Mac 1900 No FM output

captouch

Addicted Member
My Mac 1900 doesn't output any sound from FM on either channel. Everything else (other inputs) work fine, including AM.

At first I thought it might just require a real antenna, but after hooking up a rooftop antenna, still nothing. The tuning strength meter seems to work - just no output.

I read these units are prone to having some tuner issues.

Any common points of failure that I should check out?

I may just replace the caps on the tuner board to see if that resolves it, but thought I'd ask any experts here first.

Thanks.
 
Virtually nothing. If I crank the volume to max, I hear the faintest audio. Barely audible.

In all other modes besides FM, it works perfectly.
 
The tuner's ps regulator transistor is turned on and off via the mode selector. Check the schematic and measure the output from that regulator transistor.
 
If 16V to the board is OK it is probably a bad tantalum capacitor hanging it in the muted mode. Check C120, 123, and 136, as a safety measure replace all 1 uF and above caps.
 
OK, I'm starting into this finally.

The 16V (it's actually around 15.2V measured) is getting to the board OK - always seems to be there, whether I'm selecting FM or phono, etc. So I'll check C120/C123/C136 next.
 
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I checked C120, C123, C136 - all measured normal on a cap meter, but I replaced them all anyway.

I also replaced C108, C116 and C137 per Terry's input to replace all 1uF and above caps.

Still no sound from FM. I played with R148 (FM muting pot) a bit, no difference. I see R115 and R126 are also pots, but not sure what they do (one says FM level, the other isn't labelled in the schematic).

I did verify that the FM muting button on the front panel works by checking pins 11, 13, 14 (verify the appropriate ones would short out when button was pressed/not pressed).

Here's a picture of the board:

photo_2_8.jpg


C133, which appears to only be in play for the left channel, appears bulged a bit.

photo_1_7.jpg


When I touch it, I actually get a bit of static/hum sound - when I try to tune into a station and the signal strength meter goes up, no actual music/sound though. Even though it's supposed to only be for the left channel, I do get that static/hum sound even on the right channel when I touch it.

I don't know whether these type of caps bulge when they go bad, but was thinking of maybe replacing it. I did measure both C133 and C135 (right channel equivalent) in circuit with a cap meter. C135 (not bulged) reads 100pF, while C133 (bulged) reads a bit lower at 87pF - so at least based on in-circuit capacitance reading, C133 isn't grossly off or different from C135.

I'll look at the schematic to see if there's anything obvious I should be checking, but if anyone has any ideas, I'd love to hear them.
 
Disk ceramic caps normally do not go bad, I think it is time to use an FM stereo generator and a scope, sorry there is only so much one can do with minimal test equipment.
 
Is the 37 volt supply there? It goes thru the rear section of S301 and to the IF section after regulation.
 
Yes, the 37V supply is there.

Here are the voltages I'm reading out from the various pins:
1) 37.3V
2) 0V
3) 0V
4) 0V
5) -85mV
6) 0V
7) -0.66V
8) 0V
9) 0V
10) 0V
11) 0V
12) 15.2V
13) 0V
14) 0V
15) 37.2V
16) 15.2V
17-19) 0V

As far as deviations from schematic values:
Pin 4 should be 19V, but it reads at 0V - pretty big deviation!
Pin 5 should be 0V, but reads -85mV
Pin 7 should be 0.15V, but reads -0.66V.

The rest are all at schematic value (well close - 15.2V instead of 16V, 37.2V instead of 40V, but I think those are OK).
 
I probed around a little more and all the pins of IC102 are zero, whereas all the pins of IC101 measure as they should.

Maybe that makes sense of Pin 4 isn't getting 19V. I'm going to hunt for R145 next since the 19V seems to be sourced from the 40V through R145.
 
Ok, FM is fixed.

Went looking on the underside to see where the 19V was getting lost from the 40V supply.

Turned out there were two caps in parallel - a 0.05uF ceramic disc and a 470uF/40V electrolytic in parallel with that - which I don't see in any schematic.

At any rate, the 470uF/40V failed as a short, shorting the 19V node to GND.

Since it's not in the schematic, I debated just removing it and not replacing it, but decided that if it was there, I should replace it. So I did and everything fired up and FM was playing well.

Thanks for everyone's suggestions - appreciate it.
 
My schematic shows the cap but notes it was not used in early (before AN2085) units.

I am surprised the PS did not fail driving what must have been a near dead short.

This again shows why it is a good idea to plan on recaps of these mid 70 units.
 
I'll look again for that cap - I didn't see it, but I could have missed it. I've been going off an online SM, but just got my hands on the original that came with the unit. So it may be in the later schematic. This is a later unit with the impedance switch on the back.

Best as I can tell, the 40V was being taken to GMD through a 680ohm 1W resistor, so if my calculations are right, there was ~2.4W being dissipated through that resistor. It didn't appear visually stressed and I measured it in circuit close to it's intended value, but I suppose it'd be best to change that out next time I'm in there.
 
If you look in the preamp section you will see 37 volts going to pin1 of the IF-MPX board....I think the 40 volt notation is a misprint, makes more sense that the tuner voltage would come from the PS, not the main amp section.

You got it working....thats great.
 
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