I have had the opportunity to work on a TJ4A 4 Band HF Transceiver from a local ham. These are kits from a maker called Youkits and I am impressed with some details and not impressed with others. The problems with this rig began with a failing Push To Talk condition that stopped the keyer from making the rig TX. Later, the mic PTT failed as well and the rig would go into full transmit when turned on. As well the mic audio would then come out the speaker. After that it stopped turning on altogether with no power in any stage of the radio.
It was some time to figure out multiple failures and I had to start at the beginning of the DC voltage stage. Below is the main board that handles power from various sources. In my case I have only one power input and that is from my home power supply, at my operating station an Astron RS-35A station power supply of 13.5 volts dc. This power connects to CN2 on this board. CN1 and CN3 are empty. Two things failed on this board, Diode Q1 and Transistor Q1. I think that the cause of this was from the other problems occurring in the radio and this was a final result.
So now that I had power corrected I could continue to trouble shoot why the PTT for the mic failed and then the Switching to the Keyer failed.
There is a part of the circuit on the main board that manages the different types of Push To Talk commands. This circuit is managed by three transistors called Q25 Q26 and Q27. They are located on the left frond edge of the main board. This is the control center for one of the main Transmit and Receive relays when the unit powers up and when the unit goes into transmit. In this initial situation the keyer began to malfunction and would not make a transmit key down. But when one pushed the PTT switch on the mic, then the keyer would work. The keyer chip is on the lower right side of the main board and at first I thought that it had failed in some way but that was not the case. I then ran into the problem of the radio going into full transmit when turned on. No receive was happening and the power transistors were getting hot. So I then realized that the T/R relay was stuck on. This relay is the small one on the lower left at the front of the main board. I decided to remove this and to do that I used solder wick and this did a good job at removing solder enough that I could get the relay out. I downloaded the tech sheet on this relay and learned what pins to test for operation and found that nothing worked. It was no good, so I ordered two new ones from eBay and they came in a week. I tested the new one and it worked like a charm, so then this new relay was installed.
There is a capacitor that resides next to the relay and this cap controls some delay times in the keyer. Relay is in the lower right side of this image. After the relay replacement I tested the three transistors near by and they were also US as well as a diode that resides beside this relay. All were removed, the board was cleaned and then two PNP and one NPN plus a new diode were installed. The new parts were not subs but exact numbered components.
Transistors shown on the upper left side of this image. This board was installed along with the corrected DC power board and bingo, the radio started up correctly and the Mic PTT as well as the keyer switching worked properly. So I finished a minor alignment of the modulator as per manual and then returned the radio to testing service. So far the corrections are holding up properly and I can work the Mic and keyer with no problems.
This radio has a good receiver in it and signals are managed well with this radios AGC. All that I have left to do is see if I can add a mic amp to the rig as this radio does need some gain or a very hot mic to get some good talk power.
I use a Turner Super Sidekick amped mic and the mic gain is cranked to the max for normal voice response. Perhaps this radio is better suited to a handheld electret microphone. I will have to see if I can sort one out for a test. The other issue that I miss is an internal speaker but I am reluctant to install one at this time.