Rockford Punch 75 (non HD)

[Clipped]

i have an old school rockford fosgate punch 75 here (non HD) with a power supply problem.

actually i have two, one is good and the other is bad.

ive taken measurements between the two, and legs 1-4 of the tl494 are getting different readings.

good bad

leg 1 [2.26].....[0.030]
leg 2 [2.26].....[4.504]
leg 3 [2.87].....[0.056]
leg 4 [0.005]....[4.556]

supply voltage for the good amp is around 14vdc and 8vdc volts for the bad one, no matter how high i crank up the supply voltage on my power supply it always stays around 8vdc for the bad amp...this is with a resistor inline with B+. for both amps.

if i take the resistr off B+ for a split second, it pegs the ammeter on my power supply to max, which is around 100 amps.

but when i leave the resistor on B+, everything looks fine and dandy with normal current draw.
------------------------------------------------

power supply mosfets dont seem to be getting enough signal to the gate.

Good Bad

g [2.18].......[0.389-0.430]
d [11.72].....[8.19]
s [0.002]......[0.003]

------------------------------------------------

the regulators arent getting power because it appears the mosfets arent switching.

L7815

good bad

leg 1 [25.19].....[0.261]
leg 2 [0.002].....[0.002]
leg 3 [15.17].....[0.216]

L7915

leg 1 [0.002].....[0.003]
leg 2 [-25.30].....[-0.286]
leg 3 [-14.75].....[0.003]


suffice it to say all this started because i turned the bias pots just a tad bit too much.

ive changed the regulators and the mosfets in the PS, but its still dead and makes a humming noise in the PS.

anyone have a clue?

thanx

[Perry Babin]

The amp is in protect/shutdown mode.

Can you post a photo of the amp? It doesn't have to be a good photo. I simply need to see which amp this is.

[Clipped]

here ya go

[Perry Babin]

Pull the fuses to see if it will power up. If it does, replace the fuses in one chanel at a time to determine which channel is causing the problem.

[Clipped]

man you never cease to amaze me...

pulled all the fuses and it powered up, didnt measure anything though...LED came on.

the problem fuse/channel is the bottom left one, according to the above picture....with it in, it goes back into protect and the LED doesnt come on.

with all other 3 fuses in, it powers up and the LED comes on.

so its the fet connected to that fuse?

[Clipped]

hey man the cap on the end of that fuse just blew out and almost took my eye out...LOL...

now i have fur all over the board....roflmao

the fur is quite dry...

btw, i still had the resistor on B+...and that fuse was still out...

[Perry Babin]

The cap may have been defective. On some of the amps, the caps would blow when the outputs failed.

If the amp drew excessive current with only the bottom left fuse in the circuit, I'd suspect the capacitor. If it never drew excessive current unless 'both' of the fuses on the left were in place, I'd suspect the outputs, one of the drivers or the bias setting.

[1moreamp]

Originally posted by Clipped
hey man the cap on the end of that fuse just blew out and almost took my eye out...LOL...

now i have fur all over the board....roflmao

the fur is quite dry...

You definitely have blown outputs and possibly driver issues < But you on the track with Perry's wisdom.

Just to cheer you up I have had my share of RF caps pop in my face also over the years. Its wise to use a face shield on these exact amps.
Amazing what these old RF amps do when you try to service them. the caps IMO was not bad. It just had the wrong polarity across it due to shorted outputs.

[Clipped]

LOL hah....

yeah the outputs were bad, i don't know why i didnt check them first, guess i was lazy...then i was lazy again...

i changed the 540, then checked for dc on the outputs....25volts !
the 9540 was also bad....changed that...still had 25 volts...checked the 540 again, and it went bad again because i was too lazy and didnt change the 9540 at the same time...

actually it wasnt just laziness, i pulled some pads off a while back and had to insert jumper wires through the through holes and solder them back onto the traces on back....yeah actually i was being lazy

alls good now....eye protection is a good idea, i kept my eyes at a level that wouldnt fully absorb the casing from a projectile capacitor...

oh and i changed all those little Joe Meister's .... those who know will understand .. lol