Hughes and Kettner Warp 7 100W Head, Loud hum and no guitar sound until maxed

[batbob]

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.

[g1]

The next step will be applying a constant tone to the FX return and tracing the signal to see where it is getting lost.
Do you have something like a tone generator?  Like a phone app or computer app will work, or something like a portable keyboard with a key taped down.
And a signal tracer can be a simple practice amp with a capacitor probe to block DC (.1uF 200V or so should work)  Or your meter set to AC volts with same capacitor probe.
But before that there is one more quirky reading that needs checking.  T8 that is out of circuit right now, you show that there is -15V (or -7V) at the terminal that is supposed to be ground?  That should not be.  Is there a ground missing at the board?

[batbob]

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.

[batbob]

So, forgive my thinking on this, I might be 100% incorrect. I was worried about the fact that if I replace IC5 it might blow again.
I checked the voltages around IC5 inputs.
for earlier stage for Channel 1 treble/presence I found that the + input was .203v and the - input was .226v, output was .226v. All seems legit.

I checked the input voltages for the problematic op amp and the + input was -14.23v and the -input was -7.75v, output was -15.6v, looks dodgy to me, not sure why though.
I measured voltages back from the +input and it was -14.4 that side of C16 (a 220nf 63v cap), the other side where it went off to fx loop and R46 was 0v. I can't find why it would ground to 0v so now I suspect that C16 is knackered, when I capacitor test it with my DMM it shows as 230nf so it looks ok.
Just as a double check, I think that this is normal behaviour to block the DC but I wanted to check with you guys and girls that this looks normal.
If you think that the op amp IC replacement will work then I'm happy. Just worried about these weird voltages......

[nosaj]

To block DC and or to separate 2 different DC circuits.
jason

[Jazz P Bass]

Pin #3 is an input pin.

It can't possibly have +14 Vdc on it as shown.

I vote that the ic is toast.

To prove pin #3, remove the ic and see that the pad on the pcb  reads 0 volts.

[g1]

Agree with Jazz, remove the bad IC and check the pads.
Please also see the end of reply #31 above and double check the empty pads for T8.

[batbob]

g1, regarding post #31, I've just been testing that and it appears that I had my drain and source mixed up. G=-6.9v, D=-15.6v and S=0v.
For some reason I'd watched a video on JFETS and they had the source as the top and drain as bottom on the diagram. I see where I went wrong now.. Sorry

Jazz and g1, 14v sounded wrong to me, I'll get unsoldering and let you know.

Thanks,
Bob

[batbob]

IC5 removed and pad voltages are all 0v apart from 4 (-15. and 8 (16.3v)
I guess that this means it was goosed and the weird voltages were a symptom of this.
To move forward in the circuit I went back to the drain of T8 pad, that was -15.6v before. It reads 0v now so I guess that the IC was sticking some strange voltages across T8, causing the hum too.

If it is the IC as we suspect, what explains the sudden gain achieved when the signals were large? Is there any logic in that effect happening like that? I suppose that it could be the inside of the op amp starting to function for some reason...


Thanks,
Bob

[batbob]

Thanks to you all from r your help so far. I'll be receiving my new IC on Wednesday hopefully and I'll let you know how I get on.
I'll put the ic in and try it before I put TC8 back in the circuit too.
Thanks again

[batbob]

Well, it's a big thanks to you all, g1, Jason, Jazz,Phil and Tony for all of your help.
After the IC arrived today I soldered it back into the board and then tried it, and wouldn't you know it, all worked!
I banged in the T8 jfet too and it was all still working superbly. :dbtu:

It's all back together and plugged into the H&K 4x12 and sounds good, but I can't test it at the high end yet as the family were watching tv earlier and now I've just come back in from shooting its 11:30 at night!

Many many thanks again and I'll be hanging around on here from now on to see what other stuff people get up to, and look at trying some of those cool electronics/home brew pedals projects.

I've added a couple of chip points to you all as we've been going through this but it's a very little gesture compared to my gratitude.
May you all have a superb 2018 and I'll hopefully see you all on here again.

[phatt]

Great to hear you got it working,,, enjoy :dbtu:
As to why / how the chip failed but still half worked,,, well same way a bad battery terminal connection will refuse to start a car,, try a couple more times and it suddenly turns the starter motor.
If you frig around with electronics long enough you will come to understand that up to  90% of most electrical breakdowns are due to bad connections.
whether it be a rusty RCA plug or a failing connection on a silicon wafer inside a chip the result is the same.
I remember a motorcycle rider asking why his self cancelling indicators were not working?
I replied saying; It's electrical mate, like all electrical circuits they eventually self cancel.
Resolved by cleaning the contacts.
ramble over,,
Phil.

[batbob]

Cool, thanks Phil. You're a gent. I'll bear that in mind in future with any other projects I take a look at.
Regards,
Bob

[batbob]

Bollox.
Just got it up to half vol for a bit of Muse Psycho and it stopped working, 500ma fuse in rear has gone. I'll check out why it's gone and get back..
Bob

[phatt]

OK don't panic,,,Q What channel was engaged when it blew?
Phil.