Rockford Punch 60x2 DSM

Dr Zeus

Member
2010-10-17 5:12 am
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.

IMG_20101029_201909.jpg


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

Dr Zeus

Member
2010-10-17 5:12 am
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

Member
Paid Member
2003-11-20 11:01 am
Louisiana
www.bcae1.com
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

Member
2010-10-17 5:12 am
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.
 
Last edited:

Dr Zeus

Member
2010-10-17 5:12 am
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.
 

Dr Zeus

Member
2010-10-17 5:12 am
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.
 
Last edited:
Is there a D7 connected in parallel with R32?

If so remove it and measure the resistance across R32 again.

It's supposed read 0 ohms from shield to shield. This input circuit has virtually nothing in common with the PPI amps.

Have you removed it from the heatsink to see if you have any open traces on the bottom of the board (I'm assuming you would have noticed any on the top)?
 

Dr Zeus

Member
2010-10-17 5:12 am
OK D7 is out, and it fails testing on my DVM's continuity test. Its supposed to beep once, but this diode just acts shorted. 0.0 ohms actross it. For reference theres D1 near the transformer that looks the same as D7. D1 tests out fine with DVM.

I'm going to replace D7, but unfortunately I dont have spares so it will take a little while.

R32 measures just a hair shy of 1k ohms now.
 
Last edited:

Dr Zeus

Member
2010-10-17 5:12 am
I dont see any open traces.

I was able to replace D7. As it happens the Radio Shack accross the street from me actually stocks Z1 4003 diodes. It now tests good on the PCB while the R32 is also measuring 1k ohms. Unfortunately thats about all it fixed, and still none of the previous tests are passing.

10ohms from RCA shield to shield
Rising resistance from RCA shield to ground (Normal I guess)
DC voltage across terminals when powered
 

Dr Zeus

Member
2010-10-17 5:12 am
Oh my I think you are totally right. I just found a very very thin hairline trace that shows not connected - and its just half an inch from the Right RCA shield.

Take a look at this photo. Its a little blurry, but you can just barely make out the copper from the trace that blew near R103. The break I'm talking about is just up about 1/16 of an inch from R103 and looks sort of like the trace 'popped' like a fuse.

IMG_20101030_235201.jpg


I'm going to drop a new connection point from the hole just next to R104 over to an empty hole near C123.
 

Dr Zeus

Member
2010-10-17 5:12 am
Yes shields still measure 10 ohms across. I powered her up and she's making sound on both channels; and very close to 0.0v DC across speaker terminals.


Now I've gotta set the pots back. I presume this is like the PPI amps? Set them to 0.000v DC across emitter resistors? EDIT: Yup! She's back in tune and sounds pretty good. I'll go back and check it through with the OScope and go from there.

Thanks Perry!
 
Last edited: