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Can old resistors go bad?

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platinum95

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Joined 2012
#1
I'm fixing an old record player here, and I'm trying to work out what the problem is. The symptom is the resistor right after the rectifier tube is getting too hot and starting to bubble.
Now, the only 2 reasons I can see for this happening is a short somewhere drawing too much current through, or the resistor has gone bad. It is 50 years old, after all.
Of course,the most likely problem is a short somewhere, but even that must be caused by a bad resistor, as I replaced that caps.

So, how likely is it that one or more of the resistors have gone bad?

Thanks.
 
#3
Hi,

Old resistors carbon type can absorb moisture..They can change value as well,
However the chances are that as mentioned you either have a bad supply cap or coupling cap that could drive the Output tube out of bias. So the voltage drop across the resistor would now be to high and cause it to over heat.

Its difficult to comment not knowing where exactly the resistor is and if its carbon or wire wound power etc..

So do you have a schematic...?

Regards
M. Gregg
 
#4
Sorry, I seem to have neglected a lot of information in the original post.

Its a HMV 2004 model record player, 1964 at the earliest (written on the cap can). I do have a schematic, and I'd be royally screwed with it.

The resistor in question is a carbon composite resistor, of 820 ohms. Its the first resistor after the rectifier, and is situated between 2 grounded capacitors.
I thought it was a short, but after an attack probing with my multimeter, it all seems to check out. And none of the caps are shorting either, I checked them individually.
 
#5
Hi,

The first thing is the circuit is probably drawing to much current..
Beware that the caps will/could hold charge after power off..so be carefull.

You say you checked the caps..how? If its with a multi-meter then forget it..the voltage off the meter will not show leakage..If the coupling cap is leaking to the Output tube it will drive it to draw more current..and it could red plate..

Have you changed the coupling caps..thats the one onto the grid of the output tube...What is the Output tube number?

Can you post the schematic? We can guide you...

Regards
M. Gregg
 
#6
*UPDATE* Checked the resistor, and it seems like 40 years in a shed in the wet west of Ireland is not good for resistors. The resistance has gone up in most of them, and a good bit too. I.e, a 100k has turned into a 130k, and a 22k turning into a 30k etc.
I'll just repace all the old resistors, and post here on how that goes.

Quick question, what wattage should I use? I was going to use mostly 1 watts, and 1/2 watts for any 1 watts that I cant find. Is that ok?
 
#7
Remember,

Resistors have a working voltage..I would normaly use 1watt..however if its on the cathode of a power tube then Higher wattage maybe wire wound.
I still think its got a good chance of a coupling cap..thats a signal cap from one valve to another..Please take great care that the caps are dead BEFORE you touch them..

Regards
M. Gregg
 
#8
Sorry, in the time it took me to write that update, you had replied 😛
Yeah I checked the caps with a multimeter just to see if they were shorted.
The coupling cap has been changed, but I changed it 3 years ago when I had barely any knowledge of electronics, so I probably screwed that one up too.

I'll replace the resistors and the caps
 
#10
The first thing I would do,

Is change the coupling caps...If you have changed the power supply caps..

Maplin do some nice 2watt high voltage small size metal film resistors..

try the coupling caps first..This is assuming that the unit is producing sound and no actual shorts on the circuit..
If its not working at all then..caps first..then let us know how things are going..

Regards
M. Gregg
 
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#14
I agree,

Yes could be the DVM is out of calibration..its realy more likley that a cap is at fault..

Testing a cap with resistance setting on a DVM is a waste of time..OK a short will show..however leakage is a different matter.

Regards
M. Gregg
 
#16
Some hammond organ owners with product made in Denmark have found carbon comp resistors high in value 50-100%. These look just like the brown standard American Allen Bradley or Sprague parts, but obviously are not. Buying a new resistor somewhere, or cutting up an old TV or power supply, and checking the calibration of your meter is in order. But This is not unheard of for parts in Europe. Bubbling heat can drive any carbon comp resistor high value, can be just inadequate wattage by design. I have 3 $7000 organs (1967) all have bubbling of a 10 k resistor in the power supply that now reads 11.5 k or so. These were AB or Sprague parts, just undersized by the designer.
 
#17
The carboncomps are 10% and drift upwards.

When replacing them the power rating is just one factor, modern resistors usually have lower voltage ratings so check that otherwise you will have failures.

I use carbonfilm works as good as CC in most places if you go for metalfilms watch out for places where they´re suppose to stop oscillation.

Good quality film/foil caps don´t go bad without a reason.
 
#19
Right, so the plan so far is to:
-Replace the smoothing caps in the can, and the other electrolytic one.
-Replace the coupling caps and the others that I replaced before with better quality ones (metal film or the likes)
-Replace a few of the resistors in the path of the high voltage.
-cross as many digits as possible.

I'll post back here with the results when I've done all that.

Thanks again to everyone! 😀
 
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