SX-3700 Recap/Restore Thread

ohgoshjosh

New Member
First time posting, short time lurking. Found a lot of very helpful info to repair and recap my SX-3700. This all started when I noticed the speaker protection relay loudly clicking, and eventually failing completely to push power to the speakers. I thought replacing that relay would have done the trick, but if I had read these forums more closely, I would have found out my problem likely lies elsewhere.

That brings me here, where I'm going to attempt to recap the Power Amp, document my process, and hopefully learn a few things in the process. I haven't seen a ton of SX-3700 recap threads on AK, so I hope this might be helpful to others in the future. I've read the 3700 Service Manual and the Tuning Fork guide on Pioneer caps.

First, here's a draft of my cap list. I'd appreciate any feedback folks have here. I'm a bit unclear about the difference between Nichicon caps that end with 1TD,1TA vs no suffix after M[x]D.

2x ACH-215; 8200uF/50VDC; 647-UKW1H103MRD
2x ACG-004; 0.01uF/150VDC;
2x CEA R47M 50L; 0.47uF/50VDC; 647-UKW1HR47MDD
3x CEA 010M 50L; 1uF/50VDC; 647-UPW1H010MDD1TD
1x CEA 470M 10L; 47uF/10VDC; 647-UPW1A470MDD
1x CEA 470M 25L; 47uF/25VDC; 647-UPW1E470MDD
3x CEA 470M 50L; 47uF/50VDC; 647-UPW1H470MED
3x CEA 101M 50L; 100uF/50VDC; 647-UPW1H101MPD
1x CEA 221M 35L; 220uF/35VDC; 647-UPW1V221MPD
1x CEA 221M 50L; 220uF/50VDC; 647-UPW1H221MPD
1x CEA 331M 10L; 330uF/10VDC; 647-UPW1A331MPD
2x CEANL 100M 50; 10uF/50VDC; 647-UKL1H100MDDANA
2x CEANL 2R2M 50; 2.2uF/50VDC; 647-UKL1H2R2MDDANA
2x CEANL 4R7M 50; 4.7uF/50VDC; 647-UKL1H4R7MDDANA
2x CEA 4R7M 50L; 4.7uF/50VDC; 647-UPW1H4R7MDD
2x CEA 100M 50L; 10uF/50VDC; 647-UPW1H100MDD
2x CEA 220M 25L; 22uF/25VDC; 647-UPW1E220MDD
2x CEA 101M 35L; 100uF/35VDC; 647-UPW1V101MPD
 
replacing these 38 year old caps with just about any modern caps will be an improvement, for the caps, but its likely not your problem.

the relay unclicked because you either have and overload or big DC, something broke. but its fixable. the manual might be here, but it is on hi fi engine. you will need it for reference. our resident pioneer sultan may be showing up shortly to tell you exactly what broke, but if it were mine, I like to isolate the outputs, then go and verify all the different +/- voltages from the power supply area. if by any chance you disable the outputs and it clicks and stays clicked...that points to something..
 
"Quaddriver" is right - the caps won't fix it.
It may only need an adjustment to the amps. Start with the set on the DBT and read the service manual, page 19 for the procedure.
Ask questions and don't attempt any adjustments. Record only and report.
What's a DBT?
http://audiokarma.org/forums/index.php?threads/dim-bulb-tester-build-and-how-to.808399/
Build the DBT before you do anything else.
Use micro grabbers for the DMM leads. Never remove or install the DMM leads when the set is on.
Black probe to the chassis, red probe to the amp test pin(s). DC voltage, auto range is fine. Readings will be in the millivolt range.
 
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Also the power supply has some transistors that like to unsolder themselves.
Probably look at the back of the PS before moving ahead.
 
All good advice above. +1

I'm a bit unclear about the difference between Nichicon caps that end with 1TD,1TA vs no suffix after M[x]D.
I can address this question. The 1TA, 1TD, etc suffixes are only on products that come on a tape applicator designed for machine insertion. Not that you can’t use them but the suffix doesn’t apply to your application.

On the main filter caps I might tend to go up in voltage a little. You’re running at least 41v through those caps and 50v doesn’t give much margin. Higher voltage replacement will likely yield longer lifespan of the capacitor as it’s not running on the edge of spec. Another critical measurement is lead spacing. These filter caps are board mounted and need to be fairly accurate with the spacing or you risk awkward installation, leads bent too far, and bad mechanical connection.

Enjoy the project!

Mike
 
As Mark (The Sultan) would say "Danger Will Robinson!!!!!"
The SX-3700 is an NSA amp design and is a very tricky. High frequency transistors that are hard to find, although not impossible.
"Quaddriver" is right - the caps won't fix it.
It may only need an adjustment to the amps. Start with the set on the DBT and read the service manual, page 19 for the procedure.
Ask questions and don't attempt any adjustments. Record only and report.
What's a DBT?
http://audiokarma.org/forums/index.php?threads/dim-bulb-tester-build-and-how-to.808399/
Build the DBT before you do anything else.
Use micro grabbers for the DMM leads. Never remove or install the DMM leads when the set is on.
Black probe to the chassis, red probe to the amp test pin(s). DC voltage, auto range is fine. Readings will be in the millivolt range.

Thanks to everyone for getting me going here. Went out and built a DBT after work today. Using a 75W bulb, it’s very dimly lit when power is on. Following the procedure on Page 19 (8.3 Idle Current Adjustment), here’s what I’ve got:

Per the instructions on p. 19, between the test pins:
TP48-49 (L ch) 14.7mV (fluctuating between 5-14.7mV)
TP47-46 (R ch) 2.2mV

Test pins to chassis:
TP48: 6mV
TP49: 20mV
TP47: 36mV
TP48: 34mV

Visually inspected the pack of the PS. Didn’t see any alarming solder joints.

One more thing I observed: When the unit was last plugged into AC, I would often hear a subtle “vibration” noise coming from the speaker protection relay. Now that it’s plugged into the DBT, I don’t hear it at all anymore. Is that because the DBT is taking up that power that was formerly making it to the relay?
 
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All good advice above. +1

I can address this question. The 1TA, 1TD, etc suffixes are only on products that come on a tape applicator designed for machine insertion. Not that you can’t use them but the suffix doesn’t apply to your application.

On the main filter caps I might tend to go up in voltage a little. You’re running at least 41v through those caps and 50v doesn’t give much margin. Higher voltage replacement will likely yield longer lifespan of the capacitor as it’s not running on the edge of spec. Another critical measurement is lead spacing. These filter caps are board mounted and need to be fairly accurate with the spacing or you risk awkward installation, leads bent too far, and bad mechanical connection.

Enjoy the project!

Mike

Mike, thanks for your recommendations about the caps here. Once I can figure out the power issue, I’ll return to the caps. The lead spacing is something I had not considered. I guess I thought these were of some “standard” sizes. Should have known better!
 
Pull the jumpers from the back of the receiver and re read the idle millivolts once again.
Look for the white jumper and report whether the jumper is cut or not. Both channels.
Just do the check in the service manual.
One more time.
You want a fairly stable reading between those pins.
Per the instructions on p. 19, between the test pins:
TP48-49 (L ch) ?
TP47-46 (R ch) ?
 
Pull the jumpers from the back of the receiver and re read the idle millivolts once again.
Look for the white jumper and report whether the jumper is cut or not. Both channels.
Just do the check in the service manual.
One more time.
You want a fairly stable reading between those pins.
Per the instructions on p. 19, between the test pins:
TP48-49 (L ch) ?
TP47-46 (R ch) ?

Didn’t see any white jumpers, but saw two red ones that look like they had already been cut (I didn’t cut them) — they look like the same ones in the manual — are they?

Now both the power meter and the frequency displays on the front don’t light up anymore, but the bulbs on the tuner panel do. What did I do?
 
Its not in tuner mode?
Red wire right next to the resistor R319, R318?
The connectors for the outputs are very cheesy. Check for good contact and security.
Would like to see the Idles with the jumpers removed.
You can keep it on the DBT.
 
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Its not in tuner mode?
Red wire right next to the resistor R319, R318?
The connectors for the outputs are very cheesy. Check for good contact and security.
Would like to the Idles with the jumpers removed.
You can keep it on the DBT.

Even in tuner mode, it’s not lit. Neither is the watt meter.

Yep, red wires right next to R319,R318.

Idle voltages w/o jumpers:
TP48-49: 14mV
TP46-47: 2.2mV
 
Here are the readings for those pins. The board diagram from the manual doesn’t exactly line up with the arrangement of the pins on the board. I couldn’t find a pin on that board labeled 18, 32, or 24.

20: 18.35V
22: 6.1V
25: -20.2V
26: -12.9V
23: 12.8V
24: No pin

18: Cannot locate
19: 0V
21: 0V
27: No pin (empty hole in board labeled 27)

30: 25.9V
31: 26.0V
29: 26.1V
32: Cannot locate
 
Power supply is good.
The Idles are OK.

The DC balance needs to be checked. We should of looked at that earlier but the display turned our attention.
The difficulty in checking it is the relay is not engaging. so checking at the speaker terminals wont give a reading. We have to locate a point between the amp board speaker output and the relay. I don't have a set here to look at.
There's a Henry coil at the output of each amp labeled L1 for the left and L2 for the right. Can you grab at those points and check the DC Balance?
Black probe to chassis and the red probe to the relay side of the coil. Set on the DBT. The reading should be close to zero volts.
 
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Must be a protection issue.
Everything is checking out.
Although that right amp could use a little more Idle/bias. :dunno: 2.2mV's is right on the edge and no jumper to cut.
Did you find a rebuild list for the protection circuit?
 
The SPC and the Power Amp are combined in the Parts List in the service manual, so I’m making a guess based on the circuit diagram. Should I also be swapping out the resistors and diodes?

Transistors
Q21,Q22 (2SC1914A): 512-KSC2383YTA
Q23,Q24 (2SC945A): 512-KSC945CYTA ** I got this from another AK post for part no. 2SC945, missing the “A” — what’s the difference? Seems like maybe the leads are switched around for “A” models?
Q25 (2SC1384): 512-KSC2690AYS

Caps
C111,C112 CEA R47M 50L 647-UKW1HR47MDD
C113 CEA 331M 10L 647-UPW1A331MPD
C114 CEA 101M 50L 647-UPW1H101MPD
C115 CEA 010M 50L 647-UPW1H010MDD1TD
C116 CEA 470M 50L 647-UPW1H470MED
 
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