Preamp Studio Projects VTB-1 - No signal, only hum.

[stonerbox]

Update!


Not finished but I have marked the 48volt affected areas with red lines along the sides. Maybe you'll see something I don't.

[tempus]

I think we need to start by looking at the power supply. How is this thing powered? If it's an AC adaptor, did you check it to make sure that it's putting out its rated voltage? The fact that you don't have enough voltage to power the 7812 is not a minor issue.
Also, on the top left of the page you've got 12v hitting a 1000uF cap - this must be a typo, since caps block DC and if it were actually wired like this, there would be no DC voltage past that point.
You mentioned the red lines - are you measuring 48v at these points? Is Q8 also hot?

[stonerbox]

I thought that I had posted about it earlier, but I guess I missed it?? I checked the power supply some days ago and everything seems to be fine, 12+ (Acv) like it should. I'll look into the 7812 power issue tonight.

Believe it or not but the 1000uF is correct, but I missed drawing a line between +12v to D9 negative side.
Another question, what are the resistor-like yellow capacitors? I can't find any numbers or letters on them at all.

Yes, all the areas marked with red reads 48v. Q8 is not hot but the resistor connected to its emitter leg is very hot (R53)

Updated "beta" layout here:


Also, thank you so much for helping me out Tempus, without your support I would have probably given up.  

[tempus]

np - hopefully we can get it sorted out.

So the 12v is AC - I kind of suspected that, and it makes more sense now. Caps don't block AC, and I think we can assume that the 4004 diode network is rectifying the AC power to DC. Not sure what to make of the little caps with no markings.

[stonerbox]

Yeah, let's hope for the best and prepair for the..  

AC to DC convering is probably the case. I read AC on some of the 1N4004 diodes but mostly DC. The power also gradually increases from 12v to 48v throughout the diodes.


I found some new info on this circuit. The tube is supposed to be supplied with 90v and right now I only got 48v going to it. Could it be that one of the 1N40004 is broken, if so how can I check this?
http://clatrofic.blogspot.se/2013/10/vtb1-tube-circuit-does-not-apply.html

[duck_arse]

looking at your board and because they have no marked value, I would guess those yellow caps were 100nF or so monolithic ceramics as bypasses. I might be wrong though.

have you checked if Q8 is good, and not short circuit? what voltages do you read on it?

[stonerbox]

Thanks for the help on those mysterious caps, cause I don't have a slightest clue what they could be.  

Q8
Collector: 46+dcv
Base: 13.5+dcv
Emitter:46+dcv

Is it safe to change Q8 or is a new 3904 just gonna crash and burn too?

[tempus]

So the diode network is likely a voltage multiplication/rectifier circuit. You can read more about them here: http://www.allaboutcircuits.com/vol_3/chpt_3/8.html. If you want to check any of the diodes, you could desolder one leg of any diode and check it with the diode check on your meter. Not really knowing how the rest of Q8 is connected, it's hard to say if it's shorted, but you could desolder it and check it with your meter as well before trying a new 3904.

[stonerbox]

Thanks for the link on the voltage multiplication, I will check it out and maybe learn one or two things about them.


I've checked the diodes and they all seem to be just fine. I changed the 3904(Q8) for a new one and the voltages look much better now, also the resistor connected to the emitter leg no longer gets hot as a MF. All good there I guess. Still no signal out though, I have to tap into one of the op amps to pick up the signal (except op amp U5 which is not receiving any signal at all). The hum is also still present. The voltage to the tube is still too low (48v) should be feed around 90v, as I posted earlier.  Damn, this is a hard nut to crack..

The entire power part is finished in the current layout but I will head back and continue layout making. It's time to get all the tube, op amps and transistor stages ready.

[tempus]

Actually, I'd say it's going pretty good so far. Keep at it, though - when you're just kind of learning this stuff it takes a while to sort through everything, but you learn tons along the way. I'm wondering if the voltage multiplication circuit should be getting us closer to 90v to power that tube, if the article you linked is correct. In any case, be careful in there now, because the voltages are starting to get to dangerous levels. Make sure you don't touch anything with your fingers, don't where chains around your neck or bracelets, etc when you're looking at stuff.

Good news about Q8; it looks like you fixed at least part of the problem.
Do we have enough voltage on the 7812 yet?

[stonerbox]

Yeah, I guess it's harder to see the real progress when you're felling like you just been starring at the circuit board for half a week, but we are learning something here! Thanks for the heads up, 90v is best to be avoided outside of the circuit.

Solving the Q8 values was a good first step in the right direction. Now the voltages seems to be correct, even though I don't really know what the correct values are. At least things aren't getting super hot any more in that particular area off the board.

Q8
Collector: 48v
Base: 19.2v
Emitter: 19v


The L7812 is still pretty much unchanged.
In: 11.1v
Ground: 0v
Out: 8.7v

[Gus]

Have you looked for a burned PCB trace?

[stonerbox]

Gus, I've not found anything that looked burnt, not yet anyway. I'll keep my eyes peeled.

Here is another update on the layout. Some new voltage readings and corrected connections in the power part (the diodes) and tube pins. Voltages on REG 1, 2 and 3 does look very suspicious, but then again, almost everything else does so too.

[tempus]

These voltage readings are telling.
1st, the 7805 and 7905 are + and -5v regulators, respectively. If working correctly, the 7805, with 12v on its input, should have 5v on its output. There is no + or - voltage pins on the 7905, it should have a -in, gnd, and -out. According to your diagram, the +in is connected to ground, but you've got it reading -12.5v, with your -v out at 0v. There's either something wrong with the pinout on the diagram or something seriously wrong with that regulator (probably the former). Can you verify this?
Also, the TIP122 has all 3 pins at the same voltage, which is suspect. I would check to see if it's shorted by testing the transistor. Note the 100v caps connected to 2 of the legs of the TIP, which may point to what should be the 90v for the tube - although I'm not sure about that, as I don't know a whole lot about tubes.
The TIP with the zener connected may be functioning as a high power voltage regulator.

[duck_arse]

what are the ratings for the power supply for this unit? I'm not sure you should be looking for 90V, I think the TIP is regulating your 48V line with the zener, and I can't see anything that might be working that up to 90. and the 48V from the TIP goes straight to the valve.

[stonerbox]

Tempus you where right, the 7905 "+In" was a typo and the connection was also wrong, sorry. I have updated the "beta" layout above with correct names and connections now.

The values on the TIP does look strange, I will do a check with my mulitmeter (oldschool analog) if I am able.

Duck_arse,I keep checking it from time to time and the power supply is steady 12+acv.

[tempus]

duck - there has to be something wrong with the schematic we're seeing. Stoner - could you double check this? If C32 is actually where it is in the schem, there would be no voltage at all on the TIP, since it would be blocking the 48v coming from D5. There are 7 diodes shown, so it's possible that the DC is getting multiplied that high, although it is hard to tell if they're all being used for multiplication. Keep in mind that 12vAC is almost 17vDC, and if it went through a voltage multiplication of only 6, there would be 102vDC after the last diode. That combined with the unlikelihood of the company overrating the caps at 100v near the TIP lead me to believe that the tube is running at more than 48v.

All this being said, though, we've already ascertained that the problem is something to do with the phantom power, so we really need to be focusing on the power supply section in general, at least until we get that resolved. Have you measured the phantom voltage (or tried to talk through a condenser) since you replaced Q8?

I would also be checking the caps for shorts, etc.

[stonerbox]

The C32 (should be C23, now updated above) are correctly connected on the layout. It got 24+v on the negative side and 48+v on the positive side - which is coming from D7 (negative).

I tried both the DI input and with my condenser. I get signal but just like before only via tapping into U6, U1 or U2 (not yet visible on the layout) and with a lot of hum.

[tempus]

I'm not sure I'm understanding - does the DI work all the way to the output, or just up to the opamps you mentioned?

[stonerbox]

Just up to the op amps I mentioned.