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Old 29th November 2004, 06:56 AM   #1
nova_gh is offline nova_gh  United States
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Default Rf punch amp won't turn on

I aquired a non-functioning rf 250a2 punch amp a few years ago going into electronics eng. at college. I've been trying to troubleshoot it for a few years now but never really get much time to spend on it. Its some what an older amp.

The original problem i had was i wasn't getting any sound out, at school i injected some simlpe tones into the rca inputs to see if i was getting anything out using an o-scope. I think i've trailed it back to a few transistors but i'm unsure as before i could continue troubleshooting the transistors, the unit itself stopped functioning. Now the unit will not power on. I know i have power, it does not blow fuses, it just sits there. Anyone with some knowledge of these amps know where abouts in the circuit it's failing? Obviously somewhere in the power supply?

I can't seem to find anything that shouldn't work via ohm checks, atleast bad resistors. I'm unfamiliar with the circuit protection of these amps, perhaps a bad transistor would keep it from turning on? If anyone knows where i could get a schematic for it that would help a ton.

Anything info would help, thanks!
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Old 1st December 2004, 08:56 AM   #2
djk is offline djk
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The earliest RF amps had a power turn on lead that SOURCED power to the deck. To make one of these turn on, hook up a #561 dome light bulb to the power turn on lead and ground.
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Old 1st December 2004, 11:23 PM   #3
anatech is offline anatech  Canada
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Yup, make sure there is power on that lead, if it goes to ground some parts will be fried. Fixed lots of these. You should only need to draw a hundred mA or so to turn these on, try some 12V lamps in your junkbox. Basically, you are drawing power through a diode with a parallel resistor (to ensure turnoff), also through the E-B of a PNP transistor. 2N3906 will work here if it's blown.
-Chris
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Old 2nd December 2004, 12:35 AM   #4
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Quote:
I'm unfamiliar with the circuit protection of these amps, perhaps a bad transistor would keep it from turning on? If anyone knows where i could get a schematic for it that would help a ton.
If it's the very early Fosgate (no Rockford yet), I doubt they even had protection circuitry. I have a schematic for this design dated 1978. Because it's a large drawing, it's in 2 scans. I'll post them separately.
If it looks correct for your amp and you need better resolution, I can email a better scan.

Tim
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Old 2nd December 2004, 12:36 AM   #5
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RF250a2 is MUCH newer than the original Fosgate Punches. I remember those, too - back when few car stereo's had remote/antenna trigger outputs. The amp sensed the current draw by the deck and turned the amp on.

BUT.

The 250a2 is only a few years old. The most common problem with these amplifiers is that the current sources short causing the protection circuits to kick in. Some bright bulb in their engineering dept finally switched back to TO-92 devices stood off from the board after finding out the SOT-23 devices couldn't dissipate enough heat. It's been a while since I've seen the schematic on these (and usually repair them without the prints...but I digress).
I think they are replaceable with the MPSA06/56 equivalents.

However, even though the output section is screwed up, you'll still get a response from the power LED. If you don't...read on:

A completely dead power supply is indicative of anything from a dead PWM regulator (rare), Shorted PS MosFet (common) or fully shorted Totem-Pole gate drivers. The last item usually accompanies fully trashed MosFets, and rarely fail on their own.

Try this simple test. There are two dual-diodes on the power supply - they look like transistors on the heatsink, but will have a pictorial laser etched into the plastic body. Lift the center leg on each of these and turn the amp on. If the LED comes on, and you aren't drawing 10A of current, the power supply is in good shape and your focus should go to the output section. If the supply comes up, but squeals or pulses, you have to check your rectifier diodes (the dualies you just tweaked) or other components for short circuits. There really isn't much in the power supply, comparatively, so finding malfunctions is pretty simple.

Let's see where this takes you...
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Old 2nd December 2004, 02:21 AM   #6
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Quote:
RF250a2 is MUCH newer than the original Fosgate Punches. I remember those, too - back when few car stereo's had remote/antenna trigger outputs.
I guess that's a good thing since I didn't include the scans in the post!

Yeah, I can remember having to tap a remote on lead off the power switch of a few head units in the old days...

Tim
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Old 2nd December 2004, 06:08 AM   #7
nova_gh is offline nova_gh  United States
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hmm, just a little confused. I'll tell ya i got the three standard inputs, power (B+), source (power turn-on), and ground. So what if i understand you right, your saying hook my 12vdc power up, then hook a light bulb up from my source input to ground? Sounds different. Maybe i'm reading you wrong?
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Old 2nd December 2004, 09:15 AM   #8
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Rockfords fosgate 1100a2 output
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Old 2nd December 2004, 02:15 PM   #9
nova_gh is offline nova_gh  United States
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thx for the help guys, gonna try this all out, hope i figure this thing out.
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Old 3rd December 2004, 12:15 AM   #10
nova_gh is offline nova_gh  United States
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Quote:
Originally posted by EnvisionAudio


Try this simple test. There are two dual-diodes on the power supply - they look like transistors on the heatsink, but will have a pictorial laser etched into the plastic body.

Hey i'm looking at my circuit and i'm only seeing one transistor with a dual-diode etch on it. The transistor across from it has different numbers, should I only have the one would your test work the same?
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