Need help troubleshooting Tube Tech MP1-A

[innercityman]

Hi,

I'm trying to identify the issue on one channel of an MP1-A preamp. Channel one have low distorded signal; I've measured tensions at each tubes and I have an issue at ECC82 cathode. On channel 2 which is working, I have about 7V at cathode but almost nothing on channel 1. I've checked and measured 47uF caps and 4,7K 1W resistors that follow cathode and they're good. Any idea on where I should investigate ?

Thanks

 

[innercityman]

I took tension measurements carefully on every pins of both ECC82 and here's what I got...

On working channel ECCC82 I got :

- 1 a' : 372V
- 2 g' : 6,5V
- 3 k' : 6,03V
- 4 f : 10,43V
- 5 f : 0,12V
- 6 a : 372V
- 7 g : 7,29V
- 8 k : 7,2V
- 9 fc : 5,26V

On faulty channel I have :

- 1 a' : 372V
- 2 g' : 7,4V
- 3 k' : 2,46V
- 4 f : 10,4V
- 5 f : 0,14V
- 6 a : 372V
- 7 g : 0,4V
- 8 k : 0,4V
- 9 fc : 5,28V

I've checked C12 cap connected to tube grill pin 7 and I have 49V on both caps on one side but 0,4V on grid's side of faulty channel and 7,2V on working one. I've measured both C12 values and they measure good, around 345nf.

 

[radardoug]

We need to see a schematic to really comment. Normally you should have no volts on the grid, and volts on the kathode. No volts on the kathode indicates tube not conducting. Can you see the filament lit in all sections?

 

[CJ]

swap tubes, remeasure,

i believe that the faulty tube section may have a short from grid to ground,

looks like the plates are connected directly to the B+ so the cathode is lifted thus the high grid voltages,

try to measure grid-cathode voltage by connecting pins 2-3 and 7-8,

this takes the error out of the meter being across the hi z grid,
(meter Z in parallel with grid Z)

same with opamps, sometimes hard to read pins 2 and 3 on TL072 etc

speaking of opamps, i think you might have a tube opamp circuit there.

 

[gyraf]

Either a tube problem, which can be found by swapping the two ECC82's as CJ mentions

..perhaps a shorted cathode electrolytic, the 47u/63V in parallel with the 4K7/1W. Would make sense these fail, it's a hot area - although I don't see how this would pull grid down too..

The stated
- 7 g : 0,4V
- 8 k : 0,4V

..could indicate a short here - measure tube itself, and then the socket itself (at no-power) to figure out where it's shorting?

 

[innercityman]

swap tubes, remeasure,

i believe that the faulty tube section may have a short from grid to ground,

looks like the plates are connected directly to the B+ so the cathode is lifted thus the high grid voltages,

try to measure grid-cathode voltage by connecting pins 2-3 and 7-8,

this takes the error out of the meter being across the hi z grid,
(meter Z in parallel with grid Z)

same with opamps, sometimes hard to read pins 2 and 3 on TL072 etc

speaking of opamps, i think you might have a tube opamp circuit there.

That's what I suspected first so I did it and same issue on same channel. I've even tried with old tubes I had around... Same.

I've checked for a short beteween grid and ground... No short.

What I've found is that on faulty channel, I have continuity between hot, cold and ground XLR ouput socket pins, so a short between ground and both audio outs of output transformer. Could it be a faulty output transformer ? Or maybe the short is somewhere else... ?

 

[beatnik]

Perhaps the tube socket is dirty / oxidised or it has loose contacts ?

 

[radardoug]

Wow, this is an odd circuit, with the grids being pulled positive by the voltage divider feeding the 1M grid resistors. Indications are the grids are drawing current. Care to comment, Gyraf?

 

[CJ]

Pull the tube out and measure the voltages on the socket,

Just pins 123 and 678

Do it on the tube side of the socket, (this will check your soldering on the chassis side) note that this will be a mirror image so keep track of your pins,

They go clockwise when looking from bottom, so counter clockwise from the top, call me captain obvious

We don't care how the mule got in the ditch, we just care about getting him out.

 

[gyraf]

..the 4K7 cathode resistors being rated 2w points towards cathode voltages upwards of 40-60V'ish, so grids need lift. But I don't get the fixed-bias scheme, imo would have made sense to refer grid to cathode instead..

 

[innercityman]

Pull the tube out and measure the voltages on the socket,

Just pins 123 and 678

Do it on the tube side of the socket, (this will check your soldering on the chassis side) note that this will be a mirror image so keep track of your pins,

They go clockwise when looking from bottom, so counter clockwise from the top, call me captain obvious

We don't care how the mule got in the ditch, we just care about getting him out.

I've already checked continuity between each solder pad and each tube socket pins. I'll make the test later on and will give you the results.

Thanks for your help by the way !

 

[innercityman]

Pull the tube out and measure the voltages on the socket,

Just pins 123 and 678

Do it on the tube side of the socket, (this will check your soldering on the chassis side) note that this will be a mirror image so keep track of your pins,

They go clockwise when looking from bottom, so counter clockwise from the top, call me captain obvious

We don't care how the mule got in the ditch, we just care about getting him out.

So here are my measurement results with ECC82 tubes out...

- anodes (pin 1 and 6) 445V
- grids (pin 2 and 7) 20,14V
- Cathodes (pin 3 and 8) 0V

Same results on both channels

And even without tubes, hot and cold audio outs are shorted to ground on faulty channel

 

[beatnik]

And even without tubes, hot and cold audio outs are shorted to ground on faulty channel

I am not sure a faulty output transformer could cause the problems you described but it should be easy enough to swap the output transformers around and see if the problem follows the transformer.

It wouldn't be the first time I hear of a failed Tube Tech output transformer

 

[innercityman]

I am not sure a faulty output transformer could cause the problems you described but it should be easy enough to swap the output transformers around and see if the problem follows the transformer.

It wouldn't be the first time I hear of a failed Tube Tech output transformer

Obviously I thought about that but I wanted to avoid desolder pads as the unit is old and it heats a lot so, often when you desolder, the pad come out and you have to put jumpers. But II'll do it if no choice.

 

[CJ]

Are the tube sockets soldered to a PC board? Check continuity from pin 8 to ground.

 

[innercityman]

Are the tube sockets soldered to a PC board? Check continuity from pin 8 to ground.

As I said, yes they are. Every pins from tube socket are properly soldered to PCB. No continuity between pin 8 and ground on both channels.

 

[CJ]

try grounding pin 8

 

[beatnik]

Obviously I thought about that but I wanted to avoid desolder pads as the unit is old and it heats a lot so, often when you desolder, the pad come out and you have to put jumpers. But II'll do it if no choice.

Come on it's not that old, and I would hope Tube Tech uses decent quality PCB material that doesn't delaminate so easily.
You'll have to take out the soldering iron sooner or later, it ain't gonna fix itself just by staring at voltages...

 

[beatnik]

Found a gut shot, it looks like the main board is one sided so it should be a breeze to rework.

They seem to be using cheap quality tube sockets, I would definitely check those carefully.

By the way, does anybody know a source for those long pin toggle switches ? they could be very useful

 

[innercityman]

Found a gut shot, it looks like the main board is one sided so it should be a breeze to rework.

They seem to be using cheap quality tube sockets, I would definitely check those carefully.

By the way, does anybody know a source for those long pin toggle switches ? they could be very useful

The unit is more than 20 years old. I've already replaced tube sockets few months ago. Also replaced few electrolytics, diode bridges, power resistor... I gave it back to the client and it was working as it should.

Check all the jumpers I had to put because of delaminated solder pads...