[TechAssist] Re: RCA TV F20601SE CTC175A won't regulate 140V B+ [FINAL UPDATE]
- From: Gary McCartney <gary@xxxxxxxxxxx>
- To: techassist@xxxxxxxxxxxxx
- Date: Thu, 21 Jul 2005 17:44:52 -0400
I have this set running ok now. Problem is, I don't have any idea why it
is ok.
I had another set in the back room which I thought was a CTC177 so I was
ignoring it. Then one day I figured I'd try the jungle IC out of it in
this problem set. Opened it up to discover it was also a CTC175. This
was great as it allowed me to compare voltages and resistances, etc.
First I tried the jungle IC from the good set but there was no change in
the fault. I still had the set on the variac and verified that raising
the AC past a certain point, that the B+ was going over the 140V and it
kept going until it hit 146 where I backed it off. Did some comparisons
with the good set and noticed that all of a sudden the problem set was
regulating. B+ was much lower than before. Cranked it up to 125V ac and
it still regulated at 141.6V.
I really don't like when this sort of thing happens because there is the
fear of it going haywire again. I did some more solder connections and
verified that resistances and voltages matched the good set (all very
close by the way). Also tapped around for anything loose. Have been test
running it for 2 days now.
Side notes to this set: After resoldering the tuner there were no
channels below channel 51- just snow on the screen. Turned out that I
had soldered C7319, a 0.01uF surface mount, and it shorted out. This
stopped the B+ from going to the VHF section of U7301.
Also, the remote control function did not work. Scope showed 5V p-p
waveform at pin 3 of micro chip but still no functions. Checked that all
front panel switches were ok so as to not load down the micro. All ok.
Then I realized I had changed the 4 Mhz crystal by the micro IC earlier
in the repair, so I put the original crystal back in. Bingo, the remote
worked again. Maybe the original is not 4 Mhz as the manual claims, or
the new crystal was off frequency. Will post these fixes separate.
Gary
My previous message:
Thanks for the message Dan, but I believe the CTC177 is diffrent than
this. My chassis has a Darlington power transistor as the regulator output.
My standby is 153V but I have the AC connected through a variac so the
source is low, maybe 105 V ac or so.
I have double checked everything now, parts values, continuity of
traces, etc. and it is still not working. Q4153 is being driven too hard
from the jungle IC, that is all I can think of. Still, when the scene
changes on the screen, the jungle IC pin 29 outputs varying voltages to
correspond to the scene brightness, so it appears the jungle is doing
it's job. The 12V run supply to R4169 is bang on 12V. The base is at
0.688V but the collector is at 0.044V where it should be higher, like
around 0.7V according to the print. When I manually feed in a voltage
going to the base, I need to get around 1.6V on the collector before the
set will run at 140V B+. with the ac input at 120V. Of course I have
replaced the transistor but with exact same results.
Something isn't adding up but can not figure it out. Even replaced
filter caps C4153 and C4705 and added another cap across the main filter
C4007.
I'm thinking right now of either getting a new junlge LA7610 or giving
up on it. Don't know what else to try.
Oh by the way, I reinstalled the original eeprom and it seems to be
working. Parameter # 18 will lower the B+ by 2V only.
Gary McCartney
McCartney Electronics
Guelph Ontario Canada
Est. 1984
email: gary (at) number63.ca
Webster's TV wrote:
> Gary,
>
> Is your standby B+ voltage OK? Should be around 149V, if this is high, I
> found a tip I had written down for this problem on a CTC177, that says to
> change C4123 and R4125. Don't know if this will be of any help to you or
> not, but thought I'd mention it anyway.
>
> Dan Webster
> Webster's TV
> Shullsburg, Wi.
> ----- Original Message -----
> From: "Gary McCartney" <gary@xxxxxxxxxxx>
> To: <techassist@xxxxxxxxxxxxx>
> Sent: Thursday, July 07, 2005 5:34 PM
> Subject: [TechAssist] Re: RCA TV F20601SE CTC175A won't regulate 140V B+
>
>
>
>>Wayne, thanks for your help.
>>
>>I still can't pin-point the culprit, although I am now suspecting the
>>micro. Q4153 is being driven too hard. If I replace R4162 (100 ohm) with
>>a 47k, the set will regulate at 132V B+ because this forces Q4153 to be
>>driven less, allowing Q4151 base to conduct.
>>
>>I have replaced all the transistors involved- Q4150 with a TIP122 which
>>is what the number stamped on the original is, Q4151 with an NTE287 or
>>MPSA42, and I now have a 215495 from a scrap set- different area of
>>circuit- installed for Q4153. All act exactly as the originals did in
>>the circuit.
>>
>>The micro, pin 30 is exactly 3.7V as the schematic indicates, but pin 29
>>is high, ( 4.58V with B+ on variac at 130V, although goes down to 4.1V
>>as B+ raised to 139V.) which connects to Q4153 base, causing it to be
>>driven too hard. I've checked all the resistors in the circuit.
>>
>>Has the micro ever been known to cause this problem?
>>
>>Thanks,
>>
>>Gary
>>
>>
>>
>>
>>Gary McCartney
>>
>>McCartney Electronics
>>Guelph Ontario Canada
>>Est. 1984
>>email: gary (at) number63.ca
>>
>>
>>
>>
>>
>>
>>
>>
>>Wayne Vanaman wrote:
>>
>>>Monitor the collector voltages of q4153 and q4154. The voltages should
>>>increase and decrease inversely as the B+ goes up or down. If not
>>>suspect a bad q4153/4. If it does look at q4150.
>>>
>>>R4169 passes the 12v run supply to the collector of q4153
>>>
>>>q4153 215495- smd, high failure
>>>q4151 146851
>>>q4150 217309
>>>
>>>The jungle outputs a regulator voltage. The path into the micro is the
>>>141v supply by way of R4157 and r4163, in series, with r4171 and r4158
>>>in parallel to ground at the r4163/r4157 junction. You also have 39k
>>>r4127 between U1001p29 and p30.
>>>
>>>Wayne Vanaman
>>>Omega Electronics
>>>6904 W Fairfield Dr
>>>Pensacola, Fl. 32506-3310
>>>
>>>Wayne@xxxxxxxxxxxxxxxxxxxxx
>>>WWW.PensacolaTVRepair.Com
>>>
>>>850-456-5995 voice
>>>850-458-6369 fax
>>>
>>>
>>>Gary McCartney wrote:
>>>
>>>
>>>>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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- » [TechAssist] Re: RCA TV F20601SE CTC175A won't regulate 140V B+ [FINAL UPDATE]
