Hi. I have an early 80s mission 778 integrated amp (pre-Cyrus One, mosfet output devices) which I have just repaired (vol control needed replacement). It's sounding rather good but it does run hot, even at low volume levels. I've no idea if this is normal.
The power amp circuit diagram is here - Mission 778 - Manual - Stereo Integrated Amplifier - HiFi Engine - page 2 of the "schematic" link.
And you can see there is a trim pot in each output channel, with voltage measurements given. Looks like these measurements are close but not identical for the + and - rails. I've not yet measured the channels of my amp, but in general should I be looking for the + and - measurement points shown (marked as 14.6v and 15v respectively on the diagram) to be the same?
If these measurements are not the same / near the values given, would this cause the amp to run hot, or is it nothing to do with that?
I'm fairly handy with a multimeter and soldering iron but I do not understand the finer points of MOSFET power amp design.
The output devices are fairly substantial T03 can devices fitted to a substantial finned heatsink. However after nearly 40 years I guess the thermal grease may be past its best?
Any advice appreciated.
Paul
The power amp circuit diagram is here - Mission 778 - Manual - Stereo Integrated Amplifier - HiFi Engine - page 2 of the "schematic" link.
And you can see there is a trim pot in each output channel, with voltage measurements given. Looks like these measurements are close but not identical for the + and - rails. I've not yet measured the channels of my amp, but in general should I be looking for the + and - measurement points shown (marked as 14.6v and 15v respectively on the diagram) to be the same?
If these measurements are not the same / near the values given, would this cause the amp to run hot, or is it nothing to do with that?
I'm fairly handy with a multimeter and soldering iron but I do not understand the finer points of MOSFET power amp design.
The output devices are fairly substantial T03 can devices fitted to a substantial finned heatsink. However after nearly 40 years I guess the thermal grease may be past its best?
Any advice appreciated.
Paul
The two important measurements are that the DC offset at the speaker terminal is zero (and the preset will adjust that) and that the output stage quiescent current is 'correct' although no value is specified. Typically that would be 100ma per output pair for lateral FET's like these but there is no easy way to measure that unless you either break the circuit and add an ammeter or (better) break the circuit and add a low value resistor like a 0.1 ohm and calculate the current from the voltage drop.
Fwiw, 50 volts across each device at 100ma is 5 watts dissipation per device, so 20 watts in total and yes, that will generate quite a noticeable temperature rise on a small heatsink.
Fwiw, 50 volts across each device at 100ma is 5 watts dissipation per device, so 20 watts in total and yes, that will generate quite a noticeable temperature rise on a small heatsink.
Thanks for the swift response Mooly, interesting. Left speaker DC offset is +0.016v. Right speaker DC offset is +0.012v. I can try and tweak the trim pots but not sure I'd get them much lower?
After an hour switched on the heatsinks are at around 56C. Heatsinks look like this sort of profile WA 204 | Heatsink, TO-3 | RS Components
Does any of that sound too alarming?
Thanks
Paul
After an hour switched on the heatsinks are at around 56C. Heatsinks look like this sort of profile WA 204 | Heatsink, TO-3 | RS Components
Does any of that sound too alarming?
Thanks
Paul
That offset is very low and certainly doing no harm whatever... old presets... best just left alone.
When you switch on from cold you should be able to gauge one channels FET temperature vs the other just by feeling. If both seem to heat up at the same sort of rate and feel similar then its probably all normal.
Reducing the value of those 27 ohm resistors will reduce the bias current (and so the heat) but it is not possible to accurately determine the values needed to reduce it by a given amount, just that it definitely will as less gate/source potential is developed.
When you switch on from cold you should be able to gauge one channels FET temperature vs the other just by feeling. If both seem to heat up at the same sort of rate and feel similar then its probably all normal.
Reducing the value of those 27 ohm resistors will reduce the bias current (and so the heat) but it is not possible to accurately determine the values needed to reduce it by a given amount, just that it definitely will as less gate/source potential is developed.
Extremely helpful and reassuring, thanks again Mooly. Thanks again for the prompt response.
I'll check the temp gains per channel as you suggest.
Nice to keep these old classics running isn't it? I also have a Cyrus One from the late 80s. That is well built, but the 778 is better - built like the proverbial brick outhouse. Lots of high quality components too.
Paul
I'll check the temp gains per channel as you suggest.
Nice to keep these old classics running isn't it? I also have a Cyrus One from the late 80s. That is well built, but the 778 is better - built like the proverbial brick outhouse. Lots of high quality components too.
Paul
Hi.
I have owned a Mission 778 (Cambridge) since the mid 80's.
I can confirm that the unit warms up quickly and can also get hot, warmer/hotter than other amps. This is normal and has been like this since the beginning until today. Maybe it's not really relevant for the function, but to support the cooling, I just put my feet on a small pad (~ 3 mm 'high heels').
Also, I think I remember someone once saying it was a pure Class-A amp. But I haven't received any confirmation on the www yet.
That's what I want to add to the topic of heat build-up.
I have owned a Mission 778 (Cambridge) since the mid 80's.
I can confirm that the unit warms up quickly and can also get hot, warmer/hotter than other amps. This is normal and has been like this since the beginning until today. Maybe it's not really relevant for the function, but to support the cooling, I just put my feet on a small pad (~ 3 mm 'high heels').
Also, I think I remember someone once saying it was a pure Class-A amp. But I haven't received any confirmation on the www yet.
That's what I want to add to the topic of heat build-up.
It is almost certainly a Class AB amp. A single pair of laterals running on those kind of rails can't be anything else really. Rails at almost -/+60 volts would generate 24 watts of heat in the FET's at just 100ma bias current which is Class AB. The other give away is the amps small size. Class A would be many times larger, heavier and with massive heatsinks and multiple parallel pairs of FET's.
So Class AB.
So Class AB.