engl power ball head faulty tube sense circuit

[Arbutt]

Hello people
This one is doing my head in. !! I have had another post about the original problem, but this is a bit old now and its a different fault

Original fault
Came in with tubes missing and a faulty 1st pre amp tube . Replaced power tubes for test ones and fitted a new pre amp tube. Found the anode resistor o/c. Replaced and amp worked , but ch2 was low where ch1 was perfect, i found the the both ch1 - ch2 gain pots would effect whatever channel was on
Engl_powerball.pdf
Current problem
I remember at switch on a few days later that V4 power tube LED was coming on. I swapped the tubes out still the same. defiantly not a tube issue
This amp is hard work due to the fact of the board is upside down , making it difficult to trace
after about 20 sec you hear the relay click and LED comes on for V4
checked and replaced ZD7 ok
Replaced IC3
ZD11 tests ok
I'm sadly not in my work shop while writing this , so i cant give voltage reading at this time
any assistance would be grateful
thanks in advance
Arbutt

[Helmholtz]

What is the voltage across R134 (4.7R) when V4 LED is on?

[Jon Snell]

I would check C22 for leakage and maybe test R134.

[Arbutt]

What is the voltage across R134 (4.7R) when V4 LED is on?

Hi Helm
with all 6l6gc in place at switch on the 4.7R on V4 is voltage is slowly rising and gets to about 80mv then the relay switches off and it drops back off
I have the PI tube out at this point
The 4.7r tests ok as does the others

Just to confirm i have tried many different 6L6 in v4
Can there be a problem with IC3 circuit
Its a really difficult amp to work on with no component refs and all i can see is the solder side

hope this helps
thanks again

[Arbutt]

I would check C22 for leakage and maybe test R134.

thank i have checked the 4.7R all good, with the PI tube is out . I will look into the C22

[Jon Snell]

thank i have checked the 4.7R all good, with the PI tube is out . I will look into the C22

If you lift one leg out to disconnect C22, that will prove it for you.

[Arbutt]

i have been doing some checking . decided to cut the track from the IC3. the relay is not being switched on now , but the V4 Bad LED on the front panel is coming on. as the relay is not switching off V4 4.7R has around 168mv the same roughly as the other tubes
at switch on pin 14 of IC3 is at 0v but after 10 sec ish it rises to 22v, as the track is cut its not switching the relay on
hope this makes sense,
thanks again

[Jon Snell]

Very dangerous to cut tracks! That removes potection.
Replace 4.7R, C22 and repair the damage to the track.
That will repair the fault.

[Arbutt]

Very dangerous to cut tracks! That removes potection.
Replace 4.7R, C22 and repair the damage to the track.
That will repair the fault.

I have cut the track from IC3 to see to confirm the problem is coming from IC3 circuit. It is only a temporary thing. I would of rather of lifted a component , but due to the board being upside down that was difficult

as it is as the reading correctly and i have about 160mv across it and same across the others

I will take the board out again and change the 4.7R and C22
Before i do that is there any other tests i can do that will prove the IC3 circuit is working correctly ?

[Helmholtz]

Also check CE29 and maybe CE27.

There seems to be something wrong with the schematic: All positive (as well as the negative) inputs of IC2B, IC2D, IC3B, IC3D look to be interconnected.
I don't see how they could act independently.

[Arbutt]

Also check CE29 and maybe CE27.

There seems to be something wrong with the schematic: All positive (as well as the negative) inputs of IC2B, IC2D, IC3B, IC3D look to be connected.
I don't see how they could act independently.

do you have any idea why with the feed from pin 14 of IC3 that the LED on the front panel is still coming on.. The whole amp is a right pain to work on with ribbon cables everywhere
thanks again. i will try again tomorrow

[Helmholtz]

do you have any idea why with the feed from pin 14 of IC3 that the LED on the front panel is still coming on..

LED comes on when the opamp output goes high, no matter if the relay is connected.

[Jon Snell]

Also check CE29 and maybe CE27.

There seems to be something wrong with the schematic: All positive (as well as the negative) inputs of IC2B, IC2D, IC3B, IC3D look to be interconnected.
I don't see how they could act independently.

The connected lines you refer to, (inverting inputs on op amps D&E), set up the control reference voltage with the potential difference between R116/117. The non inverting inputs on D&E is set at T7s Ce voltage.
When the non inverting input on op amps C go above the non inverting input voltage (junction R113&116) the output pins 1or6 of C swings high, overtaking the potential on the inverting inputs of D, putting a high on the outputs of D which in turn pulls the relay on and illuminates LD7/8/9 or 10.
This will occure when the grid of a 6L6 goes high due to a leaky coupling capacitor a faulty 6L6.

It is a very logical and reliable circuit using comparators.
I hope that explains the mechanics of it.

[Helmholtz]

The connected lines you refer to, (inverting inputs on op amps D&E), set up the control reference voltage with the potential difference between R116/117. The non inverting inputs on D&E is set at T7s Ce voltage.

Yes, it makes sense that the inverting inputs are connected.
But my point was about the connected non-inverting inputs.
That makes all the above listed opamps see the same differential input voltage, so output conditions must be the same as well.

[Jon Snell]

Yes, it makes sense that the inverting inputs are connected.
But my point was about the connected non-inverting inputs.
That makes all the above listed opamps see the same differential input voltage, so output conditions must be the same as well.

Reading from my copy of the service manual that is updated and didn't look at the pdf on this site. I have identifying marks written on my hard copy and cannot scan it as service agents are forbidden to share documents.
My schematic has no other connections on the non inverting inputs except the first comparator. A - B and C - D.
But reading the pdf on this thread; I see what you mean, the inverting inputs are correct but not the non inverting inputs. It is a mistake on a pre production schematic!

What a nightmare if you have the wrong schematic version.