miss points or
fail to grasp concepts .. language barrier doesn't help