Tube Screamer TS-9 Repair

[swamphorn]

Hi forum,

I apologize if this is the wrong board to post to. I am trying to repair an Ibanez TS-9 which has an issue with insufficient gain. The Drive knob has little effect, not even boosting enough to clip. The Volume knob only goes down to unity gain. Only the Tone knob operates properly. I initially suspected that the op amp had burned out (the pedal developed these problems after having been left on for several hours) so I replaced it to little avail.

I have a few ideas as to what could cause these issues.

• The clipping diodes have faulted to a short
• The switching system is allowing the dry signal through

Are there any further diagnostic steps that would help in knowing how to repair the pedal?

[antonis]

<The Drive knob has little effect, not even boosting enough to clip>

In case of shorted diode(s) (or NFB cap), Drive knob should have NO effect at all..

Easiest point to check is for continuity between grounded leg of gain resistor & GND..
(followed by checking between R4 right leg & C3 left leg and/or C3 right leg & U1 pin2..)
You'll look for "something" substancially raising total resistance from inverting input to ground..
(provided, of course, your Distortion pot works by its own well..)

If the above are OK, you'll proceed to NFB loop for exactly the opposite..
(for sustancial domination of P1 + R6 resistance..)

[swamphorn]

I haven't found any continuity issues. I believe I know what the issue is. On many TS schematics (this one seems to match mine with the 2SK30A FETs) the volume pot and negative feedback go to Vref, not ground. If the Vref filter capacitor were dead it would have high impedance at AC which also explains why the volume pot doesn't go down to zero. I'm going to try placing a capacitor in parallel with the Vref cap (it doesn't seem to have shorted--Vref remains steady at 4.5V).



Edit:

It was indeed the 47uF Vref filtering capacitor that was the problem; it was a 6.3V-rated capacitor so it was running rather close to its limit. Thank you antonis for your tip to check resistance to ground; in this case it isn't DC resistance that was the issue but AC resistance. This explains why the gain was so limited and why the volume control wasn't going all the way down. I replaced the faulty part with a 47uF 16V electrolytic and replaced the 100uF power supply capacitor for good measure.

[tubegeek]

swamphorn: Thank you for circling back to tell us what WORKED - that doesn't always happen, and it's really good when people do.

[Rob Strand]

On many TS schematics (this one seems to match mine with the 2SK30A FETs) the volume pot and negative feedback go to Vref, not ground.

Back in the day (maybe 2003?) Steve and I made an effort to get the exact schematics of a few classic pedals.  Haven't seen him here for ages,  hope he is still going OK.

[antonis]

Thank you antonis for your tip to check resistance to ground; in this case it isn't DC resistance that was the issue but AC resistance. This explains why the gain was so limited and why the volume control wasn't going all the way down. I replaced the faulty part with a 47uF 16V electrolytic and replaced the 100uF power supply capacitor for good measure.

Glad to hear it.. 

Although I have an argument for another 5k (10k//10k) resistance set in series to 4k7 could lower gain so much..
(after all, it just halves specific stage gain.. - you should still have a gain of about 35dB.. )
Unless, of course, Vref is set by much higher than 10k/10k value voltage divider..

[swamphorn]

Another telltale sign was that I was seeing a large amount of signal on the Vref line. In providing a low AC impedance the Vref capacitor is supposed to shunt AC to ground, essentially isolating Vref connections from each other. With the signal being coupled to Vref before and after the gain stage a complex feedback network is present which just so happens to neuter the gain stage.