TX-9500 no AM

[satellite65]

I have a really clean unmolested TX-9500 that works great on FM/stereo but I get no sound or meter movement at all across the AM band.

Here are the voltages on Q23 HA1138 (all + DC):
Pin 1 10.46v
Pin2 2.8v
Pin3 10.48v
Pin 4 10.49v
Pin 5 3.5 - 4.5v, varies
Pin 6 8.65v
Pin 7 2.02v
Pin 8 0.88v
Pin 9 0v
Pin 10 3.68v
Pin 11 4.32v
Pin 12 11.59v
Pin 13 2.75v
Pin 14 2.63v
Pin 15 2.73v
Pin 16 0v

All are close to what is shown on the schematic...

 

[satellite65]

AM antenna continuity looks good AM terminal to gnd, Main board pin 5 to gnd, and main board pin 6 to gnd.

Continuity thru S1-3 F&R looks good (R81/85 junction to main board pins 23 & 24).

I do have a spare HA1138 that I could swap in. What else should I check before doing that?

 

[rcs16]

Look at this thread

Thread 'SX-626 AM Tuner Issues'

Mar 30, 2023

I have a nice sx-626 that I have been working on but I can’t seem to get the AM tuner working properly. The output is very low. I only pick up one or two stations. The signal meter stays on 0.

While probing the ha1138 chip the AM started working and stayed working long enough for me to follow the AM alignment adjustments and get a nice strong signal from both ends of the AM band. Unfortunately the AM only worked for a few minutes before dropping back out. I suspected a solder connection so I have reflowed the AM section of the tuner board. I have also replaced the electrolytic capacitors...

• Motnick
• Replies: 8
• Forum: Pioneer Audio

• 

Also the sx-626 schematic shows all the voltages for the ha1138. Doubtful it's the ha1138.

 

[PE9ZZ]

Do you get any noise at all? Does the local oscillator work? If you inject 455 kHz into the IF, do you get signal?

 

[satellite65]

Do you get any noise at all? Does the local oscillator work? If you inject 455 kHz into the IF, do you get signal?

Originally, I thought not, but I must have done something wrong...

There is static which varies a bit as you tune. There are spots where it increases and has an interference hum, and the strength needle goes up.
No sense of any real signal anywhere on the dial, not even a weak distorted one.

My scope shows a 1.47 MHz 428mV sine wave on pin 2 of the HA1138.

I don't have a sig gen...

I did ohm out the transformer windings around the HA1138. T7, T8, T9, and F5 all seem ok (no open coils, all single-digit ohms).

ESR is good on all electrolytics around the HA1138.

 

[rcs16]

Seems to be AM circuit day, someone else is trying to align the sx-939, the schematic online is unreadable.
Look at fig6 in the sx-950 service manual
Pin 3 is the LO, 455kHz above tuning frequency, so in the 1MHz range, 455kHz IF at pins 13,11
Hookup a long wire to the AM antenna jack in the back to get a stronger signal.

 

[PE9ZZ]

I don't have a sig gen...

Get one. Even simple function generator is fine. If it can AM modulate even better. Without it you're effectively blind. Like without a scope. RF work just needs more equipment. Nowadays it's dirt cheap. Feebay is full with simple AM signal generators. Like this one.

A propos 1.47 MHz: at what position on your dial was this?

 

[satellite65]

Hookup a long wire to the AM antenna jack in the back to get a stronger signal.

With a long wire on the AM terminal the static is more even across the band, and the strength meter stays close to 4 out of 5 across the entire band. It never dips below 3.5.

I'll have to bring my scope upstairs to check pins 3, 13, 11. (My bench is in the basement and we're at the bottom of a hill, so I get little signal there.)

 

[satellite65]

 

[bill_p]

The noise level on the AM band has risen greatly in recent years. Devices that are powered from switching regulators are largely responsible and that includes LED lighting.

The AM band is noise-filled here and only one station makes it out of the noise at all and only with the right antenna orientation. That's during the day. At night you may receive more AM stations. The four AM radios I have behave the same so it is not a defective unit here causing the lack of reception. Your TX-9500 may not be broken on AM. Try it well after dark and compare it to another AM radio before deciding it is sub-par or broken.

 

[satellite65]

With a long wire on the AM terminal the static is more even across the band, and the strength meter stays close to 4 out of 5 across the entire band. It never dips below 3.5.

I'll have to bring my scope upstairs to check pins 3, 13, 11. (My bench is in the basement and we're at the bottom of a hill, so I get little signal there.)

Now this is getting weird.
With a long wire on the AM terminal and the amp connected the strength meter stays close to 4 out of 5 across the entire band. It never dips below 3.5.
When I unplug the RCA cable from the amp the strength meter is at zero across much of the band and has spots where it rises to about 4, as described in post #5.

I can't imagine how connecting an amp to the tuner can affect the signal strength meter like that.

(Because it was handy, upstairs I'm using my little Lepai LP-2020A+ powered off of a wall wart. I don't think it acted like that when I used a Nak mini-receiver for the amp yesterday. I'll have to try that again.)

Anyway, with the dial pointer on 100 the oscillator is running at 1.45 MHz with a level just under a volt at pin 3.

 

[satellite65]

The noise level on the AM band has risen greatly in recent years. Devices that are powered from switching regulators are largely responsible and that includes LED lighting.

The AM band is noise-filled here and only one station makes it out of the noise at all and only with the right antenna orientation. That's during the day. At night you may receive more AM stations. The four AM radios I have behave the same so it is not a defective unit here causing the lack of reception. Your TX-9500 may not be broken on AM. Try it well after dark and compare it to another AM radio before deciding it is sub-par or broken.

I have also seen LED lights cause interference, but I am getting crystal clear reception on my portable AM radio in the same location.

 

[PE9ZZ]

Anyway, with the dial pointer on 100 the oscillator is running at 1.45 MHz with a level just under a volt at pin 3.

That would be 1000 kHz so 1.45 MHz is correct as it is 455 kHz above the receive frequency.

 

[satellite65]

Now this is getting weird.
With a long wire on the AM terminal and the amp connected the strength meter stays close to 4 out of 5 across the entire band. It never dips below 3.5.
When I unplug the RCA cable from the amp the strength meter is at zero across much of the band and has spots where it rises to about 4, as described in post #5.

I can't imagine how connecting an amp to the tuner can affect the signal strength meter like that.

(Because it was handy, upstairs I'm using my little Lepai LP-2020A+ powered off of a wall wart. I don't think it acted like that when I used a Nak mini-receiver for the amp yesterday. I'll have to try that again.)

This was caused by the Lepai amp. When the wall wart is plugged into the amp there is a little interference on my portable AM radio, even with the amp shut off. When I touch the case of the amp the interference gets quite a bit louder.

... any recommendations for a little amp to replace the Lepai which has been driving my PC speakers (EPI 50's)?

 

[PE9ZZ]

If it's a regular 12 VDC wall wart get a new one or build a linear PSU. No need to stabilize, just XFMR, bridge and reservoir capacitor. Easy peasy. Took me ages to get a PSU that didn't fill the AM band with noise to power the LED strips on my workbench. I was one wall wart short of building a linear PSU myself...