Pioneer A-8 hot transformer and humming.

I don't have any dummy resistors big enough to do that test as of yet but I have been thinking of getting some...

I think I will attach a temperator sensor to the transformer and measure the temperature in case I am overreacting... it is a very big transformer but I am starting to look into a replacement if neccesary... The hum and head seem to come from one side of the transformer and I have been thinking of going through the amp and doing a recap etc on it soon. Do you think it is likely a fault with the transformer?
 
Since the amp is otherwise running fine, the issue is likely and solely with the transformer.

There's something easy you can do to wipe off any doubts you may still have, and that is to lift the transformer's secondary winding from the power supply so it may run completely unloaded. Humming and temperature are likely to remain constant.
 
I'll give that a go. Will pulling Fuse 2 (1.5A) work to isolate the lower voltage secondary? Then all i need to do is unsolder and remove the three pins for the main power supply to the outputs...
 
Right; removing F2 will open the low voltage secondary winding circuit.

Regarding the high voltage output, I'd lift the transformer's red wires from the PS. Only that would completely isolate it.
 
I removed F2 and pulled the red pins and the black pin from the PCB for the transformer (they were bundled together in a plastic holder). I decided to connect it through a DBT just in case and found that when powered on, the transformer would make a 60W bulb glow. AFAIK I don't think a transformer SHOULD make a bulb glow... I fitted a 100W and there was no glow but there was the hum.

I tested it in my workshop, which has a much lower ambient temperature to where it was geting hot so there wasn't too much heat but the same hum was there...

I connected a AV volt meter to one of the red wires and the black wire and got a reading of 34VAC with the 60W bulb and 39-40VAC on the 100W bulb.

I guess this is likely conclusive that my transformer is dying? Would a new torroid be the way to go?

EDIT: I had left the transformer on while I posted - when I went back it was starting to warm up. I took a voltage reading between the two orange wires and got 16.1VAC
 
Last edited:
looks like that transformer is finished . if indeed the secondary was out of circuit .

It should have been... I had removed FU2 which disconnects oen of the orange wires. That would be enough wouldn't it?

So am I looking for a transformer with 40-0-40 and an aditional 16V secondary?

The easiest to find seem to be toroids so with that in mind and if a fully custom one wasn't available, would it be possible to "stack" a 40-0-40 and a seperate 16V toroid on top of each other or would the toroids react to each other in a bad way? Of course they would be insulated...
 
transformers being induction devices i would think you may have problems stacking one on top of another .
 
So a custom transformer it is then i guess...

So I would need a transformer rated at 500VA? with a 40-0-40 secondary and a 16V secondary?

I was guessing at 500VA correct me if I'm wrong but seemed to be the closest I could get from the rating of 470W from the back of the A8...
 
Last edited:
Indeed seems like your trafo is bad; it's showing a highly inefficient running; in a way all that heat must have a root cause, and now you were able to see it.

Regarding a toroid as replacement, is there enough room for it? Could be larger than your actual laminated core. IMO, getting a custom normal transformer built, even a shielded one, shouldn't be an issue.

As for stacking them, you can do that, as long as there's no metal closed loop binding them together (this concept is also valid for a single transformer).
 
Regarding a toroid as replacement, is there enough room for it? Could be larger than your actual laminated core. IMO, getting a custom normal transformer built, even a shielded one, shouldn't be an issue.

Do you have any ideas on what that would cost? Just as an initial idea?
 
It will vary to a great extent depending on where you get it manufactured. My latest experience (2 years back) is with a chinese manufacturer from whom I got 2 x 650VA custom made toroids for my amp project (weighing about 12lbs each!. Price was great, below $100 each, and they run flawless; no heat, no hum, almost no voltage sag even loading them next to the rated output during tests.

This is my project thread at diyAudio in case you'd like to check it out:
http://www.diyaudio.com/forums/solid-state/241562-300-300-wrms-power-amplifier-lots-pictures.html

You may also get your current transformer re winded lowering the cost (also depending on where are you located). This possibility has to be evaluated by the shop upon inspection of the transformer.
A shop, as e.g:
http://members.tripod.com/tubes_tubes_tubes/transformerrewindingservice/index.html
 
As it is the new year, I'm waiting on a few companies getting back to me about transformers. Sadly, after an inspection, I have around 110mm by 110mm for a new transformer and the one i sources for the 40V supplies was 140mm... so that won't work off the shelf...

I got the power meter today that Pete suggested and just for reference, the transformer was pulling 18-19W by itself.
 
Wouldn't bad main caps contribute to hum issues?

You need to differentiate between transformer noise (hum/growl) and speaker hum. Both have way different stemming causes. Bad or even non existent filter/reservoir capacitors do not contribute to any kind of transformer noises.
 
So to get a new transformer, I'm gonna need to know how much power is needed for the 16V supply and the 40-0-40 supply. Does anyone know how to work out what each supply will need/draw from the schematic? I think I could get halfway there.. but not all the way there...

From my initial enquiries, a 500VA toroid will be too big for the A-8 but maybe we can find an alternative/compromise...
 
The low voltage output has a 1.6A inline fuse; so probably that winding wouldn't be designed to provide much more than 2.5A.
Regarding the 40-0-40 winding, would be awesome a transformer with 3 times the amplifier rated output power (90W+90W x 3). Would still be great if you can squeeze in one of 2 times the amp rated power.

A good approximation of the stock transformer could also be calculated by any transformer manufacturer based on how much it weighs and/or its size.
 
Back
Top Bottom