RCA TV model 20541, chassis CTC203J2 with Excessive Reg B+

On Tue, 07 Oct 2008 14:11:55 GMT, snipped-for-privacy@verizon.net (Brad) put finger to keyboard and composed:

There is this old thread in the Google Groups archives:

It suggests that the fault may be in the feedback circuit on the secondary side.

There is also the following manual at eserviceinfo.com.

I don't know how relevant the above manual is to your chassis, but I notice that the manual does not specify a value for B+, which suggests to me that it may vary between versions. Anyway, it looks to me from the values of R14115 (143K), R14116 (2K), and R14126 (37.4K) that the B+ in standby mode should be about 190V. In run mode the B+ is about

If you are seeing a B+ voltage which is about 10V too high in run mode, then I'd check the +13Vr and +16Vs supply rails. Have you checked C14101 (2.2uF)?

- Franc Zabkar

Hi Franc,

C14101 in this chassis is a .82uf non-electroylitic type of capacitor which tested good.

I checked 13Vr source components (good), but not 16V yet.

I downloaded CTC203 *. RAR (part1) from

If I repair this TV, I will let everyone know what caused this problem.

Brad

On Wed, 08 Oct 2008 16:26:00 GMT, snipped-for-privacy@verizon.net (Brad) put finger to keyboard and composed:

I managed to unpack the archive with Winrar, but you need to download all 6 parts first.

- Franc Zabkar

Hi Franc,

I downloaded all parts and now I have the manual (THANKS).

I have to work on other TVs now. I will go back to that one later.

ERROR in Model Number:

The model number is J20541 (I forgot the "J").

Brad

Hi, UPDATE:

I didn't locate the source of the problem (see below) after testing many parts, but I used a "patch", after the owner gave permission, to restore the proper B+, which I assume is +130V (HV now around 25KV). Note: I do not have SM.

"Patch" to Restore Reg B+ to 130V:

Thanks to Franc Zabkar for the training manual source (see below). It has partial schematics.

I found a few others on the web that had the same problem, one had exactly the same "+154" volts, however no one posted a solution. One person said he had that problem and he said the problem lies on the secondary side of the transformer in the feedback circuit, but he didn't say what part(s) he replaced (if he fixed it?). Of course, this is what I suspected in the beginning, and I checked many parts in the feedback for the regulator, also the regulator reference circuit, and many parts around the regulator on the primary side. I checked +13V and +16V sources, which were about a volt+ high until I installed the "patch".

Next, I would have to replace semi-conductors, although I tested them in circuit, to see if one of them is the problem, but there is a limit to how much time, and the customer is willing to pay me to fix this 20" TV. I already spent too much time on this repair project.

Brad

Response from Franc regarding the source for the training manual:

On Mon, 13 Oct 2008 20:13:23 GMT, snipped-for-privacy@verizon.net (Brad) put finger to keyboard and composed:

I would have measured the values of the three resistors (R14115, R14116, R14126) and computed the expected B+ voltages in run and standby modes. The junction of the potential divider would be at 2.50V in all modes, and R14126 would be switched out of circuit in run mode.

AFAICS, a clue as to what is happening would be in whether or not the B+ rail drops when switching from standby to run mode. If not, then one or the other of the +16Vs or +13Vr transistors could be faulty.

It seems to me that if one of the components is marginal, then your kluge may come back to bite you if that component deteriorates further.

- Franc Zabkar