Tecsun PL-660 SSB BFO alignment

Alternate title: fixing the 1 kHz offset

Like a select few others I enjoy listening to shortwave radio at night when there’s a chance of picking up exotic stations, my only SW radio right now is the Tecsun PL-660, a very capable and affordable SW radio receiver.

Unfortunately in my copy, and I have heard reports from others about this as well: the SSB mode is offset by around 1 kHz. This is obvious when you switch from AM to SSB and immediately hear a loud 1 kHz carrier tone. Another way of telling is to tune 10 MHz in the Americas and 9996 kHz SSB in Europe. 9996 kHz is a Russian time-sync station which transmits in CW, meaning it outputs a carrier each second at exactly 9996 kHz. If you tune that frequency in USB or LSB mode, it should output a very low frequency beat or nothing at all. In the 660 it may output around 1 kHz beeps. This is wrong, for reasons that are beyond this post to explain. Properly tuned, you’d tune 9995 USB and hear a tone, or 9997 LSB and hear the exact same frequency tone.

This is only a SSB problem, so I’m assuming the AM tuning is correct (mine seemed to be), AM tuning accuracy is less important anyway.

To tune this without using expensive and specialized equipment, you can use a CW time station like 9996 or 4996 kHz, the important thing is it needs to be a known frequency CW station. If you have a signal generator this can also be used.

Basically, we’ll open the radio up and re-tune the SSB BFO frequency, then adjust the SSB centering, to check that we’re on frequency, you need to find a PC or mobile app with a spectrum analyzer that can tell you the frequency of the audio output from the radio. The reasons for this are clear when you know how SSB demodulation works.

First, open the radio, there are a few screws on the back and inside the battery compartment, the antenna can stay in place. The front cover with the wires for the speaker will come loose fairly easily. Just fold this to the side, you’ll need to hear the speaker output for tuning.

The adjustment points are under the LCD+button assembly, there are several screws holding this in place, and you’ll need to remove the tuning knob (just pull it off). Freeing the tuning encoder took some bending of the chassis for me, simply fold the whole assembly over. The adjustment points in the picture are now visible. Leave points 1, 5 and 6 alone unless you know what you’re doing. They don’t affect the SSB tuning.

I did this by tuning my reference CW station directly, switching to USB and adjusting point 2, center frequency adjust until the pitch of the tone got as close to 0 Hz as possible. That’s called zero beating, and is a way of getting very close to spot on frequency with basic equipment. After that, go 1 kHz down still in SSB, using the spectrum analyzer software check the frequency. If it’s not 1 kHz, adjust point 3 until it’s right on 1 kHz. To confirm tuning, go 2 kHz up (i.e. reference freq. +1) and switch to LSB. The audio frequency should be the same. You may need to go back and forth a few times to get this spot on. I didn’t adjust point 4 at all, but it seems to do something similar to point 3.

As a final confirmation, tune an AM station in AM mode, make sure you’re on frequency (they’re almost always on multiples of 5 kHz), switching to SSB you shouldn’t hear any carrier whine. If you’re slightly off you’ll hear a strong humming in either LSB or USB mode, and you may want to consider redoing the adjustment. Obviously the BFO adjust wheel needs to be centered before doing any adjustment!!

To further check you may tune true SSB stations, I like to use 5505 kHz USB, which is an aeronautical information station. You should get natural pitch without adjusting the BFO wheel. Don’t use amateur stations to check unless you know the person transmitting has a calibrated rig because many stations are uncalibrated and may be several hundred Hz off.

Source for the adjustment image: http://www.radioscanner.ru/forum/topic42851-26.html

Hints: I like to use SignalScope on iOS for this, it has a nice high res FFT and can put markers on signals of interest which is great for a proper tuning. VisualAnalyzer is a freeware Windows software that also works well.

Tooling is very basic, a small Philips screwdriver and a small flathead is all you need. If you have a piece of semi-professional test gear like a real frequency counter they often have a 10 MHz output port on the back, if you connect a piece of wire to this it can be tuned on the Tecsun and you can use that as a reference which will give a nice strong reference signal. The FE-5680A rubidium reference is another great choice if you want true accuracy, but be warned: the Tecsun is unlikely to stay in perfect cal over temperature or time, it seems fairly stable to me, but if you start measuring this sort of thing you’ll need to know when to call it good enough.