Kitty Hawk preamp repair driving me nuts!!!

[McBarry]

Hi, My M1 head died while working hard in classA.

Now it's back going but crackling like mad in clean channel - this just gets amplified in lead channel (with no input).

Clean volume and tone stack have no effect on crackling. Clean master can vary the crackling volume. So I suspect fault around V1 and V2 area.

Hardly suprising because I have a nice 282v to anodes, and 0v to pin1 of V1 and V2. Pin 3 on V1/V2 is also low(like 0.1v) and pin 8 is a nice 1.7/1.4v for V1/V4.

V3 voltages all good, as are V4.
Good supply from 12v bridge.

The 100k resistors are 99k out of circuit.. the 10nf caps measure 9.9nf each..

Have resoldered all joints around each tube/socket..

No arcing visible, sockets look clean, no shorts under board and tubes replaced.

ANY ideas??
This is driving me insane...
I reckon either I'm missing something super simple or ... I'm missing something super simple..

(I've got suspicions about the opto's and the relay too....)

Here's the schematic:
http://www.schematicheaven.com/newamps/kittyhawk_m1.pdf


Regards, Dave

[52 Bill]

Clean volume and tone stack have no effect on crackling. Clean master can vary the crackling volume. So I suspect fault around V1 and V2 area.

Yes, V1A and V2B.

Hardly suprising because I have a nice 282v to anodes, and 0v to pin1 of V1 and V2. Pin 3 on V1/V2 is also low(like 0.1v) and pin 8 is a nice 1.7/1.4v for V1/V4.

I don't understand these numbers, 0 volts on pin 1's and pin 3's of both tubes?
You should have high voltage on both pin 1's of V1 and V2. Where are you reading the 282v?

I assume that you have tried different tubes for V1 and V2?

[McBarry]

Oops, that's what posting at midnight will do.. instead of sleeping.
For the anodes: one side of the 100k has 284v.
V1pin1=28v V1pin 6=105v
V2pin1=10v V2pin6= 54v
V3pin1=190v V3pin6=188v ( within spec on c/diag)

For the cathodes;
V1pin3=0.8v V1pin8=2.5v (seem a bit high??)
V2pin3=1.1v V2pin8=3.0v (seems bit high??)
V3pin3=1.3v V3pin8= 1.35v (within spec on c/diag)

Am about to replace the 4 x 100k anode resistors and re-check, then will do the 10nf/100nf/470nf caps and theirseries resistors.
Also about to check with opto on V2b out though with amp on clean it should be open anyway.

Dave

[Enzo]

Are all the heaters running?

Have you verified the cathode resistors and their paths to ground? In other words, measure resistance to ground from each cathode.

You have reported plate and cathode voltages, how about the grids. If a leaky coupling cap allows DC onto a grid, it can cause that tube to conduct REALLY hard, and that will drag the plate voltage really low. At least as an example.

Pull those tubes from their sockets and retake the readings on empty sockets. Your plate pin voltages should then rise up to the B+ voltage. The cathodes should drop to zero. Grid readings ought to stay about the same.

REmember, it is the circuit that matters, not just the parts. SO a 100k resistor might be just fine, but if there is a break in teh circuit going to or from it, it won't do its job.

[McBarry]

Hi Enzio, thx for replying.
Did a quick supply check- 530v at point A
15k/1W resistor next is actually 3.9k/1W on my board (I've owned since new).483v out of this, and 351v on rail supplying all the 100k resistors to anodes.
Some quick figures (V1 and V2 tubes taken out, V3 insitu)

V1pin4=8.7v V1pin5= -8.7v
V2pin4=8.74v V2pin5= -8.7v
V3pin4=8.8v V3pin5= -8.69v

V1pin1=6v V1pin6=337v
V2pin1=260v V2pin6= 319v
V3pin1= 232v V3pin6=232v

V1pin3=0v V1pin8= 0.02v
V2pin3=0v V2pin8= 0.07v
V3pin3=1.88v V3pin8=1.95v

V1pin2=0.2v V1pin7=1.7v
V2pin2=2.5v V2pin7=19.21v (17v between 22p and 10n cap linking V1pin1 and V2pin7...this looks very wrong yes?)
V3pin20v V3pin7=0v

Cathode resistors all 1.49k or close.

Really perplexing me..

Dave

[Enzo]

Remember, there is no "I" in Enzo. Hey I like that....

OK, look at your chart. The tubes are out, so they cannot confuse things.

V1-1 - 6v. You should have high voltage there like the other pins 1 and 6. so either that 100k plate resistor is open, or the connections to it are broken, or B+ isn;t even getting to the resistor, or the reading is faulty. Can't get much clean signal through with that stage dead.

The rest of thoase plates are all over the place. They all run off the same B+ node and all have the same 100k load resistors. I would expect all them to have the same voltage.

V3-3, V3-8 Almost 2v. I see no reason for these voltages to be there.

And all those grids? None of that DC voltage ought to be there.

How is this amp constructed? Is it on a eyelet board like Fender? Printed circuit board? I am wondering if yo0u might have a conductive board situation as on numerous old Fenders.

If it is on some kind of board, find that 2.2meg resistor that connects to V2-7. You apparently have 17v on the tube grid end, what voltage is on the other end up by the 10nf cap? If it is a lot higher, the 10nf cap may be leaky. A lot of your caps could be leaky and thus explain a lot.

On the other hand if it is a conductive board, find that resistor, and poke your probe close next to the parts solder connection, but with the probe not touching the solder. See if there is any voltage sitting on the supposedly insulating part board itself.

[McBarry]

Hi Enzo,
changed the 10n caps at V1pin1 &V2pin1,and 22p caps at V2pin7 & across c&d of optocoupler on V2a.
With all tubes in much nicer voltages but still some noise and volts not all cured.
Supply still 526v, 474v post 3.9k resistor and 331 post 15k resistor.


V1pin1=168v V1pin6=198v
V2pin1=130v V2pin6= 128v
V3pin1= 223v V3pin6=220v

V1pin3=1.6v V1pin8= 1.94v
V2pin3=2.8v V2pin8= 2.9v
V3pin3=1.64v V3pin8=1.75v

V1pin2=0.24v V1pin7=0.32v
V2pin2=2.24v V2pin7=2.91v
V3pin2=0v V3pin7=0v

Clinically, the crackle is much less, it can be nullified by shorting V2pin7 which it couldn't before the new caps. Shorting V2pin2 has no effect.

The 47p cap in parallel with the 470k resistor after the 10n cap from V2pin1 is not installed..doesn't exist.

I'm wondering of DC is sneaking in from the relay and putting V2pin7 high, thus also elevating the output on V2pin8. Not sure why V2pin2 is high..

Re. board conductivity, it had flux all over it, some quite thick between tracks. Cleaned with electrical contact cleaner - improved. Voltages around board ON the board are amazing; 80v next to that 2.2meg resistor..Whaatt??
Is this normal?
Maybe another clean with another product??
Would this explain the crackle?
Cheers, David

[Enzo]

That is why I asked. Coatings or dirt films on boards can absorb moisture and become conductive. Many an old Fender was all out of whack because the eyelet board went conductive. heat guns, cleaners, various ways to deal with it. When you are getting voltages on cathodes with no tubes in the socket, it has to come from somewhere, and the circuit isn;t it. SO coming across the "insulated" circuit board comes to mind.

You apparently found leaky caps as well. or unsoldering the old caps and soldering in the new cooked the circuit board in their area.

You bet all that conductivity on the board can cause noises. It will mess with operation too.

You still have 2-3volts of unwanted DC on those V2 grids. It is coming from somewhere. If the coupling caps from the previous plates are not leaky, then we are looking again at maybe conductive circuit boards.

[McBarry]

Well here's the update.
I followed Enzo's advice and found a TON of stray volts around the board, mostly where I had been working.
Cleaned with methylated spirits (I think its called denatured alcohol over the pond), tested, still small volts on board and on pins 2 & 7 of V1/V2.
Cleaned some more, less volts etc etc.

But of note also 2 ceramic caps (the 22p off V2pin7, 22p across opto on V2pin2), and 2 Phillips MKT caps ( the 10n V1pin1, 10n V2pin1),also leaking.
One of the 22p had a burn mark on it's case -think it was on V2 but can't be sure.

Really don't know what the initial fault was that caused the amp to go silent while being driven hard.

While on the bench I learnt how to and then checked the bias.

While amp apart re-made a front grill board but with 2 huge cutouts to assist airflow. painted, covered with shadecloth - looks really professional.

Considering adding a fan -thoughts anyone?

Thx heaps to Enzo for all assistance, and also thanks to Derek Lark of Lark Amplifiers (can google them) in Canberra. Recommend his work, and thorough gentleman who loves valve gear and is exceedingly knowledgeable about it.

David

[Enzo]

Great. Glad it is working.

Invite your friend Derek to visit us here on the forum. We can always use additional knowledgable people on here.


I bet learning that circuit boards can get conductive was an eye opener. I remember the first time I encountered it. Voltages in the circuit just made no sense at all, and then I found I still had voltages with parts unsoldered and removed. I finally learned what was going on when I rested my meter probe tip on the board somewhere, and got a reading. There is an epiphany for you.

[McBarry]

Hi Folks,

well it's crackle time again..
it's before V1 pin2..
replaced the 100k resistor off V1pin6 in case noisy,(wasn't), the 100nf MKT cap but it was OK ( they've been culprits around V2..)
Wondering about the two 220p caps - the one across volume is ceramic and the next one between treble and 100k resistor is polyester - maybe MKT but not the quality Philips MKT like the rest of them..
But both are hideous to get to - board needs come out.. kinda hope it's track moisture like before...a-la Enzo tip-of-the-day..

stay tuned..lol, Dave

[Steve Conner]

Usually conductive boards hit one node of the circuit really hard. If it's sensitive, like the grid of the first stage, and physically close to another node carrying high voltage, there's your recipe for disaster. If you find the offending node, you can just disconnect it from the eyelet board and float it in the air. I did this on a hi-fi amp where all attempts at cleaning the board failed, and I couldn't be bothered tearing the whole thing down to replace the board.

Do you live in a humid climate? That's only going to make it worse.

[McBarry]

G'Day Steve, I'm in Canberra/Australia - coming off winter here, not really humid..
I'm gonna fire up the CRO tomorow and hunt the tracks and board between..
I've a fair idea of the area/node, and there's some B+ tracks close to some signal tracks, but where I think it is is tucked behind/under the pots - absolutely awful to get to...
Summer days are heating up and putting the chassis/board out in sun and gently heating/cooking is on the agends too.. no, I'm not gonna bake it.. just enough to try dry out any possible hydroscopic parts of the fibreglass circuit board..
Not keen on floating componentry, but if all else fails I might have to..

[Steve Conner]

Oh, it's a fibreglass PCB? They should be fairly proof against absorbing moisture and going conductive in their actual bulk, as opposed to just getting conductive crud on the surface that you can clean off.

There are some known cases of bad PCBs, though, some Marshall amp (I forget the model number) suffers from a leaky board that makes the power tubes red-plate.

[McBarry]

Yeah, last time it was crud around V2 socket/tracks giving noise between pin 6 & 7..and a 2 caps which were completey shat - on was actually burnt a little..
Now its V1 somewhere before pin2, and after V1pin7.
I really hope it's a leaky cap as I don't really wanna have to forensically clean the board like last time..a true pain in the butt exercise..
DB