Swr la series chirps on power up

[booj]

Not your average pop or click. It's a nasty oscillation type noise that lasts maybe 20 miliseconds. There's a DC component which jumps to about 5 volts then goes back to zero.
It doesn't have the noise suppresion circuit, which from what I gather is unobtainable.
I guess I could cut the ground trace to the headphone jack and install a relay delay circuit, but I wonder if it's a fixable thing.

LA preamp D.pdf

LA12 and LA15 Power Amp Rev A.pdf

LA 12 & 15 Noise Suppression Service Bulletin.pdf

[Enzo]

Isolate the problem. Is it coming from the preamp and being amplified? or is the preamp out clean and the PA generating the signal? Is it riding a power supply rail? There are both high and low voltage rails to consider.

At power up, do the rails snap right up, or are they lazy? Does one or more take longer than others?

[booj]

Isolate the problem. Is it coming from the preamp and being amplified? or is the preamp out clean and the PA generating the signal? Is it riding a power supply rail? There are both high and low voltage rails to consider.

At power up, do the rails snap right up, or are they lazy? Does one or more take longer than others?

>Thankyou for your reply, Enzo. Just needed a little kick in the right direction.
>Problem seems to come from the power amp. No noise in the preamp output. Strange thing thogh. After disconnecting the preamp signal plug and turning on the power amp, the output went straight to the DC rail. (sorry, forgot which one) Connected the preamp plug and tried again and just got the "chirp"
>Not sure what you mean by riding the power supply rail

>Both rails come up quickly and cleanly

I followed the noise all the way to the differential pair, where I couldn't see any noise. Q6 is connected directly to Q8, one of the differential pair. ALL of the pins of Q6 are noisy.
No open resistors. Voltage across R4 emitter resistor is 2.1mv for 21 ma. and 2.8mv. across R3 for 28 ma.
Bad differential pair?

[Enzo]

Sorry, when you disconnect the preamp cable, you need to ground the input ground on the power amp. Note the different ground notation there. COnect a clip wire from the input connectort ground to the other grounds in the power amp. I should have mentioned that.

Riding the rail? Scope the power supply rails. Does your unwanted noise signal show up there? You will probably see some indication on the main power amp rails since they are not regulated. But we are looking for a severe presence we wouldn;t want.

Well whatever you see at the base of Q6 sure better also be at the collector of Q8.

Your diffy pair are cheap, so why not throw a couple new ones in there. You should always change those in pairs, so you get transistors from the same batch. At least hopefuly.

But don;t overlook Q9,Q10. They act as a switch to turn on the V+ to the diffy pair and the bias string. That could be slow. We could probably defeat it to see if it was involved. SO make sure C16 is working, and watch the collector of Q9 to see if it has troube getting down to ground.

[rf7]

It doesn't have the noise suppresion circuit, which from what I gather is unobtainable.

I just bought one 2 months ago from Fender, did you talk to them? It stops the problem completely.

[booj]

Ok, I'll give them a call, but I didn't know Fender would sell to a private party. Was your problem with a poping noise or more like the chirp I'm getting?

I've tried a new differential pair. Same problem. I tried bypassing the turn on circuit of Q9 and Q10 by lifting the end of R19 that is connected to the collector of Q9 and grounding it. No good. Lifted C10 and checked it. It was ok. I also tried some cleaner on the bias adjust pot VR1 and working it a little. (Seems like there is more noise on Q2 than anywhere else). Nada.
The paths and pads on this board are delicate and the board has foil on both sides. I really consider myself lucky to have not done any damage to the board so far. I'm gittin' scared!

Maybe the Fender boys will help. sigh....

[Enzo]

ANything they have should be available through a Fender dealer or parts supplier, if not direct. Generally they don;t deal direct.

You cleaner VR1, but how is your bias SET? A little colder that dead-on any help?