WD88VA XL - heater fuse blowing

[mellis]

Would appreciate any help with this, please...

My newly finished WD88VA XL sound brilliant! But there seems to be a problem...

It seems that after approx 3+ hours of running and sounding great, the heater fuse blows. All the voltages are very much in line with the instruction manual, tho the HT is high (440V). In particular the DC heater for the small valves was spot on at 6.3V.

Allied to this, perhaps, is a question as to how hot should I expect the transformer to get? I can touch it for approx 10seconds, but no longer. Altho the DC heater fuse has blown, the main fuse hasn't and the KT88s seem to be still funning and the transformer is still hot. First time I thought the fuse blew because the amp was in the sunshine (and it was a hot day today). But tonight same thing.

So I don't know if this is really a diode/capacitor failure on the DC heater circuit, or the blowing is a symptom of something more complicated? As far as I can tell testing them, they seem ok.

See photos attached. Note that the link on the negative side of D7/D8 (tags 1 + 4 nearest C14 and pwr transformer) is underneath the tagboard and carefully sleeved/insulated and clear of the other tags.

Thanks guys for your thoughts... it is a brilliant amp and I look forward to its resurrection! ;-)
Mark.

[systemx]

Hi Mark.
Yes the mains transformer does run hot, I tried your 10 sec test on mine and agree. I did have a fuse problem and traced it to the relay board, I had forgotten to insert one of the fibre washers under the fixing screws and the fuse failed after about 1 hours use.
Since then I have had no problems at all ( the amp has been run about 10 hours a day since last Nov) and sounds great.

Terry

[Richard]

Hi Mark,

What's the fuse rating? If near to the limit these can survive a while before fatiguing and fracturing. The load is a little over 2A. Have you tried a replacement?

However, the problem may be that the diodes in your amp are the original 15V ones, 95SQ015, which may be starting to fail.

When they're not conducting the cap and TX are effectively in series which can give a piv up to 3x the ac voltage. So 20V or greater diodes would be best.

I'd change them for;

1N5822 (RS 544 4988) 3A 40V Axial
or
STPS1045D (RS 486 0216) 10A 45V TO220AC package (square 2 legs)

Stand them up off the tag strip in clear air to help cooling.

Rich

[systemx]

Hi Rich and Mark.
The fuse rating is 3A anti surge, and mine has been fine.
Rich.
At the time of my build, we were all trying to find out what had caused a mains Tx burn up, so I went for the uprated diodes (STPS1045D) from the start, as well as uprated caps in the heater circuit (RS 127-802). Had to get rid of the plastic clips so went for these (RS 543-052)

Terry

[petercom]

Hi Mellis

We are supplying STPS1045D diodes for the XL kit (yours must be an earlier one) and will be happy to send you these plus the associated tag board layout for them if you send us an email or give us a ring.

This applies to anyone else who still has the 95SQ015 diodes in the heater supply circuit of their WD88VA XL. (We have tried to contact every owner about this but it still seems there are one or two remaining!).

[mellis]

Thanks for this. Have sent Peter my address and once new diodes fitted, will be in touch if the problem persists. Mark.

[mellis]

Customer service and delivery from Peter amazing. Now working fine. Uploaded pics of blue valves ;-) Mark.

[systemx]

Jolly Good. Must give mine a dust!!