Low V supply issue Firebass 700

[lowell]

I'm only getting +6v and -7v on Vreg outputs. The fuses are not blowing, I left it on for 10 minutes. I did try to get some signal, nothing. The power light, I forgot to mention, is not on. Must be more another issue as well. What is SG26 right after the mains inlet? Is that a MOV?

[chipprogr]

Please listen... you may have tested the op amps for shorts and you may not find any shorted..thats normal. Are they not socketed? Ive asked a few times for you to pull them and power up, you wont so there isnt any chance of eliminating them until you do..and ill bet you find the 3080 is the problem. The power led is not on the schematic you have, so its probably powered off the preamp board and if you still have the pre amp board connections pulled, it wont ever get voltage to light. The SG26 is a 5 ohm 12 amp in rush current limiter -not an MOV. Its to soften the turn on power surge. You have some pretty big capacitors in there and that turn on surge can be hard on the rectifier , power switch, breaker etc.. Please pull the 3 op amps , power up and recheck voltages.

[lowell]

chipprogr, sorry, I was going to pull them, just hadn't yet and didn't think if they were bad that it would affect the power light. And yes the preamp board is connected so the power light SHOULD be on. I just pulled all 3 opamps, yes they're socketed, and the supply is still +/-6.5v.

[chipprogr]

pull your pre amp connections now with the 3 main board IC's still removed, recheck voltages.

[lowell]

Ok I disconnected the preamp board and the fuses blew again. I managed to measure the voltages post vreg before they blew and it was +/-3v... less than WITH it connected...

[Enzo]

Your 16v rails are seriously loaded down. Remove the two fuses, set your meter to AC current, and touch the probes to the two clips of one of the fuses. And have the amp powered. DOn;t leave it on loaded down to 6v for any length of time. Your meter will tell you what the amp us trying to draw from the Vregs.

Your power light is an LED running off one of those low voltage rails, isn't it? SO if the rails are collapsed, so too will your power light be affected.

When you said the op amps were not shorted, I assumed you had pulled them. Please DO remove them from their sockets - note which chip went where and its orientation. ANy internally shorted IC loading this much should get pretty warm, by the way.

If not an op amp, then we have to hunt down what is doing the draw. We will probably have to desolder a few things to get there.

ANd yes, please leave the preamp cable disconnected until we are sure the power amp is producing a solid strong +/-16v set of rails. You don;t need the power light to work on this.

[Enzo]

And don;t rule out a solder bridge somewhere. All it takes is a tiny blob of solder between two traces. And the schematic won;t guide you to that.

With power off, what resistance do you get between +16 and -16?

[lowell]

Enzo,
I had pulled the opamps previously when you were helping before with this, and they were not shorted, neither were their socket pins.

Resistance from +16v to -16v rails is 7meg ohms.

I get 2.8 amps being drawn from Vreg.

[chipprogr]

at this point im sure your getting tired of changing fuses, maybe consider temporarily soldering in a 12 volt auto lamp ( not headlight) - possibly a bayonet style turn signal lamp to act as a soft fuse here.

Sounds to me like you have a part that is avalanching. When it gets to a certain voltage, it clamps and wont let go causing your 3 volt reading. With the 3 opamps removed, and preamp removed, i would do something to act as a fuse for testing, couple big resistors ,10-16ohm ceramic or a light bulb should be close for each fuse. Then 1st part id go after are the zeners located at CR101 and CR131. Lift 1 leg of each part, then test for voltage again. If your still not up to voltage yet, leave the zeners lifted and go lift the leg on cr140, cr144 , cr102 and finally cr103.

There are just NOT that many things here that feed off the + - 16 volt lines.
Just for safety, pull off J103 and 105 to remove high voltage from the power transistors...just for now untill we get the 16 volt line fixed.

Keep us informed.

[Enzo]

Actually, this is exactly the place where a variac shines. You connect an ammeter in series with the mains and bring up the mains voltage with the variac. If the current starts to ramp, back off. No fuses blow.

I agree it is time to start lifting things. You have a dead load across these rails. The schematic can no longer lead us to the problem, but it can help identify potential spots for trouble. The diodes are a good place to lift, but please do not ignore those little caps next to U100, U102. They look like resistors. Lift them too.

[chipprogr]

Hey Enzo -if this were a Behringer MX series mixer, we would be lifting every one of those little capacitors.. hundreds of them..

[lowell]

Lifted CR101 and CR131. Put some 12ohm 2w metal oxides as fuses, powered up and fans started working for first time. Current idled at 8ma and +/-16v rails are good. Guess those are the issue. So diode checking didn't reveal those as being at fault. They aren't a dead short but must be close to it. Popped opamps back in and connected preamp board, still all good - in fact idling at around 4ma now. Will replace zeners. Those zeners are 15v for the 15v supply right? And I believe you said that CR100 and CR130 are for polarity protection and to separate the 16v/15v rails right? Anything else please let me know, thanks for the help!!

[chipprogr]

diodes 140 and 144 are more for polarity protection. 100 and 130 are basically to counteract what 141 and 145 do. 141,145 boost up the regulators , 140 and 144 drop them by about the same amount. I am surprized to not see a dropping resistor attached to the zeners. It looks to me that the zeners are in a bad position here electrically. They could potentially load to much current and causing this issue. IF the tolerance of the regulators are putting out a little to much , or the zeners are dropping a hair more than 15 volts, they will fight each other.

Glad things are working out here.. keep us informed- eric

[thesynthdr]

Hi thanks for all the info in this thread it allowed me to fix a poorly firebass 700 (lol). I also popped in tl072 op amps instead of the very noisy 5680d it came with a marked improvement in sound quality was observed on tests worth the effort. speak to you all soon. Sam

[jucarlvi]

you still have the firebass 700 schematic pdf