Rockford Punch 60x2 DSM

[Dr Zeus]

Of a batch I picked up today, I have a Rockford Punch 60x2 which I think is a DSM; Trans-ana amplifier.

Inside looks crisp and clean but of coarse its got an electrical bug or two.

Both of the channels will play - the left output much better than the right; but both have DC voltage across the speaker terminals. The left output pictued below outputs 1.2vDC, and the right channel has 6vDC. This is without speakers nor RCAs attached. This voltage also causes my test speakers to retract when the amp powers up.

I tried adjusting the pots near the left and right output transistors - and I was able to -start- seeing the voltage go down in hopes to get closer to 0, but the amp startred popping 5A and 10A fuses on the bench so I've relaxed those adjustments. All large transistors check out ok with the dvm on Diode.



Any clues? Misteriously, I have a second 60x2 DSM that does the same exact thing. Hmmmmm.

[Perry Babin]

Leave the pots fully counter-clockwise until you find all of the faults.

Confirm that you have both positive and negative rail voltage on both pairs of outputs. Also confirm that you have positive and negative regulated voltage on all of the op-amps.

[Dr Zeus]

Im not 100% sure how to check for positive and negative rail voltage on the output transistors.

On the opAmps, this amp has three opAmps - a LM339M (Quad) and two LM833 (Dual). For the two LM833s, using my o-scope shield to grouns, I see positive 13vDC on pins 8, and negative 24vDC on pins 4. The LM339M sees positive 7vDC on pin 11, and positive 0.5vDC Pin 4. Resistance from ground to pin 4 is 1kohms.

[Perry Babin]

The rail voltage will be on the third leg of the output transistors. The IRF540s will have negative voltage. The IRF9540s will have positive voltage.

The LM339 is not an op-amp, it's a comparator. The power pins are 3 and 12. 3 is ground.

-24v DC is too much. Connect the RCA shields to the amp ground and measure it again with your multimeter.

[Dr Zeus]

The IRF540s will have negative voltage. The IRF9540s will have positive voltage.

Rail voltage looks good. Pin 3 on each output transistor measures either plus or minus 26vDCdepending on IRF540 or IRF9540.

Both LM833s measure -26vDC at pin 4. If I ground RCA shields this measurement drops to -13vDC Pin 8 stays at +13vDC no matter if RCA shield is grounded or not.

The LM339 comparator reads 0vDC at pin 12, and +12vDC at pin 3.

All above measurements taken with DVM's COM lead attached to the amp's ground terminal.

[Perry Babin]

With the RCA shields grounded, do you still have the same DC voltage on the speaker terminals?

With no power applied and nothing connected to the RCA jacks, what is the resistance from the RCA shield and the amp ground?

The voltage is correct on the 339. The ground is on 12 instead of pin 3.

[Dr Zeus]

Grounding RCA shields removes DC from the speaker terminal outputs. Test speakers actually play fine in both channels under this condition.

With no power/connected items the resistance from RCA shield to amp ground is variable; almost like testing a capacitor. Varies from 20k to ~1m Ohms.

[ppia600]

The rca shield/ "ground" isn't directly connected to power ground, its not going to read a steady resistance.

[Perry Babin]

Are R32 and R33 within tolerance (nothing connected to RCA jacks)?

[Dr Zeus]

R32: 0.3 ohms (It says 102 on it's back)
R33: 10 ohms (It says 100 on it's back)

From RCA shield to shield measures 10 ohms. Isnt it supposed to be 20 ohms like my PPI amps?

I dont know what readings they are supposed to be; nor do I trust my soldering skills to remove/replace surface mount components.


Edit: Near R32 is D7 and it appears it -might- be shorted.