PLAC! in the speaker when plugging a Grundig NF20 for the first time in years.

[foliver]

Hello again.
I have a working Grundig NF20 in my living room ( will be recapped soon) after your help, and a second one that was in the shelving, from 2 years ago when i bough it.

It is rusted, but seems in good shape, looking at it has the same diagram that the one is working ( i have a third one that is different, it seems older, not working).

So today i said, let's cleaning it and test it.
I don't know how many years it's sleeping.
It has 4 diodes ( one has a more white colour.....), no selenium rectifier, one of the fuses was blown, and i add a thin hair of copper to test it.

So i did it. I don't have an AC variator, to increase the voltage slowly.

I put tubes on it, checked the 220 V, plug a pair of speakers, an when i plugged into the AC, i have heard a PLAC!, in the right speaker.
The speaker is fine.
So i unplugged inmediately.
I have feel a little smell in the amp, no smoke.
The fuses close to the Diodes are ok, the copper ¨hair¨didn't melt it.

Nothing seems to have change in a visual inspection.

What can it be the cause of the PLAC!?, May be normal? The one i have working didn't do it.
Where can i start to look at?
Here's a picture of the back of the unit, with the Diode with different colour, and the blown fuse.


Thanks again for your help.

 

[RWood]

Hi, I recommend you use a Dim Bulb Tester the next time you fire something up. It is inexpensive to build and it helps you in two ways. First, it is a visual indicator that you might have a problem (bulb glows very bright and does not subsequently dim as the amp warms up) and second, if there is a problem the tester absorbs some of the voltage, acting as a cushion to your power transformers and speakers.

Now on to your amp.....was the correct voltage selected on the transformer?

 

[foliver]

Hi, thanks. I'll read on dim bulb tester.
The voltage selected is 220V, as on my country. It's set up this way, from early days.

 

[gadget73]

could be a capacitor arcing internally, or possibly a tube. Hard to say really.

 

[foliver]

Hello gadget,
I tested the tubes in the orejeras amp,after this episode, and they are all 6 working...May be a cap?
A cap can blow a fuse?

 

[Silentnet]

Yes, a cap can blow a fuse.

 

[gadget73]

yes, shorted caps cause high current draw.

 

[foliver]

Thanks.
Hoy can i identify a shorted cap?
I know some can leak, others can increase de diámeter....but the Ones that doesn’t look bad, but are bad ?
Can i do it without a cap multimeter, or is time to buy one?

Any recomendation?

 

[gadget73]

need to test them at rated voltage using a suitable tester. Or replace them since they are 50+ years old and overdue.

 

[foliver]

copie that!
I go for the tester!
txs

 

[Erhard-Audio]

as a general rule, I would replace ALL the electrolytic capacitors! Those are very old and a lot are probably leaking...that is not a physical leak like fluid, but electronic leak. Perhaps also some of the film capacitors need to be checked/replaced.
You cannot tell how bad a capacitor by looking at it. Sure, some might bloat up, but most do not. It is a known fact though that electro caps and to some extend the bumble bee style of caps do not age well and should be replaced. Usually, ceramic and mica caps are fine, even if they are decades old.
As to resistors, most are usually OK, although some of the older carbon composition type can drift in value.

 

[gadget73]

right, honestly I'd just change them. I own a suitable tester and rarely use it. Old caps seldom pass and since my tester needs them removed from the circuit to test, I don't see the point. If its coming out, I may as well just put a new one in.

 

[primosounds]

Hello again.
I have a working Grundig NF20 in my living room ( will be recapped soon) after your help, and a second one that was in the shelving, from 2 years ago when i bough it.

It is rusted, but seems in good shape, looking at it has the same diagram that the one is working ( i have a third one that is different, it seems older, not working).

So today i said, let's cleaning it and test it.
I don't know how many years it's sleeping.
It has 4 diodes ( one has a more white colour.....), no selenium rectifier, one of the fuses was blown, and i add a thin hair of copper to test it.

So i did it. I don't have an AC variator, to increase the voltage slowly.

I put tubes on it, checked the 220 V, plug a pair of speakers, an when i plugged into the AC, i have heard a PLAC!, in the right speaker.
The speaker is fine.
So i unplugged inmediately.
I have feel a little smell in the amp, no smoke.
The fuses close to the Diodes are ok, the copper ¨hair¨didn't melt it.

Nothing seems to have change in a visual inspection.

What can it be the cause of the PLAC!?, May be normal? The one i have working didn't do it.
Where can i start to look at?
Here's a picture of the back of the unit, with the Diode with different colour, and the blown fuse.


Thanks again for your help.

That snapping sound is very likely the power supply capacitor/s arcing. Since you indicate that they are stock. Old Electrolytic caps are made with chemicals that can lose their ability to form the proper voltage barrier which causes an arc or short between the positive and negative plates. Sometimes even with a variac and a slow voltage start up from 0vac to full power over a long period of time say 6 hours more or less (i have not seen any definitive time period that was recommended) the reforming process will not work. Also once the cap has failed to reform, it is unlikely that repeating the process will make it reform. So the smart thing to do is to replace the power supply electrolytics.

 

[foliver]

Hi primosounds, thanks for your response, folowing your comment, the're the first i'll replace, and see.