Sencore SG165 400 Hz output low - diyAudio
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Old 9th March 2015, 05:09 PM   #1
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Join Date: Mar 2015
Angry Sencore SG165 400 Hz output low

Greetings fellow DIYers.

I am restoring a Sencore SG165 AM / FM analyzer that I purchased last year, and have run into kind of a tough problem.

I have re-capped the entire unit with new electrolytics and re-assembled it to where I still have access to the main RF board for testing. The power supply also had a leaky driver transistor that caused the -12V line to shoot up (down) to around -35 V (not a very fail safe design) and the unit was operated with this problem for a few minutes until it was discovered and corrected. Following this, I did the power supply calibration and now both +12 and -12 are within 0.1 V of each other at around +/- 11.6 V.

The problem that I am having is that the 400 Hz sine / square wave output at the front panel with the attenuator wide open is low and distorted. The sine wave is a little better, but the square wave is barely visible on my scope on the 50 mV range.

Tracing these signals backwards, on the schematic, the waveforms look OK on the audio oscillator board, with the sine wave at 2.8 V P-P (adjustable with control on audio board) and the square wave at just over 2.1 V P-P.

I have replaced both transistors in the output buffer on the RF board (TR 316 and TR 318) as well as both transistors in the isolation buffer (TR 308 and TR 310) with the exact replacements (MPF102 and 2N5227).

One thing that I noticed is that the board markings for the MPF102 FET do not match the device pinouts on any of the data sheets that I can find online -- drain and source leads are interchanged.

To add more confusion, a lot of information that I see online indicates that these devices can be operated with the drain and source swapped without a problem.

I don't really have a way to test the original FETs, and the leads were cut off extremely short to mount the replacements temporarily on the top of the board using the original leads.

One other point, the modulated AM signal using the 400 Hz audio looks OK on the scope, so this aspect of the audio signal is fine.

Can anyone that owns a working one of these units confirm what kind of levels they are seeing for the 400 Hz audio outputs at the front panel?

Sorry for the long-winded post - I have put this project on hold until I get some other opinions or come up with a new approach to solving this problem.

Thanks,
Charles K.
Valier, IL
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Old 9th March 2015, 11:26 PM   #2
nec3 is offline nec3  Turkey
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Join Date: Jun 2005
Location: Ankara
400Hz Modulated AM works OK because 400Hz sine wave directly goes to modulator from Audio osc.

(Assuming all the other positions of OUTPUT switch works good) The isolation buffer (TR 308 and TR 310) is also good because it works without problem with other signals such as 67Khz, 10.7Mhz.

Check signal at R363 while SW301E OUTPUT selector is at 10th or 11st (sine/square) position. You should see 400Hz.

This will tell you where the problem is.

Last edited by nec3; 9th March 2015 at 11:40 PM.
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