troubleshooting M40 Echolette with loud hum

[notea]

Hello everybody,
just bought an old Echolette M40 supposingly working but it's not..
If anyone knows this unit a bit i'll be grateful to have some help with it
I precise that my level in electronic is pretty low as well as my english!.
So here is the problem:
After powered it on it starts to make a big hum so i switched it off right back.
after blowing few fuses i made a light bulb tester and i tried to power it on again.
I just had the time to check if there was some DC on the output and there was..
So perhaps there is a bad cap somewhere that's leaking some DC,
-I want to change the caps that are connected to pin 3&8 on the 12AX7s cause there are leaking,
but i wonder if i could replace them with normal electrolytic caps, or do i have to buy special ones?
i read here and there that it's not recommended to use tantal caps for example.
-Do you know what is the purpose of the green caps 50uf (see the attached pics) as long as it's + is connected towards the ground?,
is it to dissipate the extra DC? when measuring it should'nt exceed 25 V right?
Cheers Nick

[stokes]

The caps with the + side connected to ground are probably negative voltage supply caps.The caps connected to pin 3 & 8 of the AX7's are cathode bypass caps,use any electro you want that has the proper capacitance and voltage ratings.I would suggest changing all the electros.You say you used a light bulb current limiter and "I just had the time to check if there was some DC on the output and there was.." Why did you feel you "just had time...." ? The idea of using a limiter is so you can leave the unit powered on while you trouble shoot without blowing the fuse.You say you see dc voltage at the output.Are you talking about the speaker output?If so you likey have a problem in the OT.

[notea]

Hi Stokes,
thanks for your answer, yes i've got DC at the speaker output! even with the light bulb the hum is increasing very quick and very loud so i've got to power off directly cause i don't want to ruin it. Yes i hope it's not the OT.. I think we can test it in-circuit with a meter? i will do some research about it
Cheers Nick

[g1]

Just because there is hum does not mean it is necessarily DC. Did you measure DC with a meter? If so, how much?
In tube gear with output transformers, hum is usually 60hz or 120hz AC voltage.

[notea]

Hi, well i measured at the speaker output and i got 18 VDC in less than 5 sec, and it was still increasing so i switched it off.
I measured the lug of the OT and it seems ok, i've got 3 ohms and it's a 6 ohms output, the primary (i think) is 46 & 465 ohms, not sure if i did the right measurements though,
if anyone already measured this OT i'll be glad to have some infos.
Cheers Nick

[g1]

Lots of less expensive meters will show DC voltage if AC is present. Especially if you don't wait more than 5 seconds for the reading to settle.
It is likely that you have a problem with power supply filter caps. And maybe other problems if the fuse was blowing.
I wouldn't be in any hurry to condemn the output transformer without a lot of further testing.

[notea]

Hi g1, thanks for your advices, my meter is not a crappy one but i can assure that if i wait more than that, i really feel it's gonna burn or something, the noise is pretty scary..
The power supply filter caps are suppose to be new, it's the only thing that 's been replaced on this unit.
Is there any particular way to check them if they are good or do i simply measure them and see if there is a large amount of DC?
the new cathode bypass' caps are on their way.
Cheers Nick

[oc disorder]

Echolette M40


Firstly 2 schematics.
Have to know exactly which one before continuing .
One with fixed bias (OA150 diode) with din 3 pin input ,2 x EZ 81rectifier tubes 4 x EL84 with grounded cathodes.


The second diagram 4 x adjustable bias pots (SFD108diode) Jack input, 2 x EZ81's 4 x EL84 with 1 ohm cathode resistors on each EL84.



Check the bias diode... diode check on multimeter should get approx .5v in one direction.

Step 1.
With no output valves plugged in ,measure "negative" D.C. bias voltage at each pin 2 EL84.
Pin 2 should have a 2 k or 2k2 grid resistor also.

Tell us the negative bias at each pin 2 please.

[notea]

hi Oc nice to see you again!
the unit i've got is the first one with din inputs,
-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
-while the unit was upside down i could see through the center hole of the tube socket that one of the EZ 81 was extremely red compared to the other which i didn't notice before
i 'll look also on this direction and see if i can find a new pair of rectifier tubes.
The unit could have few issues so i don't wanna burn new tubes if it's not the first part of the problem.
Cheers Nick

[notea]

Hi everybody,
i finally got the new EZ81, i replaced them + i replaced the leaking decoupling caps with NOS ones,
this didn't change anything, i still got this increasing hum unaffected by the volume pot..
this time i waited more than 15 secs but i shouldn't it was close to the electrical arc i could see before cuting off the first EZ81 glowing white/blue.

I'm affraid there is a problem with the OT, does anyone know how to check the M40's OT with a simple multimeter?
Cheers Nick

[oc disorder]

Hi Notea
You have an ultralinear connected output section where the screens are connected to
tappings from the output transformer.
To do a basic test with a multimeter connect one probe to the centretap of the
output transformer (connects to Ro.11 ,C34 ,R88) and measure resistance to
the outer windings that go to the anodes .
They should be approximately the same .
Say you can measure 120 ohms across the transformer from anode to anode connection.
From the center tap you could expect 60 ohms each side.
Could be more like 50 ohms and 70 ohms.
If one side is say 60 and the other is 20 ohms there is a short circuit in the 20 ohms "half"
of the transformer.
The UL taps that go to the screens should also have similar dc resistance readings
to each other. In my 120 ohm example they would be more like 20 ohms and 25 aprox
reading from the center tap to the screen tap where it connects to the 200 ohm screens.
The secondary , where the speaker connects will measure like a short circuit like .1Ω.
With a low ohms meter you may detect differences in the tappings but they're very small.
Here's another example with some aprox values.

[Mickey]

Hi everybody,
i finally got the new EZ81, i replaced them + i replaced the leaking decoupling caps with NOS ones,
this didn't change anything, i still got this increasing hum unaffected by the volume pot..
this time i waited more than 15 secs but i shouldn't it was close to the electrical arc i could see before cuting off the first EZ81 glowing white/blue.

I'm affraid there is a problem with the OT, does anyone know how to check the M40's OT with a simple multimeter?
Cheers Nick

Hi Nick, pull out all the valves and see if the hum is still present. The hum should now not be present. Replace just the rectifier valves, if hum is present bypass the power supply filter capacitors with known good ones, no need to unsolder anything just use jumper leads to connect the substitute filter capacitors or just bridge them across the old ones. Be careful not to electrocute yourself.
Bear in mind that because you have removed the valves there is no signal path so if the hum is being generated in the pre-amps you wouldn't hear this.
Now if there is no hum put back two of the output valves (EL84). If still no hum replace more valves but only one at a time and work backwards from the output stage.
Please let us know what you find.
Mickey

[notea]

hi guys,
Oc, thank you for your schem but i've got to do complementary researches to know exactly how ultralinear OT works, i need to visualize things cause it's not clear to me, for now it's hard for me to understand where the wires goes from the OT, cause on this one (see the attached pic)
-i can see 3 lugs on the left with a black wire far left that could be the center tap
(those 3 lugs are the primary i guess)
-and 4 lugs on the right 2 red wires with gold strip and 2 brown or sort of
(those 4 lugs are the secondary)
-i need to figure it out with some researches which is which and really get into the M40 diagram.
Mickey, i already tried this and there were no more hum but after few secs i could hear few cracking noises so i decided to stop. I will follow this time your procedure and will tell you what i found.
Cheers Nick

[Mickey]

Hi Nick,
If you have removed all the valves including the rectifiers and you can still hear that noise then your problem may be with the power transformer as there would be no HT with the rectifiers removed. If the noise is there with the rectifiers in place then measure the voltages on the anodes of the four output calves. This voltage should be the same at all anodes. If not then you may have a faulty O/P transformer. You could disconnect the HT to the centre connection of the primary winding of the O/P transformer and place a resistor in series ( try a 10K) and if this starts to overheat then you could have a short in this transformer. If it doesn't start to overheat then again measure the voltages at the anodes of all EL84s.
Do not worry too much about ultra-linear as they are taps on the primary of the O/P transformer which are usually connected directly to the screen grids of the output valves, in the M40 they are connected to the EL84s via resistors.

[Mickey]

Hi Nick,
Just looked at your photo of the output transformer and I would suggest that the wires on the LHS are the speaker windings (secondary) and the wires on the RHS are the primary. The red ones are most likely connected to the anodes of the EL84s and the other two would connect to the screen resistors.
The 2.2K resistor (red-red-red) shown would be directly connected to the cathodes of the rectifiers and one side of this resistor would be connected to the centre tap of the O/P transformer.
Can you post a photo of the connections/sockets of the output valves.