Infinity SSW-10 sub-woofer power suply

[Sierra117]

Hi all,

I have an Infinity SSW-10 sub-woofer that stopped working that I acquired from family. Upon opening the woofer the fuse and 2 resistors (R1 & R2: 430 Ohm 1W 5%) were blown. I replaced both the fuse and the 2 resistors (437 Ohm 1W 1%). The PCB was also badly burned where the resistors were which damaged some traces, so I repaired them as much as I could with jumpers (see Trace Repair). I have attached both the repair manual as well as before/after pictures of my repair. See page 38 of the manual for the schematic and pages 34 & 35 for the PCB layout. Please note the capacitors (C30, C35, and C33) on the schematic should be swapped with (C31, C36, and C34) respectively.

Trace Repair: Both the pads for R1 & R2 were destroyed. I was able to scrape the solder mask on the 31V side to access the copper trace. On the 16V side jumper wires were used to connect to C7 and C8. On the solder side of PCB-1 there is a trace that runs under R1 & R2 from C10 to J4 then J4 to R10 on the component side. I used a jumper to go from C10 to J4. I also needed a jumper from R2/C7 to J7 pin 3 (the ribbon cable).

Test Voltages:

V_c6 = +29V
V_c5 = -29V
V_c7 = +16V
V_c8 = -.6V
V_r2 = 12V
V_r3 = 28V

The +31V, -31V, and +16V rails look ok, but the -16V rail is not, and the relay is closing normally (although immediately and not delayed 1-5 seconds like the manual states). Looking at the repair manual (page 19) it mentions that a short on IC1 or IC2 could cause R1 and R2 to fail. It mentions to remove the ICs and test the voltages. Removing IC1 and IC2 only, does not produce a change in voltage across R3. According to the manual this implies a bad Zener diode ZD2. I replaced both ZD1 and ZD2 with new diodes, an still no change in voltage. By removing all 3 ICs (IC1, IC2, and IC3), the -16V rail returns to normal. This would imply that one or all of the ICs are bad, so I replaced them all (2x LF347N and 1x LF353N). However, with the new ICs in, the -16V rail returns to -.6V. I next turned to the filter capacitors C7 and C8 (47uF 50V). Since C8 has the bad voltage, I replaced C8 and still no change.

I am stuck, and any help would be appreciated. Where do I look next?

tl;dr: No load on PSU = Voltage Good. Load on PSU = Voltage Bad. Replaced ZD1, ZD2, IC1, IC2, IC3, C8

[budm]

Re: Infinity SSW-10 sub-woofer power suply

R3 (430 Ohms) for the -16VDC power supply still shows 430 Ohms?
Also measure the Voltage drops across that R3, if the power supply is loaded down so much to make the output to be only -0.60VDC that means the Vdrops on the resistor will be = 29V -0.60V = 28.4VDC which means the current flowing through the resistor is = 28.4V/430 Ohms = 66mA = about 1.88W of power so that 1W rating of R3 should gets really hot.
The design was made for 13Vdrops (29V - 16V) = about 30mA = about .040W (13V x 30mA) so they use 1W (good design will use at least twice as much power rating).

[Sierra117]

Re: Infinity SSW-10 sub-woofer power suply

Huh, R3 is showing 4.38K Ohm. R2 is showing 436 Ohm. They are both the same resistor though (4370 205A) so that must mean that it overheated when i put it in and changed its value. Time to get another resistor.

[budm]

Re: Infinity SSW-10 sub-woofer power suply

Yes they have to be the same value since they are for BI-POLAR power supply. So if the new resistor is damaged, that means you still have bad circuit some where that is drawing too much current.
BTW, in post 1:
" Upon opening the woofer the fuse and 2 resistors (R1 & R2: 430 Ohm 1W 5%) were blown." You mean to say R2 and R3, right? since R1 is the 1K resistor.

[Sierra117]

Re: Infinity SSW-10 sub-woofer power suply

Correct, that should have said R2 and R3.

[Sierra117]

Re: Infinity SSW-10 sub-woofer power suply

Thanks for the help. After replacing R3, my voltages went back to normal. I'll test the audio output tonight. I tested my op-amps (IC1, IC2, IC3) and 2 of the original ones had shorts, which would explain why R3 went bad after I replaced it the 1st time.

[budm]

Re: Infinity SSW-10 sub-woofer power suply

"The design was made for 13Vdrops (29V - 16V) = about 30mA = about .040W (13V x 30mA) so they use 1W (good design will use at least twice as much power rating)."
BTW, that is the typo, it should be 0.40W.

[Sierra117]

Re: Infinity SSW-10 sub-woofer power suply

I didn't even catch the typo. I read it correctly as 0.4W. The sub is working great Unfortunately my denon has now decided to act up

[taligent]

Re: Infinity SSW-10 sub-woofer power suply

Thanks for the write-up! I came across what seems to be an older revision of this sub, labeled as Infinity Infinitesimal Servo Controlled Subwoofer (no model indicator like SSW-10). R2 and R3 were burnt enough to flake off the paint and leave just the bare metal body of the resistors - yet amazingly they still measured around 430ohm.

A bad +/- 16v rail was the symptom here as well - the voltage drop across R2 and R3 was around 25v instead of the expected 11-14v. Removed the 5-wire ribbon cable leading to the pre-amp board to rule out IC1 and IC2, issue remained. Removed the LF347N IC3 on the amp board (on this PCB, it is labeled as IC1 but is IC3 in the service manual) and that fixed the +/- 16v rail. No shorts on the IC (except the intentional short on pins 8-9), but it was bad anyway - without the service manual I wouldn't have seen a reason to replace it based on in-circuit measurements.

I didn't have an LF347N laying around but TL074CN seems to be an equivalent substitute and I had a few of those so I threw that in and the sub came back to life. IC1 and IC2 on the preamp board seem fine, and R2 and R3 are pending replacement whenever I order parts next - but for now the sub seems to be doing just fine.

It's a bit anemic for home theater use but it seems well-suited for a smaller bedroom type 2.1 setup. At the very least it's no longer heading for the landfill - thanks for the discussion!