Tube Tech mp1a issue

Hi.

I've got a TT mp1a with a issue I can't solve. It makes a clicking noise every time i change posision on any switch(watch the video). It just suddenly starting doing so. It works. The sound isn't messed up in any way. But this can't be good in the long run. Both channels have this problem.

I had a problem a couple of years ago when the sound on the line in suddenly got whery thin. The DI was fine. I checked the schematics and thought that it was the input transformer. But after I changed the transformers the problem was still there. I changed the tubes just to check and it was good to go...

The sound on the video is from the output on the TT. No sound from camera.
https://vid.me/9HwH

I would check the chassis ground conection and front panel (are you using a grounded cable isn't?).  that kind of sound is usually because a DC voltage on the switches. so check if there is any cap leaking DC. 

Rafael

Hi.

The grounding is ok. When it comes to caps leaking dc i can't find any. But I noticed another thing. When i put the hi pass on 20 Hz the noise disappears from gain 50 and above. I can't get my head around what this can be when i look at the schematic. Kind of a semi-noob at this.

http://gyraf.dk/schematics/tubetech_mp1a.gif

check DC voltage at the red point (red wire provably) and yellow. you should have 0V DC at the red point and some volts at yellow.

you could check if there is any difference with phantom power. does the phantom power work?
it wouldn't hurt to check the instrument input jack and clean it.

one of the two decks have DC and it shoulnd't that is what I think. if you pull out the first tube and the noise is still there the culprit should be the 330nF/250V cap or a weird voltage in the front plate(sometimes a lose screw can make weird noises too).
if the noise go off when you pull out the first tube that mean that the culprit is the 15u 63V or 4,7u 35V

is the gain switch working on all positions? the HPF at 20Hz add a 330nF that filter DC from the second deck.  that should stop the noise completly if the culprit is the 330nF/250V BUT! there is an electrolitic cap from the 330nF to the cathode of the first stage.  from 50 to 70 the first deck is a short circuit so there will not be a DC change so no noise.

my conclusion: the 330nF /250V is leaking and the 4,7uF 35V failed due that. so you could have DC on both decks unless the HPF is at 20hz. you should check the 3 caps, 330nF from second stage anode, the 15u/63V and 4,7u35V.

if i'm right you owe me a beer! 

..are you sure it's a click, not tube microphonics?..

Jakob E.

I don't know if this will make it clear what the problem is, but here it goes:

check DC voltage at the red point (red wire provably) and yellow. you should have 0V DC at the red point and some volts at yellow.

Click to expand...

Got 65V DC at the yellow point. At the red point there is a bit of weirdness. I don't know if i can explain it right. When I turn it on and measure I get approx 10V DC if i set the HPF to 20Hz. If i set the sw to off or 40Hz I get 0,5-0,4-0,3-0,2-0,1-0.  But when I turn back to 20Hz I don't get 10 but 1,3V DC and rising(but very slowly).  And the thing is that the noise is gone. If i turn it off for a while and try again the same thing happens.
The funny thing is that when I am not measuring the noise does not go away when flipping the HPF back and forth.

you could check if there is any difference with phantom power. does the phantom power work?
it wouldn't hurt to check the instrument input jack and clean it.

Click to expand...

Phantom and DI is OK.

one of the two decks have DC and it shoulnd't that is what I think. if you pull out the first tube and the noise is still there the culprit should be the 330nF/250V cap or a weird voltage in the front plate(sometimes a lose screw can make weird noises too).
if the noise go off when you pull out the first tube that mean that the culprit is the 15u 63V or 4,7u 35V

Click to expand...

When I remove the tube the noise goes away. No loose screws.

is the gain switch working on all positions?

Click to expand...

Yes.

if i'm right you owe me a beer! 

Click to expand...

Why not make it two!

65V at the yellow point is a problem the cap is rated for 63V.  but it seem that it hasn't failed yet.

Got 65V DC at the yellow point. At the red point there is a bit of weirdness. I don't know if i can explain it right. When I turn it on and measure I get approx 10V DC if i set the HPF to 20Hz. If i set the sw to off or 40Hz I get 0,5-0,4-0,3-0,2-0,1-0.  But when I turn back to 20Hz I don't get 10 but 1,3V DC and rising(but very slowly).  And the thing is that the noise is gone. If i turn it off for a while and try again the same thing happens.
The funny thing is that when I am not measuring the noise does not go away when flipping the HPF back and forth.

Click to expand...

that is normal, when you flip the filter switch you are dischargin the 330nF cap(hpf) and then it start charging through a ~30K resistor (deck resistors to ground) the noise will gone when the voltage is still low.

if you measure the 10VDC with a scope then there is a problem, if you measure it with a tester sometimes when there is some AC you can have some DC measures  but still 10V is high. you should have 0DC, for that low voltage I would say that the problem is the 4,7uF 35V cap. you can cut one leg of the 68K resistor and check if it solve the problem or just change the cap.

..are you sure it's a click, not tube microphonics?..

Click to expand...

the video does not sound like a microphonic tube and that does not explain why it does not produce noise when the HPF is at 20Hz.

the more the beers the better!

you can cut one leg of the 68K resistor and check if it solve the problem

Click to expand...

The noise got worse...

that is interesting, you have re checked the red and yellow voltage while the 68k was out? if the red voltage is higher then the problem was the 330n/250V. the 68k with the 4,7u 35v form a feedback network that reduce the gain to have less distortion, so if noise come from the first deck it will be more amplified when the network is out. but my guess is that the DC voltage at the red point is now higher.

can you measure voltage at the orange, red, yellow and brown wires?

cheeers
Rafael

Voltages drops to 0 on red point and 64 on yellow when i remove the resistor. 10 and 64 when the resistor is in place.


When you say orange, red, yellow and brown wires do you mean the points on the schematics or the wires to the transformer?

schematic, I haven't seen a unit inside yet.

when the resistor is out you still have noise in the filterswitch? you could still have noise in the rotary switch because of some DC in the first deck but if the DC is 0 at the red point you should't have noise in the HPF Switch.

the problem is at the first stage at this point. you need to measure the bias point of the first stage, it is out of it's normal operation. you shouldn't have 64V on a 63V cap.

the 4,7uF 35v is a tantalum cap? if yes it have sence, they fail short many times. anyway it is leaking DC and should be replaced.

can you measure at the orange point? also the b+ voltage would be useful I calculate it as 112V or something similar but better be sure.

when the resistor is out you still have noise in the filterswitch? you could still have noise in the rotary switch because of some DC in the first deck but if the DC is 0 at the red point you should't have noise in the HPF Switch.

Click to expand...

Yes, but a tad less then before. But there is no noise on the gain knob over 50 in any of the hpf position.

the problem is at the first stage at this point. you need to measure the bias point of the first stage, it is out of it's normal operation. you shouldn't have 64V on a 63V cap.

Click to expand...

Bias point pin7 is 0V, pin 2 is 62V and droping fast as I touch it.
It says 63V in the schematics, but on the cap it self it says 100V.

the 4,7uF 35v is a tantalum cap? if yes it have sence, they fail short many times. anyway it is leaking DC and should be replaced.

Click to expand...

Looks like an electrolytic.

can you measure at the orange point? also the b+ voltage would be useful I calculate it as 112V or something similar but better be sure.

Click to expand...

Orange point: 65V
B+ pin1: 196V  pin6: 181V

if there is no remanent DC on the HPF and second deck I would say that all the noise is coming from the first stage.
it have a lot more sence that the 15uF cap is 100V and not 63V.
if you had 65V at yellow point and 65 at the orange point it mean that the 15u 63V is working correctly(not leaking DC).
no noise from 50 to 70 (gain) means that the HPF and second deck is working correctly.

everything looks correct if there is no DC on the first deck, I can't say why is poping if there is no DC on it. I could speculate that the change in gain in the tube produce a diference in current of the tube for a small amount of time, that current produce a diference in voltage on the cathode and plate of the tube. the noise produced by the anode will be inversed in phase on the anode of the second stage but will be out of phase with the signal of the feedback network so the noise should be canceled in some ratio ,provably enough low to not be a problem.

you need to change that 4.7u 35V cap and resolder that 68k leg, that should solve all your problems (with this circuit al least).

if your unit is plus 10 years old maybe would be a good idea to change every electrolitic cap on it.

cheerz
Rafael

Does one of you have the schematics of the PSU from Tube Tech? wmtunate posted it many years ago. I have problems with my MP1A with a noisy phantom power.