Another TS808 Debug

[rockhorst]

Just finished a TS808, or so I thought. Basically, with all knobs fully on I get unity gain, no overdrive.

Got a verified PCB from Newtone.
Mods: Keeley mods
AMZ fat mod and asym. clipping are both on a DIP switch. Measurements with asym on and fat off.

Bypass: normal guitar signal
Engaged: normal guitar signal, slight toneshift probably due to TS buffering
Level pot works (from zero to unity)
Tone pot works
Drive pot: no effect (does go nicely from 0 to 500K)

Q1
C = 9.3V
B = 3.7V
E = 3.2V

Q2 identical to Q1

IC:
Pin 1 = 9.3V
Pin 2 = 4.7V
Pin 3 = 4.7V
Pin 4 = 4.7V
Pin 5 = 0V
Pin 6 = 4.7V
Pin 7 = 4.7V
Pin 8 = 4.7V

D1
A (anode, the non-band end) = 4.8V
K = 4.7V

D2
A = 4.44V
K = 4.7V

D3
A = 4.44V
K = 4.8V

[jkokura]

Your numbers look good. Actually, I think they're not the problem. If your major problem is in the drive pot, I would suspect that there might be a problem in the wiring of the pot, or just of the board. If there's a voltage issue, it's after the IC and Transistors. The voltage on the Diodes seem fine also.

Jacob

[MikeH]

I'd remove the drive pot and verify that it is the right value- ie, make sure you didn't use a 50K instead of a 500K

[rockhorst]

Definitely a 500K pot. I thought the voltages matched up myself, but better save then sorry. Also triple checked the wiring on the pot.

[oskar]

Either you have an open connection in the .047/4k7 net to Vb  or a short in IC1a feedback.

oskar

[rockhorst]

Hey Oskar, I'll check that out when I get home, thanks for thinking out loud.
Could that also account for the unity having unity gain? From what I know I would expect the unit to give a bit of boost with the level all open and drive closed.

Link to the document containing the schematic btw:
http://www.newtone-online.nl/downloads/NewtoneScreamer.pdf

[oskar]

Would that also account for the unity having unity gain?

I'm not sure about the gain in a soft-clipping stage as the ts-808, but it is a non-inverting amplifier.
http://en.wikipedia.org/wiki/Operational_amplifier_applications
If you short the feedback path you'll get a voltage follower seeing the .047/4k7 net to Vb as a load and
if the .047/4k7 net to Vb is open you just get a voltage follower. Booth of these cases will give you unity gain.

[rockhorst]

You were right on the money Oskar! Continuity problem between the resistor and the cap.
Only experimented with it for half an hour, but its going to the gig tonight. Right now I favor symmetrical clipping and AMZ fat mod both engaged. Then its a pretty close match with my OCD on HP, I dare say. For a little top end boost during solos on an already cranked amp, it like it better without the fat mod. I couldn't resist and changed opamps as well: JRC4558 over TL072 (didn't like that one)...And I also had to get a Burr Brown. Its very expensive compared to the other ICs, but dang...it does sound good (I feel like an audiophile now)!

Next up: tube boost...then probably a bufferless TS just for the fun of it.

Thanks a million!

[oskar]