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BGW 150 amp seems hot

S

Studio1

I have recently acquired a BGW 150 'Broadcast Power Amplifier'.

It came with a few things that needed sorting, such as a loose gain control, change the primaries of the transformer from 110 to 230 and replace the electrolytic caps (two in each channel).

It seems to be running Ok. Power rails measure as per the schematic at +40 and -40 volts respectively.

What concerns me is the heatsinks on both channels seem overly hot - even when no signal is applied.
I measured them with a point-contact thermometer at 47 °C.

Is this normal? Anyone who has one of these, does yours run hot as well?
 
Have a BGW we bought back in 1983--don't remember the model off the top of my head, but it does run warm. So allow some ventilation above it if you rack mount the unit.

Before you do--open it up and see if there are any signs of distress or age. If it does have some age on it would not hurt to do these things:

1. You've replaced the filter caps?

2. Then look at the coupling caps.

3. Heat sink compound on the heat sinks may help.

4. If you have a scope take a look at the output without any audio. Something may be causing high-frequency oscillation which you can't hear, but is drawing current. If this is happening, you'll see it on the scope.
 
Check the bias voltage on the PA amplifers. I had one of their amps years ago get hot on one side. It was something to do with the bias being incorrect.
 
TomT said:
Have a BGW we bought back in 1983--don't remember the model off the top of my head, but it does run warm. So allow some ventilation above it if you rack mount the unit.

Before you do--open it up and see if there are any signs of distress or age. If it does have some age on it would not hurt to do these things:

1. You've replaced the filter caps?
2. Then look at the coupling caps.
3. Heat sink compound on the heat sinks may help.
4. If you have a scope take a look at the output without any audio. Something may be causing high-frequency oscillation which you can't hear, but is drawing current. If this is happening, you'll see it on the scope.
Click to expand...

I haven't replaced the filter caps but I have done the coupling caps. There are no signs of distress or age. It's in remarkably good condition. All transistors seem to have sufficient compound under them.
I will fire the scope up and take a look at the output.

OKCRadioGuy said:
Check the bias voltage on the PA amplifers. I had one of their amps years ago get hot on one side. It was something to do with the bias being incorrect.
Click to expand...

I suspected a bias issue as well, but both channels are getting equally hot so I can only presume it's aging resistors that have gone high.
There's no bias adjust preset - it seems to be done by thermal feedback transistors mounted on the heatsink. The manual does not state the bias current values either.
It's been a LONG time since I repaired a bi-polar output amp. Everything I've seen lately is MOSFET!
 
It looks like running warm/hot is normal.

Scoped the output, nothing unusual there.

Checked the bias current - both channels the same, slightly on the low side if anything.
 
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