ctc203 B+ too high

Jan 12, 2006 4 Replies

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sofie 20 years ago

CTC203J2 RCA Hospital Model #J20541

Intermittent 3 strikes symptom but "sometimes" starts up, turns on and displays great pic and good sound but B+ seems to regulate about 10 - 15 volts too high.... about 154 vdc. I found that if I reduce the B+ going to pin #2 of flyback by about 10 volts with a 27 ohm resistor that it powers on just fine, every time, so I am hoping that if I can reduce the regulated B+ to a lower correct voltage that all will be OK and will work fine as it should without the voltage dropping resistor. I checked values of resistors and electrolytics ESR all around SMPS primary (changed R14108, 0.1 ohm, 3W) and secondary without finding any significant deviations from the norm. Resoldering around Flyback, HOT, L4401, HDr xfmr, etc had been done and has been redone.

How do I reduce the B+ ..... adjust? component change?

TIA

Best Regards, Daniel Sofie Electronics Supply & Repair - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

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Jason D. 20 years ago

Sofie,

Check on secondary side, these SMPS use feedback regulation full time, TL431 is the TO-92 size via 2 1% (green bodies) resistors and regulates SMPS circuit via optisolator.

Like mine, I had one run high, was in that secondary side feedback circuit.

ATC113 uses same SMPS, horizontal and vertical designs as CTC203.

Cheers, Wizard

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dkuhajda 20 years ago

As I recall some of the early ctc203 SMPS had a goldenrod service bulletin on the transformer pads lifting and the ultra small circuit traces going open.

SMPS not regulating or regulating high, always check the feedback circuit components.

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electricitym 20 years ago

Wizard: I am having a similar problem like the one that Sofie described. I replaced both optos and the TL431 and still the B+ seems high..... it appears to be regulating OK... I can vary in AC line input from 90 to

130 volts and the B+ stays within a few volts of 156, with or without the flyback load. All other parts that I could identify in the secondary feedback circuitry... diodes, electrolytics, etc test OK.

Did you find a faulty component or did you adjust slightly the balance of R1 / R2 ? How did you get the B+ lowered? ..... what is the correct B+ voltage?

David: .... the solder joints and the circuit traces are in perfect condition, but I resoldered them anyway and checked the traces with a meter and with a magnafying glass.

Thanks for your inputs. electricitym . .

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John-Del 20 years ago

Dan, make sure the flyback wasn't replaced with a generic one at some point. Excessive hv is normal for this. I usually add a 472 2KV disc across the timing cap to reduce the HV when a generic is used.

John

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