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SG-9500 recap, but a headscratcher, at least for this rookie

hertzdonut

hertzdonut

Press Play and Stand Back
(Preface: I've fully recapped two SX-850's and a PL-630; so it's not my very first prom. ;))

My SG-9500 worked great previously. My choice to recap was simply for longevity's sake. I hit all switches with DeOxit first, and then went on to recapping once my parts arrived.

I started on the power board for this SG-9500, and have replaced 11 'lytic caps. Being a believer in "check-as-you-go-along", I plugged the unit in and flipped the power switch. No light. Hm, not what I was expecting. (fyi, there was NO noise or smoke or anything. Just...no power)

Going back to the beginning, I rechecked the values on the caps, comparing old, removed caps to the new. I then verified that each new cap was properly oriented. (When I was progressing on the install, I pulled each cap, one at a time, and compared the board to the schematic. All matched up: values, polarity, etc. The only difference was my "B" version board sported two additional caps)

Next, I double-checked my solder work joint by joint. No cold joints, no bridging, no spatter. Then, plugging the unit in, and flipping the power switch on, I carefully wiggled each cap, checking to see if the light came on or even flickered. Nothing.

I shut everything off, and decided to check the fuses. Pulled, checked, all 4 are good.

Lastly, I went to the schematic to find where to take some readings. I'll be the first to admit I don't know crap about schematics (other than identifying parts) but I saw where there should be some voltages on some pins. I grabbed my DMM, and took readings. And those were good too. (I’ve uploaded the schematic and highlighted the three areas [five pins] that should have voltage.) (I’m also uploading the foil side image, too, if that helps)

Sooo, before I tear all the caps back out, is there somewhere else I should be looking? If I’m getting proper readings at the pins, doesn’t that mean the board is performing properly?

Oh, one last thing: Thinking that maybe just the bulb itself (the power indicator lamp) was blown out, I hooked the unit back into my gear, but nope, there is no power.

Thanks for any help shared!

Jeff



 
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You are going to have to learn to read a schematic and find out where to measure the important voltages as they have printed on the schematic.
Looks to be the standard full wave rectifier and a series pass regulator arrangement. Check AC voltages at the fuse and then DC voltages after the rectifier, then regulated voltages that go to the loads.
 
Muting circuit might be messed up check for proper caps and polarity in locations C16 and C18. (?)
You need the voltages to be per the schematic. +-14.4 regulated and the 14 volts the muting circuit puts out from pin 15 (PS). That .6 volt difference is important. It turns everything on.
 
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Do as the others have suggested and start taking voltage readings at the supply. On the supply take AC readings between pins 6 and 7. Then go between pins 4 and 6 then 4 and 7.
Its easy to assume the AC is there, but need to check anyway.
 
Thanks for a starting point, gents! I will take a look tonight and see if I can make sense why there's no power. (That, and learn what a rectifier is, a series pass regulator is....)
Jeff
 
Ok, got a chance to check some stuff. I'll start with the easy one first. Here's visual confirmation of caps C16 and C18 (and I threw in C17 as well). The values, as compared to SM and the actual old caps themselves, are correct. (I've got bipolar caps in there, not for any particular reason other than that's what I ordered. My understanding is that those are acceptable in this situation. The reverse situation would not work. Meaning, polars in a bipolar spot)

Two images (taken with my phone, sorry!)




I took readings as instructed...at least I think I did.
The following were taken with red lead to fuse, black lead to chassis as ground:

Fuse 1 (SM calls it primary): 18 Vac
Fuse 2 (SM calls it secondary): 18.7 Vac
Fuse 3 (SM calls it secondary as well: 18.7 Vac
Fuse 4 (Lamp circuit): 7.4 Vac

The following were taken with red lead to pin, black lead to chassis as ground:

Pin 8: +14.4 Vdc
Pin 9: +14.4 Vdc

Pin 13: -14.3 Vdc
Pin 14: -14.3 Vdc

Pin 15: +13.84 Vdc

The following were taken from first pin to second pin

Pins 6 and 7: 37.8 Vac
Pins 4 and 6: 18.8 Vac
Pins 4 and 7: 18.7 Vac

@rcs16: I now know that a full wave rectifier converts both polarities on the AC waveform to DC pulses. (Still over my pay grade, but hey, at least I looked it up!) And the series pass regulator is a way of providing voltage regulation. Yep. Ohhhh-kay.

@zebulon1: You referenced an important .6 volt difference. I'm just not sure where that is. None of my readings indicated a difference of anything close to that. Unless...one of my readings indicates bad. So, maybe I should be seeing it, but am not.

I appreciate the help guys!

Jeff
 
Ok, so now we know there is AC voltage going into the supply board, and regulated DC at the output terminals. Now go to the main boards and check to see if you have +14 volts
at pin 18 and -14 volts at pin 20, and also the 14 volts at pin 15. These voltages should be present at both channels. Use pin 19 at the main boards to ground your meter. If you don't get proper voltage try grounding your meter at the power supply board.
 
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Thanks for the fast reply Glen!

Here's what I get:
Left board:
Pin 18: +14.4 Vdc
Pin 20: -14.3 Vdc
Pin 15: +13.87 Vdc

Right board:
Pin 18: +14.4 Vdc
Pin 20: -14.3 Vdc
Pin 15: +13.85 Vdc

Using pin 19 for ground worked great
 
Hey, I can't subtract.
It looks like the power supply is working as stated in your first post. The voltages look good at the PS pins 8,9 13, 14 and 15.
So:
Why does the power lamp remain off? It is an AC feed from the primary side of the transformer and you said the fuses were good. Pull the fuse (F4)(with the set unplugged) and ohm out from one fuse holder to the other and see if you have continuity of some value. If no continuity then the lamp is out or a wire or fuse holder(?) is loose.
Is the EQ working? The description of the muting circuit says Q6 has to turn on making +14v at pin 15 (+Vcc). It looks like it is. For a short time after the power switch turned on pin 15 is negative (-) as it is in muting. Then switches to a positive 14v's on pin 15 after the set warms up which releases the gate of Q11(s) on each amp channel . Each channel has it's own output gate so the likelihood of both being inop is remote, unless something happened that you didn't fully explain in your first post. :biggrin:
Take a reading from pin 15 as you turn on the set and see if it switches from neg to pos.
At least that's how I read the explanation of the circuit.

Late post: I'm glad to see Glen is watching/helping in this thread too.
 
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hertzdonut said:
Thanks for the fast reply Glen!

Here's what I get:
Left board:
Pin 18: +14.4 Vdc
Pin 20: -14.3 Vdc
Pin 15: +13.87 Vdc

Right board:
Pin 18: +14.4 Vdc
Pin 20: -14.3 Vdc
Pin 15: +13.85 Vdc

Using pin 19 for ground worked great
Click to expand...

Just want to back up a bit here and ask a question. You said that you checked-as-you-went -along. Did you actually try connecting the EQ into a system to see if it was passing sound?
 
zebulon1 said:
Hey, I can't subtract.
Click to expand...
No problem, I can't either. Nor add, multiply or divide!

Continuity check:
Power off, I pulled fuse F4, and put a lead to the input side of the F4 fuse holder, and the other lead to the input side of F3 fuse holder and got continuity (fuses 3, 2, and 1 still installed). Keeping the first lead on the F4 holder, I moved the second lead to input of F2 and got continuity. Moving the lead to F1 input side of the fuse holder, there's no continuity. Not sure if that means anything or not.

EQ check:
You are correct zeb: Checking pin 15 on both EQ boards does indeed show negative volts upon power up (gets to about -12 or so), then switches to positive after a couple of seconds.

Lol, I think I covered it all in my first post, so no dropped screwdrivers with power on or anything! ;)

Thanks, Jeff
 
glen65 said:
Just want to back up a bit here and ask a question. You said that you checked-as-you-went -along. Did you actually try connecting the EQ into a system to see if it was passing sound?
Click to expand...

Sure did! That was my last test before I started this thread. I was thinking how foolish if I did all this troubleshooting if it was only a bulb issue! (meaning, I'd be the fool!)
 
hertzdonut said:
Sure did! That was my last test before I started this thread. I was thinking how foolish if I did all this troubleshooting if it was only a bulb issue! (meaning, I'd be the fool!)
Click to expand...
Ok, did you get sound when you set the three position equalizer switch to its off position?
 
It sounds like it's working?
The lamp check on fuse 4 is pull fuse 4 and probe both open holders of fuse 4. One probe on each open holder. Look for continuity. If there is continuity the lamp should be working. There or other ways. Remove the lamp and ohm it out directly. Or, Check for AC voltage at some point in the lamp circuit.
 
Well, zeb and glen, apparently I can do math. In fact, here's an equation I just learned: Idiot = ME

Zeb, I put one lead on one end of (the empty) fuse holder number 4, and the other lead on the other end of fuse holder number 4, and read the result. No continuity. Hm...just a blown bulb after all this?

So Glen, your question about the switch position got me thinking. When I did my last connection test, before I started this thread, was I positive I made the correct connections, and switches properly positioned? No, I was no longer positive. I grabbed the equalizer, and added it back into the stack. Forgoing any connections to my reel to reel and RG-2 expander, I went directly out of my SX-850 (from Tape 1) into the 9500. I flipped both units on, and waited for the relay to kick on. Then, on the 9500, I made sure the Tape Monitor switch was set to Off, and and the Equalization switched to On. I grabbed a handful of sliders, listened to the music, and slid the entire left channel down. And sound-wise, the left channel went down accordingly. I slid all up to max, and yes, all the frequencies shot up in volume.

Oh. Em. Gee.

Let me say it again: Idiot = ME

I repeated the sliding test with the right channel, and it worked just fine. Obviously, when I did that original test, I either cross-connected my cables, or didn't have the switches properly positioned. Gah.

Such a bonehead. I've now ordered a replacement lamp (plus a spare) from dgwojo.

Thank you for all your help, and I feel I owe an apology to you; zeb, Glen, and rcs for wasting your time.

Jeff
 
Although I didn't have a part in this thread, I'd like to mention that admission of a facepalm in this arena garners my respect.

Problem solved, no blood, no foul (LOL).
 
I had a little scare, last month, after recapping one of these. I did not remember that these have muting circuits, and was disturbed that it did not wake right up and start singing. I had already replaced the power indicator with an LED, so I did not have the additional, misleading symptom. I was provoked to probe around a bit, myself. That novel muting circuit had me going for a few minutes, since the schematic shows the post-wake-up voltage state. Seeing a negative voltage there was disconcerting, to say the least. I reverse-engineered the circuit and determined that it was working fine.

So, Jeff, don't sweat it. Any day that you can learn something cool, is a good day.

Enjoy,
Rich P
 
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