[Johnny Law]

And also, I’m too much of a chicken to plug it straight into the wall WITH the FM circuits connected.  Although, that might be a quick way to find out what’s wrong (ZAP)…

[Johnny Law]

Oh – I forgot to mention – into the wall directly, I am seeing 21.4V AC out of the transformer.

Via the DBT, I see around 19.6VAC when it’s at 15V, but when it gets stuck lower, the transformer is reduced to 19.1VAC from the bulb.

So – hopefully that means the transformer is OK.

[Johnny Law]

After lengthy diddling over the weekend, I have decided to conclude that it’s the DBT causing the 15V power supply to sometimes not activate, but only when isolated from the tuner’s IF and discriminator circuits.

I had been using a 53W bulb – and it had behaved as described above.

Over the weekend, with the 15V PS circuit electrically isolated (i.e., everything connected to it de-soldered), and plugged directly into the wall, I see 15V EVERY SINGLE TIME I power the unit on.  I must have cycled the power on and off 50 times or so.  I saw 15V every single time.

Then I found a 100W bulb and installed it in my DBT.  Through the 100W bulb, I cycled on-off-on-off and again saw 15V every single time.

Feeling confident, I then soldered back into place the jumper leading from the power supply to the IF and discriminator boards (see attached image).

Note that the pre-amp and all bulbs are still disconnected.

With the FM circuitry hooked back up, via the 100W bulb, power on and I only see 0.02V at the 15V power supply!!

If I de-solder the jumper, strangely, the first couple times I power on (via the DBT), the PS gets stuck at ~1.8V … but off-on-off-on a few times and it comes back.  (WHY?? No idea …)

So anyway – I figure the next thing to try to figure out is what’s dragging the voltage down in the IF & discriminator circuits.

I removed and checked TR19, TR20, TR21, and TR22, all are OK and no change to the 0.02V with them out of circuit.

Onwards – I figure I’ll keep checking devices until I find a short to ground?  But any other thoughts highly appreciated …

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[Johnny Law]

I have even tried to alter values / remove grounds / try different things in LTSpice to try to duplicate/simluate the behavior, but nothing so far produces the same values.

• This reply was modified 3 weeks, 3 days ago by Johnny Law.

[Johnny Law]

Yeah, I am really at my wit’s end.  Everything tests good, measures OK, and yet it still doesn’t work.  (Well, once it works, it works fine, but it doesn’t always work unless I cycle the power off and on!)

I tried while plugged directly into the wall, but see the same behavior as through the DBT.

The rectifier B30C350 is a new B40C1500.
TR17 was originally a BC212B and is now a new BC212B (I have the original still on the bench, it tests OK).
TR178 was originally a BC119 and is now a new 2N2219 (this is a TO39 package device, the original was shorted, got red hot under operation, and had a cold solder joint that I discovered).
ZD148 is the original which works fine, although I have at various times put in a new NZX9V1 Zener diode, but the circuit does the same thing with the new or original one in place.
C145 is a new Vishay 138AML axial 470uF / 40V.
The resistors are either original or were replaced by metal film before my time. All measure correct values out of circuit.
All grounds are good.

I am totally at a loss.  I will keep poking around and taking measurements and hoping to find some inspiration …

[Johnny Law]

Wish it were that easy – turns out, when the PS isn’t activating, TR17 is only seeing ~0.4V B to E.

The attached image shows my Mastech is on the +PS, and my Fluke is on TR17 B to E.

So it’s not a TR17 issue.  It’s a current issue … for some reason, it’s not seeing enough of a drop across R150 (even though R150 measures correctly) to activate TR17.

Here’s where things get weirder … when I turn the unit OFF, TR17E-B BRIEFLY hits 0.6V+ and activates!  So while the unit turns off, the 15V PS turns ON for a moment before then going dead.

I managed to snap a pic when this happens.

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[Johnny Law]

Great idea … I had disconnected the pre-amp, but I had not disconnected it from the FM circuitry!  So I lifted various leads of all components getting fed by the + side trace of C145.

With the +15v PS completely isolated … it turns on correctly MOST of the time.

When it gets stuck, now it’s stuck at around +1.8V instead of the +0.06V I was seeing before … when this happens if I touch anything in the circuit with my DMM lead, it switches on fully.  (This obviously implicates TR17 which SHOULD activate with that much voltage at its base …)

Once it does switch on to the full +15V, I can turn it off and on again repeatedly and it always activates correctly.

I let it sit off overnight, and tried it this morning: it immediately went to the full +15V (I was wondering if maybe discharged capacitors had something to do with it … apparently not).

So from here, two things: 1 – why won’t it turn on 1-5% of the time; 2 – what’s shorting out in the FM circuit?

Onwards!

• This reply was modified 3 weeks, 6 days ago by Johnny Law.
• This reply was modified 3 weeks, 6 days ago by Johnny Law.

[Johnny Law]

Yeah, I’ve been checking it over for longer than I care to admit.

Originally, I found quite a few, notably including TR18.

Everything has been re-flowed and/or checked … no harm in checking again I suppose!

Thanks for the input … looking forward to finding the issue and shaking my head, looking back on my troubleshooting, whatever it ends up being …

[Johnny Law]

Craig: Thank you!

I should have noted the work done … everything in green boxes in my graphic (aka, every component in the circuit) has been replaced and/or tested OK.

I see the same behavior either with the original 9.1V zener or with a new replacement 9.1V zener (specifically, a NZX9V1D).

The issue (I think) is that for some reason there’s no current through R150 & R151 to create the voltage drop needed at TR17’s base, which once activated would flow current through R146 to activate ZD148.

Despite weeks of scratching my head and trying different things, I can’t determine why that’s the case …

One idea was that the 15V PS load is too small – I confirmed in LTSpice that if the load is 1ohm, the PS won’t engage.

But, I measured the resistance from the +15V PS to ground and am seeing 285ohms.  Modeling that effective startup resistance in LTSpice, the power supply should still activate.

 

(I’d post the LTSpice images, but I’m new to the forum and it won’t let me either copy / paste images, upload them in the image upload tool, or attach them as files for some reason?  Is the image tool operational on this forum?)

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