2520BC troubles... continued...

[Neeno]

I've stuffed some 2520bc opamps... and as always i'm getting troubles... :shock:

They seems to work propely, signal passes, no oscillation even at the higher gain stage, but... the BD139(16) is getting really HOT !!!

They are build following the latest suggestion i've found on the api312 post, so i used: 360R for R2, 10p for C5, 20k for R3, 20k for R4+R5, 3k9 for R7, 51R for R9 and R13 (i dont have 56R here...) and 1N4148 for the diodes...

Now i can't figure out why the 139 becomes so hot... and i don't have any value to take some measurements... so... any help would be appreciate !



Neeeeeeeeno

[/list]

 

[jrmintz]

I'm not sure what the differences are between the version you're building and the originals, but the reason they're potted is to dissipate the heat from the output transistors via the thermal compound.

 

[Neeno]

hi jrmintz... thanks for your reply...
you know... I heard something about the enclosure, but it seems it's used to shield the opamp... not for heat dissipation, but i may be wrong...

My BD139-16 is around 100°C !!!
After 10 seconds they are turned on i can't touch them with the finger...

Maybe i can use them as a mini toaster ! :grin:

 

[jrmintz]

Inside the potting shell there is a small layer of very thin, unetched PC board, with a small notch on one edge, covering the solder side of the 2*5*2*0 board. Obviously, the copper side of this additional board points away from the solder side of the 2*5*2*0. There is a thin wire connecting this shield board to a ground pad on the 2*5*2*0 board through the little notch. You have to be careful not to short it to anything else. The socketing pins are pointing up from the 2*5*2*0 board past the components, so when the whole assembly is pushed into the potting shell the secondary shield board goes first followed by the main board. When the opamp is potted and inserted in a device, the shielding board is facing out to the world and all the components are facing down towards the device board. In any case they are definitely potted using thermal compound and they will toast themselves if not potted.

 

[Ethan]

On a related note, Seth, do you know where to get those cute plastic enclosures for the opamps?

 

[strangeandbouncy]

Perhaps I cam help, since I am unable to use my 2520bc's at unity gain, or anywhere near unity gain. They are fine at higher gains, but get roasty at lower gains. Can't remember which transistors get hot, but I am sure it is at least one of the output devices(BD139/140?) I am eagerly awaitng the arrival of the new op-amps (developed under our noses on this forum!) which are apparently very happy at unity gain - It was amazing to follow this thread - thanks guys!)

Hope that this may help . . . . .


ANdyP

 

[balluda]

[quote author="strangeandbouncy"]Perhaps I cam help, since I am unable to use my 2520bc's at unity gain, or anywhere near unity gain. They are fine at higher gains, but get roasty at lower gains. Can't remember which transistors get hot, but I am sure it is at least one of the output devices(BD139/140?) I am eagerly awaitng the arrival of the new op-amps (developed under our noses on this forum!) which are apparently very happy at unity gain - It was amazing to follow this thread - thanks guys!)

Hope that this may help . . . . .


ANdyP[/quote]

Wich ones do you mean?

 

[strangeandbouncy]

??? - Wot wich wuns?

you'll have to expand on this . . .


ANdyP

 

[dasbin]

My 2.5.2.0 BC's are running cool at all gain levels... only change from schematic was C5=10pf. BD139 doesn't even get warm for me. Strange.
Don't know if you guys are using current source though, or if that matters... I'm not.

What supply voltages are you running? Also any DC offset at input or output?
I should note I'm using some extra poly caps on the supply rail as decouplers... maybe just try sticking some .1uf 's on there to ground and see what happens... perhaps something is oscillating at frequency you can't measure?

Edit: I I just noticed something strange and possibly related on mine... sometimes, just by slightly physically moving or touching the whole opamp assembly, it will cause the BD139 to heat up hugely and a very slight "ringing" sound can be heard coming from either it or the output trafo (not sure which) when this happens... also the output momentarily spikes to like 2 VAC. All this from the only change of a slight physical movement... I think maybe the length and quality of the connecting pins (and their appropriate solder points) may have something to do with this... perhaps the slight movement increases the impedance on the supply rails just enough to send the opamp into oscillation?

 

[peterc]

I had my Melcor based units up last night & I had the same overheating problems on one channel.

I found a broken resistor on my Gain rotary switch, so the1731 input that goes to ground via the Gain gcontrol & cap was floating. The signal still came through OK on the scope.

Is your gain control OK?

Peter

 

[Neeno]

Hi peterc, i'm using a 22k rev log pot... no switches and no resistors...

Dasbin:
I'm running my api's at +-16V.
I've tried sticking 2 .1uF cap from V- to gnd and from V+ to gnd...
What do you mean by current source ?

 

[jrmintz]

Can't speak to the Melcors, but the 2520s are supposed to run very hot and must be potted with thermally conductive compound or they will destroy themselves.

 

[dasbin]

The other option is just to stick a heatsink on BD139 if that's the only one getting hot.

 

[Flatpicker]

[quote author="dasbin"]The other option is just to stick a heatsink on BD139...[/quote]That's what I was going to say - no problem that a big hunk 'o 'luminum won't fix! :wink:

 

[chrissugar]

[quote author="jrmintz"]Can't speak to the Melcors, but the 2520s are supposed to run very hot and must be potted with thermally conductive compound or they will destroy themselves.[/quote]

I built some NTP100 discrete opamps and the output transistors are hot so I used some heatsinks, and they work fine:

Although I think it is something wrong if in a push pull output configuration only one transistor is hot. Verify the DC at the out pin of the DOA. It should be close to 0 volts.

chrissugar