Help with Troubleshooting SET amp no sound issue

BassKulcha

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I recently picked up a 1960s Radio Craftsmen SET amp and recapped it with help from some of the fine folks here at AK. That thread is here, with pics.

Overall it's been playing great (except for some lingering post-Deoxit pot noise and the treble pot not doing anything after 12:00—) and I've been blown away by its warmth and musicality.

Two nights ago, in the middle of a listening session, the sound just went away. I thought maybe it was the end of the track near the label, and the Q-Up just kicked in before the song was over. Nope. No sound, no jewel lights, nothing, except that I was running my AR XA though the single switched ac on the back of the amp, and that was still running.

I've checked it out visually and nothing seems to be amiss (the fuse looks ok, and there are no exploded caps or burned resistors, etc—) All the tubes seem fine—there is just no power to them, apparently. (I did make a mistake and installed an X-capacitor instead of a Y-capacitor, but I figured that would last a while until I could replace it...)

I have a Fluke multimeter, but I'm honestly a noob as really understanding how to use it. Can anyone give any tips on what I need to do to troubleshoot the problem? Hoping it's not a transformer issue...Thanks in advance—
 
I wouldn't worry about tubes or caps or resistors at the moment.

Concentrate on the power supply area and the wall plug. Usually, power problems are the easiest to solve.

You will need to learn how to use your meter for ohm, voltage, and continuity testing.
 
for starters and while it's on,
1. filaments lighting? in drivers, in output tube
2. any power to unit - check fuse
3. check power cable - pull and reseat
with power off, pull and reseat tubes.
4. check audio input signal eg signal making it into the amp?
5. everything in the sound chain turned on and connected?

if no then you'll need to go inside. power off and check continuity from power plug spade
through IEC connector through fuse to power switch and when switched on to transformer
and repeat through other power spade. then measure transformer primary for continuity
then secondary.

then you're in the power supply to continue.
 
Thanks for the input, folks.

I guess I need a new eyeglass prescription, because when I set up the good lights and double-checked the fuse I noticed that the filament had clearly melted...

So next step is to order a replacement. It's an inline fuse with axial leads, but I'm not sure of the ratings. There's quite a bit of corrosion, but I've cleaned enough off to notice markings on each end. One side says "Sightmaster 125", and the other side says (I think) "3AG 8/10TL". I'm guessing that means .125 amps?

I've already de-soldered it and think I've found a 3 AG Slo-Blo .125 Amps, 250 VAC replacement from Littelfuse (I'm going to stick with the axial-leaded fuse for now):

So what do you think—will this do the job?

(Old fuse for visual reference. Yep, it's blown...)
IMG_3685.jpg
 
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I don't know much about the details of that circuit but from the other thread this has 1x12AX7, 2xEL84 (6BQ5), 1x6CA4

For filament power alone that's 2.82a @6.3v or about 18w, and the EL84s are likely pulling near their 12w limit-- each-- with little difference between idle and full power. That's 42w for just the major uses of power, not accounting for losses or the preamp.

At 120v that's (at least) 0.35A from the wall. At least a 0.5A fuse and maybe more, depending on what you can find, and your philosophy on how close fuses should be to typical power draw.
 
Is this the fuse in the power line, or on the high voltage side ? Power line I'd say probably 800ma. If its high voltage, that could be 125ma.
 
Is this the fuse in the power line, or on the high voltage side ? Power line I'd say probably 800ma. If its high voltage, that could be 125ma.

This is a good point, I hadn't thought about this fuse being something other than the main power line. Though jewel lights are usually run from the 6.3v heater line so lights out suggests the main power is out. This could be the main fuse, in which case its more than 125ma, but it not there could be another fuse which is also blown (possibly due to the same problem).
 
This is the fuse (pre-removal/pre-recapping). There doesn't seem to be an additional fuse anywhere....

2017-08-24 16.53.07_adj 04.jpg 2017-08-24 16.57.38.jpg
 
You get one shot at fuse replacement because the original is a 125vac fuse and the replacement is a 250vac

If it holds I still would be wary but it could have been an "old" fuse
 
You get one shot at fuse replacement because the original is a 125vac fuse and the replacement is a 250vac

If it holds I still would be wary but it could have been an "old" fuse

Ok, so the 125 in "Sightmaster 125" is referring to 125 VAC, not 125ma, correct? And by your comment I'm guessing that, unlike capacitors—which can be replaced with higher voltage ratings—fuses cannot? This definitely was an old fuse, the plastic tubing that had originally wrapped it was pretty crusty...

Is this the fuse in the power line, or on the high voltage side ? Power line I'd say probably 800ma. If its high voltage, that could be 125ma.

Not sure—I've uploaded current photos to show the connections better. One of the wires from the wall plug goes up to the power switch, then back down with the yellow wire to the switchable AC outlet (I've taped a bit of blue tape there to remind me that it's where one side of the fuse goes—), then to the power transformer (via the 2-lug terminal strip—again, I've taped blue tape there). The other wire from the wall plug goes directly to the other side of the switchable AC (which I'm assuming is ground—)

Am I correct to assume that this means that the fuse *IS* in the power line side, and *NOT* the high voltage side since it comes before the power transformer?

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Hey JM
Well how 'bout you. Expert troubleshooter!
The fuse is fed by the yellow wire which comes from the off/on switch and feeds the aux receptacle and the amp. The fuse appears to be for the amp and not the receptacle.
Do yourself a big favor and get the correct fuse holder for a 3AG fuse. Soldering fuses is a PITA.
image.jpegimage.jpeg
Jimmy
 
Hey JM
Well how 'bout you. Expert troubleshooter!
The fuse is fed by the yellow wire which comes from the off/on switch and feeds the aux receptacle and the amp. The fuse appears to be for the amp and not the receptacle.
Do yourself a big favor and get the correct fuse holder for a 3AG fuse. Soldering fuses is a PITA.
View attachment 1039629View attachment 1039631
Jimmy

Ha! Thanks Jimmy—I can only hope to have 1/10th the knowledge of you pros here on AK :thumbsup: Yeah, I'd seen those in-line fuse holders, but didn't want to push my luck on finding the correct one. Thanks for the link—

What voltage and amp rating fuse would you suggest?
 
JM
The 3AG designation is the physical size of the fuse.
The 8/10 is the current rating. 8/10ths of an amp or 800ma. Yours are slow blow.
These fuses are readily available. image.jpeg
 
JM
The 3AG designation is the physical size of the fuse.
The 8/10 is the current rating. 8/10ths of an amp or 800ma. Yours are slow blow.
These fuses are readily available. View attachment 1039693

You rock, Jimmy—thanks, brother. For the VAC rating, do I go with 125 or 250 VAC? Seems both Mouser and Digikey only have 800ma glass Slo Blo fuses at 250 VAC...
 
Let's hope that the main fuse just let go from age, surge, or something simple. It doesn't look blackened to my eye but if it actually melted, you could have other problems. I hope you bought a few fuses. When you go and put the new one in, if it blows then stop right there and find out why it's blowing. Don't put another fuse in as it will just blow again.

Just FYI, blown fuses are due to too much current being drawn by a short of some sort.

Culprits would be the power transformer, a tube or power filter capacitor.

Speaker transformers also do short but it is rare so that would be the last suspect if all else were to fail.
 
Let's hope that the main fuse just let go from age, surge, or something simple. It doesn't look blackened to my eye but if it actually melted, you could have other problems. I hope you bought a few fuses. When you go and put the new one in, if it blows then stop right there and find out why it's blowing. Don't put another fuse in as it will just blow again.
Just FYI, blown fuses are due to too much current being drawn by a short of some sort.
Culprits would be the power transformer, a tube or power filter capacitor.
Speaker transformers also do short but it is rare so that would be the last suspect if all else were to fail.

Thanks for the tip. I haven't purchased them yet, but I'll get a few to have on hand.

Yeah, I'm hoping it's not a larger issue. I guess I'll find out when I replace the fuse. I just replaced the filter caps a couple months ago, so I guess the worst case scenario is something wrong with the transformer. I had planned on rolling some tubes before this issue surfaced, anyway, so I'll round those up soon.
 
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