SWR Strawberry Blonde II; Permanent Loud Hum w/Cyclical Popping

[njmondy]

Hello Everyone, this is my first post. I have a SWR Strawberry Blonde II acoustic amp, and most of the schematics. I suspect the Pwr Amp PCB, but the schematic I received from Fender appears to be for the original Strawberry Blonde amp, NOT the Strawberry Blonde II. As soon as power is applied, This really loud, constant hum starts. It has a cyclical popping occurring, also. Kind of like a consistent tremolo pulsing. I don't leave it on for more than ~ 20 seconds at a time, 'cause I don't want to blow a fuse. I've tried all of the standard visual inspections. Disconnected the Input PCB, no difference. Having a blank 1/4' jack plugged into the input has no effect. I removed the Power & Pre PCBs and checked for any obvious signs of burning, or burnt components. This PCB has four large discreet diodes to make the Bridge Rectifier, and there is present +15V and -14.7V. I haven't checked the rails for signs of AC voltage yet. I have a DVM, but no O-scope.
Does anyone have access to the newer Power PCB schematic? It has a 2004 Fender Musical Instruments label on the component side, and is cream colored. The circuit side is green with solder resist and has this part # 0061560000 Rev. 6.
Have any of you ever worked on this amp before? Any suggestions if I can't find an accurate schematic?

I removed the two big filter caps, 3300uf @ 63V, to check for shorts. I'm not saying that I don't suspect them anymore, just that they're not shorted. I resoldered them, along with what I think are the two + & - 15V zeners. There is slight scorching on the PCB at these zener diode components. I also checked the values of the two cement power resistors on these supply voltages, and they still measure as expected. Also resoldered them, and some of the other major components, but I did not resolder the whole PCB. I did resolder the whole, small speaker output PCB.

Any help would be greatly appreciated!
Thanks, Nick

[Chuck H]

I'm not posting to try and troubleshoot the problem. I wanted to say you should unplug the speaker before you blow it up. It seems you have DC on the output.

[olddawg]

Unplug the speaker. Check for DC across the output terminals. Discoloration of the board around a Zener may be normal from heat dissipation. The Zeners will quick check out of circuit like any other diode. I suspect that you have a shorted output device. You should be using a variac or at least a light bulb limiter.

[njmondy]

Thanks. I'll check for DC on the output terminals.

I'm not posting to try and troubleshoot the problem. I wanted to say you should unplug the speaker before you blow it up. It seems you have DC on the output.

"Thanks. I'll check for DC on the output terminals."

[njmondy]

"Thanks. I'll check for DC on the output terminals."

[Enzo]

And call Fender back and tell them you need the SB II instead of the original. MY files in fact do not include the power amp for that model, so specifically ask for the power amp too, in case it is separate.

[njmondy]

Fender Cust Support Not Much Help

And call Fender back and tell them you need the SB II instead of the original. MY files in fact do not include the power amp for that model, so specifically ask for the power amp too, in case it is separate.

Enzo, I did call Fender back last Fri (3/28/14), and asked for an updated Power Amp schematic. They do not have another schematic for the Power Amp other than the one listed as Combined Pre & Pwr for the original Strawberry Blonde. (The front line agents may not know how to go look for the correct schematic). I suspect that someone in Fender tech support "bundled" that set of schematics as current for the Strawberry Blonde II. Now the agents answering the phone don't go look for a different Pwr Amp schematic since they assume all of them are connected with their link to that amp.

So far, I did check the output with the speaker connected, and it is reading -25VDC to -30VDC, fluctuating. I haven't had time to check for AC voltage on the + & - DC rail voltages yet.

Nick

[njmondy]

Enzo, I have tried contacting Fender again, but still no luck getting the Power Amp schematic for the Strawberry Blonde II. Is there any way you could help me with getting the schematic? I'm dead in the water until I get one.

Thanks,
Nick

[olddawg]

Please disconnect the speaker. A solid state amp does not care if the load is open. You are taking a big chance if damaging the speaker every time you turn it on. If you have -25vdc across the speaker it is very possible that you have a +25v rail voltage missing. It shouldn't be to difficult to isolate your rail supplies without a schematic. Does this amp have an output IC or discrete components? If it is an IC, you can get a spec sheet for the number and look at the pin out diagram.

[52 Bill]

I looked up the manual and as Enzo stated, the power amp schematic is not included with the rest of the circuits. The manual also states that the power amp is the Rumble 100 circuit. I could not find the Rumble 100 schematic either. It may not exist as I think that that is one of the import units.

Olddawg is right look at the circuit and compare it to something similar. It couldn't be that different that normal troubleshooting can't lead you to a solution.

[njmondy]

Thanks Olddawg & 52 Bill, I appreciate the tips about disconnecting the speaker & comparing to another similar amp schematic. I'm not in any hurry, but would like to feel that I'm making some progress. I'll try to do some looking for a Fender Rumble 100 schematic. I have looked at some other SWR amp schematics, but haven't seen any other designs with the same output transistor's as mine. I may have to just work on it from simply a design similarity.

Nick

[Jazz P Bass]

Rumble 100 Pwr amp schematic.

[olddawg]

Now check your +/- 15v and + /- 41v rails. But I would bet you have a shorted output device. Static check them first. Every time you turn the amp on you could be causing more damage and make a simple problem more complex.

[Chuck H]

Thanks Olddawg & 52 Bill, I appreciate the tips about disconnecting the speaker

Um... Maybe we could look back to your FIRST thread response. Post #2 !?!

I wanted to say you should unplug the speaker before you blow it up. It seems you have DC on the output.

[njmondy]

Thanks, Chuck. I'm an old school tube amp guy. I didn't know you could operate a ss amp without a speaker load.