Sanyo AVM-20F5

SuntharAbout 4 min

Sanyo AVM-20F5 CRT RGB mod

Is it possible to restore a CRT that went through a poor RGB mod? Let's find out. This tutorial covers the RGB mod for Sanyo AVM-20F5 (Flat 20", Sanyo CRT). These instructions should also work for the below CRTs.

Sanyo AVM-20F5

Given that this CRT TV solely features one composite input at the front, two composite inputs, one s-video input, and an RF input at the back, it's certainly worthwhile to modify it for RGB input. Although a few adjustments are required, the modification process employs the standard mux mod.

Backstory of the Sanyo CRT

Someone who purchased two RGB modded CRT tvs from me wanted their Sanyo CRT tv looked at. Upon initial inspection there was a heavy red tint. When we tried to get into the service menu, suddenly the CRT stopped working. It went into HV protection mode. Tube and the chassis were in great condition. This Sanyo CRT tv had undergone prior modifications by an individual, but the RGB mod was executed inaccurately. It was a bit challenging to take the back cover off, since everything was glued in place with no easy way to disconnect. Upon further inspection the problem was found to be poor RGB modding, incorrect resistor values and shorting of some pins. While the simplicity of just soldering the resistors directly to the SCART connector can be admired, sometimes it can cause more headache than what it's worth. I highly recommend you order the PCB with quick disconnect, so that you can easily service the CRT when needed.

CRT safety

Caution

You can die doing this! So read carefully! CRT TV is not a toy. Do not open a CRT TV. If you don't have any prior knowledge about handling high voltage devices, this guide is not for you. CRT TV contains high enough voltage (20,000+ V) and current to be deadly, even when it is turned off.

READ THIS BEFORE CONTINUING

Plan of attack

READ THIS TO PREPARE

Theory

Sometimes it is nice to know the theory behind the mod. I have put this on a separate page. This shows how the various resistor values are calculated.

READ THE RGB MODDING THEORY

Specs

Sanyo AVM-20F5

Manufactured: Assembled in the USA
NTSC, 60 Hz
Chassis:
Tube: LG Philips A51QDJ279X
Chroma: Toshiba TB1231CN
OSD: KEC KMPA8701CMN-3B14

Service manuals

Couldn't find one for this. Please contact me if you do find one.

Calculating the RGB external resistor value

Love it when we can put theories to use! Formula from our theory page!

We find out using ohms law, this CRT is using ~0.7Vp-p. How?

Vp-p = 5V x (680/(4700+680)) = 5V x 0.1264 = 0.63V

RGB external resistor value
= (0.7 x (4700 + 75) - (75 x 5)) / (5 - 0.7)
= (0.7 x 4775 - 375) / 4.3
= 2967 / 4.3
= 690Ω

680Ω resistors should do the job. One thing to keep in mind is that the external resistor value calculated is typically very close to the resistor value removed.

Performing the mod

Now that you roughly know what needs to be done, prepare for the mod. Place the board on a comfortable place. Make sure you are not putting pressure on the flyback or other components.

STEP 1: Remove the following components

Remove the following components. RGB resistors to the ground. Measure twice and mark before you remove.

R007 (680Ω)
R008 (680Ω)
R009 (680Ω)

STEP 2: Blanking diode

To prevent current from flowing back into the OSD and causing interferences in RGB mods, it's necessary to include a diode in the blanking line. The absence of this diode is the main cause of such issues.

Replace jumper J035 with a diode 1N4148. Pay attention to the direction of the diode.

STEP 3: Connect RGB, Blanking

Refer to the picture below to connect the RGB and blanking wires. Be sure to solder them to the leg of the resistor that is facing away from the OSD chip.

STEP 4: Sync and Audio

Sync and Audio can be connected to the Video input.

STEP 5: Build your mux board

Below mod uses the RGB mux board. This is optional, but will make your mod easier and stable. You can also create the circuit presented in the schematics above without the board. Please also checkout the precalculated resistor values.

Buy your RGB mux board

On Sanyo CRT Chassis	AVM-20F5
CRT RGB inline resistor	4.7kΩ
CRT RGB ground resistors removed	680Ω
CRT RGB inline cap value	0.1uF
Add blanking diode on chassis (Replace J035)	Yes

RGB mux board	AVM-20F5
Use RGB inline diodes on CRT chassis?	No
Mux board Audio LR (R7, R8)	1kΩ
Mux board RGB termination (R1, R2, R3)	75Ω
Mux board RGB inline resistors (R4, R5, R6)	680Ω
Mux board blanking diode (R9)	1N4148
Mux board blanking ground resistor (R10)	1.0kΩ

Buy your RGB mux board

Although the calculations were helpful, a dedicated tuner board was utilized to identify the optimal combination of RGB inline and termination resistance required to achieve the ideal color balance. After testing, it was discovered that a 220Ω termination provided the finest RGB image.