HD83 Problems

[Dave Senior]

Hello Guys

I've had my HD83 for about 3 years and its always worked perfectly. A few weeks ago I replaced C1, C2, and C3, with BHC equivalents. Now it doesn't work!!

The HT voltages are all shot. I get 173ac direct from the transformer, -80dc from the first diodes, and -235dc from the second ones, but when I check voltages at R1 & R2 its 0.001. Voltages at tabs point f are the same. Heater voltage is 6.6ac.

I've double checked the caps polarity, I replaced the original caps, I replaced the diodes, I've remade all the joints in the power supply and none of this makes any difference. I'm sure its something simple but I'm at a loss where to start. Any advice would be much appreciated.

Regards
Dave

[Global]

Hi Dave

"173ac direct from the transformer, -80dc from the first diodes, and -235dc..."

If I am understanding this correctly your 173ac and 235dc look OK if current is not being drawn. Why are your HT voltages -ve and where is the -80 being measured? Is this just a reversed pair of meter probes or has the HT really gone negative?

It looks as though, despite your efforts, there is a break in the supply rail on the diodes side of R1 or the first cap is leaking badly (any heat or swelling?). It may also be worth checking that those parts of the circuit which should be at earth potential actually are, just in case you have accidently broken an earth path. Check resistances from chassis to earth points in the PSU with power off. Should be zero!

Edit - also check resistance from D1/D2 cathode junction to point A (should be 0), point b (should be 100R) amd point C (should be 4.8K).

Let us know.

Mark

[Dave Senior]

Hi

I am measuring the voltages with the black probe earhed on the chassis and the red probe on the anode of the diodes. I've checked to make sure the probes are orientated correctly in the meter.

The D1/D2 resistances you mention measure OK too. The earth seems OK too!!

Still Confused
Dave

[Global]

Ok Dave

Just to be certain. With the black probe earthed to chassis, the D3/D4 anode junction (I mean where they join) should read 0v when probed with red probe and the D1/D2 cathode junction should read approx +235v.

You have already checked for continuity from the diodes through R1 and R2 and that's OK. Does seem strange!

Excuse silly question but are the valve heaters lit ok? Could you have disturbed heater wiring? You did say heater was a bit high at 6.6v which might mean no heater current being drawn.

You need to probe for voltage right along the path from D1/D2 cathodes to and through R1, R2 and into the amp and see where the voltage is disappearing.

Mark

[Dave Senior]

Hello

I still haven't solved this one. The voltage is 0 at the junction of D1 & D2, but when I test the voltage across C1,2 & 3, it measures 163vdc, and thats without the power on after a day !!!

I've always known these caps held their charge but I didn't realise it was so high. I accidentally shorted one of the caps out with solder wick, which caused a hell of a crack and spark. Could this have done damage anywhere??

Regards
Dave

[Global]

Hi Dave

That cap voltage could have been hanging around there a LONG time - like since before the fault arose! I think it's a red herring.

Now - let me make sure I'm reading you correctly. With the power switched on you have NO voltage at the D1/D2 junction. This seems at odds with what you posted earlier. Apologies if I am misreading!

If you have AC across the transformer secondary and the same AC across the D1/D4 junction (black meter probe) and the D2/D3 junction (red meter probe) THEN measure DC across D3/D4 junction (black meter probe) and D1/D2 junction (red meter probe) which should give about 210v DC. If you get NO voltage or a low voltage then suspect faulty diode(s). If you get a good voltage then there is a break either in the ground connection to the D3/D4 junction or in the HT connection from the D1/D2 junction to the +ve of C1.

If what you said in your last posting is correct I am tipping at least one faulty diode AND a break in the above connection.

Another thought - some meters are upset by varying DC such as the raw output of a rectifier. If the bridge is OK and you have such a meter then a wiring break en route to the caps will produce an apparently bad voltage at the bridge due to absence of smoothing.

If the +ve of C1 had a retained voltage and you can't measure that same voltage at the D1/D2 junction, then there IS a wiring break! Remember that an apparently good wire can be broken within its sleeving.

Lots to think about there! Post back please.

And don't forget to discharge those caps through a 50K resistor before wielding a soldering iron!

Mark