[TechAssist] RCA TV F20601SE CTC175A won't regulate 140V B+
- From: Gary McCartney <gary@xxxxxxxxxxx>
- To: techassist@xxxxxxxxxxxxx
- Date: Wed, 06 Jul 2005 17:36:51 -0400
RCA TV F20601SE CTC175A won't regulate 140V B+
This set came in dead, with F4150 open clean (not black looking) and 12V
regulator outputting 6.2V only. Replaced these and eeprom plus power
tact switch. Now set will fire up when run on variac at 95V AC.
I can't figure out why the power supply won't regulate at 140V. It will
go higher than 140V if I crank up the variac toward 120V ac..
Have done the following:
-Replaced regulator output Q4150 with TIP122
-replaced Q4151 with MPSA42 (NTE287 exact)
-replaced C4150 10uF 200V & C4155 2.2uF 200V
-replaced Q4153 surface mount #215495 with MPSA06 surface mount
-Measured R4175, R4155, R4150, R4153, R4154, R4152, R4169, all OK.
-Measured:
-12V reg out-ok, 4.8V on eeprom pins 6, 8 -ok,
-U1001 pin 30= 3.7V -ok
-U1001 pin 29 should be 3.7V -measures 5.2V but as viac is raised, the
5.2V comes down close to 3.7V it should be
-Q4153 B= 0.688 V, E=0V, C= 0.044V. C on SAMS schematic show it should
be 0.7 but I can't see why if the base is high, why C should read much
more than zero. My measurement agrees with me, not the schematic. Wonder
if mis-print or my mis understanding? Replaced this transistor with same
results.
-Q4151 This transistor is not conducting because output of Q4153 is low.
B= .044V, E= 0V, C= 138V or whatever I have variac set at.
- Q4150 reg out: depends where I have variac set but tracks the voltages
proportionally ok. No change since installing new part.
-Q4151 This transistor is not conducting because output of Q4153 is low.
Only other thing I can think of is to check the main B+ filter cap.
Any other help appeciated.
Gary
Gary McCartney
McCartney Electronics
Guelph Ontario Canada
Est. 1984
email: gary (at) number63.ca
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