Vox Venue SS Amp - DC on output

[ggjgjhkihkjhiuyiy8788]

Hi there,

As usual, I've been totally stumped by an issue with a solid state Vox Venue Lead 30. Here's the schematic 1st off:
Vox Venue Lead 30.pdf

It came to me with the outputs blown (2x 40871's, 1xBC300, 1xBC303) and the BC550 connected to those had cracked from excessive heat and was testing shorted. The sole BC212 tested bad as well. I replaced all 5 of the BC550's and the BC212 but the rest are out of production so the best equivalents I could find were BD139 (for the BC300) and BD140 (for the BC303) and TIP41C's for outputs, which the schematic says are acceptable alternatives. While I was waiting on parts, I tested all the resistors in the output section- everything tested fine but I replaced the 0.5R, 680R, 68R, 100R, 1K5 and 1K2 resistors just to be safe. The 2 serial diodes are testing fine (diode test on meter) and the ground connections are okay.

So after replacing pretty much all of the resistors and transistors in the output section, I fired it up with a lightbulb test and it wasn't drawing excessive current so I began VDC measurements. The rails are both reading about right, if not a little high: +/-34V but there's +21VDC on the output with no load connected. Measuring the V_ec of the outputs shows that the upper one is dropping 10.5V while the lower is dropping 53V. Likewise for the BD139 and BD140 (same drop on each). So I reckon the lower TIP41C is conducting too much or the upper one not enough. Does this reasoning make sense?

Any ideas on why the huge voltage on the output? +21V seems like a lot so I suspect one of my 'substitutes' is not a true equivalent. I matched them for Hfe and maximum voltage, current and wattage ratings, but I'm not familiar enough with transistors to see if there's another parameter which is causing the problem. Instead of the BD139 and BD140, I also tried a BC462 and a 2N1893 (although the latter is not a great match for Hfe, max V, Ic or Pd) but regardless- I had the same result: +21VDC on the output.

Since I don't have any original voltages to go on (other than the rails), I'm not sure how to carry on troubleshooting. Any help would be greatly appreciated, please!

Thanks in advance,
Jaret

[Enzo]

Transistors are controlled by their bases. Look at the output transistor bases. I'd wager they BOTH sit at around +20vDC.

So you tested everything from the 1k2 and 1k5 resistors and to the right, now move one step to the left and check the two transistors there, and make sure those 68 and 100 ohm guys are doing their jobs.

[ggjgjhkihkjhiuyiy8788]

Thanks, Enzo.

Both the BC212 and the opposite BC550 test fine. The 68 ohm has +ve voltage on both sides, likewise for -ve voltage on the 100 ohm and I verified this again at the emitters of both transistors. The base of the 212 has +ve DC. The base of the 550 also has -ve voltage on it, but the collector has +21V. The 'middle' BC550 on the schematic has +21V on all three points, so you're right about the bases of the outputs having +21 on them, too.

All of these transistors test fine out of circuit on my diode tester. The 68 and 100 ohm resistors are okay and the connections of both 1K5 and 1K2 are good, too. I even subbed out the 1uf and 100p caps in this region with no difference.

So, I think I know where the problem is, but I still don't see how it can be happening. I'm going to check for shorted traces on the PCB next. That's all that I can think of if it's not a transistor, resistor or a capacitor fault!

Any more ideas or hints, please?

[Enzo]

Actually, open traces or connections would be more likely than shorted ones, unless you have a solder bridge somewhere.

That middle transistor with three times 21v is the bias transistor. It maintains a voltage "space" between the upper and lower driver bases. Then that vertical colume with the 68 and 100 and two transistors grabs that middle guy and shoves him up and down with the music, the output follows. That's how they work. Yours is stuck at +21. SO either the top transistor won;t let go, or the bottom transistor won;t turn on hard enough. Or there is a break in the current path through all that so the bottom half CAN'T pull the middle down to zero. Zero being the middle, if you get me.

Now they could all be working fine, and the diffy pair, the diferential pair of transistors at the powr amp input, are telling them to go offset like that. SO make sure both those transistors are OK. And the resistors around them.

You checked the two diodes in series I recall, so what voltage is across that pair now? Should be about 1 volt. So from -V to 1 volt less than -V. And check that lower left transistor right under them.

[ggjgjhkihkjhiuyiy8788]

Well, I've been thinking over the schematic in terms of what you said and since none of the components seem faulty and there's no open traces or bridged solder joints, I reckoned the problem is with something I did when I replaced things. I made sure the output replacements were all good and equivalent and then moved back to the BC212/BC500 pair. Here's the kicker- I had replaced the BC212 with a new BC212LB, assuming (in my ignorance) that the pinout was the same because it was a TL-092 package, same as the original. Nope! Checked the datasheets and the original had B-C-E on the pinouts whereas the LB has E-C-B! So the base was connected to the 68r/+32 rail, the collector back to the differential pair, and the emitter to the bias transistor. I replaced the 212LB with another one, but wired in the correct orientation and the +21VDC on the output is gone! Thanks for your help, Enzo- I wouldn't have found the mistake without your advice.

There's still about +150mV DC there and there's still some hum with the speaker connected so I'm wondering if the incorrect BC212 connection could have damaged any other transistors? Everything does test fine, it seems. I'm gonna run through the tests again and see if I can find where the remaining DC is coming from.

Otherwise, when I play through the amp, there's a background buzziness (like a faint second distorted audio signal) that I'm pretty sure is sourced in the preamp. The gain control doesn't affect its volume and the tone controls affect it a bit but not much. The gain potentiometer is jumpy, even after cleaning so I think I'll replace it first and then move downstream from there. I believe this is the original problem the customer was dealing with- he said he used to hit the amp and it would be fine but one time he whacked it too hard (!!!), which is when the output section failed. Maybe just a poor ground? The louder output signal is clean and sounds fine- it's just shadowed by this second buzzy signal that is less noticeable if you turn up the master.

[ggjgjhkihkjhiuyiy8788]

Nope, I was wrong- the fizzy background is coming from the output section somewhere and I'm betting whatever is causing the remaining DC on the output is the culprit. With the reverb turned up, if I hit a chord then damp it immediately, it's obvious that the ringing spring reverb signal is still causing the background noise. Since it still does it with the reverb on zero, it's gotta be happening at the output section.

So I've done some more tests on the output section and there's -0.6V between the differential pair emitters and +1.3V on the collectors of the BC550/BC212LB pair, so the emitter and collector of the bias transistor also has +1.3V on both. So I'm thinking that -0.6VDC is the source of my troubles. There's no DC leaking through the 1uF cap from the preamp (the base of the input transistor for the output section=0VDC) and subbing out each of those three BC550s (diff pair plus the third feedback(?) transistor) doesn't change anything. Maybe one of the 100p caps is leaking voltage from the +32V rail? Or the BC550 driving the bias transistor is messed up from my earlier basing mistake and not conducting enough to completely zero out in the middle? I'll sub out these parts and see, I guess.

To answer your question regarding the two series diodes, there is about 0.6 volts across each of them (~1.2 across both) so they seem to be doing the job.