24-01-2017, 02:44 PM
Hi Hayden,
Thanks for your reply, and for the 3 very interesting links, which contain a lot of useful information and some worthwhile modifications.
It is particularly good to have the redrafted circuit diagrams - I could not reliably read all the component values on the circuit from mods.dk.
Since my last post, I have managed to get hold of a working D-3800, and I have measured all the pin voltages on IC1 and IC2, for output voltages of 14 V, 10 V and 5 V. You will find those in the attached PDF.
When you get around to opening up your D-3800 again, note that it has a number of components not shown in the redrafted circuit diagrams, specifically R41-C26 (in yellow ellipse), an RC filter across the inputs of IC2D, and D16-D17-R42-R43 (in red ellipse - D16 is in the white sleeving), which modify the signal to the + input of IC2D. These components are only shown in the circuit from mods.dk, and are all soldered point-to-point on the solder side of the PCB, as shown in the attached image. (Note that C25, which is soldered between pins 7 and 10 of IC1 on the solder side, has been removed in this image.)
I am now fairly sure that the fault in my D-3800 is IC2D (LM339). It's output (pin 13) is low even though its IN+ (pin12) is greater than IN- (pin 10). This is shutting down the LM723. As indicated in an earlier post, lifting one end of D12 restores full output voltage.
Capacitors are a good place to start of course. If your fault is in the IC2D area, the only relevant capacitor is C26. Connecting across C26 is a convenient place for monitoring IN+ and IN- of IC2D. I tried removing this capacitor, but it made no difference; not surprising in view of the fact that I always find the differential voltage across IN+ and IN- to be healthy (i.e. positive).
By the way, my fault mode is not always full shutdown. When set for 14 V, the supply output sometimes wanders around in the vicinity of 7 V. Full output is restored when I lift one end of D12, and then I find a wandering voltage of about 7 V on the output of IC2D (again with IN+ greater than IN-). That also seems like a fairly solid indication that IC2 is faulty in my unit.
I will replace IC2 (LM339) next and let you know what happens.
Best wishes,
Lew.
D-3800_IC1_IC2_Pin_Voltages.pdf (Size: 82.35 KB / Downloads: 783)
Thanks for your reply, and for the 3 very interesting links, which contain a lot of useful information and some worthwhile modifications.
It is particularly good to have the redrafted circuit diagrams - I could not reliably read all the component values on the circuit from mods.dk.
Since my last post, I have managed to get hold of a working D-3800, and I have measured all the pin voltages on IC1 and IC2, for output voltages of 14 V, 10 V and 5 V. You will find those in the attached PDF.
When you get around to opening up your D-3800 again, note that it has a number of components not shown in the redrafted circuit diagrams, specifically R41-C26 (in yellow ellipse), an RC filter across the inputs of IC2D, and D16-D17-R42-R43 (in red ellipse - D16 is in the white sleeving), which modify the signal to the + input of IC2D. These components are only shown in the circuit from mods.dk, and are all soldered point-to-point on the solder side of the PCB, as shown in the attached image. (Note that C25, which is soldered between pins 7 and 10 of IC1 on the solder side, has been removed in this image.)
I am now fairly sure that the fault in my D-3800 is IC2D (LM339). It's output (pin 13) is low even though its IN+ (pin12) is greater than IN- (pin 10). This is shutting down the LM723. As indicated in an earlier post, lifting one end of D12 restores full output voltage.
Capacitors are a good place to start of course. If your fault is in the IC2D area, the only relevant capacitor is C26. Connecting across C26 is a convenient place for monitoring IN+ and IN- of IC2D. I tried removing this capacitor, but it made no difference; not surprising in view of the fact that I always find the differential voltage across IN+ and IN- to be healthy (i.e. positive).
By the way, my fault mode is not always full shutdown. When set for 14 V, the supply output sometimes wanders around in the vicinity of 7 V. Full output is restored when I lift one end of D12, and then I find a wandering voltage of about 7 V on the output of IC2D (again with IN+ greater than IN-). That also seems like a fairly solid indication that IC2 is faulty in my unit.
I will replace IC2 (LM339) next and let you know what happens.
Best wishes,
Lew.
D-3800_IC1_IC2_Pin_Voltages.pdf (Size: 82.35 KB / Downloads: 783)