You've got a problem there. It should not oscillate when touched.
Could be bad caps or grounding arrangements or whatever. But it's something.
The amp had a couple of problems, hum and instability.
I have read numerous threads over the years of them just blowing up.
Here is a page concerned with hum problems.
Maplin 150W MOSFET module hum modification
Did Mooly find models for those old Hitachi mosfets? I struggled and gave up. Good job - you've been vindicated! Now don't do it again.
I thought you'd forgotten ECX10N20, ECX10P20 models post 9
If they worked fine in the past then either something when wrong on both PCBs (unlikely) or something went wrong when you re-boxed them.
Would you be able to take some close-up photos that show your wiring arrangements, including grounding paths and inputs? If the ground paths are not quite right this can easily cause instability.
I soon discovered that I could not find any models of the original transistors: 2SA872, 2SC2547, 2SC1080, 2SJ48 and 2SK133. nor the FETs. So I gave up!
I soon discovered that I could not find any models of the original transistors: 2SA872, 2SC2547, 2SC1080, 2SJ48 and 2SK133. nor the FETs. So I gave up!
Its a very old design. I built my first one in 1980.
When I did my redesign of it I replaced all the transistors with modern equivalents.
So, With regards to my original question, regarding Transistor temperatures, I had the chance to take some measurements today.
The amps with the Bias set to 50ma, and an ambient room temp of around 20°C settle to around 30°C after about 30 minutes, this reading is taken from the top of the output transistors.
Feeding the amps with a 1Khz Sine wave for an output of 25v pk – pk into a 7.5R Dummy load the temperature of the Output Transistors gets to about 61°C after about 15 – 20 minutes, actually quite uncomfortable to hold your finger on for very long.
The difference between the temperature of the Transistor Cases and the Mounting Bracket is about 10 degree’s lower, so maybe in my effort to be tidy and avoid too much heatsink compound squeezing out everywhere from under the Transistors I possibly skimped a little, and this difference might be improved by adding a little more compound or at least check to make sure the entire area has an even amount of compound on it.
The biggest difference is between the Heatsink Mounting Bracket and the heatsink itself, The temperature of the Heatsink is 19°C lower than the Mounting Bracket, So while the Transistors are at 61°C the Heatsink is only 32°C.
In my original post, I mentioned that the amp is bolted to the heatsink through the side panel of the case, which is 3mm thick aluminium, I thought this would help give a good surface area for heat to dissipate, Wrong ………
It looks as though I will have to cut a slot in the side panels to allow the Heatsink Mounting Bracket to be bolted directly to the Heatsink and hopefully this will lead to a much better transfer of heat and see the transistors run cooler.
Both amplifiers driven, start to clip at just a little over 75v, I’ll let someone else work that wattage out, the only formula I ever had and understood I lost and there are so many conflicting formula’s on the net.
The amps with the Bias set to 50ma, and an ambient room temp of around 20°C settle to around 30°C after about 30 minutes, this reading is taken from the top of the output transistors.
Feeding the amps with a 1Khz Sine wave for an output of 25v pk – pk into a 7.5R Dummy load the temperature of the Output Transistors gets to about 61°C after about 15 – 20 minutes, actually quite uncomfortable to hold your finger on for very long.
The difference between the temperature of the Transistor Cases and the Mounting Bracket is about 10 degree’s lower, so maybe in my effort to be tidy and avoid too much heatsink compound squeezing out everywhere from under the Transistors I possibly skimped a little, and this difference might be improved by adding a little more compound or at least check to make sure the entire area has an even amount of compound on it.
The biggest difference is between the Heatsink Mounting Bracket and the heatsink itself, The temperature of the Heatsink is 19°C lower than the Mounting Bracket, So while the Transistors are at 61°C the Heatsink is only 32°C.
In my original post, I mentioned that the amp is bolted to the heatsink through the side panel of the case, which is 3mm thick aluminium, I thought this would help give a good surface area for heat to dissipate, Wrong ………
It looks as though I will have to cut a slot in the side panels to allow the Heatsink Mounting Bracket to be bolted directly to the Heatsink and hopefully this will lead to a much better transfer of heat and see the transistors run cooler.
Both amplifiers driven, start to clip at just a little over 75v, I’ll let someone else work that wattage out, the only formula I ever had and understood I lost and there are so many conflicting formula’s on the net.
A bias current of 50mA is at the low end of ideal for laterals, but is OK.
The Maplin design was intended for 8 Ohm speakers. a single pair of devices will suffer into 4 Ohms, due to low device transconductance, especially the P channel.
Where is the output inductor, mosfets still need one for stability into some loads
The Maplin design was intended for 8 Ohm speakers. a single pair of devices will suffer into 4 Ohms, due to low device transconductance, especially the P channel.
Where is the output inductor, mosfets still need one for stability into some loads
You say that the case is aluminium. Before you go cutting big holes in it try removing the paint at the junctions between the bracket and the case and the case and the heatsink and apply a light coat of heatsink compound to the joining faces.
Heatsink compound should only be applied thinly, it is only there to fill in imperfections in the two mating surfaces. If the two mating faces were perfect mirrors (which they can never be), there would be no requirement for it at all.
You will always get a temperature difference across a thermal junction but it should be possible to make that quite small.
Heatsink compound should only be applied thinly, it is only there to fill in imperfections in the two mating surfaces. If the two mating faces were perfect mirrors (which they can never be), there would be no requirement for it at all.
You will always get a temperature difference across a thermal junction but it should be possible to make that quite small.
Sorry, only just noticed this reply, I think my problem Heat wise is down to a bad idea in the way the amps are mounted (please see the post above).
I don't have any photos of the wiring, but what I can tell you is that the +/- Supply rail wires are kept as short as possible, but long enough to allow me to lay the side out should I need to work on anything or unsolder the supply wires.
The supply wires and 0v are not soldered to the pins, but to the Track Itself about midway up the board, and the speaker grounds are taken back to the 0v at the smoothing caps.
Up until today, I had only had chance to view the outputs on a scope, I gingerly connected a pair of head phones and realised I do have some Hum, but connecting 0v to Chassis Ground cures this completely, although I know this shouldn't be necessary.
What problems would you be looking for in the wiring?
I think the problem with the heat sink mounting is twofold,
1) The outer face of the side panel (The side that mates with the Heat Sink) is brushed aluminium so not really a good mating surface.
2) Over the years it seems the Heat Sinks themselves have bowed very slightly, so where they are tight in the centres where they are bolted, at either end there is a gap large enough to slide a sheet of paper in, The side panel actually gets hotter than the heat sink.
The 10°C difference between the transistors and the Mounting Bracket may be perfectly acceptable, I'm hoping there maybe someone who knows a "General rule of thumb" as to what the difference on average is.
Both amplifiers driven, start to clip at just a little over 75v, I’ll let someone else work that wattage out, the only formula I ever had and understood I lost and there are so many conflicting formula’s on the net.
75v pk/pk is 37.5 pk. Divide that by root 2 (1.414) and you get the rms voltage which is 26.5 volts. Power into 8 ohms is 26.5 squared divided by 8.
So about 87 watts.
I built the Maplim mosfet amp around 1982 and added a heatsink to the TO-92 driver. Also tried adding a second pair of 2SJ49/2SK133's.
It was never unstable or oscillated for me.
It's got plenty of punch but most annoying was the hum and grit.
I found RF comes in on the speaker leads/ground/mains and the mosfets seem to demodulate/amplify this.
The RF (mostly AM radio) can be seen on the mosfets' source terminals with a scope.
Common-source amplification I think was going on.
My opinion the design needs some RF decoupling caps and ferrite beads and common-mode chokes etc. to tame.
It was never unstable or oscillated for me.
It's got plenty of punch but most annoying was the hum and grit.
I found RF comes in on the speaker leads/ground/mains and the mosfets seem to demodulate/amplify this.
The RF (mostly AM radio) can be seen on the mosfets' source terminals with a scope.
Common-source amplification I think was going on.
My opinion the design needs some RF decoupling caps and ferrite beads and common-mode chokes etc. to tame.
Attachments
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.