Death of my WD88VA XL - World-Designs-Forum

NickW · #1 13th October 2007, 05:53 PM

Hi, hope someone can help me.

My WD88VA XL died last night. I had been using it for several hours at quite high volume, but no where near full volume. For the record, I reckon it has around 300 and 400 hours on it.

Anyway, I went upstairs to do some work on my computer and left it running downstairs. The CD that was playing duly ended, but I didn't bother going down to put something else on cause it was getting late. When I eventually did go down (about two hours later), I found that the amp had died. No big bangs you understand, but nevertheless; dead. Front small signal valves all off, but KT88s still glowing. Turned it off, and had a quick sniff around the amp. I could just detect a smell of something having burnt, but didn't investigate further until this morning.

Opened it up and pretty obvious what was wrong. The red heater wires from the mains transformer that go to the small signal valve DC heater rectification system had melted. And when I say melted, I mean insinerated - all the way back to the transformer. One wire had got so hot and brittle going into the rectifier diodes, it had broken.

Ok, to cut a long story short..... I spent this afternoon investigating. I established that the small signal valve heaters were still ok, that the rectifier diodes still worked, that C14 and C17 were not short-circuit and that the 45V step-up was still ok. I concluded that the (red) heater transformer winding wasn't short circuit or open-circuit - albeit that around 1 inch lengths of both wires nearest the diodes were so britle they would not bend - only snap!

Anyway, given no red plastic left, I threaded some wide cable screening plastic over the remains of the wires and hooked it all back together. Did some quick voltage checks and then re-fitted all the valves. Switched it on and waited for smoke, but it appears to work again. But I don't trust it yet!

In all honesty, I think the mains transformer will need replacing as the red DC heater winding wires are VERY badly damaged. However, has anyone got any thoughts as to what went wrong?, and could it happen again?

Notes.

1. There is no obvious burning of any other wires. e.g. the brown and grey DC heater wires all look ok.

2. R30 looks ok.

3. And so the only evidence of a fault is the complete melting of the red plastic around the transformer winding wires. They got so hot that they did melt themselves into the black AC heater wires in places, but without breaking through.

4. The tags at the rectifier diodes are quite damaged. The old solder is nearly impossible to remove, it is so badly damaged and over-heated.

Anyway, please can anyone help me?

many thanks, Nick

Richard · #2 13th October 2007, 07:35 PM

Hi Nick,

If it's ok now then something must have caused an intermittent short. Possibly the cap after the bridge, they can short then burn open again.

I've only got the magazine circuit showing C3, so don't know what any later dc arrangement is. You mention C14 and C17, is there a pi filter on the later amp and what voltage are the actual caps you have?

Anyway, use a 25v one. When I modded Kit88 to include a pi filter I found a 10v one caused the (5A!) mains fuse to blow at switch on so went to 25v rather than risk even 16v.

Other thoughts would be a clipping of wire or solder across the valve pins or terminals etc that may have cleared itself.

Rich

Shian7 · #3 14th October 2007, 12:19 AM

Hi Richard,

There is indeed a pi filter on the XL version....

NickW · #4 14th October 2007, 10:10 AM

Hi Richard, Shian

The decoupling is a CRC filter with 10,000 uf 16V caps as shown in Shian's circuit. The caps seem ok, but who knows?

The really odd thing is that the red wires from the mains TX must have got really hot. They were tightly twisted together and the plastic got so brittle it all fell away when I pulled it apart. The tags on the tag board to which the red wires were attached were black! There was no sign of damage on the other side of the board i.e. at the output of the diode bridge. If something had gone s/c downstream, I might have expected that the diodes, the output tags, and/or the 0.47 ohm resistor in the CRC filter might have also shown signs of overheating? This lack of any damage on the output side of the diodes leads me to believe it must have been a fault in the red wiring or maybe the winding itself? Is this a possibility?
Many thanks, Nick

soulminer · #5 14th October 2007, 10:28 AM

How tightly were the red wires twisted together? Is it possible that the insulation was compromised?

NickW · #6 14th October 2007, 12:45 PM

Quote:

Originally Posted by soulminer

How tightly were the red wires twisted together? Is it possible that the insulation was compromised?

Hi Lee,

They were closely twisted but not so tight that the insulation could have broken. I think their death was slow rather than a sudden short-circuit and bang! You see the entire length of the red sleeving on the wires has melted - right back to where they exit the Mains TX. This suggests they got really, really hot before one wire became so brittle that it came away from the solder tag. looking at the wires now, they are now very stiff and brittle all the way back to within 2 cms of the mains Tx, where they become flexible strands again. I can't believe they are now safe as they are.
regards, Nick

JerryT · #7 14th October 2007, 12:47 PM

Quote:

Originally Posted by NickW

The really odd thing is that the red wires from the mains TX must have got really hot. They were tightly twisted together and the plastic got so brittle it all fell away when I pulled it apart. The tags on the tag board to which the red wires were attached were black! There was no sign of damage on the other side of the board i.e. at the output of the diode bridge. If something had gone s/c downstream, I might have expected that the diodes, the output tags, and/or the 0.47 ohm resistor in the CRC filter might have also shown signs of overheating? This lack of any damage on the output side of the diodes leads me to believe it must have been a fault in the red wiring or maybe the winding itself? Is this a possibility?
Many thanks, Nick

Hi Nick

Wiring fault at the board - yes. Winding fault - no.

You've identified where the source of heat was - in the last few cm of wire closest to the board. The blackening of the tags, the brittleness of the wire and the 'crisping' of the insulation all point to this.

I presume the fuse didn't blow because the fuse is mains side of a step down transformer and therefore saw only a small current increase.

Jerry

NickW · #8 14th October 2007, 01:24 PM

Quote:

Originally Posted by JerryT

Hi Nick

Wiring fault at the board - yes. Winding fault - no.

You've identified where the source of heat was - in the last few cm of wire closest to the board. The blackening of the tags, the brittleness of the wire and the 'crisping' of the insulation all point to this.

I presume the fuse didn't blow because the fuse is mains side of a step down transformer and therefore saw only a small current increase.

Jerry

Hi Jerry

I agree that the blackening of the solder tags all point to this as the location of the fault. The amp has been runing for between 300 - 400 hours up till now so I don't think it was a wiring error, more likely a breakdown of something - but I'm still puzzled as to what? I was wondering whether a dry joint or two could have led to a big increase in resistance at the joints, or something like that?
regards, Nick

Paul Barker · #9 14th October 2007, 02:22 PM

Dry joint wouldn't cause this.

Shunt capacitor shorting to ground (but it wouldn't recover) or the two twisted wires shorting to one another.

Could now be a latent problem with the transformer through the heat, we may not have heard the last of this.

JerryT · #10 14th October 2007, 02:27 PM

Hi Nick

It is a mystery. A lot of board members run similar circuits and haven,t met this problem. The only time I've seen anything similar was in poorly wired old three pin plugs where a high resistance was made at the live screwed terminal. Then the higher resistance caused many of the symptoms you saw.

If you hadn't been very sure that all the diodes were functioning correctly (you have checked them out of the bridge circuit haven't you?) I'd have been looking for one of those to have gone closed circuit and/or the first capacitor to have done the same. This would have drawn significant extra current for a long period of time but the circuit would still deliver a voltage albeit with a somewhat larger ripple.

Have you contacted WD and asked them for their opinion.

Jerry