Hacker AL42 Smoothing Cap Voltage Reading

[PJay1966]

Hi

I am starting work on an AL42 amp for my GP42 player and this morning checked the voltage on the smoothing cap in the unit. Having removed the 2 ECL86's and leaving just the EZ80 in place I turned on and noted a steady rise to what seems a high reading of 358.2. This is over the WV of the cap so I wanted to seek some guidance on what I should check next or if this is acceptable. The voltage dropped steadily over the next 35 mins with the cap reading 78V after about 25 mins so it seems like its not overly leaky.

There is a noticeable hum after the volume pot is turned up about 3/4 but other than that it seems to work ok.

appreciate any pointers or advice..

[Station X]

Off load voltage will always be high as there's no volt drop in the smoothing resistor.

Of more concern is the drop to 78V. This does not equate with the cap "not being overly leaky" does it get hot?

[Nuvistor]

Beware, that smoothing cap could explode at the bung end, don’t have your face over it.
As Graham asked, does it get hot?

Without load the voltage will rise to the peak RMS value less drop in the transformer and valve so 350 or so volts seem about right. The valves warm up together so by the time the rectifier is conducting so will the other valves. Not completely in sync but good enough to keep the voltage within spec.

[PJay1966]

Not particularly hot but warm to the touch after about 30 mins.

[Nuvistor]

With only 78 volts across it with no load it seems there is a problem to fix.

[ms660]

Perhaps with no load connected 78 volts HT after switch off after a period of time is feasible and indicates that the HT capacitors are ok so far as leakage goes?

Lawrence.

[PJay1966]

Originally Posted by ms660

Perhaps with no load connected 78 volts HT after switch off after a period of time is feasible and indicates that the HT capacitors are ok so far as leakage goes?

Lawrence.

Thats what I thought, it read 78V after about 25-30 mins (I wasn't timing it too accurately). I noticed that one of the ECL's wasnt lighting so it must be duff, I had a spare one and installed it they all now glow nicely but I dont know if the other one is dodgy or maybe they need to be identical pairs? Could that be the source of the hum? I've just checked most of the resistors and they are bit out but seem largely in tolerance. It has the nice mustard caps as well on the top of the main tag strip.

[Station X]

Are you saying that the voltage gradually drops after the player is switched off with the ECL86's removed? If so this is perfectly normal and suggests the smoothing caps are good.

I had assumed that the voltage dropped with the player switched on.

[ms660]

See if the cathode voltages are more or less the same on the pentode section of the ECL86's.

EDIT: Post crossed.

Lawrence.

[Nuvistor]

I assumed it was switch on, shouldn’t assume but I keep doing it.

[Station X]

The trouble is if you don't make assumptions you just end up asking questions.

I'll leave this one to Lawrence.

[ms660]

Originally Posted by Station X

I'll leave this one to Lawrence.

But you might have some further insight about something I might assume....

Lawrence.

[Station X]

Very good!

[PJay1966]

This is what happens...

ECL's removed and unit powered on with my DMM connected to the red terminal on the smoothing cap and to the chassis. The voltage rose steadily to 358.2 and roughly stayed at that reading. I turned the unit OFF and observed the voltage drop, which faded steadily over about 20-30 mins the reading I mentioned of 78V was after it was off for about 20-25 minutes. As there is a slight hum at high volume I assumed the CAP could be dodgy but from what I have read that seems pretty normal?

[PJay1966]

I've read a few posts about ECL86's causing hum when they're failing. As one is completely dead I am wondering if the other one is faulty. Visually it looks fine and it lights ok, perhaps I should consider buying a new pair?

[ms660]

I don't know off hand what the normal level of hum would be, the last time I would have serviced one of these would have been in the 1960's or the 1970's at the latest, in theory the effect of the ripple component on the HT that's fed to the anodes of the pentodes more or less gets cancelled out by the action of the output transformer in push pull if the valves are reasonably matched, measuring their electrode voltages will normally indicate anything untoward in terms of operating conditions (excepting heater cathode leakage)

I wouldn't worry too much about the unloaded HT being a few volts above the capacitors maximum working voltage during warm up, as said earlier with no load the capacitors (with very little leakage) will charge up to approx. the peak voltage of the AC feed to the rectifier, eg: AC volts (RMS)*1.414, the schematic gives the AC voltage feed to the rectifier as 250 volts per anode so at that voltage it's approx. 353 volts peak, in other words 353 volts DC across the capacitors.

Lawrence.

[PJay1966]

Thanks Lawrence...

I just need to trace the source of the hum which is only there at higher volumes. I will try plugging it in to a sound source though and see how it sounds.

[Michael Maurice]

The capacitor should not get warm on its own, it will warm up as the chassis warms up but thats it. Hacker capacitors are generally very good.

[PJay1966]

Still working on this AL42 and just been testing components. I have found that the smoothing cap resistor which should be 200ohms is reading nearly 300, certainly out of tolerance in my view. I have a few 10W 200 Ohm wirewounds which I think I may swap the old one with does that sound reasonable?

[Station X]

If the original resistor is wirewound it would be unusual for it to go high in value. They generally fail open circuit.