API 550a Opamps getting very warm. Troubleshooting tips?

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Helsing

Well-known member
Joined
Jun 7, 2004
Messages
133
Location
San Diego, CA
Im racking up a pair of older API 550a eq modules, the fully discrete version with the two 2520s. Both modules are of the same vintage with the same vintage 2520s. Im running them at +/- 15vdc. They both function fine but the 2520s on one of the modules get very warm,(not hot though) while the other modules 2520s get just slightly warm. Im a believer in "if it aint broke dont fix it", but Id like to avoid frying those opamps. Any tips on what might be the problem and what to test for?

Thanks

Helsing
 
I tested the modules for current draw. Had the unit on all day. Working fine, but with extra warm 2520s in one of the modules.

"Warm" 550a module current draw: 69.5ma at idle

"Cool" 550a module current draw: 45ma at idle

Any ideas?

Helsing
 
Hi Helsing,

Is it same label on all 2520 amps ??

If some of them have be change, the temparature can be different because the quiescent current in the complementary output transistors can vary between the 2520 series around the years.

Here is some of the different 2520 series:

API 2520, Melville NY, around 1972-1975
API 2520, Huntington, from 1986 and up
API/Datatronix 2520, Reston VA, around 1980 to 1985

Check also if there are high offset voltage on the output (before the capasitor) on this 2520 that is more hot than the other.

BTW, is it the input or output 2520 that is hot ??

--Bo
 
Bo,

They are all Melville opamps in both modules. Both input and output 2520s get warm.

Ill check the offset and report back.

Bo wrote:

"API 2520, Melville NY, around 1972-1975
API 2520, Huntington, from 1986 and up
API/Datatronix 2520, Reston VA, around 1980 to 1985

Check also if there are high offset voltage on the output (before the capasitor) on this 2520 that is more hot than the other."

Helsing
 
Getting closer.

DC offset Measurements:

Warm 2520s

Input 2520: .006 vdc
Output 2520: flucuates between .041 to .053 vdc ****looks like the cuplrit but why?****



Cool 2520s

Input 2520: .015 vdc
Output 2520: .019 vdc


Helsing
 
Helsing,

10 to 20 mv offset is ok, and even more in discrete op-amps when it not offset adjusted with a preset outside. (IC op-amps are a lot better because they are laser adjusted under manufacturing)

Check one thing more on the hot unit, connect a oscilloscope on 2520`s outputs and check so there not are some oscillation in the HF band around 100kHz.

--Bo
 
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