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5000X "Unserviceable"? :(

archibael said:
Right now on my phone I have the following tabs open to refer to:

Sansui 5000X Service Manual
A guy building his own AM radio from scratch
Tuner tweaks just short of a full alignment, which I'm not following but which I find okay for reference material
Details of FM stereo modulation scheme

There are probably books I can buy that would give me the right information, and certainly we covered the basics of some of this stuff 25 years ago in school, when I was much smarter but also much dumber, but really, the interactive effect of kevzep and dr*audio and maxhifi and Hyperion here, not to mention the guys on the tuner thread, have been priceless and inspirational.

Also, I picked up a used copy of this book years ago and it sat on my shelf a quarter-finished until recently. Highly recommended for its use of layman's terms, from a simpler time.
Click to expand...

My favourite book on tuners is "Troubleshooting, Servicing, and Theory of AM, FM, & FM Stereo Receivers" by Clarence R. Green and Robert M Bourque.

It also happens to have some of the best introduction to basic audio electronics including amplifier and power supply circuits I've ever come across. Written by community college profs but their descriptions of semiconductor phenomena is physically accurate and the way they explain problems puts most Berkeley or Stanford profs to shame! The physics side is accurate and they stay away from poor analogies (which is a problem I find with almost all introductory texts relating to physics attempting to make things simpler, by saying a bunch of stuff that ultimately isn't strictly correct and causes more long term harm to understanding than good).
 
On a side note, I'm looking at having some transistors on stock in case any of the ones in the AM path have gone bad (that in itself is another topic).

Basically the entire AM circuit in this unit is built on 2SC380 of two different hFE ranges. In looking for a replacement, it looks like KSC1675 is comparable in those specs, but that brings to the table: what should I even be looking for in transistor specs for a replacement?
 
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smurfer77 said:
My favourite book on tuners is "Troubleshooting, Servicing, and Theory of AM, FM, & FM Stereo Receivers" by Clarence R. Green and Robert M Bourque.

It also happens to have some of the best introduction to basic audio electronics including amplifier and power supply circuits I've ever come across. Written by community college profs but their descriptions of semiconductor phenomena is physically accurate and the way they explain problems puts most Berkeley or Stanford profs to shame! The physics side is accurate and they stay away from poor analogies (which is a problem I find with almost all introductory texts relating to physics attempting to make things simpler, by saying a bunch of stuff that ultimately isn't strictly correct and causes more long term harm to understanding than good).
Click to expand...

Ha. Maybe I need that.

On the early stages of the AM unit, I'm seeing only the basic oscillator signal after the mixer transistor. Prior to the mixer, after the RF amplification, I'm seeing very little.

Maybe I'm expecting too much... the DC offset of the signal at the test point (3C) is the 12V power supply, so if there is a tiny 2mV RF signal it's likely swamped by that. About how high in amplitude should an RF amplified antenna signal pre-mixer be? The oscillator is putting out +/- 1V, so I was assuming something of that order of magnitude, but as I think I've illustrated many times in this thread, my understanding of these issues is touchy at best.

EDIT: it occurs to me that I could probably probe on the ground side of the tuning gang for that circuit and thereby remove the offset, leaving only the characteristic RF (if any). Would this actually work, or is probing the ground side just going to get me... ground? Would I need to throw a resistive load in the way? Does anything I'm saying resemble sense? :)
 
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As soon as you touch anything in an RF circuit around the Front End and the IF stage, the probe loads the circuit and detunes whatever it is you are measuring, you can use a X10 probe to get a 10meg impedance but then you wont see much on the scope.
So its a fine balancing act when you get in there and start probing around.
 
Fair enough. I thought the extra impedance might impact the signal, but I thought there'd be *something* there, even if shifted in phase or frequency as there is in the oscillator circuit. Seems like to mix well you'd have to at least have order of magnitude comparable amplitudes post RF amp, but maybe not.

The AC coupling setting of the scope would seem to be what's needed to take out the bias. But it's a flatline as far as I can tell. Will try again soon.
 
archibael said:
On a side note, I'm looking at having some transistors on stock in case any of the ones in the AM path have gone bad (that in itself is another topic).

Basically the entire AM circuit in this unit is built on 2SC380 of two different hFE ranges. In looking for a replacement, or looks like KSC1675 is comparable in those specs, but that brings to the table: what should I even be looking for in transistor specs for a replacement?
Click to expand...
KSC1674 is what you want. I have a stock, often the pinout is wrong for most Japanese tuners EBC instead of ECB......but they are a good general purpose RF transistor....
 
Thanks for the reading list and the links.

My 'schooling' was '63-'67 and I have trouble remembering the instructors, much less the content. It is like a Smith Chart - I learned it once, then never used it again.
 
Sorry guys I was on vacation with no internet access.
AM IF frequency is 455KHz. Your LO is ok so we need to look at the IF. But if the rf amp is blown there will be no IF because the LO has nothing to mix with.
So first probe the base and collector of TR304 while turning the tuner knob. Where there is a station you should see a 455KHz signal. If you see the higher frequency LO we don't care about that, we are looking for the 455KHz IF. It will be smaller at the base and much larger at the collector. If you had an AM generator or a function generator capable of 455KHz it would be helpful.
 
dr*audio said:
Sorry guys I was on vacation with no internet access.
AM IF frequency is 455KHz. Your LO is ok so we need to look at the IF. But if the rf amp is blown there will be no IF because the LO has nothing to mix with.
So first probe the base and collector of TR304 while turning the tuner knob. Where there is a station you should see a 455KHz signal. If you see the higher frequency LO we don't care about that, we are looking for the 455KHz IF. It will be smaller at the base and much larger at the collector. If you had an AM generator or a function generator capable of 455KHz it would be helpful.
Click to expand...


Never apologize for vacations, especially when you're doing this out of the kindness of your heart when your student doesn't even have the proper equipment and doesn't yet know how to use the stuff he has.

TR304 base and collector have waveforms of about 450kHz when I find various sweet spots on the dial; I'm assuming that's close enough given that I've added a disruptive signal path with the oscope probe. I had never seen this before-- likely I was tuned to dead air on the dial and had only recently figured out how to AC couple the scope.

At any rate, that looks decent to my inexpert eyes. The base of TR305 has the signal, too, though the collector has nothing. Which pretty much implicates TR305, I think. It's not electrically open or shorted, but I'm seeing no AC signal on the collector except in the 1mV range. Although I suppose the coil at T305 could be so detuned that it's filtering out the output signal?
 
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archibael said:
Never apologize for vacations, especially when you're doing this out of the kindness of your heart when your student doesn't even have the proper equipment and doesn't yet know how to use the stuff he has.

TR304 base and collector have waveforms of about 450kHz when I find various sweet spots on the dial; I'm assuming that's close enough given that I've added a disruptive signal path with the oscope probe. I had never seen this before-- likely I was tuned to dead air on the dial and had only recently figured out how to AC couple the scope.

At any rate, that looks decent to my inexpert eyes. The base of TR305 has the signal, too, though the collector has nothing. Which pretty much implicates TR305, I think. It's not electrically open or shorted, but I'm seeing no AC signal on the collector except in the 1mV range. Although I suppose the coil at T305 could be so detuned that it's filtering out the output signal?
Click to expand...
Measure the dc voltages on TR305 BCE. Sounds like it's bad but lets measure twice and cut once.
 
dr*audio said:
Measure the dc voltages on TR305 BCE. Sounds like it's bad but lets measure twice and cut once.
Click to expand...

13V collector
5V base
0V emitter

Which is kinda electrically open, contrary to what I said earlier: at 5V base we should see some current flowing through the 470ohm emitter resistor putting it higher than 0V, no? The resistor is present and measures 470ohm.
 
archibael said:
13V collector
5V base
0V emitter

Which is kinda electrically open, contrary to what I said earlier: at 5V base we should see some current flowing through the 470ohm emitter resistor putting it higher than 0V, no? The resistor is present and measures 470ohm.
Click to expand...
Bingo! Bad transistor.
2SC380
NPN
VCBO 35V
Ic 50mA
PD = 300mW
HFE 40 -240
Fc 100 - 400MHz
Basing ECB
The Fc is the most important spec to meet or exceed, with HFE being the second.
At Digikey I found
https://www.digikey.com/product-detail/en/on-semiconductor/KSC945CGBU/KSC945CGBU-ND/1047198
This should work fine. Note that the basing is different.
 
Yes I have used the 1674'a in place of the 2SC380 before with good results, the pinout is different as the guys have pointed out....
 
Thanks. I'll try those tomorrow. The 2SC536s on the FM MPX board look more challenging as some are toward the higher hFE values, so I'll just hope I don't have to mess with any of those.
 
Yay! After a stupid transistor reversal (stupidly read pinout as live bug when I KNEW it was dead bug!!!) I found my error and we now have AM! :banana:

TR305 was indeed the culprit and when KSC1674 substituted in the circuit I have sound on that band for the first time on this receiver. I'll have to align it at some point, but for now I'm moving back to FM. :)
 
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