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Old amplifier blowing fuses too often, Faulty transformer or other component?
ChronoSphere
post Apr 10 2013, 00:21
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Hello,

I have an old amplifier from the Soviet era, which I am trying to get into a working condition, just like the speakers, for fun. It has the habit of blowing the fuse (1.6A small cylindrical fuse) either immediately when turning it on (most of the time) or after a random amount of time. Volume doesn't seem to be a factor.

I tried using fast/middle/slow reacting (is that the term?) fuses and all of them seem to be acting nearly the same. I disassembled it and the cable that goes "through the fuse" goes to the transformer. Could it be faulty and not producing stable output? How could I test that? If it's the transformer, can I build one myself (or repair this one)? If yes, how do I find out it's specs and how much would it approximately cost? I have the instruction manual with a complete electrical scheme for the amplifier, which I could scan and upload, if that would help in any way.

I'm basically a newbie who likes to tinker with stuff, so I am completely clueless about the difficulty of this. laugh.gif
Soldering is not a problem.
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Arnold B. Kruege...
post Apr 10 2013, 03:07
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QUOTE (ChronoSphere @ Apr 9 2013, 19:21) *
Hello,

I have an old amplifier from the Soviet era, which I am trying to get into a working condition, just like the speakers, for fun. It has the habit of blowing the fuse (1.6A small cylindrical fuse) either immediately when turning it on (most of the time) or after a random amount of time. Volume doesn't seem to be a factor.

I tried using fast/middle/slow reacting (is that the term?) fuses and all of them seem to be acting nearly the same. I disassembled it and the cable that goes "through the fuse" goes to the transformer. Could it be faulty and not producing stable output? How could I test that? If it's the transformer, can I build one myself (or repair this one)? If yes, how do I find out it's specs and how much would it approximately cost? I have the instruction manual with a complete electrical scheme for the amplifier, which I could scan and upload, if that would help in any way.

I'm basically a newbie who likes to tinker with stuff, so I am completely clueless about the difficulty of this. laugh.gif
Soldering is not a problem.



Most common source of this problem is probably bad output tubes shorting out or incorrectly biased.
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MikeFord
post Apr 10 2013, 03:33
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QUOTE (ChronoSphere @ Apr 9 2013, 16:21) *
Hello,

I have an old amplifier from the Soviet era, which I am trying to get into a working condition, just like the speakers, for fun. It has the habit of blowing the fuse (1.6A small cylindrical fuse) either immediately when turning it on (most of the time) or after a random amount of time. Volume doesn't seem to be a factor.

I tried using fast/middle/slow reacting (is that the term?) fuses and all of them seem to be acting nearly the same. I disassembled it and the cable that goes "through the fuse" goes to the transformer. Could it be faulty and not producing stable output? How could I test that? If it's the transformer, can I build one myself (or repair this one)? If yes, how do I find out it's specs and how much would it approximately cost? I have the instruction manual with a complete electrical scheme for the amplifier, which I could scan and upload, if that would help in any way.

I'm basically a newbie who likes to tinker with stuff, so I am completely clueless about the difficulty of this. laugh.gif
Soldering is not a problem.


I would suspect load before source. What is the line the fuse is on? Keep in mind the fuse is a fault protection device, so each time you replace it and it fails again some circuit is operating outside the safe range, eventually other items will start to fail.

When "tinkering" with tube circuits I follow the keep one hand in a pocket rule, you never want to touch high voltage, but in tinkering it WILL happen, and if the other side of your body has a connection to ground the current goes through your body (heart) and can kill you.
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ChronoSphere
post Apr 10 2013, 11:49
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Yes, I am aware of the dangerous voltages, thanks for reminding. About the only difference I can see in the line is that in Germany, the line is rated 230V and back in Latvia it was 220V. Could this be the reason?
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pdq
post Apr 10 2013, 12:56
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One common cause of fuses blowing is what is known as "inrush current". When you turn off the power, the line voltage is at a random point in its cycle. If it happens to be near a maximum then it leaves the core of the transformer magnetized in one direction. Then when you turn the power back on, if the line voltage happens to be near the opposite maximum voltage then you get a large surge in current to rapidly reverse the magnetization.

A well designed product will have a device in series with the transformer primary (I forget the name) that initially has a high resistance to limit the inrush current. This device then quickly heats up and its resistance drops to a much lower value once the inrush current has passed. Probably your amp doesn't have one of these.
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ChronoSphere
post Apr 10 2013, 20:05
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I guess that would explain why upon turning it on, the "overload" LEDs sometimes flash momentarily. Sometimes this is followed by it blowing the fuse. Is there any way to add such a inrush current protector? I heard something about a varistor, is that the component you are talking about?

What about the voltage? I asked my father and he said that the voltage back then was rated at 220-230V, because they couldn't provide a stable value. Maybe the amp can handle peaks of 230V but gives out at constant 230V?

Would using a power bar with a surge protector help against the inrush current?
edit: Just checked wikipedia, varistors do NOT provide protection from inrush current. So I guess a surge protected bar wouldn't, either.

This post has been edited by ChronoSphere: Apr 10 2013, 20:08
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MikeFord
post Apr 10 2013, 20:10
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QUOTE (ChronoSphere @ Apr 10 2013, 03:49) *
Yes, I am aware of the dangerous voltages, thanks for reminding. About the only difference I can see in the line is that in Germany, the line is rated 230V and back in Latvia it was 220V. Could this be the reason?

I said line, but meant the part of the circuit where the fuse is located. Is it the mains connection? I was guessing it was part of the B+ internal to the amp.

Unless the line voltage was more than 10% higher I doubt it would be an issue. Much more likely something internal to the amp is drawing too much power on its way to croaking.
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ChronoSphere
post Apr 10 2013, 20:30
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I scanned that part of the electrical scheme (full scheme will follow if needed) and uploaded it here
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Porcus
post Apr 11 2013, 07:55
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QUOTE (MikeFord @ Apr 10 2013, 21:10) *
Unless the line voltage was more than 10% higher I doubt it would be an issue.


Where I grew up, electricity companies had by regulation a +/- ten percent leeway on the voltage. Fortunately, at our place it was so way down in the 190's that they could not get away with it and had to set up a new transformer. (Then grandpa could actually bring his electric razor when visiting us.)


But tube amps? Likely need an adjustment or new tubes (and likely an adjustment due to new tubes).


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MikeFord
post Apr 11 2013, 11:24
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QUOTE (ChronoSphere @ Apr 10 2013, 12:30) *
I scanned that part of the electrical scheme (full scheme will follow if needed) and uploaded it here

This is the mains input fuse. When it blows the amp is drawing too much power, doing so under idle conditions makes me think you should test and replace components before replacing the fuse too many more times. I don't see it as safe to proceed as it is.
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ChronoSphere
post Apr 11 2013, 12:33
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QUOTE (Porcus @ Apr 11 2013, 08:55) *
But tube amps? Likely need an adjustment or new tubes (and likely an adjustment due to new tubes).
Adjustments? I see some potentiometers on the board, are those used to tune the amp? Here is a photo of the amp with the cover removed: click. I'm guessing those two big capacitors are the tubes?

QUOTE (MikeFord @ Apr 11 2013, 12:24) *
This is the mains input fuse. When it blows the amp is drawing too much power, doing so under idle conditions makes me think you should test and replace components before replacing the fuse too many more times. I don't see it as safe to proceed as it is.
Well, not exactly idle conditions. Most of the time the fuse blows when turning it on, which, as pdq said, might be due to the inrush current. The other times when it blows it after a random time is when using it at around half the volume.

How do I proceed with the testing? On the full scheme, they list voltages (tolerance +-10%) on certain points, should I start from there and try to find where the voltages deviate too much?



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pdq
post Apr 11 2013, 13:14
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Looking at the photo, I see nothing that looks like tubes, and the heat sink on the back is not something that you would expect to see in a tube amplifier.

Then looking at the schematic, I see lots of transistors, and what looks like a transistor power amp circuit, but again no tubes.

If, as I suspect, this is actually a transistor amplifier, then it might be something like poor insulation between the power transistors (on the back panel) and the heat sink.

If you had tubes then they would be very obvious. They would be standing upright on the circuit board with nothing above them other than a metal top cover with perforations for ventilation. You could look inside and see the glow of the filaments inside the glass envelope.
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ChronoSphere
post Apr 11 2013, 13:20
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Well, Porcus said something about the tubes so I got confused tongue.gif
It should be a transistor amp then. I will check the heatsink for now.

Started browsing the Russian net, many mention exchanging the capacitors/transistors/resistors on that thing to get it back to a stable condition. Also, testing which part blows the fuse by uncoupling the parts from the transformer. Some say that those big 15000F capacitors don't unload upon turning it off, that being the reason for overload on power on. Others also suggest completely replacing the power cord. Someone said to solder a light bulb into the line where the fuse is and to see if it flickers when turning it in... or putting a 2.0A fuse instead and rapidly turn it on or off to see if it blows too. Russian diagnostics at their best biggrin.gif

This post has been edited by ChronoSphere: Apr 11 2013, 13:21
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pdq
post Apr 11 2013, 13:40
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OK, let's look at the various suggestions.

Capacitors are certainly a limited life item, and the large filter caps could cause problems like blown fuses. Replacing electrolytic caps in old equipment is usually a good idea.

Transistors are also a possibility, though I would first check the insulators as I said. Resistors should almost never need to be replaced.

The filter caps should not retain significant charge for any length of time, and even if they did, that would decrease the inrush current rather than increase it.

Forget the power cord. That is before the fuse so could not possibly cause one to blow.

Forget the light bulb idea. Larger fuse would be a possibility if for some reason the one that was chosen was marginal, but if that is not the case, and there really is something wrong, then you would be increasing the danger of causing more serious damage.

Of course, for less than $200 (at least here in the states) you could get a modern receiver without all of those problems and lots of new features. Just saying...smile.gif

Hope this helps.

This post has been edited by pdq: Apr 11 2013, 13:40
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DonP
post Apr 11 2013, 13:59
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QUOTE (pdq @ Apr 11 2013, 07:40) *
OK, let's look at the various suggestions.

Capacitors are certainly a limited life item, and the large filter caps could cause problems like blown fuses. Replacing electrolytic caps in old equipment is usually a good idea.


Electrolytic caps often fail if they have been unused for a few years, so consider that.

The rectifiers are another place to look.
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ChronoSphere
post Apr 11 2013, 14:13
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Yes, I know it's less hassle to just buy a new one, but where is the fun in that? laugh.gif
I currently have a working one anyway, so if I can bring that one back, it would be a spare one.

I think replacing the capacitors first should be a good steps. There aren't that many of them anyway.

The insulation at the back part seems to be fine. There are also 2 fuses at each of the 2 backplates, and all 4 of them are intact. According to the scheme, it's the YM-1/YM-2 part, the "power amplifier" (literally translated, I have no idea what the correct term is)

I saw some people arguing about what type of capacitor to use, with people saying ceramic ones introduce distortions while film ones being more linear. Any suggestions on which brand/type to take for replacement?
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pdq
post Apr 11 2013, 14:29
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The capacitors that you will be replacing are in the power supply, and there is no other choice than electrolytic.

Choose replacements that are physically compatible (i.e. will fit in the space, and the contacts are the same spacing) and at least the same voltage rating. Try to get nearly the same capacitance, a little less is OK, but avoid getting ones with much higher capacitance.

Edit: The fuses to the power transistors being intact means that the transistors and insulators are probably fine.

Edit2: I am assuming that the old capacitors are screw terminal type rather than snap-in. It looks as though those will run you about $20 each, for 15,000 microfarad at 63 volts.

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ChronoSphere
post Apr 11 2013, 15:46
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Yes, the old ones are connected with screws: click
Does the 63V come from the fact they list 40V as the voltage and 63V is next closest to it?

I have decoupled everything except the transformer and the fuse doesn't get blown anymore. Going to continue adding the blocks one by one and see when it starts blowing again.
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hlloyge
post Apr 11 2013, 16:19
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Offtopic: can you show some pictures of the amp and speakers? I am very interested in how it looks like.
You can link them in first post, or you can send me link via PM.
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ChronoSphere
post Apr 11 2013, 17:09
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IT LIVES! laugh.gif
System stable for the past hour. And unless I am having a heavy placebo effect, this speaker/amp combo is better than my "new" system I have been using the past years.

About the only thing I changed is removing those 5 capacitors immediately before and after the transformer, so those seem to be toast (none of them is conducting though, so it shouldn't have been a short circuit). Once I removed them the instant death on power of was gone. Tried about 10 times, no fuses blown. So maybe replacing those will be enough.

@hlloyge: I will once I have assembled everything.
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Porcus
post Apr 11 2013, 21:58
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QUOTE (ChronoSphere @ Apr 11 2013, 14:20) *
Well, Porcus said something about the tubes so I got confused tongue.gif


Sorry. It was already mentioned, so I didn't read thoroughly. Good to see that your debugging works.

And yes, the 63 V in place of the 40 V is likely the next above. That is how much voltage they can handle, so a higher value is not a problem.

But I am surprised to hear it sound good without caps ... and what do you mean by "none of them is conducting" you haven't ... uhm ... connected them serially to a lightbulb and tested with ... a ... battery? wink.gif


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ChronoSphere
post Apr 11 2013, 22:17
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From what I understood, those 5 caps are functioning as a stabilizer, since back there the power fluctuated a lot. If you look at the scheme I posted previously, they're positioned at the power switch and each of the transformer spools. I think only the caps in the amplifier itself affect the sound, those 5 are "only" to stabilize the power supply.

QUOTE (Porcus @ Apr 11 2013, 22:58) *
And what do you mean by "none of them is conducting" you haven't ... uhm ... connected them serially to a lightbulb and tested with ... a ... battery? wink.gif
No, I have not xD
Maybe what I did was silly, but I just put the multimeter in diode mode and looked if I get any value. That's how I usually find out which cable is which when they're unmarked. Not sure that would do anything for the caps though.

I am sure the amp is now much more stable than before though. Back when I stopped using it, it would pop about a fuse per hour and usually needed 1-2 fuses to turn on. Today it worked for almost 3 hours before I turned it off, on one and the same fuse biggrin.gif

Here's a picture together with the speakers btw: click
No LEDs are actually alight except the green power one. Which actually not a LED but a light bulb. I wonder why they put LEDs everywhere else but there...
They're a little battered with a few dents and scratches here and there... comes from the people who helped us move in having "arms growing out of their rear" as the Russians say.
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MikeFord
post Apr 12 2013, 08:06
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The caps I saw on the input side are typical for reducing RF noise that house wiring can pick up, and the ones on the other side of the transformer are to smooth the AC ripple depending on where they are relative to the rectifier. Nobody puts extra unneeded parts in amps, replace them with same value known good devices.
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stephan_g
post Apr 12 2013, 10:07
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Caps C1, C2, C3 must be special mains-rated types (X and Y class (sorry, only the German Wikipedia entry covers this in detail)). Specifically, it would be some X class here. Reichelt carries WIMA MP3-X2 series 22n and 47n types, for example. Possibly the original types weren't such caps and could no longer sustain the voltage spikes that occurred.

I'm kinda surprised the designers didn't put spark extinguisher caps or R-C combos across the power switch (those would have to be Y class devices, I think). That tends to result in burned contacts in the long run.

In case the problem should come back, the big filter caps would be the next suspects. Nominal 40 V types operated very close to their rated voltage even at 220 V may not be that happy about slightly higher mains voltage. Some old Rotel amps are affected by this quite regularly, there they even tend to leak.

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ChronoSphere
post Apr 12 2013, 14:37
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QUOTE (MikeFord @ Apr 12 2013, 09:06) *
Nobody puts extra unneeded parts in amps, replace them with same value known good devices.
I am intending to replace them, sorry if it wasn't clear. I just didn't notice any effect on sound from them not being there.

QUOTE (stephan_g @ Apr 12 2013, 11:07) *
Caps C1, C2, C3 must be special mains-rated types (X and Y class (sorry, only the German Wikipedia entry covers this in detail)). Specifically, it would be some X class here. Reichelt carries WIMA MP3-X2 series 22n and 47n types, for example. Possibly the original types weren't such caps and could no longer sustain the voltage spikes that occurred.
Those have an additional grounding wire, the amp doesn't have any grounding at all. So, for C1-3 I just take the X2 class ones. What about C4-5?

Here's how the original ones looked like: click
They're rated 630/400/400/250/250V respectively. Ignore the damaged insulation, they were soldered in a way you couldn't simply remove them (ends wound around the the contact a few times) so I had to pull and twist to get them loose dry.gif

I'm completely fine with German btw, been living here long enough ^^

QUOTE
I'm kinda surprised the designers didn't put spark extinguisher caps or R-C combos across the power switch (those would have to be Y class devices, I think). That tends to result in burned contacts in the long run.
Mainly because they didn't care about safety/longevity that much. As a matter of fact, the power switch is dead and I had to connect the wires directly, so the amp turns on the moment I plug it in =/
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