Fender Frontman 212R major issues- PLEASE HELP!

[ggjgjhkihkjhiuyiy8788]

Hi there. I have a Fender Frontman 212R that was humming when nothing was plugged into either input or the power amp in (FX return). Otherwise, it worked fine and the guitar signal was as it should be. Here's a schematic for reference:
Google Docs

While I was testing transistors in the 'Mute' section (Q6,7,8), I accidentally shorted the legs of either Q8 or Q10 (next to each other). It was only momentary, but now the amp hums all the time and the guitar sounds fizzy and distorted through the output. I tried replacing all of Q8-Q13 as a shotgun approach, but no luck. Since the hum is really loud through the 2x12" speakers (4 ohm total), I connected a 10ohm dummy load while testing voltages. However, the dummy load gets very hot, suggesting to me there's a LOT of current running through it. This same dummy load takes the output of a 100W tube amp without getting warm, so I'm thinking this is abnormal.

By shorting legs of Q8 or Q10, what components could have been fried? I don't have parts for Q14-21 so I'm really hoping it's not in there! The hum seems to be worse after replacing only Q13- the original tested bad for hFe. Perhaps the replacement is incorrect, although I'm using the equivalent, according to NTE (2SC2383>NTE31: http://nte01.nteinc.com/nte/NTExRefSemiProd.nsf/).

I'm up sh*t creek with my inexperience with solid state- hoping someone can throw me a paddle!

Thanks,
Jaret

[ggjgjhkihkjhiuyiy8788]

Here's the schematic right-way up:
Google Docs

[Enzo]

Get rid of the load - no speaker and no resistor dummy. Your amp is putting DC on the output - check with a voltmeter. Do not put a load on the amp until the DC is gone from its output.

Q18-21? Like it or not, check them for shorted condition.

Please find something other than NTE for parts. When you cross a 2SC2383 to NTE, you are not getting a 2SC2383, you are getting one of their parts that they decided is reasonably close to the original part. MCM has the real part for 29 cents.

When you are testing transistors, you can check for shorted while still on the board - if they seem shorted, remove and retest off the board. But if you are testing for gain or hfe, you really shouldn;t do that on the board, if you did, that might explain a low reading.

While transistor gain can be an issue, I generally find that simple junction tests with my meter weed out most bad ones.

[ggjgjhkihkjhiuyiy8788]

Thanks for your help, Enzo. I was going with NTE because I'm in Australia and the only company I could find that supplied the necessary transistors used NTE (non) equivalents. I'm checking to see if MCM would ship to Australia (they ship to UK, so I don't see why not). We have Farnell here (which distributes MCM) but they don't have the components I need.

I've been checking the transistors with the diode tester on my meter and Q18-21 seem fine, although the readings were a bit odd since they form 2 darlington pairs (read somewhere that this complicates things?). No dead shorts for sure, though. The original Q8-13 are back in there, and they actually test fine (with diode checker), although Q13 tested very low for hFe when I had it out of circuit. Q22-26 seem strange on the other hand- get .5V from base to one leg (on NPNs), but >2 ("OL") to the other (sorry- can't remember which was emitter and collector). These are all preamp parts, so I'm thinking they wouldn't be giving DC on the output, right? Q6+7 seem bad, too, which may have been causing the original hum I started out with.

What else could be causing the DC on the output? Bad caps? And is it safe to pull transistors and power up without them in place (like pulling tubes on a valve amp)?

[Enzo]

No, you can't power up with missing transistors - other than the redundant output stages. The whole thing is DC coupled and totally fed back upon itself. One big loop.

WHy not add your location to your profile?

Q22-26 are not in the signal path at all, they are the switching control circuits. They are not making DC outputs on your amp.

Q8 has to be on for the amp circuit to function. Q6,7 do that. Any residual voltage in the circuit will confuse your meter.

[gbono]

RS Australia | World Leading Distributor of Electronics, Electromechanical and Industrial Components

I've used these folks when I was in the UK.........they have an Oz distribution channel too.

[ggjgjhkihkjhiuyiy8788]

Thanks- I'll check RS when I know what I need.

I'm measuring about -350mVDC on the +ve speaker terminal, which I've traced to the base of Q17. The 15K resistor off the -42V rail looks a little dodgy. I ran out of time today, but it'll be first on my hit list tomorrow, along with the corresponding R on the +42V side. That and the Base-Emitter cap (C54). I re-checked Q6-21 using the diode fxn and all seem fine except (again) Q6+7.

[Enzo]

If it is under half a volt, put the load on it and see if the offset disappears. When I say get rid of DC on the output, I am more concerned with a bunch of volts than I am a fraction of a volt.

[ggjgjhkihkjhiuyiy8788]

Right. I think the massive DC on the output was a result of using the NTE sub for Q13, because the massive hum on the output speaker has disappeared since I put all the original transistors back in. With the speakers connected, the hum is now gone and I only measure about 100mVDC on the output.

Now, the original hum I started with is gone, but the output is dampened and distorted. If you really nail the guitar strings, the sound comes through distorted and fizzy and quickly dies away (on both inputs, both channels and when direct into the Power amp in jack). Could this be something as simple as a bad ground or does it sound like a bad transistor? I've checked nearly all the grounds plus a lot of resistors and diodes in the power amp section and all seem fine, tho. The speakers are fine, BTW- I tested them on another amp. I'm running out of things to test here!

I'm wishing I had a 'scope to trace the signal path!

[Enzo]

Let the amp sit there running a while. Stay with it. We want to find out if it gets hot. It is normal to heat up when you play it hard, but does it get pretty hot just idling I am concerned there may be oscillation way above audio. R113 22 ohm 2 watt. Over on the right, at the output. Make sure that resistor is not open.


yes, a scope would be the thing to be using.

Make a signal tracer. Google up "signal tracer". Ther are commercial units, but really all it is is a probe - like a scope probe - connected to the input of some little amplifier. There will be a cap in the probe to block any DC from the little amp. Now you can probe circuit points and listen to what the signal sounds like there.

And for that matter, google "RF probe", it is another very simple accessory for your meter. It is made of a diode and a cap more or less. You could tack one together with a coupple parts just for one test. You could then measure the output of the amp and if there is hi freq oscillation that was beyond the response of your meter, this little probe will detect it.

[ggjgjhkihkjhiuyiy8788]

Ok- R113 is definitely okay and C58 has a good ground.

With the amp on and no signal through it, R119, R143, R144 and R145 all get pretty hot. R114, R115, R109 and R112 all stay fairly cool or just warm.

Are C71 and C72 the main filter caps (4700uF) and would a leak in one of them cause the fizziness I'm experiencing?

Thanks for the ongoing advice! I really appreciate it.