Repairing an Electrohome G05 19" Black and White Vector Monitor
By, William Stephens
(clicking any picture in this article will bring up an enlarged version)
We have an Asteroids in for clean-up and resale... the cabinet and control panel are beautiful. The electronics was real shakey. After the shop took the job as far as they could it came to me at the bench for a few details.
The main thing was that the monitor had half a horizontal line on it, no! originally it could not be operated without F600 blowing. First thing I did was check the power transistors for a short, and I found one at Q609, a 2N3792. I replaced the transistor and put in a new fuse, powered it up and now the half a horizontal line problem showed up, ah... progress.
Next, I decided to replace all the power output transistors (Q608, Q609, Q708 and Q709) because sometimes they read good statically but under dynamic load, they fail. After this, I had a complete horizontal line on the screen, better than half a line but not anywhere near good enough. It appears that the vertical amplifier has more extensive damage. I started off by checking D608 and interestingly enough it crumbled to pieces (see picture 2). These things are black anyway and it's hard to tell by visually examining them if they are burnt or not, in this case, it was. I replaced this diode, powered up again and no change!
I continued backwards through the amplifier checking transistors, diodes, capacitors and resistors using the horizontal channel parts as a comparison.(for every 600 [Y axis] series part identifier there is an exact copy of that part in the 700 [X axis] series part identifiers). I found that the big white power resistor R620 was open circuit (see picture 3). This is a 2 ohm 10 watt resistor which is not common, fortunately I had a parts chassis available so I removed the resistor and swapped it out. Picture 4 actually shows the burn on the part but this is not easy to see in real life, let alone this picture. Now when I powered up I got a double feature... first no picture at all and second, smoke rising from the resistor I just put in. I killed power immediately and checked the resistor. Fortunately it was still the same value and didn't look burned, but it was hot!!! hundreds of degrees, you would burn yourself like touching a soldering iron.
Obviously something further is wrong with the amp that's smoking that resistor, got to be a collector-emitter short somewhere near. Further up the circuit I found that Q605 (see picture 4), a little-bitty 2N3609 indeed, had a C-E short. I replaced that and upon power up I got the test pattern in full horizontal and vertical deflection, looking good. Further, no more smoking resistor, looks like this one's fixed!
I ran it for about 1/2 a day to burn in anything that needed burning in and it performed perfectly, I would have to call this one, fixed.
The saga continues...