1980 FM tuner dead

[jtee1340]

Hello all,
I have a 1980 that the FM tuner is dead. No sound at all, no FM light when selected. The am works.
I have tested my power supply voltages and all are ok. 13 volts to tuner board is good.
I have the schematic, and suspect an issue with the IC chips on the tuner board. I am just not sure how to trace through the tuner board to find the issue.
I repaired a 1280 tuner board that had a bad PA3001 chip and I know they are hard to come by.
I have a complete untested tuner board from a parts unit, and I thought about just swapping out the complete board.
I removed a couple of the pins to keep from unwrapping them, but wanted to see if maybe someone here has some advise before I go and change the board out.
I have not gone through the switches yet, but I have zero issues with any of them as of now.
Areas that are common failures and areas to look first would be appreciated. Not much info out there on tuner repair on these things.
Thanks,
J.T.

 

[txturbo]

i don't think the fm selector light is tied to any IC. I would look at the selector switch.

 

[zebulon1]

For sure, check for power at the tuner board.

 

[jtee1340]

Thanks guys,
Zeb, I have checked all the voltages on the power supply and where it goes to the tuner board (13 volts). I have read that if the power supply has an issue and more than 13 volts goes to the tuner due to power supply, those chips suffer damage pretty easy.
I will give the selector switch a cleaning and look txturbo, thanks.
This unit came off of the auction site and I don't know if the tuner was worked on. The tuning meter is full to the right even with power off, so that will have to be swapped out. It makes me think something happened to the tuning board to cause that.
The tuner section is not something I wanted to have to mess with, but it all comes with the territory I guess.
This thing is so massive, it is not easy to just flip over and work on. Have to do most of it on its side.
Any more suggestions are totally appreciated and welcomed.
Regards,
J.T.

 

[txturbo]

Looking at the schematic..

Check the voltage between pins 46 and 47 on the function selector board for the FM lamp. If you don't have 13.5 then I would start there.

If I read the schematic correctly the FM section is always powered. It's effectively shutdown by the two pins, IF killer (pin 32) and VCO Killer (Pin 23). There may be others. It does not look like a single power line that is cut off.

If you have power and the Killer pins are not at 13.5 then the IC's may be suspect. I am sure someone with more experience with this will chime in..

By the way is AM working?

 

[jtee1340]

Yes, AM is working fine.

 

[markthefixer]

I work on them with them on their side, on a lazy susan so turning is effortless.
I use a recessed case screw (case screw AND bonnet mounting hole) and a wide wood foot to keep it stable and upright.

Voltage measurements are only valid AT the board.

Measure the voltage across that sprung tuning meter's connections at the board. Even with power off.

Previous posts are good advice. MOST of the chips are shared with other models,
so replacement parts won't be an extreme hassle (except the pa1002, fingers crossed on that one) but
they will come from donors.

 

[jtee1340]

Thanks Mark,
That is how I am working on it also on it's side. Thing is a beast.
I pulled the sprung meter out and I could not get it to move at all. I took the plastic cover off and the meter was frozen to the one side.
I have one to replace it with, but don't want to replace it until I check the voltage to it like you said.
I examined the back side of the tuner board and it appears it has been worked on before.
Couple of non-factory solder areas.
Thanks again,
J.T.

 

[jtee1340]

When measuring between pins 30 and 31 on the tuner board (outputs to the tuning meter) I get between 6.5DC and as low as 2 volts as I turn the tuning knob. It fluctuates up and down as I turn the knob. Nothing with power off.
No movement on the signal strength meter at all as I tune.
J.T.

 

[markthefixer]

both meters run off the pa3001 chip, and the tuning meter should have 0.000 volts across it (pins 30 and 31, NOT to ground!!) in tune, and perhaps a maximum of 100 mV +/- when way off

That chip or the circuit around it is sick, where are the non-factory solder areas? what components?

 

[jtee1340]

I have attached pictures of the non-factory areas.
From the component layout it looks like the double tuning filter F9 (which I don't see listed on the component list) was soldered on. Also R90.
Schematics say r90 should be 47K, but I read 10K, Brown black orange? There are 3 47K resistors in that area.
I also measured the voltage across pins 46 and 47 as Txturbo suggested, and I have 7.75dc volts across the two pins, not 13.5v. Cause of no light maybe?

Thanks for the assistance Mark.
J.T.

 

[rcs16]

The tuning meter is full to the right even with power off

That has to be addressed first, pin 10 on the PA3001(Q4) is the reference bias pin, it is a fixed voltage ~5.7V and pin 7 is the afc pin, which varies based on the IF frequency deviation from nominal. Adjusting the quad coil lower core should be able to vary the AFC voltage to be equal to the ref bias pin, which represents centering of the tuning the meter. steps 23,24 in the procedure.
This is done with no input to the PA3001. WIithout this being correct it will never work properly. So either, the chip is toast, the quad coil is toast (check for a broken core) or some comp around the PA3001 is causing the problem.
You can tie the IF input pin 34 to ground,pin 35 with a 330 ohm R to make sure there is no RF IF signal to do the quad coil centering adjustment.
On the pcb pins 30,31 are your afc ckt measuring points
Good luck.
F9 has nothing to do with FM operation, it is the AM IF filter.

 

[jtee1340]

Thank you for the input rcs16, I appreciate it.
Just to be clear, are you saying put pin 35 to ground through a 330 ohm resistor?
As far as adjusting the quad coil lower coil, this will require tuner adjusting tools correct? I don't have any at the moment, but can look into getting a set.
After reading what Mark stated about there should only be 100mv or so on the tuning meter for full swing left or right , I suspect my 6-2 volts it is getting on pins 30 and 31 fried the coil on the old meter to the point it would never move again.
I have inspected the power supply closely and see it has been repaired also. The voltages all read good at the power supply board, and the 13 volts reads good at the tuner board.
I am a little gun shy about adjusting the quad coil lower coil without the proper tools. I will probably have to have this tuner aligned after this work anyway, but I would rather not get it totally wacked.
Does anyone have a source for a set of plastic tuning tools? I am going to check MCM tonight. All-Spec a possibility too.
Thanks for the help.
J.T.

 

[rcs16]

Just to be clear, are you saying put pin 35 to ground through a 330 ohm resistor?

yes, but you can first try by shorting the RF/antenna input to ground as well.

As far as adjusting the quad coil lower coil, this will require tuner adjusting tools correct?

yes only a non-metallic, nylon/plastic tuning tool or you will damage the fragile cores.

After reading what Mark stated about there should only be 100mv or so on the tuning meter for full swing left or right , I suspect my 6-2 volts it is getting on pins 30 and 31 fried the coil on the old meter to the point it would never move again.

It is the difference in voltage (~100mv AFC) between pins 30 and 31 that drives the meter. The schematic shows those voltages referenced to ground but the meter is not grounded, it is connected between those pins.
Digi-key sells tuning tool kits, 243-1016-ND
Have you measured the resistance of theold tuning meter. Is it open ckt? So you are saying the DC voltages of pcb pins to ground is 6.2V? That might be okay, spec is 5.7V.

 

[jtee1340]

I measured the resistance of the old meter at 659 ohms across the connectors. It reads exactly the same on my replacement meter. So it seems the coil is ok, but the pivots for the needle are frozen and will not move. Strange.
I did not measure DC voltages to ground on pins 30 and 31. I measured between 30 and 31 and read 6.2 to 2 volts as I turned the tuning knob. I do get very small AC mv fluctuations as I tune also, but with a digital meter, it just jumps around and never steadies out.
Ok, so now I measured DC voltage from pin 31 and 30 to ground.
31 to ground fluctuates between 12.3 to 7.8 volts DC as I turn the tuning knob
30 to ground is pretty steady at 5.8 volts DC as I turn the tuning knob.
That's funny, those are the tools I ordered from Digikey.
Thanks,
J.T.

 

[rcs16]

Knowing that there is such a big difference in DCV, it would surely pin the tuning meter needle.
Okay then, once you get your tuning tool kit, try to adjust the quad coil lower core to see if you can get pin 31 to be the same DCV as the ref pin 30 or 0VDC across them. If you can not, I can only assume the chip or the quad coil, other comps in the ckt are faulty.
I am not 100% sure the tools will have the correct hex mm size, but I recall they were ordered by another user in the past and I did not here back that they were the wrong ones. good luck

 

[jtee1340]

Thanks RC,
I will be back as soon as I get a chance to check this part out.
I have another Tuner board from a parts unit. Doesn't seem to have been worked on, but the power supply from that unit was not installed when I got it, so I don't know if it fried chips on that tuner board. I could swap just the chip as I do have desoldering experience and a PACE desolder station. Just don't like the idea of heating up a chip that may be tough to get later. May just swap whole board to avoid that. However the parts unit is an unknown also, could be jumping from the frying pan into the fire. lol
I will pull the function switch board while waiting for tools to clean and test the switch as not getting the "FM" light seems to point to that switch since I don't have the 13.5 volts to the bulb on pins 46 and 47 as txturbo suggested I test?

Thanks,
J.T.

 

[rcs16]

Looking at the schematics, they have mistakes in the schematic on page 70 = idiots, it is a 8V supply, not 13.5V on pin 13
From PS(AWR-134) pin 16 B+(8V) to (wire a') GWS-129, pin 13, through R47(10) to S1(B), then on to pin 47, through FM lamp, back to pin 46.

 

[markthefixer]

if the AM light (usually the LEAST used lamp) works, it means pin 13 has 8v on it and R47 (10 ohms 1/4 watt) are OK, then all we have is a dirty switch, a jumpered pair of holes where a resistor can be inserted, and then the trace to pin 47 to interrupt the Lamp Power.

 

[jtee1340]

How about a blown bulb? Imagine that, a blown bulb in a 30 year old piece of gear. I thought I needed 13.5 volts at the bulb but only needed the 8 volts which I measured out at 7.8v. Keep it simple, but noooooo. I didn't ohm out the bulb.
All the other ones worked......and I thought I didn't have enough voltage. Well .
J.T.