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SX 450 No AM/FM

micrometer

micrometer

chameleon
I picked up a SX 450.

1. DBT pass
2. DC offset R- 8.4mv L- 31.3mv
3. Bias R- 56.5mv L- 49.2mv
4. Cleaned controls with CRC -QD elect. cleaner/D5/F5
5. All 5 internal and 1 external fuses show continuity
6. Took apart and cleaned the dreaded ALPS on/off speaker selector
7. Hooked up speakers on speaker A
8. Aux - iPod both channels even and good volume
9. Tape Play - iPod both channels even and good volume

When I select AM or FM I get no sound when moving the tuner dial. I get constant static on FM when FM mute button is depressed. On the AWE-074 board I get 13.9 vdc at TP 20 and 35.6 vdc at TP 2.

I'm at a loss. I would appreciate any assistance.

Regards,
Mike
 

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Just covering the basics, but since your power supplies seem to be in order, hang a 31" piece of wire onto one of the FM 300Ω antenna inputs, and another on the AM external antenna input. If there is no signal meter movement when moving the tuner VC, power down and check the tuner VC sections for shorting between the gangs or plates using your ohmmeter. Also check the trimmer caps on top of the VC assembly for shorting, but try not to disturb the positions when probing the trimmer screws (TC1 through TC5).

upload_2021-12-9_8-39-4.png
 
Grumman F6F Hellcat was the best US WW2 fighter with a 19:1 kill ratio. She produced the most US aces during this time and the most kills. What a beauty!
 
...And Hamilton McWhorter was the first one of those. He passed a few years ago, but I got to meet him at Oshkosh back in the '90s. He had a really good perspective on the whole affair.
 
Watthour said:
Just covering the basics, but since your power supplies seem to be in order, hang a 31" piece of wire onto one of the FM 300Ω antenna inputs, and another on the AM external antenna input. If there is no signal meter movement when moving the tuner VC, power down and check the tuner VC sections for shorting between the gangs or plates using your ohmmeter. Also check the trimmer caps on top of the VC assembly for shorting, but try not to disturb the positions when probing the trimmer screws (TC1 through TC5).

View attachment 2416865
Click to expand...

I attached an antenna to the 300 and 75 ohm connections and turned on the unit. Selector was set to FM and then AM. I moved the dial all the way across and noted no deflection on the signal meter.

I'm not sure how to check for shorting of the VC and TC's. Do I turn it on and set to FM and probe between the fins checking for voltage? I've worked on several receivers and not come across the tuner section not producing sound so I'm treading on new territory here. I greatly appreciate you taking your time to share.

Cool story about Hamilton McWhorter.

Thanks,
Mike
 
Power the unit off and test resistance between the stationary and moving gangs of fins in each section of the VC.

I'm making a guess that nothing is getting through the front end of either the AM or FM section, and the VC is one of the few items common to both. You've already astutely checked for proper voltage to the board, so that's off the list (for now).
 
And:
The +B is 13.7 volts at pin 20. Got voltage?
If you have close to the 13 volts, check the HA1137 IC for the pin voltages.
Use a probe that is insulated to the tip - only leaving a small portion exposed, limiting the chance for shorts.
Report what you find in the format below.

Pin 20 = 0.0?
IC Pin:
1=?
2=ect....
 
zebulon1 said:
And:
The +B is 13.7 volts at pin 20. Got voltage?
If you have close to the 13 volts, check the HA1137 IC for the pin voltages.
Use a probe that is insulated to the tip - only leaving a small portion exposed, limiting the chance for shorts.
Report what you find in the format below.

Pin 20 = 0.0?
IC Pin:
1=?
2=ect....
Click to expand...

TP 20 - 13.7 vdc
ha 1137
1 - 1.97 vdc
2 - 1.94
5 - 1.6
7 - 5.9
8 - 5.4
9 - 5.4
10 - 5.6
11 - 12.1
12 - 5.12

Thanks
 
I hooked up my $7 function generator to the AUX input and my $20 o-scope to the speaker output. Turned on the unit and have sine wave output on the o-scope for each channel. When volume is increased the amplitude on the scope goes up.

Can I inject a sine wave through the FM circuit and check the scope to see when/where my signal drops?
 
Do you have another receiver hanging around that works?
I've taken the RF out from one front end and injected it into a suspect tuner assembly.
Had both receivers powered up and used a single, solid core wire to connect them. Bonded grounds using a jumper between them.
For some reason I thought it was funny that it was working like that.
I don't think you need to disconnect any components or wires. Find the RF out of the good receiver and connect it to the RF in of the 450.
 
Last edited:
zebulon1 said:
Do you have another receiver hanging around that works?
I've taken the RF out from one front end and injected it into a suspect tuner assembly.
Had both receivers powered up and used a single, solid core wire to connect them. Bonded grounds using a jumper between them.
For some reason I thought it was funny that it was working like that.
I don't think you need to disconnect any components or wires. Find the RF out of the good receiver and connect it to the RF in of the 450.
Click to expand...

Sorry, no available receiver present.
 
Any radio would work. It’s a matter of figuring out how to connect it.
They make RF probes. Cheap on eBay. From Ukraine. Add a 12 volt wall wart and your up an running?
 
Watthour said:
Power the unit off and test resistance between the stationary and moving gangs of fins in each section of the VC.

I'm making a guess that nothing is getting through the front end of either the AM or FM section, and the VC is one of the few items common to both. You've already astutely checked for proper voltage to the board, so that's off the list (for now).
Click to expand...

I did the resistance test on the VC and got zero ohms.
 
micrometer said:
I did the resistance test on the VC and got zero ohms.
Click to expand...

That should be checked on a low resistance range. Measurement of the AM side (greater number of plates in the VC) sections should show from about 2.5-4.0Ω. A convenient measurement point may be the rectangular metal tab/flag adjacent to the trimmer cap atop the VC assembly and its associated moving plate section.

I just pulled a spare working SX-550 out of the rack to verify that, and it has the same ACK-012 VC as the SX-450.
 
micrometer said:
I pulled C161 and it shows 10.38 nf and 0.5% Vloss.
Click to expand...

That's a near-perfect measurement. Nothing to worry about there.

If C161 is still pulled, you can try to connect a long wire to that input of the IC (Pin 8) and see if the AM section will produce even a hum or AM "hash" noise from local RFI. If you were really lucky, you might even get some noise from a local/strong AM broadcaster nearby.
 
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