Sansui 1000X Tuner trouble

[KeithD]

Just picked up a Sansui 1000X. Noticed it was very weak at pulling in stations, so went through the alignment procedure. I fed the signal through the 300 ohm FM antenna connections. I set 88 and 108 MHz dial positions using my THD meter for minimum distortion adjusting controls per manual with several iterations. Then went through the procedure for 90 and 106 MHz per manual. This all went well. Signal meter moves as expected when tuned in. Put in a stereo signal and set for max separation per manual. Everything looked good. On a mono signal I was down to about 0.5% THD.

However, when I connect an antenna in a good location in the house, I can get the stronger stations, but no stereo, and signal meter does not move as though the signals are very weak. I've not experience this with aligning my other tuners. After alignment all works as it should. What am I missing on this one?

 

[Punker X]

Did you see the same low sensitivity on the signal generator. Often times tweaking the front-end for minimum THD can result in poor sensitivity. I would gotthrough it again and try peaking the for highest signal strength from the front end. Don't worry about THD until you have the best sensitivity. Lower THD with IF and discriminator adjustment.

 

[KeithD]

Thanks for the reply, Mike. It looked like I was getting good sensitivity with the signal generator. I'm using an HP 8657A for the signal generation and an HP 8904A for the signal modulation and stereo generator. When I changed the signal amplitude on the 8657A over the range of 10 - 90 mV I was seeing the signal meter move accordingly. I've posted the FM alignment procedure below. I actually did not adjust the IF or Discriminator, and so did not do steps 1 or 2. These call for sweep signals, and have not figured out how to do them with my generator. Can you tell me how to do those adjustments with my equipment? I've also got a Tektronix oscilloscope and a Keithley 2015 THD multimeter for looking at the receiver output. So, I did steps 3 - 8, but not 1 or 2.

 

[RossW]

The 8657A doesn't have sweep functionality built in, but there was discussion of a workaround on the antique radio forums:
https://antiqueradios.com/forums/viewtopic.php?f=8&t=296067

I would definitely try to do an IF and discriminator alignment, I had an MR67 with similar symptoms and the IF section was way out of whack.

I don't see any mention of adjusting based on THD, in my experience if you get everything adjusted well THD should more or less take care of itself (i.e. it is more of a verification)

How are you taking take of impedance matching? Most test equipment is 50 ohm output and you're feeding a 300 ohm input.

 

[KeithD]

Thanks for that link, Ross. Is using a sweep signal the only way to set IF and the Discriminator on these older tuners? I've had the impression from other threads that not everyone does this method.

Makes sense that something fundamental is off here. The stations are coming in at the right point on the dial, they are just weak and no stereo.

As far as adjusting based on THD, here again, this is the advice I've been given in other threads. Also, I've also had the impression that impedance matching is not that important for making the adjustments. Is that not correct? If not, what do I need?

Thanks!
Keith

 

[RossW]

You can usually do it with a straight 10.7MHz signal, looking for maximum output rather than "best wave form", although doing the discriminator that way can be a bit touchy.

There are many ways to do an alignment, some tuners don't seem to care, others are very particular. Following the service manual is usually a good place to start. If service info is not available, then you need to get creative.

For matching you can use a 6:1 balun or I use the circuit used by Sound Technology with their 1000A:

Is it totally necessary, probably not, but it's good practice.
If the tuner has a 75ohm input you could also use that and it's probably close enough.

Also, 10-90mV is a lot of input, it should be more like a few mV. Too much input can hide all kinds of problems.

 

[KeithD]

You can usually do it with a straight 10.7MHz signal, looking for maximum output rather than "best wave form", although doing the discriminator that way can be a bit touchy.

There are many ways to do an alignment, some tuners don't seem to care, others are very particular. Following the service manual is usually a good place to start. If service info is not available, then you need to get creative.

For matching you can use a 6:1 balun or I use the circuit used by Sound Technology with their 1000A:

Is it totally necessary, probably not, but it's good practice.
If the tuner has a 75ohm input you could also use that and it's probably close enough.

Also, 10-90mV is a lot of input, it should be more like a few mV. Too much input can hide all kinds of problems.

OK, I can give it a try with a straight 10.7 MHz signal. So, just looking for the max voltage output from the tape out RCA jacks a normal?

I forgot to state that I am using a generic 300 Ω matching transformer between the HP signal generator and the receiver antenna terminals. Just one of those RCA brand ones you can buy at a hardware store.

With respect to the signal amplitude, this is an issue with all of the service manuals. Many of them have no mention of the required signal strength. As you know the HP has many options for output amplitude. I can certainly dial it down, but I noticed it does not seem to respond below about 10 mV. I've followed what some others have recommended for signal strength on other tuners where they use something in the neighborhood of 40 - 60 mV for calibrating the dial pointer. Raising it higher was used for setting the signal strength meter maximum, as I recall.

 

[RossW]

If you're not getting response below 10mV then there is a problem, you should be able to get a usable signal with tens of uV (although usually its easier to align with hundreds or a couple thousand). If there are a lot of stations around you it can be easier to set the dial pointer with a bit of extra power (to drown out the other stations), but IF is all about finesse.

You need to look at the output locations specified in the manual, the 10.7MHz is only in the IF section (i.e. it's not present at the regular outputs).

 

[KeithD]

If you're not getting response below 10mV then there is a problem, you should be able to get a usable signal with tens of uV (although usually its easier to align with hundreds or a couple thousand). If there are a lot of stations around you it can be easier to set the dial pointer with a bit of extra power (to drown out the other stations), but IF is all about finesse.

You need to look at the output locations specified in the manual, the 10.7MHz is only in the IF section (i.e. it's not present at the regular outputs).

OK, got it. I see the IF has specific injection and monitoring points. It's requiring a 10pF ceramic capacitor for the IF step at the injection point and a 0.02 µF ceramic capacitor at the measure point. Discriminator needs 0.02 µF and 0.05 µF ceramic capacitors, respectively. I'll have to see what I've got. I know I have some small ceramic caps, but might not be those exact values.

Will dial down the output amplitude and see what I get. Wouldn't the 300 Ω matching transformer drop down the amplitude of the signal from the generator when I'm injecting to the antenna terminals?

 

[Ylli]

Yeah, 10 mV is a *huge* signal. You will get a bit of loss feeding the signal from a 50 ohm source though a 75:300 balun, but only a dB or so. Receiver specs:

Check that input FET.

 

[KeithD]

Yeah, 10 mV is a *huge* signal. You will get a bit of loss feeding the signal from a 50 ohm source though a 75:300 balun, but only a dB or so. Receiver specs:
View attachment 1943722
Check that input FET.

Thanks Ylli, don't know why I never connected the Sensitivity specs with the input voltage at the antenna before, but that sure makes sense.

How do I check the input FET?

Checked my stock, and I do have 5, 10 and 1000 pF and 0.01 µF ceramic caps. Do I need the exact values called for to check the IF and discriminator, or can I use the 0.01 µF in place of the 0.02 and/or 0.05 µF caps called for?

 

[Ylli]

For the front end, I would take that signal generator, set to 100 MHz, and connect it through the 1 pF or 10 pF cap to the indicated points. Points 2 & 3 will have a bit less gain than point 1, but point 4 should show a significantly greater gain.


I'd have to look close to see if you could use the 0.01uF in place of the 0.02uF and 0.05uF caps. Generally, the smaller the cap the better, as it disturbs circuit operation the least.

 

[KeithD]

For the front end, I would take that signal generator, set to 100 MHz, and connect it through the 1 pF or 10 pF cap to the indicated points. Points 2 & 3 will have a bit less gain than point 1, but point 4 should show a significantly greater gain.
View attachment 1943859

I'd have to look close to see if you could use the 0.01uF in place of the 0.02uF and 0.05uF caps. Generally, the smaller the cap the better, as it disturbs circuit operation the least.

OK, thanks. I'll have to see if I can figure out where those points are.

If I can't use the ceramic caps I have, I'll have to wait until I get some to test the IF and discriminator...

 

[Ylli]

Yeah, may be tough getting to those points. They would be in here:

The top looks even less accessible.

 

[KeithD]

Yeah, may be tough getting to those points. They would be in here:
View attachment 1944178
The top looks even less accessible.

Yeah, I have no way of knowing what is what in there, so I don't think I'll be able to do that.

Even the signal injection point for checking IF and discriminator is unclear. The picture shows an arrow pointing to TP101 on the underside, but the image resolution is not high, and that is actualy some kind of fiber card that has multiple solder strips along the side. I can narrow down to 2 or 3 strips it might be, so it's a bit of a guessing game. I may give this a try, but it is going to take some thinking for how I am going to connect everything. For output can I just measure voltage on a DMM, or do I need to put the signal into an oscilloscope? I'm just not sure I have all the necessary connectors on hand to make it work.

I did try checking signals at much lower amplitude. I can get a signal out starting at about 100 µV input amplitude on a 100 MHz signal - lots of distortion, but it's there. On a stereo signal, stereo light comes on at about 900 µV amplitude. Below that I can see the signal out, but stereo light goes out.

 

[Ylli]

My indicated points 2 & 4 are stator terminals on the variable cap. You may be able to identify those. You should have much more gain at point 4 than at point 2. For that part, just use your ear and adjust the signal generator for a just discernible signal.

 

[KeithD]

My indicated points 2 & 4 are stator terminals on the variable cap. You may be able to identify those. You should have much more gain at point 4 than at point 2. For that part, just use your ear and adjust the signal generator for a just discernible signal.

OK, will give that a try later today.

For injecting the signal for the setting the IF in step 1, what signal strength should I use?

 

[Ylli]

You want to be able to see a fairly noise free output signal on the scope, but you also want to be sure you are not into limiting, so it's a bit of trial and error.

 

[KeithD]

Yeah, I was thinking I'd start at 10 µV and work my way up from there.

Should I just try to set the IF first, or should I test the FET at the two points you indicated first? For the latter, I assume I should be sending a modulated FM signal into the antenna terminals like you do for setting the dial (steps 3-8 in SM FM alignment)?

I'm hoping to be able to do some tests later this afternoon.

 

[Ylli]

If you hear good strong interstation 'hiss', then the IF strip is probably OK. If the hiss is subdued, then the Mixer/IF might be suspicious. I'd try injection on the input side and output side of the FET first. (Points 2 & 4). You would use an FM modulated signal fed through that 1 pF or 10 pF cap to the indicated point. It should require much less signal at point 4 than it does at point 2 for you to hear the signal.