Platine Technics SL-P377A sometimes blows fuse

[Depaj]

As the tittle says, I have a Technics SL-P377A that works great and sounds pretty good (after I replaced most caps) but sometimes when I turn it on the fuse blows. When I got it the fuse was blown, I started by just swapping it and the player worked fine for a few albums but the next time I turned it on the fuse blew instantly. I changed the caps but that didn't change a thing as I suspected. I changed the 4 first rectifier diodes by common 1n4004and the regulator (LM2940T 5V) but still nothing. I checked all the solder joints and they're good. Again the player worked fine for a few hours and again the next day but later it blew the fuse again. The voltages from the +/- 8v supply are a bit high (+9.8/-10.2V).

 

[scottjoplin]

The internal fuse? T125mA. Are you using a slow blow?

 

[Depaj]

Yes the internal fuse, and yes it's a slow blow. across the fuse terminal (with fuse in series with meter) and I get about 3 mA when the player is working.

 

[scottjoplin]

Since it is only a surge at power up it could be an increase in the leakage current of the now aging caps (I appreciate you've changed most of them)

 

[Depaj]

Well when I say most of them it's actually almost all of them, especially in the power supply. I can't imagine some small value cap in the servo circuit is causing such an inrush of current.

 

[scottjoplin]

I take your point. The +/- 8v supplies look to be unregulated, I'm not surprised they're high seeing as the transformer is putting out over 9V ac ??

 

[Depaj]

Yes 9.4v ac to be exact. All the other voltages test good and the fact that it doesn't blow every time is making things hard to troubleshoot.

 

[scottjoplin]

It looks like that supply is just for the headphone amp and is regulated before it, so, no concern. Yes intermittent faults are the hardest to find, and to confirm it is only on power up?

 

[Depaj]

I didn't check that before but yes. Headphone circuit works fine otherwise.
It never happened while playing and it has played for sometimes 4h non stop and then blew the fuse the next day on power up.

 

[scottjoplin]

Have you measured your mains supply voltage or the secondary voltages? Also I'd be tempted to disconnect the headphone amplifier to remove some of the load since it's the only way to lower it without affecting the player's function

 

[Depaj]

I'll check all of that tomorrow and report back.

 

[scottjoplin]

Ok

 

[Mooly]

Has this problem only occurred since you recapped the player, or is the player an unknown quantity you acquired from somewhere ?

Also, have you replaced like for like with regard to the main reservoir cap values ?

Equipment such as this can be 'close fused' meaning there is little leeway for any change in surge currents.

 

[Depaj]

Hi Mooly,
I bought the player with a blown fuse, the seller told me one day it just didn't turn on.
All caps have been replaced with same value, sometimes a little higher voltage but same capacitance. There are two SRU N15quick blow fuses just after the 9.4v ac rectifier section and they are ok.

 

[Depaj]

Well it definitely seems to be some capacitor inrush current problem because I discharged all the caps from the power supply, turned the player on and watched the little fuse go open in a flash. would it be safe to get a 150 or even 200mA fuse or am I looking for trouble by doing that ?

 

[scottjoplin]

I wouldn't foresee that being a problem

 

[Depaj]

A 250mA fuse survived the power on with completely discharged caps. I will try and see what is the closest value I can get to the original to keep it safe.

 

[Mooly]

Apart from going against the manufacturers recommendation, I to wouldn't foresee a problem going a little larger.

Many many years ago one of the Sanyo Betamax recorders had a close rated fuse fitted, it was something similar to yours in value, and the model suffered random fuse blowing. I seem to recall there being an official update to increase the fuse to a T800ma.

I would be happy with a T250 or T315ma fuse as long as I was sure there wasn't some other issue somewhere (unlikely if you can prove the higher fuse repeatedly survives on/off cycling).

All you can do is try it and see how it behaves.

 

[Depaj]

It has now survived a few times with the 400 mA fuse so I'm guessing that was the problem, I think I will indeed go down a bit maybe 250mA and see what happens.

It's not a great player but with its new caps it doesn't sound too bad at all.

 

[Depaj]

I've been away for a while but yesterday I got the 250mA fuse in and it didn't blow. I have powered it up a few times after discharging all the caps and the fuse is doing fine. I will keep testing but I thing this is a result .

Thanks to all