• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Sencore SG165 - AM/FM Analyzer - Repair

The blue caps read ok, but will be replaced anyways:
3cSciHs.jpg


B7lg4S6.jpg


I think I found the problem, the 2uF caps are reading almost twice as high, and check out the ESR:
5hem0a5.jpg


BE4iMBj.jpg


The small caps are definitely a problem, will start with the PSU recap. I'll try to get all of the components, transistors too, just in case.
 
mrk229 said:
View attachment 2018121

Based on your picture vs the schematic I have you have C105. 106 marked wrong. They are 1000uF it looks like they should be C103 and C108 which would have the unregulated voltages on them.

C105, 106 would be the small blue caps and should have the regulated voltages on them. If the schematic is correct and you were actually measuring C102,108 then when unplugging the load the unregulated voltages would rise.
Click to expand...

Thanks for the reply! The troubleshooting guide says measure the + side of C105, on the schematic + is tied to the 12V rail for C105. The guide asks to measure the - side of C106, which is tied to the -12V rail. I should be seeing no more than 12V +/-, yet I've got 28V +/- and 31V +/- with the RF board unplugged. Hoping it's the caps and not the transistors.
 
MarantzRefrb said:
Thanks for the reply! The troubleshooting guide says measure the + side of C105, on the schematic + is tied to the 12V rail for C105. The guide asks to measure the - side of C106, which is tied to the -12V rail. I should be seeing no more than 12V +/-, yet I've got 28V +/- and 31V +/- with the RF board unplugged. Hoping it's the caps and not the transistors.
Click to expand...

Perhaps I misunderstood what you wrote vs the picture where you labeled the large silver capacitors as C105, 106. And wrote: "I'm getting 28V + and - on C105 and C106, more than double the expected value."

The schematic clearly calls out those 1000uF as C103 and 108. Not C105,106. They are the unregulated side of the supply and should have 28 volts. The small blue caps C105, 106 are on the regulated side of the supply and should have the +- 12 volts. By removing the output load "Molex to RF" the supply becomes unloaded and the +-28 volts will rise a bit.

Your picture clearly shows the PC board silkscreen capacitor numbers. (minus 100)

Those older small value caps generally have higher ESR than new ones. Older meaning manufacture date not old as in worn out/bad or heat damaged. New caps have much better guts than the old ones did. However, I would replace them anyways while you have it apart. Also as a general rule of thumb, smaller electrolytic caps have higher ESR.

sc.jpg

The other CR you were looking for appears to be one of the yellow boxes, probably the left one.

Be careful of that C4 cap (orange box) make sure that lead does not touch the screw/nut on that heatsink. It might let the smoke out!
 
Unless we are not on the same page? The chart is correct.

sgp.jpg

Did you measure for +- 12 volts at on C105,106 at the orange boxes or the yellow boxes? From the image it looks like you marked the silver caps as 105, 106 and they are
actually C103,108.

Were you maybe indicating the C105, 106 were next to them?

Can you see where my questions/confusion are coming from?
 
Now I understand. I didn't mark those caps - I just got this SG165 in the condition you see above. I did go by those numbers thinking that was factory, thank you for catching that.
 
I'll put the stock caps all back in and make measurements from the correct location now :thumbsup:.

L28BwVh.jpg


L28BwVh.jpg


And this does look to be the zener diode, at cr5:
ih46MFe.jpg


Thanks again!
 
MarantzRefrb said:
I'll put the stock caps all back in and make measurements from the correct location now :thumbsup:.

And this does look to be the zener diode, at cr5:
ih46MFe.jpg


Thanks again!
Click to expand...

You are welcome, good luck. And I re-read the post where they referred to checking the rectifiers and I think they meant CR1-4 which are right in front of that Molex.
 
I found something strange when I went to put the caps back in.

Here is the PSU board as I received it, C107 appears to be backwards according to the print on the board:
W4v4f7S.jpg


bz9D6eO.jpg
 
mrk229 said:
You are welcome, good luck. And I re-read the post where they referred to checking the rectifiers and I think they meant CR1-4 which are right in front of that Molex.
Click to expand...

I checked the rectifier diodes earlier, CR1 and CR2 were both about 14mV, CR3 and CR4 were about 30V. I'll put this back together and see what it looks like again.
 
MarantzRefrb said:
I found something strange when I went to put the caps back in.

Here is the PSU board as I received it, C107 appears to be backwards according to the print on the board:
W4v4f7S.jpg


bz9D6eO.jpg
Click to expand...

Boards are not always correctly marked. Odds are pretty good it is correctly installed. Clues can be found on the solder side, did it look like a factory solder job?
Does the cap look similar to others of its type and size?

Now have a look at the schematic and trace it out. (Schematics are not always right either) If this schematic is correct then the - end connects to R116 and the positive goes to the base of TR105.
 
On the rear side , C107 was oddly protruding at the anode, longer lead left behind - did look factory though. The other 2uF cap that matches it, follows the correct convention marked on the PCB.
 
I know one of the caps on the power supply board was silkscreened wrong, but can't remember which one. It may not be true for all builds either.
 
Wrong or right, it's back together the way I got it. And looks to be working correctly.

C105 +
je5rq1r.jpg


C106 -
JglTjEt.jpg


What a long detour that was. At least I know the PSU is not to blame and can move on to the next steps. Much thanks mrk229!
 
ConradH said:
I know one of the caps on the power supply board was silkscreened wrong, but can't remember which one. It may not be true for all builds either.
Click to expand...

Just found and confirmed the incorrectly screened polarity on the PSU of a Run 15, it's C107.

AKA C7:
bz9D6eO.jpg


Thanks!
 
It may or may not be marked incorrectly. Your photo shows the POSITIVE terminal on the electrolytic marked, not the negative terminal as is more typical.
 
I'm not really that worried about the silk screening being incorrect, because I know that is, when compared to the other 2uF cap just like it. You can't have two identical caps, and one of these be correct, and the other be labeled opposite, and have them both be right. One of them is wrong.

Moving on.

I'm here in the troubleshooting flow chart:
iRN91eT.jpg


When I pull the RF Phono plug, the output to All Signals Out, is completely dead. I was able to test MPX Output and 400Hz Sine Wave output, those were working before I pulled the RF plug:
lbg0ceT.jpg


Also, the caps that are attached to the chassis are in bad shape too, one dented:
O2hyKNs.jpg


One growing a tumor:
d3x0kXG.jpg


I'll need to drill out one of the stripped bolts holding a front panel knob on, in order to get this all apart, and down to the TR316 on the RF main board:
0UTKnPX.jpg
 
You must log in or register to reply here.
Back
Top Bottom