fender pro 185 sold state amp problem

[amptech]

Hi all,


Has anyone came across this fault with a fender pro 185 befor.....

first problem, the thermal inrush limiter was burnt away from the pcb on one leg! then after being repowered back up there was a frying noise that could be heard from the speaker. after rechecking o/p stage all is well mm but, the power dropper resistors are heating up to quickly to the preamp stage!
all opamp's have been changed for new one's and it's still driving me nut's.....

if anyone could help that would be great many thanks

[52 Bill]

Please explain, the problem is a frying noise in the output?

As for the dropping resistors, what are the voltage readings on both sides of each resistor and on the pre-amp power rails? I don't remember for sure, but isn't there like a 40v drop across those resistors? They do get hot.

[Enzo]

I've seen inrush thermistors burnt off boards on many amps that have them. Replace it and move on.

I think the two resistors you mention are the droppping resistors serving the zeners that make +/-15VDC. Those do indeed get hot. Check the solder to them. I suggest replacing them, and instead of just soldering the wire right there at the pad, sand the green coating off the traces, and lay the excess wire lead length down flat on the bare traces for a distance. SOlder along that length. Now the resistor is mounted by a large area of wire and solder that acts as a heat sink. it will be less likely to unsolder itself.

If this amp has the two metal can caps sticking up from the middle of the main board, resolder them, and glob some silicone sealer or some hot melt between them and around their bases.

AS to noise, isolate the problem. Turn all the controls up and down. Any controls that affect the noise are after its source. controls that do not affect it are before its source. Send the preamp out signal or the effects send signal to another amp. Is the noise present there? If so the preamp is noisy. If not the power amp is noisy. Likewise, plug the guitar into the powr amp in jack or the effects return jack. If it is still noisy, then the power amp has the issues.

ANy noise coming out the speaker should be visible on a scope for signal tracing.

[amptech]

fender pro 185 problem.

thanks for your advice so far....

I've changed the dropper's from 270 ohm's to 300 ohm's 10 watt type's doing so has helped a bit, how ever the fault is still there and i've even tried the ac lamp test has a engineer said. ie when powering up the ac lamp should quickly flash and go out but if it's stay's bright it's show's heavy loading within the unit.
I even unpluged secondary feed's to the pcb and powered up and the lamp was still bright. I've checked the o/p by it's self and all's well it's only when it's all connected the low level frying nosie is there.?

it's driving me nut's even fender are stumped!

[Enzo]

No offense meant, but it sounds like you are out thinking yourself.

Disconnect the speaker loads. Get rid of the lightbulb, remove the mains fuse, and connect your AC current meter in place of the fuse. I assume it can handle 10 amps? Now power on the amp and see what current it draws. I assume you are on 240v mains? WIth no loads, I'd expect that amp to draw somewhere in the 1/4-1/2 amp range. Actually maybe even lower. I don't expect more than maybe 50 watts at idle for a SS amp.

Your amp seems to largely work, it just has a noisy preamp stage somewhere. Worrying about excess current draw - whuch the bulb might detect - is wandering down the wrong path.

If the current readings are reasonable, then restore the fuse to its holder, and move on to finding the noisy stages.

[jasonguitar]

PLEASE HELP!

I've seen inrush thermistors burnt off boards on many amps that have them. Replace it and move on.

I think the two resistors you mention are the droppping resistors serving the zeners that make +/-15VDC. Those do indeed get hot. Check the solder to them. I suggest replacing them, and instead of just soldering the wire right there at the pad, sand the green coating off the traces, and lay the excess wire lead length down flat on the bare traces for a distance. SOlder along that length. Now the resistor is mounted by a large area of wire and solder that acts as a heat sink. it will be less likely to unsolder itself.

If this amp has the two metal can caps sticking up from the middle of the main board, resolder them, and glob some silicone sealer or some hot melt between them and around their bases.

AS to noise, isolate the problem. Turn all the controls up and down. Any controls that affect the noise are after its source. controls that do not affect it are before its source. Send the preamp out signal or the effects send signal to another amp. Is the noise present there? If so the preamp is noisy. If not the power amp is noisy. Likewise, plug the guitar into the powr amp in jack or the effects return jack. If it is still noisy, then the power amp has the issues.

ANy noise coming out the speaker should be visible on a scope for signal tracing.

I'm having trouble trying to repair the same amp for a friend. The 2 filter caps had dried up and the 2 resistors mentioned(R175 & 176), the 2-270 ohm 7watts had been charred. I replaced the caps and put in 2-270ohm 10 watters and the amp sounds terrific but the resistors start smoking after 10 mins. Any suggestions?

[Enzo]

Hi, this thread has been dead for two and a half years, you really ought to start a new thread for your amp.

Resistors burn up because excess current flows through them. WHat voltage is dropped across each one? And then how much power are they trying to dissipate? And since they serve that pair of 5w zeners, are both of them sitting at about 16v as they should? Are any of the ICs in the amp getting hot?