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Sencore SG165 - AM/FM Analyzer - Repair

I seem to have all of the signals expected at TR316, or the 47uF cap lead:
atJFwFL.jpg


Next up is the FM RF signal, which the eBay seller said was completely out.
 
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I was told by the seller that the FM output is not working at all. I hooked up my scope to All Signals Out, and setup the following:
sVN1wun.jpg


In the video below, this is what I see when dialing 88MHz to 108MHz with the RF Tuning knob:

The overhead lighting ruined the picture quality, but that is FM RF output I'm getting. Not sure what to think here. I'll try to run an actual test alignment.
 
MarantzRefrb said:
I was told by the seller that the FM output is not working at all. I hooked up my scope to All Signals Out, and setup the following:
sVN1wun.jpg


In the video below, this is what I see when dialing 88MHz to 108MHz with the RF Tuning knob:

The overhead lighting ruined the picture quality, but that is FM RF output I'm getting. Not sure what to think here. I'll try to run an actual test alignment.
Click to expand...

Quick and dirty test, If it is working you should be able to tune it in with a radio. Crank the microvolt to 100 or more and hang a cliplead off the end of the bnc center pin and have it near the antenna input of the receiver.

With the modulation rockers off the receiver should tune and quiet down. Modulation on you should hear the tone. Pilot switch should turn the stereo light on and off. Vary the RF freq and see if it tracks on the receiver dial.
 
I can't get it to affect the receiver for some reason. Here's how I've got it setup:
e2JqO99.jpg


bj4nGGk.jpg


This is how the manual says to set it up, but I get no station blanking or any response from the receiver.
 
MarantzRefrb said:
I can't get it to affect the receiver for some reason. Here's how I've got it setup:
e2JqO99.jpg


bj4nGGk.jpg


This is how the manual says to set it up, but I get no station blanking or any response from the receiver.
Click to expand...

Then probably not working correctly. What is the top mHz of your scope? If it is 100mHz you can probably see and display the frequency measurement with the scope readout.

No modulation tone and no pilot should display a nice stable sinewave that varies in frequency with the RF dial, start at the lower end of the dial.

If you have a solid waveform operate the level controls on the SG to see if they work and don't cut out. Did you clean any of the switches?

Otherwise, you will probably have to dive into the guts with the troubleshooting charts.
 
I'm using a 200MHz scope. The FM waveform is consistent, but is not a clean looking sine wave - as seen in the video above, the waveform does change shape with RF Tuning knob.

I found out the AM is working, I hear the tone at the appropriate station, and it corresponds on both dials. Very happy about that!

I didn't see a troubleshooting diagram for the missing FM RF output. I'll see what I can come up with.

EDIT: I have not cleaned any of the switches yet.
 
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MarantzRefrb said:
I'm using a 200MHz scope. The FM waveform is consistent, but is not a clean looking sine wave - as seen in the video above, the waveform does change shape with RF Tuning knob.

I found out the AM is working, I hear the tone at the appropriate station, and it corresponds on both dials. Very happy about that!

I didn't see a troubleshooting diagram for the missing FM RF output. I'll see what I can come up with.

EDIT: I have not cleaned any of the switches yet.
Click to expand...

Does your scope readout show the frequency and the level? The sinewave should expand and contract horizontally with frequency change. And it should be clean
 
MarantzRefrb said:
My scope does show that the FM frequency, on All Signals Output, is from 30MHz to 109MHz:
DC5CmmY.jpg


CgXt7l8.jpg


That doesn't look clean to me.
Click to expand...
Nope it does not. And correct no trouble charts for NO RF. Page 47 and 48 of the manual has a description of the osc and diagram. Shows where power comes in to turn it on. Check that point with the scope for clean power. You can go all the way back to the main PS and look for garbage on the supply lines.

Not sure where that 30mHz is coming from and the waveform is not good. Scope the power supplies and look for ac garbage.
 
This cap with the wart reads HI-C on my tester also:
d3x0kXG.jpg


Would you recommend I recap this first? At least the PSU, and this bad pair of silver caps?
 
I notice this SG165 has been dropped on the far right Microvolt pot, the knob is cracked, and the knob only affects output during the last 10% of travel. I can only hear the AM RF tone through the receiver, if the fine Microvolt control is set to 8-10. One more problem to add.

Ordering caps now.
 
MarantzRefrb said:
This cap with the wart reads HI-C on my tester also:
d3x0kXG.jpg


Would you recommend I recap this first? At least the PSU, and this bad pair of silver caps?
Click to expand...

That one cap is certainly bad and not knowing where they are in the circuit it is hard to say if they would/could cause an output. Following the wires back you can probably determine where they are in the circuit.

You would probe the plus and minus 12-volt test points at the power supply where you had the meter before. (small blue caps) The scope would be set for AC coupling and a fairly low voltage per division. We want to see if the power supply is clean of ripple and noise.

You can probably see what I am talking about if you put the probe on the test points of the supply where you measured the + and - 28volts. (Big silver ones) You will probably see some AC ripple there, especially if the caps are weak or bad.

Overall I would certainly recap the power supply and replace those in the image above, that one has leaked.
 
MarantzRefrb said:
Thanks! I noticed that the caps with the dent, and wart are non-polarized 1000uF:
O2hyKNs.jpg


I got these for the (2) big PSU caps:
https://www.mouser.com/ProductDetail/594-2222-120-18102

I'm having a hard time trying to find 1000uF non-polarized caps. Is there a replacement you could recommend?

I'll get after the PSU with AC coupling mode on my scope.
Click to expand...

Holy toledo those PSU caps you ordered are pricey. Ah, I see why they are 125C rated. These would have worked fine. https://www.mouser.com/ProductDetail/

Quick search I don't see any large Non-polarized (Bi-Polar) caps with axial leads. Not surprising. You can use these: Caps or these Caps
 
I was wondering if I should just use radial for the bi-polar caps. I'm almost done building the order list at Mouser. Thanks again!
 
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