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

[batbob]

It was on WARP when it went. It was working earlier when I had the volume a bit lower. Turned it up to halfway had a play on clean and then switched to warp then got about a minute into A muse song then it went off.

[phatt]

Maybe pop in a new fuse and plug into the *Line in* to check if the power stage is still working.
I'd be inserting a radio signal or similar into Line input to check the power stage is stable and taking voltage readings as you turn up the volume.
I just use the headphone output of and old radio cassette for that stuff.


Now although the preamp is not in use while using the line input the *Warp select* does engage the CS resistor R75. (CS stands for Current Sense) which increases the gain of the power amp.
Power amp gain is changed via *slc2* (little box next to R37)

There is the possibility that *IF* R75 was shorted the amp would run fine on clean but pull too much current in warp mode.
Although R75 is only .33 Ohm you might think it would make little difference but if the speaker negative terminal is accidentally grounded to case or similar situation then it would quite likely blow the fuse because the warp channel also switches on R75 turning up the power stage gain. (I'm guessing by quite a bit)

Regards checking voltages, these are shown at bottom of sheet 2.
That 16 Volts node between R50 and R67 can't be right,, should read more like 18~19 volts. If it was 16 Volts then D20 and D21 would not be able to regulate the 15 volt rails. Zener diodes need 2~3 volts above the zener voltage to work properly.

Other point that confused me for a while, It seems that IC4 is marked wrong way round.
Those 3 *slc hi/lo boxes* the pins are likely correct but looks like all 3 are marked wrong way round.

IF the CS resistor is shorted then it will be at ground in both clean and warp mode.
Checking low value resistors can be a trick but I just self test first.
Switch to low Ohms,, short the leads  scrape them together till you get a stable reading,, jot that down ,,NOW hard press either side of R75 and deduct the shorted lead reading will get you close. If it reads .1 Ohm then go hunting for the short.

Check the Speaker neg wire that runs from pin 6 of J5 (HP socket) does it ground back to the case anywhere?

There may well be other problems but that would be a possible reason for what is happening.

I once had a CS resistor shorted out via a badly replaced speaker socket which I accidentally grounded and your problem reminds me of that stuff up.
Amp worked ok until you crank it up then poof,, no sound.
Phil.

[g1]

500ma fuse in rear has gone. I'll check out why it's gone and get back..
Bob

You are on 230V mains?
Did you at any point adjust the bias trimmer when you were working on the output transistors?
Should probably check the idle current anyway, what is voltage (DCmV, no signal) across the 5W emitter resistor, R69?  R70?

[batbob]

Thanks g1 and Phatt, We are on 230v, I didn't adjust bias trimmer.

I can't test anything really as the fuse went in the rear of the unit, 500mA. I changed it and then the 2 inside 2000mA went too. I've rigged up a current limiting lamp in the middle of the power lead and it's at it's brightest.

When I put an ordinary cable in, it blows the 500mA fuse in the rear. Even when the leads from the Transformer are disconnected from the board.

I think that the transformer is a dead 'un for some reason.

I've measured resistance across the primary at 22 ohms, as it's a 0.44A primary at 230v, I reckon it should be 522ohms.
The secondaries are 0.4 ohms and 0.5ohms (0.9 across both). Sounds like the transformer is now a deceased transformer.

I'll have to take a look around for a 230v 0.44 primary, 28-0-28 1.5A Secondaries transformer.

I'll let you know what happens from there. This might be the point I stop spending on it to be brutally honest.....
Thanks for your help too.
Regards,
Bob

[batbob]

Ok, I unsoldered the connections from the transformer and I'm getting the right voltages out, so it's not knacked.

I removed R47, R48 and R43 to try and narrow it down and then remembered that the +40 and -40 rails come off before that so then I started looking at what could short out in the poweramp stage again and tracked it down to transistor T6, a TIP 147 that I thought was buggered originally but seemed to be working ok once the IC had been changed.

I've checked it in circuit and saw a short between emitter and collector, thinking it might be something in the circuit other than T6 I unsoldered it to check it, it's a dead short on the desk so I'll need another TIP147 by the look of it.

I don't know if this is the last fault with it but I may try to find a decent TIP147 and bang it in. That will deffo be the last attempt. I'm reluctant to spend any more on it...
Anyone have any good contacts for TIP147's?

[phatt]

Oh dear,, I'd just replace T3,T5 and T6 as they are likely stressed and could cause another failure.
Fire it up again  (with limiter) check all is well then remove limiter and check bias as per *G1* posted above.
Phil.

[batbob]

Thanks Phil, you may be right.

What's everyones thoughts on replacing the TIP 147/142's with TIP 147T/142T's?
I know early on someone said I shouldn't do that, but looking into the spec sheets, they're the same, except for Package Type.
I ask this as I can't find any TIP147's and TIP142's that are the same size as these two in the amp without paying through the nose and waiting for too long..

Cheers,
Bob

[phatt]

The data sheet I'm reading tells me that they are different.
The TIP147T is only 90Watt while TIP147 is 125Watt.

Try searching for a power Darlington with the same specs or higher should work.
100V, 10A, 100W Darlington, but check pin out.

T3 is a BD238
which is PNP 100V, 2A, 25W unit, again anything with those ratings or higher should work.
The main specs to look for;
Max Collector Voltage, Volts
Max Collector Current, Amps
Max Collector Power dissipation, Watts

I notice that Onsemi.com data sheet for TIP142/TIP147 states that after 2012 they are changing package to TO-247 which have same pinout which would fit.  :tu:
Phil.

[g1]

Yes, use the TO-247 like phatt suggests.
Cheap and readily available at Farnell or RS components, or other UK suppliers.
Not sure if you will need different mica insulators to fit, but they will be cheap as well.
The smaller package "T" are lower power and not suitable (they will burn up).

[batbob]

Parts ordered from CPC, now awaiting the TIP147,TIP142 and BD238.
I'll let you know how it goes.

I was looking at the TIP147 and 142 with the new package but thought that they'd be a similar problem as the TIP147T, now I've been through and checked the ratings I see why they're ok.

On Farnell the 147 had an hfe of 1000 and the 142 had an hfe of 500. The knacked transistor has a 500 so I bought two the same, I figured that it's not good to have two different gains through the circuit...

Cheers, Bob

[g1]

hfe spec. is dependent on test conditions.
Farnell was a bit sloppy there using the 2 different specs.
You can see from page 3 of the attached spec sheet that both specs are valid for both parts, but under different test conditions.
Tip142 & 147 are a complimentary pair, so you should be fine if both have the same suffix (package type).

[batbob]

Hi all, got the two TIP's (142 and 147) all soldered in and working on the headphones like a good 'un.

Just as a final check to make sure I don't blow it again tomorrow after work when I thrash it a bit, is there anything about the connection to the Cab that could cause an issue?
I've measured resistance at the in and out of the cab, both 7.1 ohms, the Amp is an 4-8amp output so I don't see there being an issue.
Just wondered if there was anything I should look out for, someone mentioned DC on the output. I havent checked it yet but is there an acceptable level or should it be 0v dc?

Thanks for all of your guidance and support!
Bob

[phatt]

YES always check the DC after heart surgery or expect,,:trouble :trouble.

The basic concept is that it's a DC Amp (Directly coupled, No decoupling caps) which means each section passes it's DC offset onto the next.

Because it's split supply then the output should be very close to Zero volts.
But these things are never perfect so there is always some slight DC offset at the output. +/- 50mV of DC is a respectable number for such a basic amp.

If the whole circuit is balanced well then you should see ~ 50mv DC at T1 Base, T2 Base, and speaker output. don't expect all 3 to be exact,, that's just a ball park number for you.

My concern for the unit was WHY did it burn out in the first place but now that I've re read page one I see it was you who inserted the wrong transistor which burnt out R66. 
So I assume there was no fault in the power amp stage only the dead IC5.
But anytime you replace components (especially on high current power stages) you always check the DC conditions are still stable.

That is how it works; You set the DC conditions so that the AC signal can pass and be amplified to the maximum ability of the DC potentials that are available from the components used.
If any DC part is not right then something will burn out.
Phil.

[batbob]

Thanks Phil, that's an ace answer and easily understandable.

I'll check the DC on the output tonight when I get home.

If it is out, I'm guessing its the trimmer pot that would affect the Bias?

Thanks and I'll keep you informed as to whether it blows again!.
Cheers,
Bob

[phatt]

500ma fuse in rear has gone. I'll check out why it's gone and get back..
Bob

You are on 230V mains?
Did you at any point adjust the bias trimmer when you were working on the output transistors?
Should probably check the idle current anyway, what is voltage (DCmV, no signal) across the 5W emitter resistor, R69?  R70?

Hey *batbob* don't forget this post from *G1* 8|
Phil.