Sencore SG165 - AM/FM Analyzer - Repair

[mrk229]

Could I make the proper replacement out of a 10K ohm, with resistors to bring it in to spec:
10K OHM DUAL 2 WATT LINEAR CLAROSTAT POTENTIOMETER LONG SHAFT NOS 1 PCS.

Or just use a 100 ohm pot, with a resistor like xraytonyb said he did:
100 Ohm 5 Watt Wire Wound, Long Shaft Potentiometer - Centralab WW101

While ideally it would be great to have a 75 ohm match, the 100 ohm solution should be good enough. The idea is to keep a fairly constant ohmic value (impedance) as you go from one end to the other.



This is from the SG165 schematic. Look at what happens to each element at the shaft is turned. Full clockwise R10A is bypassed but R10B is full resistance.
Go counterclockwise and you get the opposite effect. In the full CCW position, R10B is a short to ground so no output.

 

[MarantzRefrb]

The thing I can't understand is, aren't I going to need (2) pots like the original design is, to accomplish this same circuit. How was XrayTonyB able to use a single long shaft pot to complete his?

Thanks!

 

[MarantzRefrb]

Got all of my replacement transistors for the SG165:
(MPF102 = NTE451) TR316, 308, 312, 313
(SE3002 = NTE108-1) TR301, 302, 303, 314, 315
(2N5227 =NTE159) TR310, 318, 322
(2n5172 = NTE289A) TR304, 306, 307, 323, 324
(MSP5172 = NTE123AP) TR320, 321

I'm fully aware of the NTE product, and opinions thereof, but there is no other equivalent that I know of.

Also bought and pulled apart a Clarostat 750 ohm board, to replace the rear board that was burned in this L pad, new rear board is already installed:

If that won't work, Plan B is a 100 ohm pot, with a resistor to match 75 ohms. This is the way XrayTonyB, and others, have said they did theirs.

Will complete the L pad, then replace the transistors, and then order caps for the SG165 RF board.

 

[MarantzRefrb]

The original 75 ohm L pad had a burn mark on the surface of the rear rheostat; when the contacts would touch this spot, or beyond, the resistance would fluctuate wildly.

Disassemble stock L pad, rear cover off:

Burn spot:

The disassembly of the new 750 ohm Clarostat replacement pot, was difficult because it was sealed with a high temp glue. Used a heat gun to loosen up all the glue and remove the 750 ohm board.

Stock L pad chassis and 80 ohm board, with new 750 ohm rear board, and new rheostat contacts:

JB Weld epoxy was used to put the new 750 ohm board on like it was, stock:

 

[MarantzRefrb]

Back together:

Installed:

Transistor replacement for the RF board is next.

 

[MarantzRefrb]

Found two dead-shorted transistors in the first 10 minutes today, TR320 and TR321, both FMPS5172, both now replaced with NTE123AP:

These may be the only two dead ones. I'll only replace the shorted ones, but I plan to pull and test every last one.

 

[MarantzRefrb]

TR318 is also bad, a 2N5227 has been replaced with NTE159:

The rest are all seeming to checkout.

 

[rBuckner]

What a saga. You get an A+ for effort!!

 

[MarantzRefrb]

This SG165 is really complete and in fairly good condition, couldn't let it go to waste. It's so close to being done now, pretty sure it was just the buffer that took too much voltage and blew it out, along with the L pad. Just need to order caps now.

 

[MarantzRefrb]

Put the correct ground strap for the L pad to chassis, back in:

Reassemble with stock washer:

 

[MarantzRefrb]

Replaced all SE3002, with NTE108-1, at TR301, 302, 303, 314, 315.
Circled in green:

Wondering if it was this cap that caused the problems with the transistors directly behind it:

Will replace this 2200uF 50V, with the original value, 2000uF 25V. Parts will be here tomorrow.

 

[MarantzRefrb]

Installed the new non-polar 1000uF 16V caps, replacing the 1000uF 15V:

Both the stock caps were dented, and measured about 1500uF:

With this last chassis detail finished, I can quickly replace the caps on the main RF board tomorrow, and reassemble and test things finally.

 

[MarantzRefrb]

Finished the RF module:

Replaced the 10uF 15V with Kemet tantalum, and Vishay 2uF 50V electrolytic:

Put the oscillator case back together:

Ready to go back together:

 

[MarantzRefrb]

Replaced the L pad, with a 100 ohm longshaft regular pot, added a 330 ohm 2W resistor to get 75 ohm on the button:

Replaced all transistors:

(SE3002 = NTE108-1) TR301, 302, 303, 314, 315
(2n5172 = NTE289A) TR304, 306, 307, 320, 321
(MPF102 = NTE451) TR305, 308, 312, 313, 316
(2N5227 =NTE159) TR310, 318, 322

I was able to blank an FM channel and send that channel a tone. It works!

 

[MarantzRefrb]

This SG165 is working correctly now, time to seal it back up:

Stripped the screw in the stock knob, temp in place:

The FM RF, and all functions are working correctly now.

This project is completed, thanks for following along.

 

[mrk229]

This SG165 is working correctly now, time to seal it back up:

Stripped the screw in the stock knob, temp in place:

The FM RF, and all functions are working correctly now.

This project is completed, thanks for following along.

Nice job, way to stick with it. Love that temp knob
While it may not be the best analyzer out there it is better than no analyzer. And should prove a useful tool.

 

[MarantzRefrb]

Thanks, mrk229! It was a fun project.

The SG165 is actually not that bad, everyone seems to dump on it for not being an SG80, but I've heard it said the SG165 is more than adequate in every scenario, if not comparable, you can find this mentioned in the Antique Radios.com threads, a few times.

Now I want to fix another one .

EDIT: I think my repaired L pad works properly, but I was expecting a clean FM waveform, and falsely equated the problem with the L pad, I then put in the 100 ohm pot, and shotgun replaced all the transistors. I may go back and reinstall the L pad at some point.

 

[rBuckner]

Awesome! Great job!!

 

[MarantzRefrb]

Awesome! Great job!!

Thanks, RB! Would love to see some pics of the modifications you made on your SG165. I know it's a hassle to open it all up though, but the idea of injecting a cleaner external input signal, for a better THD internal measurement, sounds amazing. Isn't that how you did yours? I like the idea of BNC instead of RCA, throughout, like you did also.

 

[rBuckner]

Thanks, RB! Would love to see some pics of the modifications you made on your SG165. I know it's a hassle to open it all up though, but the idea of injecting a cleaner external input signal, for a better THD internal measurement, sounds amazing. Isn't that how you did yours?

Yep. I used a BNC jack to pipe it in. I just hook up my HP8903B and set it to 1V. I'll see if I have pics of the mod.

Edit: Nope, no pix.