Well well well, thanks for the great idea of constant tone tracing. I did look into this last week but figured I wouldn't have the kit to do it. I made a probe out of an old screwdriver, a 10uf 400v capacitor (nearest I had) and an old guitar cable. I downloaded a tone generator app for my iphone and plugged it in. I connected the "probe" to my old Park 10w practice amp, not my 800w Line 6 L2T and started playing.

I found that the signal seemed quiet at the speaker out and phones out without too much bother but not amplified at all. I guessed this meant that it was further back in the circuit. Everywhere I probed sounded the same, on the Base and Emitters of T5 and T6 then back further into T1 & T2, I checked at R51 which is the first component after the "To The Poweramp" section change.

I knew that the signal was good until the FX Send as it sounded good at that point so I looked between the FX Return and the "To Poweramp" C16 still sounded clear and the same at Pin 3 of IC5 (op amp chip MC33078P). Changing the voltage on Clean and Warp channels to this point sounded nice.
Probing Pin 1 of IC5 showed up a quieter signal, weird. I turned up the volume and found that it was quiet and then crackling and noisy as per the output on the phones and speaker out.

Checking pins 8 and 4 for voltage gives me a 16.8v on 8 and -15.5 on 4 which is in the operating voltages for this IC. (+- 18v)
Checked Resistors R35 and R36 both read 10k so it might not be a grounding issue but actually an issue with the IC itself.

I went and checked the same IC5 which has pins 5,6 &7 connected for the op amp after Channel 2's Treble and Presence Pots, signal sounded good on all three pins so I think that it's definitely the IC internals that are used for pins 2, 3 and 1

Now I've ordered a replacement for this IC (and two spares), I can't help wonder if it'll blow again once I put it back.
Can you see a reason why T8 out stops Hum if the op amp in IC5  is knackered?
Does the problem in IC5 mean that it's the issue and not related to T8, remember taking out T8 stopped a loud hum but didn't alter main issue of signal breaking through at high gain.



Thanks again for all of your help with this, I think we've nearly got this cracked.

Hmm, I've just re-soldered every single connection on the board, there doesn't look to be any cracked tracks that I can see anywhere across the board.
The only components I've not changed in the poweramp stage are T1 and T2 but they don't appear to have too much wrong with them looking at the voltages.

I've plugged a battery powered radio into the input of the amp and I can hear it faintly through the phones. When I turn it up and lift gain and master over halfway it crackles so the louder bass notes trigger the amplification but the quieter notes stay quiet. It feels like something isn't getting enough signal to start working but I don't know what or where...

If I max out the volume on the radio and then lift the gain and master up on channel two it amplifies everything but it's obviously mega distorted and way too much for the headphones. I can't plug it into the 4x12 as it's midnight here and my wife would be rearranging my genitalia if I started making loud noises like that. It's the same as the other day though when the same effects were witnessed on the head into the headphones and then head into the 4x12.

Cool stuff, Canada eh? Thanks for taking the time you guys to help me out.
I've rechecked voltages on T3 with amp on and no input, double and triple checked pinouts.
T3 Emitter > Base was 511mv (.511v)
E>C was 41.3v
C>B was 40.9v
This sounds like the T3 was behaving normally.

T2 Base = 0.006v, Collector 44v, Emitter -600mv

With the headphones in it breaks through at higher signals (higher gain & volume)
With the 4x12 Cabinet plugged in it also does the same thing.

With the talk of dry joints and other broken joints I'm just going to heat up every single connection on the board to ensure there are none left.

The pinouts I've been working from have been scavenged from the web and are as follows:


Thanks again.

Thanks Phil I'll take a look later.
In terms of output the sound is the same through the headphones or the speaker cab which is a seperate 4x12.
I'll take a look when I finish work and get home. Thanks for the help.

Cool. I'll get on those further tests when I get home from work.
With regards to whether readings were taken in circuit and amps on/off
The amp on was t3 e-b .53
The other voltages in that post are t3's readings in circuit and amp off.
I'll also inspect the tracks from output backwards to see if I can spot a crack or dry joint
Thanks all for your help.
Whereabouts are you guys based?

Phil,
I rechecked the voltages around T3 and there must have been an issue with me not connecting correctly. The Collector was 2.5v with Base and Emitter both at 44 v

g1, I measure the voltages on T3 Emitter-Base whilst the amp was on and it was about right, at 0.530v.
I did the diode check on the DMM on T3 too to see if that identified anything, there was an oddity in the Collector to Emitter of 1.75v (whilst amp was switched OFF)
The rest were (- means black lead and + means red lead on my Fluke 79III)
B- > C+ =0.53v
B- > E+ = 0.53v
B+ > C- = OL
B+ > E- =OL
C+ > E- = 0.53v
C- > E+ = 1.75v

I also rechecked T5 and T6, traced the circuit to make sure I was right on the E,C and B.
The measurements with the amp on are consistent with the above post apart from the  emitter on T6 should have been -1.1v (you're spot on), with the amp off E>B drop of each is 1.1v

There's 30mv DC on the speaker output.

This has me foxed....

No I left it out and took voltage readings on the connections where it should be.
Not putting it back until the main issue is resolved.
Don't know what the next move is though.
Thanks all, for your advice and guidance.

Hey,
That voltage was indeed a bit weird. I've not got confused with the BCE pinout but the voltages are different than I measured the other day.
T3 Base is 44v, Collector at 44v and Emitter at 2.5v
Not sure why that happened unless I hadn't grounded the meter lead properly or the probe was not touching the solder.
Doesn't help though really does it I guess?
This is how it stands now.

Thanks Jazz. I'll double check it after work to make sure I've not got my pin outs wrong. I was working on the underside of the board and getting confused as to which was e,c or b. I think I got it right but will double check.

The resistor I've had out of circuit and it read ok but I'll chexk it again too.

I'll update later today. Thanks again.
Bob

g1, static DC voltage checks, not sure if it's what you meant but I switched on the amp with nothing in the input and measured DC voltage at each base, collector and emitter to ground.
The results are attached in the PDF with the voltages written next to each leg of the transistor.
I've stuck the img in here too to save opening up the pdf.

Thanks again, Bob

g1, you're a star for helping me through this. Thanks.
With the guitar into the FX Return, I can hear it, but it's very faint.
When I plug into the input and use clean channel I can hear the guitar build up volume as I turn up volume but it's still quieter than I imagine it should be.
Once the volume pot goes so far, the drive of the string picks jumps in volume to something really loud and distorted and clippy. When I come back down the volume it goes from noisy, clippy and distorted to clean and quiet.
I've added a sound clip of a mic in my headphones, I can't do it from the 4x12 cab as the sudden burst of power makes the whole family jump, even if they're in the other rooms!!

I'm not sure what else it can be, almost every other component in the Poweramp section has been taken out and tested or tested in place. I'e also attached a photo of the circuit that I've been using to identify stuff and everything highlighted has been tested to the best of my ability.

Thanks for your help.
Bob

Cool, Thanks g1. I've removed the jfet T8 and now the hum has stopped but the plucking a string gives a loud sound and is then cut off almost immediately. It's the same on both channels, if I do it on channel 2 and give the low E a pluck it is deafening and then instantly cuts off to a lower level of sound.
Should I get another JFET to go in there or is it something like R45, C37, R44, D24 or R43 which I have already checked and all appear to be ok?
Thanks again for all of your assistance.
Bob

Ok, so I had the jfet out, T8 (BF245B) and I did the 2 multimeter test on it.
one multimeter across Source and Drain on Resistance showing 4-6 Mohm
other on Diode setting across the Gate and Source to turn on the channel, it dropped to a few ohms, not 0 ohms but a few.
with the second dmm reverse polarity and it didn't turn on the channel, just stayed as it was, 4-6Mohm

I also had D24 IN4007 Diode out and tested that with diode checker, it checked out fine..

Then I checked C37 as I hadn't tested that, that checked out too. 4.7 uf

Only thing I haven't changed, but I do have some are the BC5468's T1 and T2, worth a try?

Thanks

Thanks Tony, I'll give that a go this evening and report back.
From my understanding T8 is a JFET Transistor so I'll google how to test one of those with my DMM.
Thanks again.

G1, thanks for the fast reply.
In truth, and ashamed to admit it but I didn't realise they were different (i.e. TIP147T)  until after R66 went. After replacing R66 and refitting the original TIP142 and TIP 147 it's gone back to the original state.

I put the guitar into the FX Return and heard nothing but humming. If I put my iphone on max volume into FX Return using a cable and 3.5mm to 1/4 jack I can just about hear the music but it doesn't alter with Master Vol ( i didn't expect it to either)

I've attached a sound file recorded from my headphones by my iphone so it's not great quality but it should give you an idea of the sounds it's making.

It starts with Chan 2, max gain and volume producing humming and crackling interference, with guitar being played on. then second half is clean channel with high e string and how it cuts off, then back to chan 2 again at the end.

It sounds something like there's not enough signal to drive the poweramp stage unless you turn guitar all way up and hit that string as hard as you can, then when the string slows the sound cuts off.