Soundstream Reference 700s

Dr Zeus

Member
2010-10-17 5:12 am
Hello world! Its been YEARS! (STRETCH!)

I've unfortunately sold most of my repair equipment. Still have a couple of Fluke meters though!

Soundstream Ref 700s. I got it with all outputs and one of the output driver boards blown (A1479) driver. Most of the gate/bias resistors on the outputs also blown open. The little Bias? transistors under the board between the outputs also blown.

I removed both driver boards, and ALL TIP102/107 output transistors from both channels, clipped off anything blown shorted, blown open, that I could find. With the little knowledge I remember; I got the amp to the point where most points measure similarly between channel 1 & 2, and nothing else seems to be shorted on the audio side of the pcb.

None of the 6x power fets are blown open/closed. Gate resisters are all good. Nothing seems shorted.

So next I check power. Again, no REAL equipment. 7.5a fuse inline with a small 12vDC lead battery. Also some 30ga clip leads. I get this amp to power up as careful as I can without the proper equipment.

Upon power-up, the High Power light lights up and stays ON the whole time. Within seconds, the amp draws power down from 12.5vDC down to about 7vDC at the amp power terminals. My clip leads start to get slightly warm, but the 7.5A fuse doesn't blow. I think my clips leads may be pretty bad. They are cheap from Amazon; but still this amp seems to be drawing way more than it should even through these cheap test leads. I do not want to put larger power wires on it at this point - it might pull down amperage and blow more parts.

I get rail voltage. First it starts off at 32vAC across the -&+ rectifiers but it drops to about 20vAC within about ~5 seconds of runtime/draw.

Nanny voltage regs start at about 10vDC and get dragged down to 8vDC.

I pulled the 4x Rectifiers and amp does the same thing. The rectifiers off the board measure good.

I have not let the amp sit powered like this for more than about 10 second. I smell something slightly like a circuit that might want to let the smoke out or maybe trying to get warm (Or the amp just hasn't powered up in forever), but nothing gets really warm at all.

Thanks for your time. Hope to fix this one for my personal use!
 

Dr Zeus

Member
2010-10-17 5:12 am
No. No difference.

There are 4x 1ohm large resistors near the transformer. Those are getting warm but expected I guess. Looks like they've been heat cycled so much so that their legs are pretty weak and the pads on the pcb look to have been slightly cooked. They're measuring about 1 ohm.
 

Dr Zeus

Member
2010-10-17 5:12 am
Amp powers with larger/thicker wire and stays on, with Rects in circuit. Minimal voltage drop. 7.5A fuse stays in-tact.

Couple things I'm noticing on this amp.

1. R176 gets really hot, starts to smoke - Not normal. Its a medium size 10ohm resister
2. Referencing ground, theres +8.6vDC on the RCA shields.
3. Not sure if the voltage regulators are working correctly.
U5 is measuring 9.4, 9.4 11.2 vDC
U6 is measuring -7, -11.2, -8.3 vDC
 

Dr Zeus

Member
2010-10-17 5:12 am
Yes I did. There are no changes that I can tell.

So the primary section of the transformer has 4 legs. Each leg has a small bundle of 3 sets of wires. All four of these legs are measuring 0-ohms. They appear to be tied all together at the PCB.

The secondary section has 2 legs. Each leg has a small bundle of 2 sets of wires.

Resistance is measuring in the millions between the primary and secondary legs.

Do you think the transformer may still be shorted? I could remove it.
 

Dr Zeus

Member
2010-10-17 5:12 am
I took both regs out. One is a a Motorola lm337t. The other is a National lm317t. The National looks kind of fake.

Now this the amp powers with +- 43vdc at the rectifier drains and the resistor stopped smoking.

The regs off the board are not measuring shorted or open.
 
Was the resistor smoking with no RCAs plugged in?

Does switching from balanced to unbalanced on the input make a difference on the RCA shield voltage?

The secondary ground is the point where two of the secondary windings on the power transformer are directly connected and also directly connected to R176. The other side of the resistor is designated as the signal ground on the diagram. Post the DC voltage on both sides of that resistor with the black probe on the main ground terminal.