New addition to the shack

I just adopted a new pet – a fairly priced piece of QRP Japanese engineering to my shack. My hopes is that it will keep me outside (in the fields) more!

 

Thanks to UPS for prompt delivery and to Ham Radio Outlet store in NH for (as usual) outstanding customer service.

73!

Chinese QRP VHF-FM radio restoration – continued

The replacement transistors BG1, BG2 (3AA9, 3AA7) are obsolete and next to impossible to find as it’s been 40 years since the radio was built. However, given the known voltage/power/frequency parameters and a friendly help from experienced radio-engineer ham, a reasonable replacement was identified: 2SA1930 from digikey.

Later, I found more data online at http://www.114ic.com/

Model: 3AA7	Model: 3AA9
Manufacturer: China semiconductor companies Production of materials: Ge-PNP Nature: RF / high-frequency amplifier (HF), power amplifier (L) Package: in-line package Limiting Voltage: 35V Maximum current allowed values​​: 0.5A Maximum operating frequency:> 120MHZ Pins: 2 Beta: 30 Maximum power dissipation: 5W Possible substitution: B772	Manufacturer: China semiconductor companies Production of materials: Ge-PNP Nature: RF / high-frequency amplifier (HF), power amplifier (L) Package: in-line package Limiting Voltage: 25V Maximum current allowed values​​: 0.5A Maximum operating frequency:> 120MHZ Pins: 2 Beta: 30 Maximum power dissipation: 5W Possible substitution: B772

The parameters were exactly as predicted. After installing new transistors, the radio came back to life. However, predictable the output power is too low – less than 100mW.

The bias/feedback seems to be wrong as the bias voltage is different for silicon (0.6V to 0.7V for silicon vs 0.2V to 0.3V for germanium). Also, the operating frequency (50MHz) is too close to the 200MHz limit which also limits the gain.

It may require readjusting the circuit / voltage dividers to ensure proper bias and gain.

While googling the radio, I also found some decent translation of the operating manual: 884_manual_translation

As the radio works on 45-50.1 MHz, it only slightly touches the lowest CW portion of the amateur 6m band. Re-tuning it to 50-55 MHz may not really worth the effort so I may put it on eBay. Would be nice to fix the finals though first.

The full schematics are here:

 

 

Of importance of RBN and radials.. and not so of SWR.

I worked on my vertical antenna today installing a 4:1 unun transformer I had removed for the winter. Without the unun, the vertical worked perfectly on 40m (only) and miserably on any other band. Now, with the unun, it is supposed to work fairly decent on all 40 – 10m bands. In theory.

In reality, the results were quite the opposite.  After few CQ calls no responses and no spots on RBN on neither band… On top of that, high RF in the shack – full scale deflection on field strength meter…. while SWR on my antenna tuner is 1:1.

So back to the antenna.

To make story short, the culprit was a wire going to the radials which has disconnected itself from the unun. The unun was okay, but, this way, the vertical became an end-fed antenna with no radials…

After re-tightening the wire terminal on the unun (and re-adjusting the tuner to 1:1 SWR) the station is now heard as expected:

 

But still not much on 20m even though the band is supposed to be open – just 2 spots:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Chinese VHF 6m backpack 884 radio – bad finals (updated)

Had some time today to test the radio. The receiver works – it’s quite loud. It was surprising to find 2-3 very loud music broadcasters on 45-50MHz band. Apparently, there is no volume (AF gain) control on the radio – I even checked the schematics but no, there is no one.

I guess that’s considered normal for army radios?

Everything seems to work fine except there is no RF output! After closer inspection, I noticed that the RF finals transistors are missing:

 

According to the schematic, the RF amplifier uses 2 bipolar germanium PNP transistors: 3AA9 and 3AA7 in TO-3 package: