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500B No Power

potoooooooo

potoooooooo

New Member
So i picked up this 100% original 500B, and it was working. I performed a brief basic function test before I reworked it with the metalbone kit. Now it no longer has power. The rear AC outlets work, so the power switch and fuse are fine, but the rest of it I'm not so sure about. I quadruple checked all of my work, and everything looked fine. The only thing i saw was C85 MAY have been shorted. The pins were uncomfortably close, but I'm not sure whether it was or not.

Does this sound like something you've ever experienced? I suck at troubleshooting so please be gentle :(

Note: In the photo below, I undid the bias caps mod as a sanity check to make sure that that wasn't the problem. It didn't change anything.

Edit: Here's a less compressed version of the image on imgur: https://imgur.com/a/ptY4cxZ
IMG_2867

IMG_2867

Fisher 500B underside
 
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I see a new bridge rectifier, new silicon diodes, new Sonic coupling caps, orange-drop coupling caps and a modern electrolytic or two. So what else did you replace?

I wouldn't power up again until you get (or build) a dim bulb tester and/or a variac. Then you would need to find someone that is comfortable with the high voltages inside a tube unit like yours to slowly bring it up on voltage and measure voltages to see where is the problem. Those voltages can really hurt you bad so you need to know what you are doing in order to be safe about it.

I also use a Kill-A-Watt meter to measure the line current which can tell you a lot on power up. (Dim Bulb tester can sorta do the same if the bulb doesn't light or gets really bright and stays bright.)

So is your fuse good and did you put in the right fuse (3.2A slo-blo, I think)?

Is your power switch working? Those switches have a high failure rate. When you did power up last time, did any of the tubes light?
 
Tim D said:
I see a new bridge rectifier, new silicon diodes, new Sonic coupling caps, orange-drop coupling caps and a modern electrolytic or two. So what else did you replace?

I wouldn't power up again until you get (or build) a dim bulb tester and/or a variac. Then you would need to find someone that is comfortable with the high voltages inside a tube unit like yours to slowly bring it up on voltage and measure voltages to see where is the problem. Those voltages can really hurt you bad so you need to know what you are doing in order to be safe about it.

I also use a Kill-A-Watt meter to measure the line current which can tell you a lot on power up. (Dim Bulb tester can sorta do the same if the bulb doesn't light or gets really bright and stays bright.)

So is your fuse good and did you put in the right fuse (3.2A slo-blo, I think)?

Is your power switch working? Those switches have a high failure rate. When you did power up last time, did any of the tubes light?
Click to expand...

I've been using a Kill-A-Watt to watch for anything. 0 Watts/0 Amps. The fuse is good, and the power switch is good. I replaced C85 (near the PSU section), C74 (towards the front, near the MPX section), and C210(In the MPX section) with new electrolytics. C57, C58, C60, C61 were replaced with sonicaps, and the rest of the mylar caps were replaced with orange drops. I also added 10 Ohm resistors to the output tubes from pin 5 to ground to act as fuses if there were a short. That's it, all pretty standard stuff as far as I can tell.

When I powered it up after the work, no light in any tubes. Zero signs of life. No power draw, no heat, nothing.
 
Do the tube heater filaments get any voltage? I see that you cannot observe the filaments getting hot, but what about a voltage measurement?
 
If you're getting no current when powering up, then something very early in the chain is not working. I haven't looked back over the thread so forgive if you've already checked. Are you getting any voltage at the rectifier or at the filter caps? Any negative voltage coming out of the rectifier?
 
Watthour said:
Do the tube heater filaments get any voltage? I see that you cannot observe the filaments getting hot, but what about a voltage measurement?
Click to expand...

0 Volts between pins 2 and 7 of the output tubes. Also 0 between pins 4 and 5 on the phono preamp tubes and tone control amps tubes, V7 through V10

thornev said:
If you're getting no current when powering up, then something very early in the chain is not working. I haven't looked back over the thread so forgive if you've already checked. Are you getting any voltage at the rectifier or at the filter caps? Any negative voltage coming out of the rectifier?
Click to expand...

0 volts between the + and - pins, going both ways. I agree that it's something early, I just don't know what it could be. Almost seems like the transformer, but I'm not really sure.
 
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Def check the transformer. Research online how to do that. It's been discussed a few times in this forum too if you will search. Something about checking resistance between leads.
 
thornev said:
Def check the transformer. Research online how to do that. It's been discussed a few times in this forum too if you will search. Something about checking resistance between leads.
Click to expand...

Yep, what Thorne said. Check the secondary windings on the Power Transformer for non-infinite/open circuit resistance. Funny that you are getting zero volts on high voltage plus also zero volts on filaments. Rare that both would go bad. Fuse or switch or primary side problems are suspect.

You might add your location to your avatar. There might be an AK member local that could help with basic troubleshooting.
 
thornev said:
Def check the transformer. Research online how to do that. It's been discussed a few times in this forum too if you will search. Something about checking resistance between leads.
Click to expand...

Tim D said:
Yep, what Thorne said. Check the secondary windings on the Power Transformer for non-infinite/open circuit resistance. Funny that you are getting zero volts on high voltage plus also zero volts on filaments. Rare that both would go bad. Fuse or switch or primary side problems are suspect.

You might add your location to your avatar. There might be an AK member local that could help with basic troubleshooting.
Click to expand...

Ok, so I took some measurements. I have some functional knowledge of transformers (thanks college). These sort of make sense, other than being lower than I anticipated. I measured after de-soldering each lead. I didn't measure between leads, it didn't occur to me at the time.

Orange+Brown 1.9 ohms

White + Blue 0.7 ohms

Green + Yellow 0.7 ohms

Red/Yellow + Red 4.7 ohms

Black + Black = 1.8 ohms

By the way, I appreciate the help so far guys. This stuff can be overwhelming, but your suggestions have all been very helpful so far.
 
Don't just visually look at the fuse. METER IT! Fuses DO blow at the ends, and it leads you to think the fuse is good.
 
larryderouin said:
Don't just visually look at the fuse. METER IT! Fuses DO blow at the ends, and it leads you to think the fuse is good.
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Good suggestion, I don't recall if I had checked it after everything. But it is in fact still good. The list of things that could be wrong grows ever shorter
 
In your first post you say that the AC outlets still work. The main power switch is DPDT with one pole powering the outlets and the other powering the amp itself. Sounds like the amp pole may be broken. You may need a new on-off switch.
 
Stratakit66 said:
In your first post you say that the AC outlets still work. The main power switch is DPDT with one pole powering the outlets and the other powering the amp itself. Sounds like the amp pole may be broken. You may need a new on-off switch.
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Most people don't use the outlets on the back. If it turns out to be the switch you could just use the other half by re-wiring the switch.
 
Stratakit66 said:
In your first post you say that the AC outlets still work. The main power switch is DPDT with one pole powering the outlets and the other powering the amp itself. Sounds like the amp pole may be broken. You may need a new on-off switch.
Click to expand...
Stratakit66 said:
Right you are Tim, but if one side of that switch has failed the other may follow soon.
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Ding ding ding! We have a winner! I checked for continuity and sure enough, only the outlets were going through. I never would have noticed that it was a dual pole switch if you hadn't mentioned it, thank you so much!

I switched the wires around and sure enough, fired right up. I didn't do a full test because I'm tired, but at the very least I am a lot closer.

Thank you so much guys! You've been a great help, I literally couldn't have done it without your help.
 
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