Sunday, January 24, 2021

New Toy: HP 8656B Signal Generator


It was time to upgrade my repair bench with a new piece of equipment. I bought this HP 8656B (w/ Opt. 001) to replace my HP 8640 and Heathkit SG-8 for customer repairs. This is a very nice signal generator and easier to use than my older models. I placed it under my spectrum analyzer since both units are used to adjust radios for optimum performance.

If you have a shortwave radio or ham radio that you would like repaired, then please send me a message.






Vizio M50-C1 Smart TV Repair


One of the other projects over the holiday's I managed to fix is my Vizio M50-C1 smart TV. This TV is one of the early 4K models introduced in 2015. The TV has been great and is on for 15 hours a day on average. It was used for playing games and watching shows/movies.

One day, it stopped working and wouldn't power on. My immediate thoughts were bad capacitors in the power supply since I've repaired several flat screen TVs with similar issues. 

China doesn't offer schematics for this model so troubleshooting had to be performed the traditional way. The Vizio M series TVs from this era all used a similar, modular board design where each major function was contained to a separate board. For example, there is a power supply board, a scaler board (GPU, sound processor, and inputs), inverter board and T-Conn board.

Fortunately, the power supply was fine after checking all voltages and signalling. I noticed the scaler board had a bi-color LED and would turn purple when a power on sequence occured and turn red when the TV was off.

I figured that the board must have a fault, but wanted to rule out the inverter and T-Conn boards. I disconnected the inverter board from the scaler board and applied ground to a SCR (Silicon-controlled Rectifier) input on the board. I was able to see the backlight turn on, but no picture. At least the backlight and inverter board were working to some degree.

I disconnected the T-Conn board and ran some tests. It seemed to be working fine and would only affect how the picture was displayed on the screen. If it were faulty, then I would see only part of the screen or solid colors.

I removed the board and saw this when I flipped it over. Nasty flux corrosion on the board! Talk about subpar garbage coming out of the Chinese factory.

 I moved on to the scaler board as the culprit and headed over to the Badcaps forum for assistance. I quickly learned that this model along with the others from 43" up to 75" had similar issues. The main cause is cracked solder joints on the BGA chips under the heatsink.This occurs over time from power cycling the TV causing expansion/contraction from hot to cold. I attribute this to poor engineering, poor design, and shoddy assembly more than typical wear and tear.

My options were to either buy a replacement scaler board, send the board out for repair, reball the BGA's with a rework station myself, or try an experimental heat gun reflow.

I looked around for a replacement scaler board and came up empty handed. Unless I wanted to spend upwards of $200 for a board (1/3 cost of the TV). I opted to try the heat gun reflow approach.

The design of this board and massive heatsink made it very challenging to perform a heatgun reflow.


I ended up using a temperature probe and Harbor Freight heat gun to perform the reflow. I preheated the board from the back side and brought it up to 230 F over several minutes. Then I increased the temperature to about 400 F and heated the board evenly paying special attention to the BGA chip area. I kept the board at this temperature for about 15 minutes to reflow the solder balls, then let it slowly cool down to about 120 F. 

I let the board sit for 1/2 hour and place it back in the TV. It powered up and worked! The TV has about 80 hours of "TV time" without any issues.

Please note this is most likely a temporary fix and not a replacement for a proper reball of the BGA chips. I added some 40mm brushless fans to the heatsink and cut holes on the TV cover to reduce the 200+ F temps down to 160 F while using the TV.

If you have similar issues, please head over to the Badcaps forum for assistance. You can also watch this video for performing a heatgun reflow yourself. If you want your board professionally repaired, then I suggest Nick's TV service.



Tektronix 465B 100MHz Oscilloscope Repair



I previously mentioned that my old analog scope needed repair. I use this 465B on a regular basis to diagnose and repair electronics and was disheartened to see it failing. All radio projects came to a halt as I had to bring the trusty 465B into the operating room for revival.



The scope was operating normally until I was using it to diagnose a TV board. The front panel lights would turn off and back on at random times. The CRT would also blank out when this occurred. Turning the scope off and on would sometimes remedy the situation for 5 to 20 minutes until it completely died.


I removed the covers and measured voltages at the test points indicated in the manual. All voltages were correct +/- 1% except for the 5 volt rail. My initial thoughts were the old rectifier capacitors were shorted. I tested the capacitor in circuit and it wasn't shorted so I moved on to the bridge rectifier (CR1551). I measured 55 volts DC as expected.

Next, I moved on to Q1556 (MPS-U45). This transistor appeared to have a solder blob shorting the base and collector pins. No idea how that happened unless another component got hot? I removed the transistor and tested with my LCR meter. It tested as resistor. I ordered a new transistor and installed it. Tested for 5V and still no voltage present.



I removed one leg of R1559 to see if there was a short to ground from Q1556 or Q1558. Nothing appeared to be shorted.

I measured voltages of the pins on the U1554A IC chip (MC1458 Dual Op Amp) and noticed pin 3 had no voltage. It should be 5 volts. Pin 8 was fine at 15 volts and pin 4 was fine at -8 volts. Pin 1 measured at less than 1 volt (it should be approx 6.7 volts).



The MC1458 Op amp was internally shorted. I ordered a replacement at Mouser p/n 595-MC1458P and installed it. The scope came to life and is working great!





Icom 728 PLL-VCO Repair



Hello Radio Friends.

I had an opportunity to pick up an Icom IC-728 transceiver over the holidays. I used to own the this model years ago before trading up for a Kenwood TS-2000. The 728 was a fun and simple to use radio and got me deeper into enjoying HF communications.

Initially, I offered to repair this radio for a fellow ham, but he decided to sell it to me at a great price instead (I guess his Icom 7600 meets his needs, haha!).

The 728 is a basic HF transceiver covering all HF bands/modes and general receive with up to 100 watts output. It utilizes a triple conversion receiver and selectable band pass filtering. This radio does not include a built-in antenna tuner and most of the functions are accessible on the front panel buttons to simplify the deep menu driven learning curve of modern gear. The IC-728 came to the market in 1992.

I met the local ham who said this radio was owned by a silent key and he obtained it in its current condition. Everything worked as it should except no TX/RX on 8MHz to 15 MHz. No receive or transmit on 20-30 meters. The ham suspected faulty BPF circuitry.

 I brought the radio home and started to look it over. Examining the schematic, I noticed the filter board is embedded deep inside the rig. Suspecting a bad coil or relay; I removed the top and bottom covers and dug in. I checked the relays and voltages to each filter stage and everything was in spec.


 I worked my way backwards and checked the VCO section. Bingo! This radio splits the VCO into 4 sub circuits which activate their respective band pass and low pass filters.

I checked the voltages on each VCO and found #2 out of spec at 4 volts. I adjusted trimmer cap C46 and brought it back to 7 volts. The radio started working again between 8 and 14 MHz.

Reading a bit on these rigs, I discovered that the trimmer caps are notorious for going bad or out of spec over time. The proper way to fix this issue is to replace the capacitors with new ceramic ones.

I ordered some caps from Digikey and replaced all four of them. Here are the part numbers and values in case you run into the same issue.

VCO1 0.5-7.99999  C35 Trimmer Cap #4610001130  CVSSA1001  10pf
VCO2 8.0-14.99999 C46 Trimmer Cap #4610001130  CVSSA1001  10pf
VCO3 15.0-21.99999 C59 Trimmer Cap #4610001000  CVSSA0701  7pf
VCO4 22.0-30.0 C70 Trimmer Cap #4610001000  CVSSA0701  7pf