Proscan PS27123 (CTC169CG5) - need proper value of R3U

[William Buchholz]

The resistor R3U in the primary of the PS is burnt out, can only make out
the first 1 (?) and 5% tolerance marks. Measures open circuit when
removed. Any help is appreciated. Thanks

 

[Edd Whatley]

On the RCA CTC169, et al RCA chassis, they use R(4)xxx resistiv
designator assignments on their schematics, with the 4 being omitted o
the chassis PCB screened markings .So, I'm definitely not seeing an
R3U assigned as a valid designator on that line side raw DC powe
supply's resistive parts list . Never- no- somehow, upon visua
examination of an old "parts" retiree '69 chassis, now I do see tha
R3U marking, all off by itself, just to the side of the SMPS ferrit
cored power transformer T(4)102, on its side towards the PWM controlle
IC, U(4)101. Otherwise, there are four 1 watt film resistors used i
that SMPs's closed area. Two are mounted in the X-Y plane of the PC
and two are set askew a bit. Two being in blue bodied casings and tw
in pale green. Using your "given" initial digit 1 visual, the only tw
of the 4, that might fall in that category, would be R(4)149 which i
a 1.2 ohm 1 watt unit. It supplies the run voltage derived from th
SMPS's secondary (after initial start up) to the discrete totem pol
driver xstrs for the chopper xstr, and also, the full time run voltag
for IC, U 4101's Vcc supply.
How- some- ever, I can't see its failure (an overload opening) excep
in the case of C(4)118 a 470 ufd/35VDC being bad, or the crunching o
its derived supply rectifier diode CR(4)101 that comes off term #13 o
chopper power transformer T(4)102.
The second of the four resistors that would have a primary 1 digi
would be the blue "askew" resistor that is floated just above th
aforementioned resistor. It is the chopper transistor (Q(4)101'
(mounted on the adjacent small heat sink) emitter current limitin
resistor. It is a mere .18 ohm resistance of a 1W metal film type. Fo
further ID info , you might check the individual end terminatons of th
mystery res and pass back the info as to R/C/L parts designators tha
are also commonly connected to its two end junctures.

73's de Ed

 

[William Buchholz]

Here is a very crude digital photo of the circuit board foil side. The
resistor is marked in blue, the red arrow is pointing to the "R3U"
designation.

http://members.shaw.ca/wmbuchholz/ctc169cg5_r3u_detail.JPG

One side of R3U connects to pin 10 of T102 and + of C7K. The other end
connects to both R126U and R110U I think.

The set came in completely dead, no blown fuse. I get a raw B+ when
plugged in but listening carefully there is no faint hissing that shows
the PS is running in standby. Q101, U101, C108, C104, C102, C118 and
CR101 were all replaced from a previous repair (not mine, date unknown).
Q101 and CR101 test ok out of circuit. All the big resistors besides R3U
test OK. The only bad part I can find on the primary side is this R3U
which measures as open circuit. Either from the actual failure or just
normal wear, the paint has all fallen off. If you can be so kind as to
measure the resistor in you chassis and post it here maybe I can make some
progress on this beast. Thank-you for your help so far.

 

[William Buchholz]

Here is a very crude digital photo of the circuit board foil side. The
resistor is marked in blue, the red arrow is pointing to the "R3U"
designation.

http://members.shaw.ca/wmbuchholz/ctc169cg5_r3u_detail.JPG

One side of R3U connects to pin 10 of T102 and + of C7K. The other end
connects to both R126U and R110U I think.

The set came in completely dead, no blown fuse. I get a raw B+ when
plugged in but listening carefully there is no faint hissing that shows
the PS is running in standby. Q101, U101, C108, C104, C102, C118 and
CR101 were all replaced from a previous repair (not mine, date unknown).
Q101 and CR101 test ok out of circuit. All the big resistors
besides R3U test OK. The only bad part I can find on the primary side is
this R3U which measures as open circuit. Either from the actual failure or
just normal wear, the paint has all fallen off. If you can be so kind as
to measure the resistor in you chassis and post it here maybe I can make
some progress on this beast. Thank-you for your help so far.

 

[[email protected]]

http://www.eserviceinfo.com/index.php?what=search2&searchstring=proscan

Download the link on that page, run the exe to extract, then the exe
for the 169 chassis.
This should give enough information.

R3 is 33K and is part of the start up.
Do a google search and check the capacitors that are commonly listed
with your esr meter, c4108,4102,4104, etc.

David

 

[William Buchholz]

To update: I replaced the resistor with a 33k ohm 2W. I plugged it in
and heard the faint hiss of the standby power. I press the on button and
the set powered up for an instant. Then there was a loud crack and a
burning smell and the set was dead again. I then noticed I forgot to
connect the CRT grounding wire. I reconnected it and tried again, however
the discharge must have blown something as the set is still completly dead.

 

[kip]

Best call in a Pro ,when a resistor goes open
usually something has caused it .
Rebuild that SMPS section and disconnect the HVB
whilst testing .Also look for coolant leaking.

kip

 

[Edd Whatley]

Looks a little late on my up grade info but:
"OK with your further info on the resistor and the provided photo as t
its interconnect. This time pulling out and consulting the fich
schematic instead of just the bare chassis visual, RCA designates tha
resistor as being R(4)003, so if the 4 is left off and th
insignificant 00's left off, that would be R3 . That along with a hal
dozen other components in the front raw DC input stage.
That unit must have been quite hot to have opened and discolored tha
initial first orange band to look as a brown and left the center tw
unreadable. If you just figure a worst case of a full 175VDC being on
end and the other end shorting to ground that would only account i
little less than a watt being dissipated in that resistor unit....wit
a 2 watt rating on the unit..and it opening up? On a 169, in doing a
initial topside visual of components, I don't remember ever seein
that 33k in anything but a pristine new look with no signs of heatin
stress or discloration. Had that unit been plugged in and had a dea
short on its other side for weeks/months.....possibly, as there i
power thru it as long as the AC line loop is complete.
And yes, RCA specifies it as being a 33K met film unit at 2 watt pw
rating. As you now may be able to see by the schematic reference
graciously provided to you by David.
That resistor is the bridge to initially power the SMPS IC and it
internal totem pole drivers until the power supply kicks in an
supplements the Vcc supply thru the term 13-14 of the SMPS transforme
and its CR(4)101 diode . Also looking at all of the tie points of tha
resistor, the very lowest impedances to ground would initially be tha
C(4)178 470 ufd/35 V electrolitic if dead shorted. Following that woul
be the path to ground through R(4)149 a 1.2 ohm 1 w res thru a shorte
CR(4)101 diode direct to ground. After that, the series resistive path
encountered would even further isolate the loading down...e.g. ..lik
the 22 ohms isolation provided if the SMPS IC should be shorted, etc.
This chassis typically will fail in two modes, like a hard crash wit
the failure of the SMPS chopper transistor or the SMPS IC in which cas
parts would be crunched and also take the line fuse.
An equally common failure mode ...typically with time...and this I
and old chassis... is the gradual
onset of high ESR in C(4)102 at the left side of the IC which filter
the voltage sense line for the standby voltage function of the IC
Adjacent to that cap is C(4)104 which filters the current sense line t
the shut down function of the IC. Off towards the heat sinks interna
center side is C4108 that is coupling the drive signal into the choppe
transistors base and filtering shot noise from the zener diode that i
shunts. And lastly, you know about the power supply filter, C(4)178 .
The last scenario of changing out of the caps may result in
restoration of normal chassis operation with no prime power component
crunched, since those electrolytics do take a continual hammering.
Keep us posted on progress. "

Had you changed all of the aforementioned electrolytics, and is th
power
supply even trying to come up now or did some of the power component
crunch.And is the hot component /smoke traceable ? And ,naturally
disregard the coolant leakage , as a 27123 DV set no havee.
73's de Ed