Mid-Week from ‘3rd Line’

I’ve now dismantled the CHARGE test rig so I can get some space back in my office. Only one task remaining and that is our spare CHARGE controller has a defective AVX capacitor somewhere on the PCB. The controller works fine but across the +5V the resistance is 37 ohms which we can live with on a spare for now, should be around 150 ohms. I’ve tried a new technique but yet to prove it, using this temperature gun.

The gun works fine so in principle a low resistance AVX will get warm/hot so I should be able to detect the faulty capacitor. Problem is that some adjacent TTL chips all get hot under normal operating conditions so could mask a faulty capacitor. One day I’ll revisit this controller and see if I can detect the problem before unsoldering each AVX in turn until I strike lucky.

Harrison X150 Amplifier (Boost Blast):
As mentioned last Saturday I have removed the installed X150 Amp as it looked decidedly tatty (the LCP must be looking pretty good if I’m doing this sort of thing). Here is a photos collection for you to refer to.

Harrison X150 and X300 Top Cover
X150 Front Panel
X150 Internal
X300 Front Panel (original)
X300 Internal (original)
X300 Front Panel (modified)
X300 Internal (modified)

The case of the spare X150 has previously been opened but I assume it is
serviceable so will take it to Cosford on Saturday and await we will give it a run. We also have two X300 amplifiers which could be used in an emergency when fixed as they have U/S labels on them. Also, both X150 and X300 amplifiers regularly appear on eBay.

One X300 is interesting as the original innards have been stripped out and two ‘new’ ILP audio amplifier modules (one for each channel) have been fitted; it has unacceptable hum while the other has one channel U/S. I can test once I
get some test leads organised.

We do not have component-level schematic diagrams for these amplifiers yet, only the functional schematics on the lids; perhaps someone out there can locate copies for us.

Pete H.

One thought on “Mid-Week from ‘3rd Line’”

  1. Mick Shore
    Wychwood Ash Lane
    Stone Staffs
    ST15 0NJ


    Hi there

    Just read one of your Bloodhound posts and my experience may be of interest. In the first days of the LCP at Newton we had problems with one pair of Argos 200 logic boxes, they were rather unreliable. We found that the 6 volt supply was anywhere between 4.2 and 4.6 volts we checked the connections back to the power supply no obvious problem. There was a separate fused 6 volt supply to each pair of Argos boxes. At the one in question 6 volt one side of the fuse 4.4 the other ??? I was a High voltage valve man so never considered current. One of the smart arsed instructors pointed out high current may have an affect. So I checked the Fuse perfect, it read short on an Avo then, ‘click’ does not need much at 6 volts. So I got a Wheatstone bridge from test equipment and the fuse read 1 (ONE) Ohm “That’s OK” Then big light , 1 ohm 1.5 volts only needs 1.5 amps change fuse no more problems.

    On a similar vein. When Working supporting the US Army in Germany we had responsibility for their Air Traffic Control Equipment in my case the Voice switching. Each base had a Nortel FSC92 switch Each switch had 3 24volt power supplies capable of 25 amps and the equipment normally ran on about 12 amps. Each power supply had a 30 amp output fuse and their carriers were starting to disintegrate. We originally thought it was faulty material and replaced the whole fuse carrier. After about a year they deteriorated again . Our test rig normally only only ran during our working hours so we ran a continuous test and checked temperature, the temperature slowly rose by about 15 deg C over 3-4 weeks We then noticed that the 30 amp fuses end caps did not have the shiny nickle look of others. We eventually found some aircraft fuses that looked better and used them. No heating and no more problem. We sent some fuses off for testing and the answer was they had some strange coating on them which produced a thermal resistance effect the warmer they became the higher the contact resistance and at 12 amps it does not take much resistance to get 100s of watts of heating which decomposed the composit material of the fuse holder.

    This switch was ZX80 controlled and built of boards of about the same age and construction as your ‘new’ LCP boards. Theses were supposed to be repaired by the manufacturer but pipeline times to the states were making life difficult for the User. They were getting non back. I was asked if we could repair some of them and find the problem State side. We tested some and found that most of the faults revolved around 2 of the chips and some Electrolytic capacitors. The chips connected to the highways and any supply interruption (miss handling) blew one or both. On replacement we fitted DIL sockets so next time it was a quick change job. Then we noticed it was always the same 3-4 capacitors (they had a red end) that were failing and on a 6-7 year life cycle. The standard US military replacement was a much better capacitor . Fault finding and retest was taking 5-6 hours so as soon as a card came in we changed all the red ended capacitors and cut turn around time to about an hour and almost eliminated further failures..
    The problem In CONUS (Continental United States) was the repairable units were routed via an Air force unit in California The Army unit located there disbanded but their address was still valid So delivery was made and that’s where they remained. Eventually return was arranged and we received 174 sub-assemblies on behalf of out customer, we actually managed to repair all of them.

    T86 Roof The T86 at Newton started to leak over the transmitter Chips Rafferty an I were told to find out why and fix it. We removed the Ram to find the alloy roof looked like a lace curtain underneath. Rain Water had become trapped under the ram and the copper mesh on the RAM with the alloy roof and slightly sulfur acidic water formed batteries which ate away the roof. We re-skinned that part of the roof treated with chromite etching primer (dangerous) glued a thin sheet of rubber?? to the underside of the RAM and re-attached the ram to the roof.

    Mick Shore of the OLD LCP


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