Resurrection of my WD88A XL

[NickW]

Hi Guys,

Been working on the rebuild all weekend and have just finished. It works again.

I have measured all the voltages inside again, and the only one that worries me is the DC Heater supply. For the XL kit, the original instructions and test procedure call for the voltage across C14 to be 5.9V to 6.3V.

Now WD have changed the diode type to UF5408, and I measure 5.64V across C14, and 4.80V across C17. The 0.84V drop occurs across the 0.47 ohm filter resistor of course. So the small signal valves actually see 4.8V across their heaters - which sounds a little on the low side to me.

I've wired the amp for 240V mains, and most of the other voltages are on the high side. For example:

The AC output of the Mains Tx (pink wires) measures at 332V as opposed to the test spec of 325V.

The DC HT measures at 425V as opposed to the test spec of 410V to 420V.

The anode voltages and cathode voltages all lie within spec.

So its only the DC Heater supply that seems low.

So my question is - is 4.80V acceptable?

Many thanks, Nick

[systemx]

Hi Nick.
Yes it does sound a little low. I have just ordered this amp and I think I will stock up on some low value resistors and adjust to get the correct reading What is the power rating of the filter resistor?.

Terry.

[NickW]

Hi Terry,

Its 6 W, and differes in value depending on whether you have an "Integrated" or a "Power amp" version.

regards, Nick

[systemx]

Nick.
Thanks very much, it is the integrated I have ordered, and I'm looking forward to it's arrival. I might get some of the original diodes as well. I think mine with the fuse mod, we will see.

Thanks again Terry.

[Richard]

Nick,

Schottkies are more efficient (lower forward voltage drop) and would give nearly 1V more. It would seem that with standard type diodes there isn't really the voltage available to use any series resistor.

If you have the fuse now fitted the supply will be safe. You could go back to schottkies to correct the voltage. I'd try those TO220 10A ones mentioned in the other thread and mount them standing up from the board with air between them to prevent mutual heating.

Rich

[John Caswell]

Hi all,
If you have changed the diodes from the 1SQ*** variety to the UF**** variety then you will have to change the series resistor as well, to about half its original value, and aim for 6-6.3v at the heater pins.

John - Dr John

[systemx]

Richard.
I agree, some quick "back of a fag packet calculations" back this up. I have ordered some STPS1045D (RS-486-0216) for my amp, as you say, with the fuse these should be fine. Will let you know.

Terry.