Radioshack 22-306 frequency counter teardown and repair - Page 1
Forum section: electronics / repair

author: muvideo
author_rank: Frequent Contributor
timestamp: October 19, 2012, 02:09:00 pm
content: Hello, recently I was playing with some cheap videotransmitters I had around to spot the working ones.I needed a fast way to see if there was RF out of the TX,and to check the channel frequency, so I remembered thatI had this:it's a radioshack 22-306, a friend of mine gave it to mebecause it was not workig, the reason is easy to spot,it sat unused for long and the batteries made a mess inside:fortunately the corrosion was not that bad:here is the front side of the pcb, there is the micro, thatis marked TANDY TCR8001, I havent found any info aboutit, probably it's an asic but if anyone knows more it welcome.There is the main cristal that feeds the micro and also is usedas timebase, the trimmer capacitor is used to trim the frequency.On the top left of the image there is the prescaler, an MB506 fromfujitsu.This is the other side of the pcb, nothing interesting here, only theLCD driver IC:The unit was repaired before, here is a mod made to the backlighton the side of the LCD:Seem that the 50 Ohm input was overloaded, the termination resistorswere badly burned:The corrosion is starting to eat under the LCD driver:I desoldered the switches and the lcd driver, after washed thepcb with water and degreaser, and with TCE to eliminate glueand flux residues. The backlight is two leds in series connectedto 5V, so I modded the connection to have the leds connectedin parallel and mounted white smd ones.Soldered everything back together and it is working again,fortunately the problem was only battery leackage.First try with a RC transmitter, FM 35,2 MHz nominal:This is the result with a video transmitter:Now some accuracy test, feeding with Rb oscillator:The Rb ampitude is too low to test counter's lowfrequency range, so I used the 1992's OCXO output,the unit seem still under original specs of 1ppm:I touched a little the trim cap to read the correct frequency,naturally the quartz used is a normal unit that isvery temperature sensitive, so the last digits will floataround with small temperature variations:The counter is useful for spotting RF on the field, itsreasonably sensitive, is able to show up to 1.3GHzmaximum, it would be very useful it it was able to go to 2.5Ghz.The prescaled datasheet tells that the MB506 is ableto go up to 2.4GHz and prescaler's pin 2 is tied to Vcc,so it should be possible to double the dividing factor.Another option would be to insert an external x10 divideron MB506 output to make easier spotting the frequency.I tried to check the crystal temperature stability:heated it with the hot air gun set to 100°C while readingthe 10Mz reference. The reading at first raised upfrom 10,000,000 to 10,000,037 and then startedgoing down rapidly under 9,999,980.So the temperature dependency is not bad.I didnt measure precisely the temp but it seem thatsomewhere in 30 to 80°C interval there is a flat tempcozone, so it would be possible to try to stabilize thequartz temperature around 50-60°C. But this willkill the batteries, so I dont know if it will be usefulFabio.

author: muvideo
author_rank: Frequent Contributor
timestamp: October 19, 2012, 03:19:05 pm
content: Quote from: muvideo on October 19, 2012, 02:09:00 pmRereading myself, I'm making a mistake here:really I dont know how high the firmware can counton this device, probably it will count higher than1300MHz, the main problem being input sensivity.In the case the sensivity will be enough and thefw will limit the maximum frequency the prescalercan be modded. Infact MB506 has two pinsthat control the prescaling factor: pin 3 and 6.In the counter the pins are both tied to Vcc thatmeans 1/64, this explains also the 64mS/640mSsmapling time.I tried connecting pin 6 to GND to obtain 1/128and the unit shows half the frequency.Now I dont have any CW 2.4GHz source, the trasmittersI have emit in small bursts so I cannot tryhow high the counter can go.Fabio.

author: saturation
author_rank: Super Contributor
timestamp: October 19, 2012, 09:19:52 pm
content: Nice work! Thanks for the teardown and repair stories. Love the 1992, they don't make counters like that anymore, did you get your working fully well or did it need any work to get it going? I use a similar handheld, a BKPrecision.

author: muvideo
author_rank: Frequent Contributor
timestamp: October 19, 2012, 10:03:40 pm
content: Hello Saturation, glad you liked it.After replacing the switches Racal Dana workslike new. I keep it on standby so theheater is always powered. From february,apart power outages, I've never shut it down.Sometime I check it with the rubidium oscillators,and every time, after powering up, in few hours itstabilizes within the same thousands of hertz of last check.Maybe the interface is a little crude (setting upPWM measurement is a little tricky) but I likethe front panel full of buttons and digitsFabio.

author: Lightages
author_rank: Unknown
timestamp: October 20, 2012, 05:19:18 am
content: I have one of those! I have no idea if it is still accurate or not. Damn, another calibration source to buy!