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 Post subject: Magnavox AMP 150 and CR 730 Tuner Popping Fuses, 5U4GB flash
PostPosted: Mar Tue 12, 2019 1:09 am 
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Location: Henrietta, NY
Hi guys,

UPDATE: Flash has been isolated to fuse catridge. Think I had a case of improper fusing. Though confused why one low rated fuse held on for a few weeks....

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My amp and tuner have been recapped and have provided 5-10+ hours or more of enjoyment already. Today was no exception.

This afternoon I tried to connect a preamp and turntable to the phono input as substitute until I get my built in changer working.

Anyway, connecting the preamp (plugging in the wallwart specifically, seemed to have kept popping the fuse I installed in the AMP150). SO I disconnected everything.

Still popping fuses. I notice in the dark theres a flash coming from one of the 5U4GBs. It happens so fast before my fuse blows that I can't see where its coming from specifically, notime for smoke).

I subbed that tube out with a 5U4G and no flash. OK, but then for further test I swapped positions ( I put the other original 5U4GB in the flashing tube position and the 5U4G in the otherwise good position). Flash from the same culprit socket and POP went the fuse.

Not sure at all how to begin troubleshooting this. Suggestions? I'm wondering if one of my new electrolytics let go, and how I might test them.

Also, I'dlike to isolate it to the amp or the tuner. Can I somehow power the amp with no tuner plugged in?

EDIT: I pulled every tube in this AMP except the two 5U4GBs. -Same Flash from the one socket it seems, and POP. When I say socket, I'm saying I see a flash in/around the one rectifer, no smoke or anything obviously wrong with socket itself.

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Last edited by radiopicker on Mar Tue 12, 2019 5:39 pm, edited 1 time in total.

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 Post subject: Re: Magnavox AMP 150 and CR 730 Tuner Popping Fuses, 5U4GB f
PostPosted: Mar Tue 12, 2019 2:39 am 
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Location: Albuquerque, NM 87123
You need to determine if the problem is too much current draw or a bad socket (you mention a flash at the socket). Disconnect everything the B+ goes to as a test of the filter capacitors, then one by one start reconnecting 1 channel of the amp then another. I recommend using a variac and an AC ammeter.


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 Post subject: Re: Magnavox AMP 150 and CR 730 Tuner Popping Fuses, 5U4GB f
PostPosted: Mar Tue 12, 2019 3:57 am 
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I ohmed around the PT leads and rectifier sockets. Nothing unusual short wise.

Here’s the kicker. 8 2A fuses later, the 9th is holding. This is embarrassing but I’ve a surplus of them so was troubleshooting various combinations of tubes in and out and swapped positions.

Anyway, I’ve no idea what the trouble is/was but I’ve got 3-4 on off cycles in and it’s been playing for 20 minutes now. No fireworks. Nothing looks unusual or red plating.

Doesn’t make sense because it really was working then stopped without me touching it. Maybe something vibrated loose and fell into short mode and I just knocked it out?

Or I’m wondering if the 5U4GBs are becoming hit and miss. I’ve read rectifiers going isn’t uncommon. This set uses GAs but GBs are installed. If I am to order a replacement set in the event I need, what’s the difference between a GA and GB 5U4?

Stay tuned. Not sure I’m out of the dark yet because 8 consecutively blown fuses is concerning.

Didn’t think I was turning it on/off fast either. Thanks!

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 Post subject: Re: Magnavox AMP 150 and CR 730 Tuner Popping Fuses, 5U4GB f
PostPosted: Mar Tue 12, 2019 12:34 pm 
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No this is blowing my mind. Fired up on the 9th fuse and held last night. Played for nearly an hour.

Just turned it on this morning many hours later. Popped so fast dial lamps can’t even light.

If it were the filter caps wouldn’t it have not worked at all? It doesn’t like starting. And it can’t be the fuse because at 117V it should be drawing less than 2A and I’ve got 2.25A fuses popping now.

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 Post subject: Re: Magnavox AMP 150 and CR 730 Tuner Popping Fuses, 5U4GB f
PostPosted: Mar Tue 12, 2019 12:50 pm 
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You need to do a little more troubleshooting than just replacing fuses. You will eventually cause damage by continually powering up with this fault.


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 Post subject: Re: Magnavox AMP 150 and CR 730 Tuner Popping Fuses, 5U4GB f
PostPosted: Mar Tue 12, 2019 1:22 pm 
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I think I’ve isolated the trouble to the power supply as the fuse still blows with no tubes except the rectifiers.
I don’t believe it’s the AC bypass cap either as I’m not seeing a short to ground from either prong.

That’s as far as I am right now. The rectifier sockets look fine. Everything APPEARS fine.

See what I can do.

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 Post subject: Re: Magnavox AMP 150 and CR 730 Tuner Popping Fuses, 5U4GB f
PostPosted: Mar Tue 12, 2019 1:56 pm 
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Could you elaborate on best way specifically to test the input filter? Or any of them for that matter. I’ve never encountered a new cap go bad before. New troubleshooting to me.

Ohmmeter shows less than a meg and dropping ohms from rectifier filaments to chassis. So I think my input filter is preliminarily okay but unsure how to test further.

Edit: I slowly ramped it up with a variac and it’s drawinf 200W. Tag on the back says 205. It sounds wonderful and has been playing for 20 minutes.

Is it possible I’ve an intermittent short in the rectifiers on the inrush current?

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 Post subject: Re: Magnavox AMP 150 and CR 730 Tuner Popping Fuses, 5U4GB f
PostPosted: Mar Tue 12, 2019 3:16 pm 
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I find it interesting you said the fuse pops immediately, so fast the pilot lights didn't even come on. A 5U4 does not conduct immediately, there is a warm up time of a half second or so This would indicate your trouble is on either the tube socket or the transformer since it is blowing before the tube conducts. The filter sees no voltage until the rectifier conducts.

Another thing, you say a 2 Amp fuse yet the load is 205 watts. That seems like no allowance for inrush current. Is this a fast acting or slo-blo fuse location? Which type are you using?


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 Post subject: Re: Magnavox AMP 150 and CR 730 Tuner Popping Fuses, 5U4GB f
PostPosted: Mar Tue 12, 2019 3:47 pm 
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Not sure the timing of things but the flash and pop was definitely before the filaments at the top of the tube were visibly warmed up, so yes I'd say it was happening instantaneously.

Figured it might be the socket as well, but it's a molded plastic kind, and its all very clean. I can't really easily tighten the pins as theyre buried inside the socket, but nothing seems suspect.

I just fired it up cold start from the wall, original rectifiers and a 2A fast blow fuse, all OK and plays fine.

I understand the 2A fuse doesn't allow much margin, but what boggles my mind is that it has worked for weeks, and then I suddenly hit a string of 8 fuses that couldn't take it? Strange.
The fuse elements are vaporizing though. It’s not a subtle break. The elements are vapored on the envelope.

Yes, it's drawing about 200W at 120V where the tag says should be drawing 205W at 117V, so 6 in one, half dozen this seems OK.

Im narrowing it to either rectifiers that intermittently short at surge or maybe my fuse just isn't cutting it and I've gotten just plain lucky with the one fuse that could handle it.

Can't find my Kill-A-Watt

5U4GBs prone to trouble?

Thanks!

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 Post subject: Re: Magnavox AMP 150 and CR 730 Tuner Popping Fuses, 5U4GB f
PostPosted: Mar Tue 12, 2019 5:38 pm 
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I managed to isolate that flash to the fuse itself, so all the talk about 5U4 flash is by the way side now.

I found a can of 2A Slow blow fuses upstairs, and I've had 2 consecutive powerups without a blow. 2A still cutting it close I know, but slowblow already seems to have helped. Fingers crossed.


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 Post subject: Re: Magnavox AMP 150 and CR 730 Tuner Popping Fuses, 5U4GB f
PostPosted: Mar Tue 12, 2019 8:38 pm 
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An amplifier that draws 200W would normally be fused with a 3A slo-blo fuse. Try that.

-David


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 Post subject: Re: Magnavox AMP 150 and CR 730 Tuner Popping Fuses, 5U4GB f
PostPosted: Mar Tue 12, 2019 9:51 pm 
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Found 2A slow blows and thats working so far, but if it happens again in short order I'll up it again because really it's all find and well and on the variac was drawing the current its supposed to. I see now. Sorry for the hubbub but learned a lot today. Hopefully I'm out of the dark on this!

What I'm more interested in is why the 1st fuse I installed in this set after recapping held on for weeks, and 8 identical fuses there after blew consecutively. Makes me think they're not all created equal even when they're supposed to be.

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 Post subject: Re: Magnavox AMP 150 and CR 730 Tuner Popping Fuses, 5U4GB f
PostPosted: Mar Wed 13, 2019 3:07 am 
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It's called nuisance blowing caused by having the fuse rating too close to the operating current. It happens more or less randomly. Part of the problem is caused by the inrush current of the power transformer which may or may not happen. It depends on the residual magnetism in the transformer core when the set is turned off and the phase of the AC power when it is turned on next time. If the residual magnetism opposes the current when it is turned on, then there is no inrush. If the magnetism is the other polarity, then there is a large inrush. Some transformers make an audible thump when turned on because of the large current.

The inrush current caused by the tube heaters and filaments doesn't help either. Depending on the impedance of the transformer, this can be 8 or 9 times the operating current of the tubes for a second or so.

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 Post subject: Re: Magnavox AMP 150 and CR 730 Tuner Popping Fuses, 5U4GB f
PostPosted: Mar Wed 13, 2019 12:42 pm 
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The power supply schematic for the AMP-150 seems to indicate that the circuit didn't originally use a fuse. Your picture looks like the pigtail type fuse holder was added by someone. I found a few pictures of the underside of the chassis and didn't see that fuse holder installed. I did see one with a chassis-mounted holder but the wiring to it looked homebrew. How did you determine that a 2-amp fuse is the correct value? I agree that adding a fuse to this amp was a good idea and that the correct value should be a 3-amp slow-blow as David said above.

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 Post subject: Re: Magnavox AMP 150 and CR 730 Tuner Popping Fuses, 5U4GB f
PostPosted: Mar Thu 14, 2019 4:03 am 
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Hi Dave and Jim-
Thanks for all the advice. Committed to my note bank.

I added that fuse myself. I know it’s imporoper looking but the fuse is easily changeable and I’m running off my dads surplus of automotive inline holders. (As an aside anyone know where I can get lighter AWG in-line holders? The chassis mounts are pretty but not always convenient.)

It’s in the hot side of the main line of the transformer primary. Hopefully this is the only protection I need? Could I do better? Not sure why these didn’t come with from factory!

Anyway, I chose the fuse based on my understanding of operating current and my obsession with marginally rating fuses. I understand now the surge here is too much. I’ve gotten away with this on my radios but apparently the quick draw is way more than my fast blows can handle.

Right now a 2A slow blow is in there and has been back to normal again. I will perhaps up this to the recommended 3A slow to reduce nuissance down the line. I imagine perhaps fuses get “tired” so maybe it’s only a matter of time before the current one I have installed goes too.

What’s very strange to me is the original fuse I installed was also 2A fast blow and that lasted for weeks. But every identical one after couldn’t handle the surge until I increased the amperage. First one must have been an anomaly/tough guy.

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 Post subject: Re: Magnavox AMP 150 and CR 730 Tuner Popping Fuses, 5U4GB f
PostPosted: Mar Thu 14, 2019 8:20 am 
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Reread my previous post. The size of the inrush can be random.

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 Post subject: Re: Magnavox AMP 150 and CR 730 Tuner Popping Fuses, 5U4GB f
PostPosted: Mar Thu 14, 2019 1:09 pm 
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I agree with your theory about some fuses getting "tired" especially after operating close to their current rating for a period of time. It probably contributes to the concept of "nuisance" fuse blowing. I just think that your choice of using a 2-amp fuse for this impressively large amp, especially if also powering the matching tuner, was a little too close to the edge even though your original fuse lasted a long time. I wonder what the tolerance of fuse ratings is? I'm surprised that Magnavox didn't use a fuse originally. What were they thinking?

Dave


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 Post subject: Re: Magnavox AMP 150 and CR 730 Tuner Popping Fuses, 5U4GB f
PostPosted: Mar Thu 14, 2019 9:50 pm 
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I'm not surprised -- I have very rarely seen fuses on consumer equipment with vacuum rectifiers. I guess the assumption was that they would not fail to a short. On the other hand, almost all equipment I've seen with silicon rectifiers is fused, recognizing the fail-short prevalence of this type of device.

-David


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 Post subject: Re: Magnavox AMP 150 and CR 730 Tuner Popping Fuses, 5U4GB f
PostPosted: Mar Thu 14, 2019 10:38 pm 
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dberman51 wrote:
I'm not surprised -- I have very rarely seen fuses on consumer equipment with vacuum rectifiers. I guess the assumption was that they would not fail to a short. On the other hand, almost all equipment I've seen with silicon rectifiers is fused, recognizing the fail-short prevalence of this type of device.

-David


The way a rectifier fails is the key here. Now before someone tells me this is unsafe, I agree. Iam only outlining how a rectifier tube will fail if no other factors are considered. In a real world situation the power transformer would go up in the magic smoke long before the tube or the rectifier diode failed.

But to compare a tube with solid state, consider, suppose an input filter fails shorted. With a 5U4 current of course increases and would cause tube failure or transformer failure in short order. So sake of illustration we will assume a transformer that can supply infinite current since we are only concerned with the rectifier. . So let's consider only the tube; a rectifier tube has internal resistance in that there is a limited amount of current that can be passed between cathode and plate (internal resistance) This will put a limit on how much current a short can pass. Of course after a while the cathode coating is stripped by the excessive current and the tube fails to conduct. The circuit now becomes open and current flow stops.

But consider the solid state rectifier. First of unlike the tube there is very little internal resistance. The rectifier diode does not have the inherent limiting of a cathode / plate in a tube; shorted it can pass much higher current. The current will generate excessive heat. This heat eventually will cause the rectifier to go shorted internally. Now we have a dead short across the transformer. (Rectifier shorted, as well as the shorted input filter cap. ) So what limits current flow? Answer nothing, the short will continue until the wiring itself melts down or the fire starts since as we stated at the outset our hypothetical transformer has unlimited output current potential The fuse becomes much more important with a solid state rectifier than with a tube.

Getting bback to reality though, a fuse in either is important since the infinite transformer in the example does not exist. in reality the transformer would fial in short order if the situation developed in either tube or solid state rectifier. The example is only to explain why the SS rectifier it is more critical.


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 Post subject: Re: Magnavox AMP 150 and CR 730 Tuner Popping Fuses, 5U4GB f
PostPosted: Mar Thu 14, 2019 10:45 pm 
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I had the same symptoms with a Nobles NS130 stereo power amplifier. The factory installed fuse holder label says 2 amp, it did not specify what type of fuse. After restoration I put a fast blow fuse in and it worked great for a few days and then blew. After doing this about 5 times I put a SB fuse in and it has been working great for over 5 years.


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