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The TF2008 was the last word in clever circuitry before the advent of digital technology and the design is extremely clever, both electronically and mechanically... but it's pretty fearsome when it comes to fixing faults. I've had a couple of problems with my example and the latest was loss of the highest frequency range which is 360 to 510 MHz. Checking the manual suggested the problem was in Unit A24, the doubler unit which generates output frequencies of 360MHz to 504MHz given an input of 180MHz to 252MHz from adjacent Unit 25. Access to the RF Units is quite straightforward when you figure out how it's done (at the bottom of this page are lots of pictures) After removing the outer case you will see a black metal box which sits on an RF gasket and totally seals the RF Units. First, hinge down the power supply panel after detaching a bracket and two screws then detach the metal screening box under which you will see there are three layers of metal cased units which are numbered sequentially left to right and vertically. Removing the two screws at the end of the layers of units allows one to lift up the top and middle layer, so that you can remove the lid of the unit requiring attention. Inside each unit is a small circuit board resting on ridges at the sides of the case. After removing four securing screws the circuit board can be lifted out to a distance limited by the cables connected to it. As I decided to remove the circuit board in Unit 24 for bench testing the cables needed to be unsoldered. There are three at one end; coax input and two pink power supply leads and an output coax cable at the opposite end. Lifting away the board revealed a VHF/UHF circuit with several coils and a small daughter board carrying a pair of BFY90 transistors, a couple of potentiometers and sundry small components. It was fairly easy to check decoupling capacitors... none was short-circuit, and the two transistors.. both seemed OK. Resistance checks showed resistors were intact, but something was odd. The two transistors are balanced by a 100ohm pot but there was no resistance path from the BFY90 emitters to the negative power feed. For technical reasons the chassis of the TF2008 is connected to the DC positive power feed with negative feeds live so the BFY90 emitters should have been connected to one of the two pink wires carrying negative power supply feeds. The fault was in the balancing circuit which ensures the two transistors operate as doublers. The wiper of the potentiometer was open circuit. Usually twiddling will sort out a bad pot but even switch cleaner failed to reliably get it back in business so I dug around and found a suitable replacement which I fitted with some difficulty as there wasn't much room, and parts have changed mechanically somewhat over the last 30 odd years (mine is dated 1980) Fitting the circuit board and testing before screwing everything back together showed all was now well and the top frequency range was working again. During fault diagnosis and subsequent testing I used my spectrum analyser. Much more revealing than any other instrument. In fact so revealing I discovered something rather odd. There are two output connectors on the TF2008. One is connected to the attenuator and the second, which provides a fixed amplitude output, is for connection to an external frequency meter. The spectrum analyser showed the latter output was rich in harmonics. Maybe the output is getting squared off somewhere but I was able to see the third harmonic of 440MHz at 1320MHz at quite decent amplitude. The story doesn't end here alas. Refitting the black metal screening box demanded some juggling and I inadvertently trapped a coax cable in the RF gasket. A fact that remained unknown until I tried the equipment once it was back in place. Read on. One of the shortcomings of the TF2008 is the fact that one cannot easily see if it's switched on. True, there are meters for carrier and modulation levels, but the way the designers intended is a red background to the on/off knob which is revealed when the mains switch is turned on. I decided to fit a lamp. This was not too easy as nearly every square centimetre of the front panel is already occupied, but after puzzling a bit and peering behind the panel I spotted between the two meters a small hole drilled in the metalwork. The plastic front panel didn't have a corresponding hole so the designers presumably decided not to bother fitting anything. Maybe they originally intended to fit a carrier level potentiometer (now accessed on one of two power regulator boards buried inside the case) or maybe an LED? An LED seemed a good idea so I drilled a small hole in the plastic front matching up with the hole in the metal panel and fitted a green LED in a screwed holder with a chrome bezel. I ran a pair of wires from the LED to the power supply panel which holds the mains transformer, rectifiers and a couple of large electrolytics etc. The capacitor on the right had about 30 volts present across its terminals so I fitted a 750 ohm wirewound resistor to provide about 40mA which makes the LED glow brightly. Final testing showed the new green LED to work nicely but as I mentioned above... no RF output. Absence of RF is indicated by the carrier level meter showing maximum reading and as the carrier level meter was the closest thing to the new LED, I thought initially that this was connected with the problem. Could I have shorted something with the soldering iron perhaps? I investigated further and found three separate units (A18-A28-A32) could be involved as the carrier meter feed passes through three different areas. In one box I discovered a BC108 with a short-circuit emitter. Removing the transistor, with some difficulty, showed it was perfect. I had forgotten to set a front panel switch back to manual from "Wide" (where it places a short across the meter input). After refitting Unit A18 I peered at the innards of the equipment, looking for inspiration I suppose, and suddenly noticed one of the interconnecting micro-coax cables linking the units together was squashed in the RF gasket. After prising it free the carrier level meter read mid-scale and RF output appeared. The cable seemed to have recovered so I didn't replace it. Putting everything back together again proved the original fault had been fixed and a nice green lamp showed that power was switched on. At this point I decided to investigate the unexpected presence of a 1300MHz signal at the RF output reserved for connecting an external frequency counter and sure enough I found the sine wave was distorted with the top being decidedly flatter than the bottom. As I switched from band to band I noticed the carrier level meter dropped about 5mm on some ranges. This turned out to reflect a drop in amplitude of the output signal from 40mV to 7mV peak to peak for odd numbered ranges and range 2. Even ranges except for 2 were OK. Back to the fault-finding chapter, but not today! |
