Hi, all, my first post here. I have an old Telequipment Oscilloscope which has served me well down the years. Have had to do the odd repair now and then, including a nice bodge to replace a shorted capacitor in the EHT multiplier unit.
Anyway, the model number has disappeared off the front of the scope, looking around online it looks similar to the D67 model, though I think it may be a higher model than that. Anyway it uses the same input circuitry, which is where my fault lies.
Channel 1 has decided to die on me (actually it's been like this for a while). Since there are 2 identical input channels I have managed to find the fault by swapping over. The problem is the input FET (s) is an odd dual device (matched pair in one package). Diode comparisons vs the good device from the other channel are similar, however there is a 15K ohm leakage between 2 pins on one side of the faulty device, I believe it is the S-D connection. My problem is trying to get data or information on this FET to locate a suitable substitute (hopefully from my scrap parts collection) The device is marked TFD1551. Looking at the circuit it appears it is a matched pair of N Channel FET in one package (J-FET probably?)
I'm not totally concerned with perfect calibration or anything, can anyone suggest a suitable alternative (or bodge?) If I can find some info on this FET I can probably substitute 2 single devices in it's place. I will also check the protection circuit just in case it failed due to overload, though I think it just wouldn't work one day I came to use it.
Any advice anyone, or maybe someone has one of these laying around somewhere?
Anyway, the model number has disappeared off the front of the scope, looking around online it looks similar to the D67 model, though I think it may be a higher model than that. Anyway it uses the same input circuitry, which is where my fault lies.
Channel 1 has decided to die on me (actually it's been like this for a while). Since there are 2 identical input channels I have managed to find the fault by swapping over. The problem is the input FET (s) is an odd dual device (matched pair in one package). Diode comparisons vs the good device from the other channel are similar, however there is a 15K ohm leakage between 2 pins on one side of the faulty device, I believe it is the S-D connection. My problem is trying to get data or information on this FET to locate a suitable substitute (hopefully from my scrap parts collection) The device is marked TFD1551. Looking at the circuit it appears it is a matched pair of N Channel FET in one package (J-FET probably?)
I'm not totally concerned with perfect calibration or anything, can anyone suggest a suitable alternative (or bodge?) If I can find some info on this FET I can probably substitute 2 single devices in it's place. I will also check the protection circuit just in case it failed due to overload, though I think it just wouldn't work one day I came to use it.
Any advice anyone, or maybe someone has one of these laying around somewhere?
sorry for not getting back sooner.
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