Starting this thread for documentation/notes etc.
Amp had blown PS fets, RFP50N06 x 12 all shorted gate to drain.
22 ohm gates, but there is also a single 1/4w 10ohm resistor before the gates from the SG per side. That resistor got a little burned up but still w/i tollerance.
Directly driven from an SG3525A
Nothing seems shorted in the output section at all.
Board seems to be triple layered.
When I first tested the amp, REM did nothing, but the amp did draw 0.5A of current with or without rem attached. I put my scope on the gates of the bad 50N06 and noticed either a PWM drive like I hadn't seen before, or the drive was very wrong looking.
I proceeded to removed all PS fets, and then installed just one IRF3205 into each bank. I may drop those to IRFZ44/48 however.
Still not getting anything out of REM, but the amp is still drawing slightly at 0.02A with or without rem. Most of the pins on the SG3525A are at zero or less than 0.5vDC. Pin15 is seeing only 0.5vDC and so its not getting much supply voltage there either. I may pull this IC especially if it shorted out, not sure yet.
U8 is an LM2903. It sits between REM and, a diode, and 1~2 resistors. Do these voltages seem to be ok?
U8 LM2903
1: 13.35
2: 13.58
3: 12.26
4: 0
5: 12.58
6: 13.35
7: 13.36
8: 14.44
The ONLY other IC in the PS seeing anything more than about 0.5vDC on any IC pins is U2, a LM2703 which has 12.26vDC only on pin 1.
Thank you
Amp had blown PS fets, RFP50N06 x 12 all shorted gate to drain.
22 ohm gates, but there is also a single 1/4w 10ohm resistor before the gates from the SG per side. That resistor got a little burned up but still w/i tollerance.
Directly driven from an SG3525A
Nothing seems shorted in the output section at all.
Board seems to be triple layered.
When I first tested the amp, REM did nothing, but the amp did draw 0.5A of current with or without rem attached. I put my scope on the gates of the bad 50N06 and noticed either a PWM drive like I hadn't seen before, or the drive was very wrong looking.
I proceeded to removed all PS fets, and then installed just one IRF3205 into each bank. I may drop those to IRFZ44/48 however.
Still not getting anything out of REM, but the amp is still drawing slightly at 0.02A with or without rem. Most of the pins on the SG3525A are at zero or less than 0.5vDC. Pin15 is seeing only 0.5vDC and so its not getting much supply voltage there either. I may pull this IC especially if it shorted out, not sure yet.
U8 is an LM2903. It sits between REM and, a diode, and 1~2 resistors. Do these voltages seem to be ok?
U8 LM2903
1: 13.35
2: 13.58
3: 12.26
4: 0
5: 12.58
6: 13.35
7: 13.36
8: 14.44
The ONLY other IC in the PS seeing anything more than about 0.5vDC on any IC pins is U2, a LM2703 which has 12.26vDC only on pin 1.
Thank you
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It is at least 3 layers.
Traces corrode under the primary PS filter caps.
See if the attached diagram helps.
Traces corrode under the primary PS filter caps.
See if the attached diagram helps.
That schematic says for an AG1000 but most of the component numbers are not matching. However, I think I may have found a break somewhere before pin 15.
Do you happen to possibly have another Zapco schematic, perhaps one which shows no fet driver transistors? This amp defiantly has no ps fet drivers.
Try this one. These diagrams came from Zapco so you might try contacting them to get the right diagram.
I've emailed them. U4 was not seeing any +12v on pin 8 so I ran a temporary jump from Pin 8 on U8. The SG then received power on pin 13+15, but the waveform is very weak on outputs. I tried a new SG and that didn't help.
REM is still not working at all. Amp is trying to come on with +B.
Do the voltages seem correct on U8 above?
REM is still not working at all. Amp is trying to come on with +B.
Do the voltages seem correct on U8 above?
Slight update. The original SG3525A was definitely bad. With a new SG I'm seeing actually OK waveform on the gates of the temporary fets. The amp is struggling however and drawing excessively.
Still, REM not working. Also U4/U8 pins 8 tied.
Still, REM not working. Also U4/U8 pins 8 tied.
Not really. Kind of a mix between the two, this amp is.
I replaced U8, 2903, with a brand new LM393. The amp is powering on REM now w the u8-u4 jumper removed. Still drawing excessively so I need to comb through more.
I replaced U8, 2903, with a brand new LM393. The amp is powering on REM now w the u8-u4 jumper removed. Still drawing excessively so I need to comb through more.
Looks like I just needed to kick up my external PS to about 4A. Amp powers fully and idles at 1.4A. Not sure if anything else at the moment. Gate waveform looks to be OK.
Do you think IRF3205 will work in this amp, off the SG3525A with no drivers, and 22 gate + 10 ohm 'pre-gate' resistors? Or would IRFZ44/48 work better? X12 of them.
Do you think IRF3205 will work in this amp, off the SG3525A with no drivers, and 22 gate + 10 ohm 'pre-gate' resistors? Or would IRFZ44/48 work better? X12 of them.
Also do you think the LM393 will hold reliably instead of an LM2903? LM393 is used in a bunch of amps I fixed and is what I had available.
Amp is working great with irf3205 and lm393.
Is there a service manual anywhere for these style of amps? Wondering how to set the board potentiometers. In these Zapco amps it’s not precisely a bias potentiometer.
Is there a service manual anywhere for these style of amps? Wondering how to set the board potentiometers. In these Zapco amps it’s not precisely a bias potentiometer.
Well, to start this amp has no discernible notch. The pots don’t seem to do a whole lot across emitters reading voltage. Also this amp is power starving. Draws a lot of current even at 1/3 throttle. It’s not getting terribly warm. Peak draw is 120A at clipping which is about right but it likes to draw 50-60A under dynamic music loads. Probably the most power hungry amp I’ve seen like this.
Sounds amazing. I’d put this one up near Soundstream class A quality. Seems the amp is “trying hard” at sounding perfect.
Sounds amazing. I’d put this one up near Soundstream class A quality. Seems the amp is “trying hard” at sounding perfect.
Some amps have limited range on the bias pots.
Does the heatsink behind the PS FETs heat up faster than the heatsink behind the outputs?
Does the heatsink behind the PS FETs heat up faster than the heatsink behind the outputs?
I set bias pots to about 1/3 the way up from CCW. There was no additional amperage draw at this point from idle. As stated no noticeable notch on the scope. Heat-wise under full load the amp warms up evenly as far as I can tell. The amp sounds very nice. Stable to clipping. I ran it a mid-level for about an hour on resistor loads also. I'll leave it right where it is set.