Retevis RT-3 Battery Eliminator Problems

 In the car, I enjoy using my hand held radios to listen to DMR and FM over local repeaters or via a pi-star hotspot that I have in the car. I use a battery eliminator instead of a battery, so that I always have full voltage at the hand held and removing the need to charge batteries.

These clip on to the back of your hand held in place of the battery pack. This one worked for years until it recently started to power the radio for the first 60s after plugging it in and then the voltage would drop out, come back, drop out, etc. repeatedly every second or so. Very annoying.

These are normally sealed units, designed to be thrown away after they stop working. However, with some force, they can be broken open along the plastic welded joint that holds the front half to the back half.

As we can see, the layout is made up of a buck converter to reduce the voltage from 12-24V down to the 8V that the radio requires.

The caps buffer and smooth the voltage as it enters into the radio. If the caps go bad then they can not buffer the 8V long enough for the next duty cycle of the buck converter and this is what was resulting in voltage coming and going. The replacement caps were fitted.

After repair, the battery eliminator worked perfectly. A perfectly smooth and constant 8V supply voltage again. 

Yaesu FT-7900 with distorted audio on transmit

 This FT-7900 was brought in with a complaint of very distorted audio on transmit.

A check on the Rhode & Schwarz CMT-55 showed that the audio was terribly distorted but that the 1750 Hz tone to open a repeater was very clean with no distortion at all. This was a clue as to where the problem might lay. A look at the circuit diagram showed the position where both signals come together and enter the microphone amplifier circuit Q1040 on pin 6.

Unfortunately, all those components are on the underside of the main board. So the radio was opened and the main board removed.

The signal was recorded at the Jumper J1005 as being 7.2Vpp, however it was immediately attenuated at Q1042, which is the MIC MUTE transistor. This is fed from the CPU and when PTT is pressed, the TP1028 drops from 4.8V to zero. However, despite the mute signal being correct at the transistor, the MIC signal was still being sent to ground. I removed Q1042 using a hot air gun and the signal came back up to normal.

Once the radio was re-assembled and tested, it had perfect modulation on all bands.

I will not order a replacement, as this DIP package is way too small for me to solder. The OM will try the radio and see what side effects it has. On my test set, I could not find any. Let's see...

Another happy customer!

Albrecht AE 5890 EU - Background Hum on Transmit

 This radio is also interesting to Ham Radio enthusiasts as it can also be used on the 10m band with 10W AM, 30 W FM and 30 W SSB. The EU at the end denotes that it meets all the European CB Radio standards.

This one came in with the owner complaining of an occasional background Hum on Transmit. The first thing to do was to  reproduce the problem being reported. At first, the radio seemed just fine but after turning on some nearby noise sources in the shack, I could hear them being picked up by the microphone cable. If I wrapped my hand around the microphone cable about half way down and then reached out towards some nasty LED lights, that I keep in my shack to reproduce ambient noise, I could hear the hum clearly being amplified.

At first, I suspected that the microphone shielding had perhaps come off, so I checked that first. However, shielding was intact but the noise was being amplified like crazy, as if no shielding were present at all.

A quick look at the circuit diagram, suggested the smoothing caps C419 and C425 might be toast:

Also, following the mic connector upward, there is another smoothing cap C20 at the microphone input to IC10:

The old caps were removed, and tested. Indeed all were leaky.

New caps were fitted and the radio retested. The hum was gone!

Looking at the other caps in this radio, I imagine it will not be long before a full recap is needed.