Kustom Kasino head preamp board

[Euthymia]

I have another Kustom Kasino in the shop, this time it's one that someone traded me for a credit on a repair.

For the purposes of this issue, I am working from this schematic:

http://music-electronics-forum.com/a...om-pc5129-.pdf

Trying basic signal chasing, what I see is that my sine wave gets no further than the base of Q1. The DC voltages on the terminals of Q1 do not match the schematic, either.

Q1 e=1.07VDC b=.7VDC c=11.6VDC

What I have done so far is replace CR1, CR2, C1, C2, C4, C6, R1, R3, R7, R8, and R10.

R2, R4, R5 measure within tolerance. R9 is specified as 200 on the schematic, but is installed as 2K2.

Q1 and Q2 check out good using my Hickock in-circuit tester and using the diode function on my Fluke.

I've also replaced all tantalum caps with new electrolytics.

Interesting note: the other preamp PCB,

http://music-electronics-forum.com/a...tom-pc5128.pdf

....displays the same flat line after Q1 as well as similar DC voltages. I tried replacing its Q1 with a 2N3904 with no change in behavior.

One slightly weird thing about this input circuit is that the sleeve of the input jack is not explicitly tied to ground on the schematic. I've tried variations of inserting a cord to make sure everything was floating, using an ungrounded battery-operated signal generator, etc, with no change in behavior.

Where would you go from here, gang?

[telecaster]

Check C5

I would expect Q1's emitter to be biased negative, can you check and see if C5 is leaking DC? If it is, that could be generating the + voltage at the emitter and preventing Q1 from turning on.

[Euthymia]

I would expect Q1's emitter to be biased negative, can you check and see if C5 is leaking DC? If it is, that could be generating the + voltage at the emitter and preventing Q1 from turning on.

Actually, I replaced C5, but forgot to list it.

Since I have nothing else to try, though, I will lift one end and see if it is the problem....

....nope, didn't change anything.

Thanks for the suggestion, though.

[Twist]

Look up the data sheets for the MPSA18 and 2N3904 and make sure they have the same pin out. Also check all the resistors of low value (5K or less) out of circuit.

[olddawg]

Are you sure you have a negative rail voltage? Put your scope in DC coupling and make sure that the +/- supplies are correct, filtered, and semetrical.

[DivAud]

Q1 Base emmitter junction needs to turn on by current flowing from the -12V rail, through R11, through R 10, Through R4, through the BE junction, through R2, through R3, to ground. First check your voltages on both sides of R11 to check the supply rail. If you have you -12VDC on both sides of R11 then read the voltages across the resistors I mentioned to see if we have any current flow throiugh this circuit.

The diodes CR1 and CR2 are for surge protection and can be removed until the circuit is fixed ( lift one end each leaving them in place, you wont have to worry about polarity later ).

[Enzo]

Amen. I'd be looking for sopmething about R11 or maybe a seriously challenged C8.

MPSA types and 2N types are both on the American plan - EBC.

And Erik, avoid the temptation to just replace all the parts. What if it was a cracked solder joint of a broken trace? When all those voltages are skewed, it is appropriate to find out why.

I don;t follow your input jack detail. The schematic doesn;t show the jacks at all. What ground reference are you looking for?

[Euthymia]

Alllll right!

olddawg (and DivAud) win the "good question!" award.

I had checked the higher voltage rails, but not the -12V.

I did check the zeners in the +/-12V shunt regulator, and they checked good, but I didn't spot the dropping resistors tucked between the giant cap and the preamp board. Two flat 200R wirewound ones stacked atop each other, and the one on the bottom was open.

I only obtained the documentation for the PCB's in the amp, and neglected to ask Enzo for the chassis schematic, which is where the power supply components must be.

Kinda reminds me of the old joke about the guy seeing his friend on his hands and knees at night in a parking lot, underneath a lamp. He asks the friend what he's doing, and the guy says "I dropped my keys when I was getting out of my car." He points out that the guy's car is 50 feet away from where the guy's searching, and the guy says "yeah, but the light's better over here."

I mean, big freakin' clue that the same thing is going on on both preamp boards, right?

[Euthymia]

Erik, avoid the temptation to just replace all the parts. What if it was a cracked solder joint of a broken trace? When all those voltages are skewed, it is appropriate to find out why.

I don;t follow your input jack detail. The schematic doesn;t show the jacks at all. What ground reference are you looking for?

Well, I didn't just shotgun them; I noticed that the voltages were wrong at the transistor terminals, measured the resistors, saw that they were outside tolerance, and replaced them. When that changed nothing, I picked a couple of strategic caps and swapped them out.



With these old Kustoms, I do go through and swap out all the tantalums, because they are often zapped.

What I meant by the input jack on the schematic is that it's shown as a 3 terminal. Hi input, low input, and ground. The "low input" connects to the sleeve of the input jack, but the input jack is not isolated from the chassis. No biggie; it just seemed kind of odd to me that the connection would be made only at the jack.