troubleshooting M40 Echolette with loud hum

[notea]

Hi,
it sounds like a nice saturday evening thanks to you guys!
(even if i burnt two rectifier tubes and blowed the fuse of the house cause i switched on the unit like a moron with the screen wire disconnected from the tag strip of the O/P, i didn't saw arcing but the rectifiers were glowing white!) anyway...
the short is located! I soldered all the connections on the tag strips of the output transformer, hooked the meter between the center tap and the chassis so i could see the 22ohms,
then i started to play with my wooden stick following the screen wire, i slightly jiggled screen resistors and tube socket lugs always staring at the meter and here it is the Eureka moment!
there is a loose pin9 on the second tube socket (but i don't know the order of the output tube sockets on the chassis), as to be Rö6 or 8.
So next step is ordering brand new tube sockets and eyelet pliers, hope it will solve the problem,
I would also like to replace C25, is it possible to put a 47uf 25V electrolytic cap instead of the 50uf 25V cap, cause i can't find the same value?
cheers Nick

[Jazz P Bass]

Quote" I would also like to replace C25, is it possible to put a 47uf 25V electrolytic cap instead of the 50uf 25V cap, cause i can't find the same value?"

47uf is the now standard value.
It will function fine.

The 50 value has not been manufactured in decades.

[notea]

ok thanks a lot!, i will order this then

[notea]

Hi guys, how is it going?
i'm back on the M40 after few weeks waiting for the right size of tube sockets to come..
We found in the previous posts that the 22ohms between the center tap and the chassis was created by a short which i thought it was a loose pin 9 on the second El84.
After replacing the tube socket the 22 ohms was still here and i realize that it was actually due to a bad insulation of the 220 ohms metal film resistor (connected to pin9)that was touching another resistor and creating the short..
What a waste of time!..
Anyway i switched on the amp and it was quiet for few seconds as i started to hear a hum in more higher frequency than before. I switched it off waited for few minutes, turned the volume control down and switched it on again.
No hum!, it looks like there is no more DC in the speaker as i can control the volume of the hum. But after 1 minute the PT started to smoke..
i disconneted all the leads of the secondary, plugged the HT and try to measure the voltage at the heaters lead but i couldn't read anything (i might be wrong doing this).
- i tried to measure the diode with the ohm test and i've got 36 Meg on one side and 17 meg on the other which is pretty high, The OA 150 could be an open diode.
Do you know if i can find an equivalent of this diode nowadays?
I also read on the internet that a PT smoke could be due to a short in the heaters wires..
Cheers Nick

[Mickey]

Hi Nick,
The OA150 diode can be replaced with any 400V 1 Amp diode such as a 1N4004 as it's purpose is to provide a negative bias voltage for the output valves.
Seldom do the filament windings become problematic. If the valves aren't lighting up I would think that you may have introduced a fault in the wiring similar to 220 0hm resistor as mentioned above.
Again if you think there is a fault in the filament winding, remove all the valves and see what happens. You can measure the resistance from this winding to the HT windings but measuring it to ground will not be conclusive unless you seperate the winding from where it comes from the transformer. Filament windings are usually grounded on one side or through a centre tap.
Cheers, Mickey

[notea]

Hi Mickey,
thanks for the diode equivalence, but now i'm not sure that it's the only issue, i tried different configurations:with the diode disconnected, with the HT lead disconnected at the filtering caps, with and without the rectifiers, every time i switched it on i end up with a burnt smell coming from the PT.
The thing is when i put all the tubes back in, they 're all glowing red and visually I can't see any kind of short in the heaters circuitry either, i could also measure 196VDC at the cathode of the rectifier..
When i desolder all the leads of the secondary except the 12Ax7 heaters (h1), it seems that there is no burnt smell. As soon as i solder back the plate leads and the EZ81/EL84 heaters (H2) the PT is close to burn out..
i'm affraid there is an internal short in the PT.
Cheers Nick

[Mickey]

Hi Nick,
If the transformer had an internal short it would get hot regardless of whether the valves were plugged in or out!
I would check around the EZ81 and EL84s as these are the heavy current drawers and the valve sockets can carbonise if arcing is or has occurred.
With your measuring etc you may have disturbed the wiring so I would check carefully for any inflicted damage.
What happens when you remove just the rectifier, is the short still there?
As I said previously remove all valves except for the rectifier EZ81 and establish if the power supply is working. Then move on from there.
Regards, Mickey

[notea]

Hi Mickey,

when i remove the rectifiers, the short is still here, i can hear a "current noise" i tried to power on the unit in pitch black to maybe see an arc somewhere but i can't locate this noise.
- with all valves remove;
-i measured 1.1 ohms at the PT secondary for the 2 heaters wires (H2), the heaters are carrying this 1.1 ohms through each pin4&5 from the EZ81 to the EL84.
-the H1, heaters for the 12AX7 are carrying 30ohms each.
-i measured 52.1 ohms at the PT secondary and at the first anode pin 7 of the EZ81.
-For the second anode of the EZ81, at pin 1, i'm measuring 33.7ohms, this wire is connected on the primary side of the OT! (see the attached pic)
i can see under that lead a black deposit like blak melted varnish.

-I measured also the ground and there is a 1.0 sometimes 0.9 ohm residual on the chassis, i don't know if it's normal (maybe it's because it's a fixed bias..), but what i can see is that this 1 ohms residual is also passing trough the mounting screws of the OT as long as this one is not isolated from the chassis. On contrary, the PT seems that it's has to be isolated as long as is carrying cartridge/rubber washers between the chassis and the mounting screws.. But there was missing one of this, so i fixed that up and now the PT is isolated from the chassis and this 1 ohm. Don't know if i should isolate the OT also as long as this one originally doesn't seem to have cartridge washers..
I'll continue the reasearches on this
Cheers, Nick

[Mickey]

Hi Nick,
I hate this site, it often doesn't allow messages to be posted !!!!!!
Anyway I don't think that there is a short in the filament windings of your transformer!
There are two filament windings, HT1 has hum cancelling trim-pot and feeds filament voltage to all the pre-amp valves. Locate the hum cancelling trim-pot.
HT2 feeds the rectifier and output valves.
Disconnect all the filament wires from the transformer and see what happens when it is switched on. Should be OK.
If the transformer still has a "current noise" check and make sure there are no connections to both of these filament windings, all valves should also be removed. If with all of this checked and there still is a current noise then you may have a shorted filament winding.
If everything is OK measure the filament voltages on the transformer, there should be two at 6.3VAC.
If OK so far reconnect HT1 and see what happens, if this causes a problem check the trim-pot and the wiring to the valves to see if there is a short to ground.
If HT1 is OK then reconnecy HT2 and see what happens, again look for shorted wires or burnt valve sockets etc.
Let me know what happens.
Cheers, Mickey

[oc disorder]

Notea
While you're checking the heater windings you might as well work out "whats what" .
i.e. map out - identify all the terminals it might be useful.
With nothing connected we hope it still works ok.
With your light bulb limiter in series with the transformer reconnect one by one
and see which one or pair "lights up" the L.B.L.
I hope that new 50uF capacitor has its positive to ground.
That bias cap connects to a negative supply.
Don't worry about transformer isolation. It should be grounded at one point.
And don't forget the pilot light (power on 6.3 light and it's wiring).

Like you found with the resistors touching .. it's often something stupid and not that complicated !

We all curse ourselves sometimes by overlooking something !

[Jazz P Bass]

Hi Nick,
I hate this site, it often doesn't allow messages to be posted !!!!!!

What is this supposed to mean?

You cannot post messages from where, when, how???

[notea]

Hi,
unfortunately when i desolder H1 and H2 the PT is still overheating.. I desolder each wire at the primary keeping the ground wire on and nothing happen 'till i solder what i think is the HT.
You're right Oc i 've got problem to determinate what is what, on attached pic this is what i understand so far with the secondary side, as for the primary,i might be wrong but i feel quite sure that one of the wire (cercled in green in previous pic #53) goes directly to pin 1 of the EZ81 according to the ohm 's reading. It seems that the PT is still producing current but overheat very fast. I think the problem was here before i replace the tube socket it's just that before there was this hum and i had to switch off the unit quickly anyway so i didn't realize that it was overheating.After few minutes it's still extremely hot and i can hear some cracking noise inside, could the PT produce a delayed fire? i don't want to burn the house while i'm sleeping, i'll put the unit on a concrete floor in case of..
Last thought, is it possible to have a short before the PT, like with the power switch, or in the power cord or current selector region or even a bad ground that connects to the lead of the PT?
I could put the PT on the table and check the ohms but it doesn't tell you if it has an internal short right? I wish i have a variac to test it, but still this PT seems dead..
Cheers Nick

[oc disorder]

Well I don't think it will cause a delayed fire unless you leave it on.. but extra precautions are a good idea.
Couple of confusing things again probably translation... the second last wire on your transformer cannot be a screen (valve/tube screen).
Maybe it goes to the 5 k resistor (R89 i think) that supplies ac to the bias circuit.
Secondly (a minor point) the primary of the transformer is the input 230- 240~ the secondary is the stepped up or down output of the transformer.
I do think you need to remove all wires from the secondary.. You will have to decide if you can remove them cleanly with a solder sucker to reterminate later or if you have enough slack- spare wire i.e. if they are long enough cut them and re-strip the ends.

Now find out definitely which wire is which - where they go to - and the resistance to ground (the load ) of each one.

I would then suggest as by now you probably have enough spares... to experiment building a temporary power supply using 1 socket 1 EZ81 and a filter cap near the original 50uF 375v or similar. This will help understanding ! If that works just connect back the minimum wires 2x HT 1 x GND and 2 x 6.3 or Heater 2 (H2).

Now just plug one EZ81 in after disconnecting the cathode wire that goes to the first 50uF in the power supply C34 . You may find that one EZ81 socket has only one wire from the cathode at pin 3 where the other has two and harder to remove. From that pin 3 connect up your loose 50uF 375v cap you experimented with previously and turn it on. Hopefully you can measure at least 300v DC there.

I think here we have to simplify . Transformers with internal shorts get hot with only the primary connected if they work at all.
Normally they blow fuses so I still think you have a "silly" short somewhere in the HT line. The rectifier valves/tubes glow because they are straining to supply
anything ! Is it the same two you are still working with ? Too much "strain" while testing may wreck them too. Make sure you have one you know is good.

"I could put the PT on the table and check the ohms but it doesn't tell you if it has an internal short right? I wish i have a variac to test it, but still this PT seems dead.."
I'm suggesting remove all secondary's check each wire for resistance to ground (not transformer) then after tracing the wires which will be much much easier without the transformer connected you should then be able to work out what each transformer terminal actually is. 8 wires. Then with an AC voltmeter you can measure the voltages at the transformer. without what I think is a short not in the transformer.

Low voltage connections don't have much wire i.e. their coil in the transformer is very short so there is not much resistance to measure .. it looks like a short but it isn't. Often a fraction of an ohm.

[notea]

Hi,
so it's ok i didn't burn myself last night! You're right Oc i was meaning the leads that's connect to the 5k resistor,
i will think of your experiment for building a temporary power supply which is a very good idea, for now i now more about what is goin on with the PT.
First i desoldered all the connections of the PT secondary and took off the PT from the chassis as i got spare wires, i switched on the unit and still overheating, i can hear current noise in the PT.
So i took the string off the wires to finally locate where goes where, till i realized that i got the continuity function on my meter... This can save some time.
- so pin1 of the EZ81 is where i thought it where, located on the primary side but isolated from the other leads according to ohms reading.
-So i moved on with this continuity test, and each of the 7 leads left on this primary side is connected to each pin of the current bridge selector, so no short
-After this i found that there is continuity between pin4&5, the H2, on each tube socket from EZ81 to EL84, there is also continuity between H2 to ground , after some researches i found that it's normal but this part is hard to get for me, that's mean that while it's powered on, the 6.3 V are feeding the heaters while they get absorbed at the same time by the chassis..
Anyway as for the PT it's look like there is a short in it, even with the plate(pin1) of the EZ81 unsoldered, it's overheating..

* this is the ohms readings of the wires disconnected from the secondary:
-H2: 1.7 & 1.1 ohms
-H1: 50.4 & 58.8 ohms
-Pin7 EZ81: ---
-to 5k: 34.7 ohms (R89+R90 which is actually a 33k)
-ground: 1.1 ohms
-pin 1 EZ81: ---

the primary is still confusing for me, i will try to figure it out which is which between the 7 leads left, and how there are connected with the current selector.
I can see on the schem that there is 2X n°4, maybe it's a unique lead on the primary and that's make 7 leads + 1 for the anode of the EZ81..

[oc disorder]

Ok H1 is ok . The two heater wires have a trim potentiometer across them and the wiper (moving part) is connected to ground.
The "pot" is 100 ohms so if the wiper was in the middle you would measure 50 ohms per wire. This is the hum reduction pot to the pre-amp valves/tubes.
So those readings make sense looking at the schematic.
H2 however is very low ! Nowhere can I see it connected to ground. There is a 7 volt 0.3A light globe/bulb (on indicator) across H2 which will read very low between the two wires but from what I can see there should be no connection from H2 to ground.
Is it possible the ground wire in the "string" is touching one of the heater wires? One of the heater wires measures 1.1 ohms to ground which is exactly the same as you measured from the ground wire that originally grounded the centre tap for the HT winding.

Have a look around that light . Is the "body" of the socket touching ground? Remove the light and remeasure. If one side still measures low ohms to ground there is a problem somewhere.
I.e. one of the output tubes socket pin may have a "blob of solder short" or maybe one of the rectifier tubes has a blob ????

At this stage dunno if I would have removed the 240 primary wires.... The two number 4 terminals on the primary are connected together internally.`
There may be only one physical terminal visible.

If somewhere H2 is grounded it would explain the EZ81s getting stressed.