Technics SA-700 ‘smokin’ hot

[dustmaker]

I think best to wait until you get done with installing new stuff before doing anything else. It would be interesting to see if the outputs are still good, or got taken out by whatever is going on.

Spark 1 - do you have any issue with the capacitors I picked?

 

[spark1]

They should be fine. I assume you intentionally upped the capacitance value?

Are they board mounted? That would be unusual for big filter caps. If not, I have a tendency to use snap-ins nowadays because they seem to have the widest availability as regards sizes, voltages and capacitance. I then solder ring terminals to the pins, and make connections to the ring terminals. I use little alligator clips as heat sinks when attaching the ring terminals and when connecting wiring to the ring terminals.

When buying filter caps, I start with the diameter I need, then filter for desired specs. I don't like having to modify mounting for them.

 

[dustmaker]

They should be fine. I assume you intentionally upped the capacitance value?

Are they board mounted? That would be unusual for big filter caps. If not, I have a tendency to use snap-ins nowadays because they seem to have the widest availability as regards sizes, voltages and capacitance. I then solder ring terminals to the pins, and make connections to the ring terminals. I use little alligator clips as heat sinks when attaching the ring terminals and when connecting wiring to the ring terminals.

When buying filter caps, I start with the diameter I need, then filter for desired specs. I don't like having to modify mounting for them.

These are the only only ones I could find that are 40 mm, 63v or higher, and 15,000μF or higher. The specs call out 15,000μF. I would have upped it if I found one that would fit.
I haven’t fully figured out how to install them yet. My initial thought was to dremmel out slots where they would fit and then solder in place. The SA-700 has metal rings that screw through the pcb into a metal frame to hold them. My second thought was to install them upside down and attach 18Ga wire to the terminals and run the wires down the sides of the caps, underneath and through the holes to solder. Again the metal bracket will hold all in place. My question here would be, would 6” of wire affect the function of these caps?
I’m wide open for other solutions on how to attach.

I’m not familiar wi snap-ins, but I apparent didn’t find any that would fit

 

[spark1]

Wire would not affect function, but is (obviously) not very aesthetically pleasing. I've never seen an arrangement like the one you describe, so can offer no help.

 

[dustmaker]

My order came quicker than expected. As you can see from the picture, there is no way I can use these caps “as is” in the board. It won’t even work to dremel out the board. I have a plan and I’ll post tomorrow after I’ve had a chance to create a “fix”

 

[dustmaker]

The Good and Bad

Good
I installed the new e-caps - what I did was bend the pins to 45° in opposite directions. I attached an 18Ga wire to them and used a thick rubber washer to provide stability. The 18 ga wire went through the pcb to solder. When I attach the metal hold-down bracket, it looks like an original installation. I also installed the new rectifier diode. Since I have the bottom bracket off, I replaced all e-caps and badly burnt resistors.

Bad
I obviously have a short somewhere as my 60 watt DBT is medium orange with a variac adjusted to 60volts. resistor R639 smokes after 5 seconds. I did some very quick measurements and I’m not reading any AC current in the circuit, but it is running hot. The newly replaced R639 has already blackened and I had previously replaced the original 1/4 watt with a 1/2 watt resistor.

Suggestions?

 

[dustmaker]

Additional info
I adjusted my variac to around 40volts so that the receiver doesn’t smoke. Measuring the current at the new rectifier diode, I get -15v dc. and +7v dc which then counts down to about 4 volts. I had similar readings on old rectifier which is why I replaced it, so my issue is obviously down stream

 

[dustmaker]

I removed R639 and R641 resistors and the short went away. In case they are still good, I’ve also removed the pins to the power transistors. I removed all transistors on the left channel and I am going to add the resistors back in. If the short is there, I’ll remove them again and continue trying to narrow down the faulty part(s). I currently have pins pulled for amp in/out and I’ve pulled the fuses for speakers to eliminate any other area

right now it looks like my problem is on the main board and/or the tone amp board.

 

[dustmaker]

Update -

I realized I over did it in removing components. Since it was only the positive side that was problematic, I only needed to remove capacitors where they were connecting between positive current and ground in search of a short.

The short resides with the output transistor board. I’ve since reinstalled all components I had desoldered (except transistors which will all be new), and I am able to connect to live outlet without issues.

Two of the NPN transistors are open. The PNPs seem to be ok. This board also has 8 x 0.82Ω/5watt resistors and only one measures near 0.8 Ω.. one is over 20 Ω and another is over 2000 Ω. One is O.L.. Remainder are around 1Ω. I’m writing my order for 8 replacement 0.82Ω/5W resistors.

here are the DMM readings when I tested the outputs. I’m only including the 6 that appear to still be working
2SD427A
B-E. 500. B-C. 471
B-E. 500. B-C. 470

2SB557A
B-E. 516. B-C. 507
B-E. 498. B-C. 480
B-E. 469. B-C. 468
B-E. 449 B-C. 448

Question: are these still good? Since I already have this unit apart, should I replace all or would you recommend I keep 2 pair of the old ones in? Each transistor is over $7 so I am torn between replacing all or saving $30 and only replacing half.

also, since I am placing another order, what other parts do you recommend replacing? I am doing all e-caps and transistors. What about the diodes? Fusible resistors slightly darkened? Thermistors? Trimmers? Any other components known to be problematic?

 

[dustmaker]

I’m looking at replacing the diodes. The SA-700 has the following
D601-D602 SVDMA26-1. Replace with 1N4108? - found on another thread
D702, D901-D902. SM112. replace with 1N1007? -found on another thread but can’t find diode
D702 SVDMZ336B. 36v Zener. Replace with ? 1N5258B? TZX36C?
D703. SVDMZ330B. 30v Zener. Replace with ? 1N5256B? TZX30C?
D801. SVDMZ214A. 14v Zener. Replace with ? 1N5244B? TZX14C?

recommendations on replacements? I’ve searched AudioKarma and can’t find a substitution listing

 

[milo63]

Attached photo shows bottom of receiver with metal bracket. It’s held by 10 screws. 4 from pcb to this bracket, 2 from transformer to bracket and 4 from frame to bracket. I could remove 9 and couldn’t figure out how to remove last screw. The screw head was underneath pcb and it went through bracket into frame. I could only access the screw threads. I solved the problem using a Dremmel and I cut a slot on the thread side of screw and was able to remove it using a small standard scewdriver

I had to remove all screws on back plate and pop out plastic pins to provide clearance to remove the bracket

Gracias, I'm fighting a power supply issue and that dang plate is in the way. Nice solution.