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

Started by batbob, March 02, 2018, 11:22:59 AM

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batbob

Hi all,
Thanks for stopping by this post, I hope you find it interesting and can offer me some advice to get this bad boy working.
It was working a year ago and it's not been played since, turning it on this week and it produces a loud hum in the 4x12 cab or headphones.

The schematics are attached and I have worked my way through them to try and track down the problem, without the aid of signal tracers etc..

The guitar plugs in and in clean channel 1 you can hear nothing but the hum. in channel 2 (Warp) the only time you hear guitar is when maxed out on the Gain and Master.

If phones are in FX Send socket it sounds like the preamp and all pots and channel switching works, you can hear the effects of the pots etc so I think it must be post-preamp and in the poweramp section somewhere.

I've checked power supply for all voltages as per page 3, all good. page 2 bottom, right to left, all voltages are spot on too.
I've removed lots of the resistors and capacitors too to check them

The issue appears to be in the poweramp stage so I changed the two large power transistors  T5 (TIP142) and T6 (TIP147), checking the removed ones with a DMM on diode setting shows there are no issues. I changed them for some off the web, but they were smaller in size and when I powered up with these and then R66 released some magic smoke and then the transistors no longer checked out correctly.
I put the original TIP's in and replaced the 39ohm R66 and I'm still having the same issue.

Can anyone see anything obvious that I'm missing? I've checked for dry joints and have tested all resistors, some caps, some diodes but I think I need advice now on where to narrow my focus into from people such as yourselves who might recognise the symptoms and immediately have a suspect component.

Any help will be massively appreciated and I thank you in advance for your time and support.

batbob

Looking at the Power transistors a little more closely, I think I substituted the TIP147 and TIP142 for a TIP147T and TIP142T, which look smaller, have a different hfe (gain?. I wonder then, if that's why R66 popped. Should I get some transistors without the T suffix?
The originals did check out with the diode tester on my multimeter though. Weird.

g1

Don't use the T suffix, they are lower power spec. and not suitable.  Not sure why you changed them, but now there may be an added fault.  This is why it's best to isolate the problem first rather than just replacing parts.
Is R66 still burning?  If not, try your guitar into the FX return.

batbob

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.

tonyharker

Check T8 BF245 & D24 the slow start circuit.  Sounds like T8 is on all the time.

batbob

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.

batbob

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

g1

I'm a little iffy about that T8 testing or your identification of the terminals?
T8 drain to source resistance should be low (on) until it gets it's turn off voltage applied to the gate.
Maybe just try the amp with T8 removed.  If you get sound you know the problem is either T8 or it's not getting it's gate voltage.

batbob

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

g1

The T8 fet is just a mute for the turn on thump.  Leave it out until the rest of the amp is fixed, then you can go back to the mute circuit.
What happens now when you go into the FX return jack?

batbob

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

g1

Well, there are only 6 transistors in the actual power amp section, so how about some static DC voltage checks (no signal applied).
Base, collector, emitter, for T1 thru T6.

batbob

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

Jazz P Bass

There is something seriously amiss with T3's voltages.

You have 44Vdc on one side of R53 (1K) and 43Vdc on the other.
That is to be expected.

But 43Vdc through R57 (100) seems amiss at 130Mvs.

I would check R57 and T3.

batbob

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