WOPL 400 4-Fin Blew the -15v Pico Fuse

[Boater222]

I may need some assistance. My 4-Fin White Oak 400 has been working great for over a year until earlier this week. This unit has the Rev E Control Board, Rev C Backplanes, Cap Upgrade, LED Light Board, and DCP board. When I turned it on everything appeared normal, but the protection relay would not enable. I checked the rail fuses and both were good. This is a later 4-Fin where the capacitors and bridge rectifier mount to the transformer.

I opened it up for a quick inspection and found the -15V was missing at the test point on the Control Board. The minus side pico fuse was open on the right channel backplane. I pulled the Control Board out for inspection. I performed a quick DMM diode test on the -15 power supply semiconductors and all passed and were the same as the +15 side. I performed a quick resistance check of the resistors and no problems were identified. I also looked for questionable joints and connections and none were found. Next I went back to the BOM to ensure the resistors were indeed correct in the voltage regulator section and the entire board. Everything looked good.

I bridged the blown -15V pico fuse with a 0.5A AGC fast blow fuse in series with a Fluke 89 meter reading milliamps. I slowly brought the unit up with a variac and dim bulb monitoring current through the fuse. I also monitored the +15V and -15V test points. The unit came up perfectly the the protection relay enabled as it should. Readings were:
+15V: 15.10V
-15V: -15.13V
Fuse Current: 18.51mA

Next I set the Fluke meter to record the maximum transient current during power on. When switched on the transient current through the -15V fuse recorded 370mA.

I powered everything down. Removed the fuse and measured resistance from the control board terminal 11R to ground. Next I manipulated the backplane boards and the control board to see if something was shorting from movement. The resistance did not change so I don't feel something is shorting to ground.

My question to the experts is: What would you advise next? I suspect the in-rush current is fatiguing the pico fuse over time.

 

[Gepetto]

Hardware does fail. The inrush is below the fuse ratings, the steady state is 25x below the fuse rating. Fuses are designed to hold 135% of the 500mA rating (or 675 mA) for an hour before blowing. The steady state current you measured will peak up another 12mA max if you drive the amp to the rails.

When replacing the pico, be careful not to heat the solder joint too aggressively because that can wound small components like these.

Replace and observe would be my recommendation. Others can chime in.

BTW, what is your normal listening style? Concert level rock or more casual listening?

 

[wattsabundant]

Fortunately the relay board sensed the DC offset and did not energize. Otherwise the least of your concerns would be replacing a fuse.

If the answer to Joe's question is rock concert levels it would suggest thermal cycling as the cause for blown fuse. Possibly the board may show signs of heat stress.

 

[George S.]

Joe adds a extra pico with the kit. Good thing because I found one DOA when I built mine. Replaced it and ordered spares. Have had no more issues. I see the latest revision uses fuses.

 

[Boater222]

I am not an aggressive listener and do not push the WO 400 hard at all. Most of my listening is done a lower volumes. I will replace the fuse with the spare that Joe provided in the kit. I will pay attention not to overheat and stress the fuse during installation. I will heat sink the fuse leads will small flat alligator clips as I solder them. Thanks for the recommendations. I appreciate the help.

 

[Gibsonian]

How bout some pics of this pico fuse? I have a weird happening sometimes when cranking my 400. It will stop playing intermittently and this stop/go thing then will go away when I turn it down to more sane levels. Wonder if my issue could be related to same as this thread?

 

[George S.]

Loose storage cap screw maybe? If it it ever loudly pops that's probably a arc at that connection. Yeah, I once got interrupted and forgot to install a screw! Played fine at low volumes.

 

[George S.]

The green resistor shaped component at the far bottom left, there's one for each rail.

 

[George S.]

"Check your pico fuses" used to be seen on this site quite often.

 

[George S.]

How bout some pics of this pico fuse? I have a weird happening sometimes when cranking my 400. It will stop playing intermittently and this stop/go thing then will go away when I turn it down to more sane levels. Wonder if my issue could be related to same as this thread?

I left a screw out of the buss bar. Also look for cold joints on the buss bar.

 

[J!m]

Also look for cold joints snacks on the buss bar.

Fixed that for you.

 

[Gibsonian]

Thanks George. Are the picos a true fuse that is either intact or broken? i.e. could this be responsible for my issue or ruled out as once it blows it's gone? Also busbar is this right?

 

[George S.]

Thanks George. Are the picos a true fuse that is either intact or broken? i.e. could this be responsible for my issue or ruled out as once it blows it's gone? Also busbar is this right?

View attachment 65522

From what I see, looks very good. Pretty sure the Pico's are good or blown, no in between.

 

[Gepetto]

Thanks George. Are the picos a true fuse that is either intact or broken? i.e. could this be responsible for my issue or ruled out as once it blows it's gone? Also busbar is this right?

View attachment 65522

It is a fuse, If it blows, it is like any other fuse...open

 

[Gibsonian]

So not the Picos. Anything else to consider? Amp works perfect in every other aspect.

 

[George S.]

Do the meter lights dim when it loses output?

 

[Gepetto]

The thermo switch?

 

[wattsabundant]

If you cycle the AC input and everything returns to normal, that is characteristic of the relay board tripping. That’s suggest DC offset.

 

[Gibsonian]

Do the meter lights dim when it loses output?

Never noticed that but I will look for it next time it occurs.

The thermo switch?

I don't think so because it always stops the intermittent output when I turn it down, immediately. If thermo I'd think there would be a time delay until cooler.

If you cycle the AC input and everything returns to normal, that is characteristic of the relay board tripping. That’s suggest DC offset.

But this is not a relay board tripping as it is not output off until I cycle it. It is intermittent output, like digital skipping, but goes away once I turn it down (volume on preamp)

 

[MusicSteve]

Never noticed that but I will look for it next time it occurs.


I don't think so because it always stops the intermittent output when I turn it down, immediately. If thermo I'd think there would be a time delay until cooler.



But this is not a relay board tripping as it is not output off until I cycle it. It is intermittent output, like digital skipping, but goes away once I turn it down (volume on preamp)

Measure the dc output offset, At the new location which will be located at the speaker output going into the Input Relays (Rt and LT) and the Negative lead to Speaker ground . Also that is a good spot for the scope lead. if 15vdc than it could be one of the Op-amp are bad.