Little Dot MK8SE / MK6 Super Mods (All verified mods are on first page)
Oct 20, 2016 at 5:03 AM Post #2,101 of 4,154
Tiny splat or even a thread of heater wire over the wrong pin which can be highly probable IF the heater wiring was replaced or moved..
Resistors usually never short,
They only burn open open.

Check the socket pin solder connections on top board side for DEBREE between socket and board


Right I think I will have to desolder the socket to see underneath. At least we know that the problem must be between that 2k resistor and the pins somewhere.
 
Oct 20, 2016 at 9:38 PM Post #2,102 of 4,154
Desoldering socket is not a great idea unless you have a new one in case of damage from removal.
 
Nov 1, 2016 at 7:48 AM Post #2,104 of 4,154
 
How are you getting on Baron?

 
Fine thanks. Just been away awhile. I've got some work to do outside then it's back to the amp!
 
What's the situation with your mods, are you doing it yourself? Keep us posted
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Nov 8, 2016 at 12:28 PM Post #2,105 of 4,154
Normal service has been resumed, I'm now back working on the amp!
 
I have now tested the circuit with one leg removed of all the modded components. Also I have removed the opamp. Then I tested tube pins 3 and 5 and, yes, they're still grounded. The only thing I haven't done that you recommended is to check the tube pins. I found it impossible to see underneath so it looks like I will have to desolder.
 
What else can I do, and what else could be causing it other than a fault in the board?
 
Edit: Oh and one of the black wires has come off on the reverse, I believe it has only just come off due to being very brittle.
 

 
The wire between the green and yellow wires toward the centre.
 
Also I haven't tested voltages on the opamp, I may have to remove other components before I put it all back together again so that will have to wait. If there's nothing else I can do myself the last resort is to take it back to a professional and hope he can fix it
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Nov 12, 2016 at 9:45 AM Post #2,106 of 4,154
Normal service has been resumed, I'm now back working on the amp!

I have now tested the circuit with one leg removed of all the modded components. Also I have removed the opamp. Then I tested tube pins 3 and 5 and, yes, they're still grounded. The only thing I haven't done that you recommended is to check the tube pins. I found it impossible to see underneath so it looks like I will have to desolder.

What else can I do, and what else could be causing it other than a fault in the board?

Edit: Oh and one of the black wires has come off on the reverse, I believe it has only just come off due to being very brittle.




The wire between the green and yellow wires toward the centre.

Also I haven't tested voltages on the opamp, I may have to remove other components before I put it all back together again so that will have to wait. If there's nothing else I can do myself the last resort is to take it back to a professional and hope he can fix it :frowning2: .

I dont like how those cathode resistors were soldered to board.
They supposed to be over the vent holes.
Also those caps leads are squashed into the hot resistors.
Make sure there is no touching of the cap leads onto those resistors.
For longevity of unit, have this redone as suggested in beginning thread unless you like potential heat issues,
Edit: especially for those small PIO caps, high heat from Cathode resistor not a good thing.
 
Nov 12, 2016 at 11:50 AM Post #2,107 of 4,154
I dont like how those cathode resistors were soldered to board.
They supposed to be over the vent holes.
Also those caps leads are squashed into the hot resistors.
Make sure there is no touching of the cap leads onto those resistors.
For longevity of unit, have this redone as suggested in beginning thread unless you like potential heat issues,
Edit: especially for those small PIO caps, high heat from Cathode resistor not a good thing.

 
Sorry Maxx, that pic was the best I had to point out the black wire I was talking about, the cathode resistors were resoldered after you pointed out the heat problem before, they are in the correct position now.
 
I'm just thinking that if I don't have any more leads about the next step to take I've no option but to try the professional again, I seem to be in a dead end.
 
Nov 12, 2016 at 7:18 PM Post #2,108 of 4,154
Well, you could check under socket for possible solder or extra wire filaments from old heater wiring that you replaced, or board trace pad detached, or socket pin irregularities
 
Nov 16, 2016 at 8:32 AM Post #2,109 of 4,154
Well, you could check under socket for possible solder or extra wire filaments from old heater wiring that you replaced, or board trace pad detached, or socket pin irregularities

 
I've done that as far as possible and it's revealed nothing, and the pads are fine. Can't find any pin irregularities, don't know what you mean really. Also I've given the whole area a shot of compressed air.
 
I've checked the CB resistances, the grid resistances, all good. I've checked the circuit trace connected to pin 3 to find out why pin 3, (and pin 5), are grounded and found 3 resistors, including the 2k resistor are also grounded. The  2k resistor also has continuity over itself but I can't find out why the grounding issue persists. Having removed the 2k resistor and the grounding is still there, and the solder points of the 2k resistor still have continuity with each other which I don't understand. I'm at a complete loss and I think I'm at my limits now, there doesn't seem to be any obvious cause.
 
Also I removed the opamp as Coin suggested and still the grounding issue is there so I don't think there's much point looking at voltages. Maxx, you said the opamp/WCF area could be problematic?
 
If neither of you have any more suggestions then it's going back to the engineer as a last resort.
 
Nov 20, 2016 at 11:00 AM Post #2,110 of 4,154
baronbeehive:

Pin 3 on the output tube is cathode, I'm assuming it's the top cathode (looking at the schematic I posted).

This node is connected via 56k (the dummy load) to the other side top cathode, meaning other output tube of the channel, and from there via that sides 2k to ground. Could this be the route?

Is the resistance 0 ohms or something else?

Pin 5 is the bottom triode anode, which (again, looking at the schem) is connected to top triode cathode. So if the above mentioned route is there, then this pin is grounded as well.
 
Nov 21, 2016 at 1:40 PM Post #2,111 of 4,154
baronbeehive:

Pin 3 on the output tube is cathode, I'm assuming it's the top cathode (looking at the schematic I posted).

This node is connected via 56k (the dummy load) to the other side top cathode, meaning other output tube of the channel, and from there via that sides 2k to ground. Could this be the route?

Is the resistance 0 ohms or something else?

Pin 5 is the bottom triode anode, which (again, looking at the schem) is connected to top triode cathode. So if the above mentioned route is there, then this pin is grounded as well.


Thanks for your help MrCurwen, I was just on the point of contacting a professional about this when I got your post!
 
Just to go over what you said: pin 3 (cathode) is connected to pin 3 (cathode) of the other tube in the channel via the 56k resistor and to the 2k resistor and ground. Yes that is the route but as far as my testing has gone, pin 3 of the other tube in the channel is reading OK, ie not grounded, I haven't tested the 56k resistor. The 2k resistor was not reading as 2k until I removed it from the circuit then it tested correctly. Before that it was fluctuating. There was continuity over the 2 contacts on the PCB for the 2k resistor with it removed which I found strange. Also both legs of the 2k resistor were grounded when it was in place, against only 1 leg in the good circuit.
 
The resistance over the cathode resistor was 330 ohms, across the burnt anode was 345 ohms. The resistance for grid pin 1 to ground is infinite, and across grid pin 4 to ground is 550 ohms. Both of these results for the grid pins read the same as for the good circuit so I assume they're good.
 
Yes, pin 5, like pin 3 is grounded.
 
Does this answer your question? Let me know if I can provide any other measurements.
 
I find it odd that having found what I thought was a short in the 2k resistor possibly causing the grounding issue in the tube pins that having removed the resistor the issue remains.
 
Nov 21, 2016 at 9:47 PM Post #2,113 of 4,154
Last I checked he's using a beeper which means it would be close enough to zero to register the beep. 
Pin 3 and 5 are both the output to the white cathode follower.
From your pictures and statements the board seems fine. 
The output wire from the board to the pre-amp jack and then to the output of the amplifier. Desolder the wire from the output of the cathode follower and then check to see if it is still shorted, this is the most likely suspect area at the moment.
 
Nov 23, 2016 at 7:01 AM Post #2,114 of 4,154
I'm using the continuity setting, not the resistance setting on the multimeter, yes. Just to check I've just used the resistance setting on pins 3 and 5 and the reading varied from zero to above zero, fluctuating continuously, which I suspect is normal, it did the same on a check for ground with both wires on the chassis.
 
I've removed the WCF cap from the bad circuit and pins 3 and 5 are still grounded.
 

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