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SX-737 tuner AWE-043

As a hint, in FM mode there should be NO voltage applied to Pin 2 of the HA1137 IC. Applying voltage there basically "kills" the incoming signal on Pin 1 by flipping a transistor in the mirrored pair at the input, per the HA1137 device schematic.

The voltage at Pin 4 of the AWE-043 is also connected to Pin 33 of the board, and that will "kill" the VCO time constant for stereo decoding.
 
Thanks @Watthour for your hint.
@rcs16 mentioned that before, “This voltage is switched to keep the noise down from the IF system when in AM mode”
What I learn from that, with 13V on HA1137-pin 2 the tuner runs only in AM mode.

There was another hint as well about the “reference bias pin 10 must be ~5.6V”.
We didn’t look in that area yet.

OEM – Datasheet (page20) shows 5.6v not only on pin 10, it should be as well on pin 6 and 7.

I don't have the expertise to determine where this voltage is regulated and if this could be part of the issue.
HA1137-pin 6 is also connected to HA1156-pin 2 Pin.
Could it be narrowed down to one or both of this ICs?
 
That is what we are trying to determine, with another possibility that the RF front end is not delivering any IF signal to Pin 1 of the HA1137.

Your audio probe at Pin 6 of the HA1137 or C27 produced no audio signal, and there is a very remote possibility that either C27 has a problem or C26 is grounding any signal.

There is also another very remote possibility that one of the IF ceramic filters upstream of the HA1137 (Pin 1) has a problem. This is very rare, but I cannot say this never happens because I recently had a ceramic filter fail in another unit.
 
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Did the audio test again on both pins of C27. No radio signal to be heard.
The only thing I noticed, touching the pin 6 or c27 minus, reduced the noise in the headphone.

Shall I replace C27 and C26 or all three in that area?
 
If you can disconnect both C26/C27 and test for audio at the IC again, that would probably eliminate any of these remote potential problems. Of course, measuring resistance of those caps (for shorting) would also be advised.
 
Removed C26 and C27. C26 measured 30pF, C27 is 13µF, 2.3ohm ESR and 5.5% Vlos.
Means nothing critical.
The audio test was negative again, no audio signal.
No luck today.
 
My suspicion is that there is nothing getting into the HA1137 from the front end. That will probably require a relatively fast oscilloscope and signal generator to diagnose.

However, the upstream signal COULD be probed with your constructed audio tracer cable if you happen to have a detector diode like a 1N60 available (or 1N294, 1N541, or CK706) to add to the probe. The 1µF capacitor might be just large enough to create some amplitude so that the "probe" can pick it up, but that is no certainty.

I am not near my bench today, but may be able to duplicate and verify this later. I pulled a SX-550 from storage previously to follow along with your thread as a comparison. It uses the same type FM front end as the SX-737, including the HA1137, HA1156, and dual filtered FET/BJT front end. Anything happening in the SX-737 is likely happening in the SX-550.
 
Have never heard about a detector diode until today, sounds very interesting and will probably go to my next order list.
That are the only diodes I have. 1N 4148, 1N 4001 - 4007, 1N 5817 – 19, 1N 5404.
But if we can build something with different regular parts, I always willing to do that.

It’s already a bit late here in Good Old Germany and was an exhausting day. Will follow your advice tomorrow ASAP.
 
You can bypass a ceramic filter with a small ceramic cap to determine if it is open ckt. Sometimes your finger touching the CF can couple some signal.

You can also test the RF front end and the IF separately.
Is Q4 bias as show?

There was another hint as well about the “reference bias pin 10 must be ~5.6V”.
We didn’t look in that area yet.
If the HA1137 is not biasing properly, with proper supply and ground, then it is a dead duck, the spec says pins 1,2,3 are to be 1.9V, pins 6,7,10 are 5.6V
pin 5 grounded is unmuted, >1.4V mute on pin 6
 
The Q4 voltages are
E = 1.29 vdc
C = 13.83
B = 1.95

C seems to be a bit high (SM = 12.6v), but what do you think?
 
Made the test with one finger touching the ceramic filter F1 and F2, still no change or audio.
All removed caps are back in, c26 ceramic is the old one, c25 c27 are new, c28 (.47µF) is replaced with film.
Still have the originals on hand, if that swap was a bad idea.

How can I test the RF front end and the IF separately? Don’t want to spoil the alignment too much, so better ask for more information.
 
Q4 collector is at the supply voltage you have a proper Vbe voltages
You can take the IF out of your receiver and feed it into another tuner IF chain to verify it is okay. You do not need to worry about alignment since it is just a test to see if the section is working. But your issue is with the ha1137 since you are not measuring the correct voltages.
sort that out first. Pull the ha1137, put in a socket so you can try and change the part
 
That means I have to order a few HA1137w for testing and a DP-16 Socket as well. Never had them on my workbench before.
If there is anything I can to, test or replace helping me on that tuner issue in the meantime, please go ahead and tell my. Every hint is welcome.

@Watthour, would be interesting, if you can duplicate and verify some of it on your SX-550.
 
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If the appropriate equipment were available, the second ceramic filter (F2) could be lifted and IF signal could be checked at that point in the circuit, and then checked backward all the way to the antenna.

We might be barking up the wrong tree entirely, and find that no RF is even coming out of the front end after the VC and trimmers, or after T5, etcetera.
 
Watthour said:
If the appropriate equipment were available, the second ceramic filter (F2) could be lifted and IF signal could be checked at that point in the circuit, and then checked backward all the way to the antenna.

We might be barking up the wrong tree entirely, and find that no RF is even coming out of the front end after the VC and trimmers, or after T5, etcetera.
Click to expand...

What equipment is necessary for that? I can get may hand on a HAMEG HM-205, but have never worked with one.
 
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pa_200 said:
My tuner worked before I started the rebuild with the power supply, protection board and the rest. But after finishing all the other boards, with the first test on DBT, the tuner stays totally dead. The AWE-043 has not been touched.
Click to expand...

We also should not lose sight of where this all began. The tuner worked before, and was reportedly untouched when the problem arose.
 
As mentioned in Post #17, an oscilloscope could be useful. It would need to be at least 100MHz with an appropriate probe. Faster would be even better. The ultimate setup would be an RF generator with FM capability and a means to modulate a signal, but that is getting into the realm of full RF alignment equipment.
 
If the HAMEG HM 205 is the equipment you looking for, it would be quite a challenge if you can explain to my how to operate the HAMAG, I will follow every single step of your instruction.

That's what I can use from tomorrow on. Hameg HM205-3 Oszilloskop 20 MHz
 
Unfortunately, 20MHz will be too slow for FM work, at least in the front end. It could work for the IF tracing, however, so it may be handy to have.
 
Got the Hameg HM205 today and a short introduction from my friend. Together we did some testing with the probe between T5 and F1, hocked up an R15. Then moved the Tuning wheel and every time we tuned in to a strong radio station, there was a small but noticeable change on the displayed signal. Was like a very small sinus wave.

What would you advice for further testing to check out the front end working ok?
 
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