Help me fix a vintage "808" - No output ! FIXED !!!

[MartyMart]

Building/de-bugging seems to come easier than "fault finding" for me, so I could use
some help/advice.
I have an original TS-808 to look at, which works in bypass but has zero level/control
when engaged...... just silence !
Here's what I know so far :
It's un-modified and looks to have never been touched inside.
LED lights up and power is reaching the board.
There are no broken/missing wires.
There is no evidence of "burned out" components or any cap residue
Op-amp voltages look about "right" with a 9.1 volt battery:
pin 8 - 8.98 volts  pin 4 - 0 volts the rest approx 7.3 - 8.3 volts

So where to look first ?
Switching fets perhaps gone bad and not "engaging" the effect ? ( 2SK30A's )
Checking the trannies voltages ( 2SC1815's )
Is it worth changing the electro's as its 26 years old ?

Thanks in advance for any tips/common problems or known issues

Marty.

[Steben]

Hey Marty,

have you probed yet (bypassing the fet's)?

Are you sure the IC voltages are about "right"?

The inputs should be around 1/2 Vss no?

[MartyMart]

Thanks for the quick reply, that would be pin 3 and pin 6 right, in that case they are too high:
Pin 3 7.32 volts
Pin 6 8.31 volts   !!!

M.

[MartyMart]

The two switching fets (K44 DOK ? ) source and drain only change by 1 volt when
engaged/bypassed ?
All read 5.8 volts engaged and 4.8 volts when bypassed
Gates are 0 volts in both states.
This seems wrong to me, voltage drop should be bigger here ?

Marty.

[brett]

Hi.
There's something quite wrong with those FET voltages.
IC voltages - pin 8 = 9V (supply), pin 4 = 0V (gnd), all others should be 4.5V

IMO, the best way to de-bug is to use an audio probe to follow the signal through to the fault. 
All you need is a high-voltage capacitor for input, a simple 386 amp like the Ruby, and a cheap speaker.  Cost $5 to $10.  Probably saved me 20 to 50 hours.
cheers

[Melanhead]

Hi.
There's something quite wrong with those FET voltages.
IC voltages - pin 8 = 9V (supply), pin 4 = 0V (gnd), all others should be 4.5V

IMO, the best way to de-bug is to use an audio probe to follow the signal through to the fault. 
All you need is a high-voltage capacitor for input, a simple 386 amp like the Ruby, and a cheap speaker.  Cost $5 to $10.  Probably saved me 20 to 50 hours.
cheers

That's what I do too ... follow the signal 'till it dies. That'll give you a good idea of what section of the pedal is the problem ... Visual sometimes works too... I had a TS9 that had no low output and one of the pots had turned and a lug was shorting against the casing of the pot beside it ... easy fix.

[R.G.]

With the opamp pin voltages that high, something is wrong with the reference voltage being fed to the opamp, or the opamp is shot. Check the voltages on the input and output buffer transistors and see if they're also being fed too high a reference.

Check the reference voltage and get it right first.

[MartyMart]

Here's the voltages for both I/O buffer trannies
C 3.97
B 9.0
E 4.71       !!!!!
Houston I think we have a problem !
Looks like perhaps 1/2 supply is shot !

Marty.

[analogmike]

I have found a few with a bad 4.5V power filter cap, check that one out.

[MartyMart]

Thanks for the replies chaps, so here's where I am :
9v comes into the circuit and hits the reverse protection diode and 100uf cap.
Then goes straight to Input buffer collector and pin 8 of the op amp.
Then follows the VR setup of 2x10k resistors and a "new" 47uf cap !
VR measures 6.08 volts !
9v goes on to output buffer collector and the diode/resistor for the LED.
Should I change the 10k's at VR for a new 1% metal film pair ?
I had my pinouts on the buffer voltages wrong earlier, so  it's :
C 9 v
B 3.79
E 4.71

Thanks,
Marty.

[TheBigMan]

Sounds like a good place to start.  If Vref isn't right then it'll definitely affect the flip-flop.

[MartyMart]

SHOOT !!
Changed 'em and I still have 6.07 volts VR   ggrrrr 

M.

[johngreene]

The Base should not be lower in voltage that the Emitters on the buffers for one. It's possible that you have just swapped the readings for the Base and Emitter. The transistors in the Tubescreamer are BEC not EBC.

Chances are that whatever is wrong is pushing back onto the Vr causing it to go up to 6 Volts. I'd start removing the connections to the Vr until it returns to 4.5V. So lift one end of the 10Ks going to the input and output buffers, the 510K going to the opamp, etc. measuring the voltage each time.

--john

[gaussmarkov]

The Base should not be lower in voltage that the Emitters on the buffers for one. It's possible that you have just swapped the readings for the Base and Emitter. The transistors in the Tubescreamer are BEC not EBC.

ok, but even with that correction Vbe is too big.  the usual range is .5-.7V.  marty has .92V.

[johngreene]

The Base should not be lower in voltage that the Emitters on the buffers for one. It's possible that you have just swapped the readings for the Base and Emitter. The transistors in the Tubescreamer are BEC not EBC.

ok, but even with that correction Vbe is too big.  the usual range is .5-.7V.  marty has .92V.

True, but since both are the same and only .22V off from each other but centered close to where they should be, I'd be inclined to dismiss it to measurement error and concentrate on the 1.5V difference being seen on Vr. If it was only one of the two, I'd think otherwise. Anyway, by removing the bias resistor from these two devices you will have eliminated them as the cause if Vr is still 6+V.

--john

[gaussmarkov]

True, but since both are the same and only .22V off from each other but centered close to where they should be, I'd be inclined to dismiss it to measurement error and concentrate on the 1.5V difference being seen on Vr. If it was only one of the two, I'd think otherwise. Anyway, by removing the bias resistor from these two devices you will have eliminated them as the cause if Vr is still 6+V.

sorry, i wasn't objecting to your strategy of straightening out the voltage divider first.  that makes complete sense.

[johngreene]

True, but since both are the same and only .22V off from each other but centered close to where they should be, I'd be inclined to dismiss it to measurement error and concentrate on the 1.5V difference being seen on Vr. If it was only one of the two, I'd think otherwise. Anyway, by removing the bias resistor from these two devices you will have eliminated them as the cause if Vr is still 6+V.

sorry, i wasn't objecting to your strategy of straightening out the voltage divider first.  that makes complete sense.

No offense taken, I was just explaining why I didn't pay too much attention to what you noticed. 

It may or may not be important later, but is definitely worth noting.

--john

[MartyMart]

Forgive me for not following this, so its possible that I have voltage "getting in" to the VR line
even though VR is created imediately following the 9v input/buffer&ic pin8 ?
It's bleeding back through from further up the circuit ?
May this come from the op-amp or another "failed" component which connects to 9v ?

Thanks,
Marty.

[gaussmarkov]

Forgive me for not following this, so its possible that I have voltage "getting in" to the VR line
even though VR is created imediately following the 9v input/buffer&ic pin8 ?
It's bleeding back through from further up the circuit ?
May this come from the op-amp or another "failed" component which connects to 9v ?

i guess that's right.  here's an example that i found on the internet and i ran in superspice.  the basic problem is that the voltage divider only works when the current is the same in the two resistors (R1 and R2 for this example).  here's one way you can mess that up:

here's my source.

[R.G.]

so its possible that I have voltage "getting in" to the VR line even though VR is created imediately following the 9v input/buffer&ic pin8 ?
It's bleeding back through from further up the circuit ? May this come from the op-amp or another "failed" component which connects to 9v ?

And that's good for a free cigar!

Yes, anything that touches the VR line can bleed current back into it. Example: the buffer transistors; they have collectors to +9V, bases connected to Vr through resistors. What happens if that transistors shorts collector to base? It puts 9V on the base and feeds current back through the bias resistor to the Vr generator. One side - or both - of the opamp may be internally hosed up and feeding current back. Or there could be a solder thread on the board shorting something random to the VR line.

The difference between the builder and fixer mentality is the builder thinks about how it could work if all the parts are good. The fixer needs to think about how it would work if one or two things were bad in a number of possible ways. You're learning about fixing things now.

The approach to take is that any one (more than one is an advanced class) of the parts on the board, including the board itself may be faulty. Then you take measurements and guess from the differences in what you measure versus what it ought to be what might be faulty. Then you do measurements and experimental tests to subdivide the circuit down into smaller and smaller areas where the fault could lurk until you find the bad one.

F'rinstance: you have a circuit that generates a reference voltage from two resistors across 9V and ground, smooths it with a capacitor and then sends that voltage through resistors to four or five places. Could be the reference itself that's bad - if one of the resistors is open, the voltage heads for the opposite extreme. If the cap is shorted, it pulls Vr to ground. If Vr is too HIGH, then either the upper resistor is too low in value (and we know from experience that resistors very, very rarely do that) or the lower resistor is too high in value (that's common) or there is something feeding current back into the Vr generator. Replacing both resistors eliminates them as a cause (if and only if the fix did not introduce a new problem, which I do to myself inadvertenly sometimes) so by elimination, something else is feeding current back in.

My next step would be to measure the voltages across all of the resistors leading away from the Vr. One of them logically should have a higher voltage away from Vr than nearer Vr, since current, like water, always seeks a lower level. If you find one resistor that's positive the wrong way, it's a fluorescent pointer that somethign is wrong on the other end of that resistor.