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Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders. |
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10th Nov 2006, 4:04 pm | #1 |
Dekatron
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 5,997
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Tektronix 454a trigger problem.
The symptom is the trigger only works with a very large input signal. Having gone through the circuit (from BAMA) I think I know what's going on.
The characteristics of the tunnel diode have drifted and due to the way the differential amplifier works, it only generates sufficient current to trip the tunnel diode if the level control is set high. I suspect I am not the first person with this problem and wondered if there was a recommended circuit change as I suspect the ageing process might have occured in any NOS items too?...Peter |
10th Nov 2006, 5:08 pm | #2 |
Dekatron
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 5,997
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Re: Tektronix 454a expert needed!
Mmm...bit more testing and the common emitter 1.5K 5% reads 1.65K. I have paralleled a 10K to make it 1.4K and it now works perfectly. I still suspect there has been drift in the tunnel diode characteristics.
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10th Nov 2006, 5:58 pm | #3 |
Retired Dormant Member
Join Date: Dec 2003
Location: North London, UK.
Posts: 6,168
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Re: Tektronix 454a expert needed!
Might be worth asking in the Tektronix forum on yahoo groups.
http://tech.groups.yahoo.com/group/TekScopes/ |
10th Nov 2006, 8:08 pm | #4 |
Retired Dormant Member
Join Date: Mar 2003
Location: Bishop's Waltham, Hants, UK.
Posts: 939
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Re: Tektronix 454a expert needed!
Hi Peter,
tunnel diode currents are quite critical, so it could just be the resistor. When they work, tunnel diode triggers are excellent, and will easily trigger at the highest speed of the scope, and often well beyond it (I have heard of people looking for fast glitches just by setting the scope to a low sweep speed, and single shot - when the glitch comes, it triggers the scope, and at least you can tell if it is coincident with other problems). The 454 is an excellent little scope, and should serve you well. The Tekscopes group is a valuable source of information, and it is worth subscribing for a lot of the non-model specific help that comes along (including hints on replacing tunnel diodes). Jim. |
10th Nov 2006, 11:25 pm | #5 |
Dekatron
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 5,997
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Re: Tektronix 454a expert needed!
Thanks all, looking at the circuit, there is not much allowance for tolerances so resistance drift or incorrect PSU settings could be enough. The A trigger works well now but the B sweep is triggering all by itself which looks like it is probably down to bias on a tunnel diode again.
The A trigger is fantastically stable. Using the delay you can magnify an edge way down the trace and it stays as solid as a rock. Frankly, I might never use the B trigger but I hate it when things don't work right...Peter |
11th Nov 2006, 12:03 am | #6 |
Retired Dormant Member
Join Date: Mar 2003
Location: Bishop's Waltham, Hants, UK.
Posts: 939
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Re: Tektronix 454a expert needed!
In all old Tek scopes, it is always best to check ALL the PSU voltages first (they are usually interlinked....). Assuming the PSU is correct, then it is worthwhile running through the calibration section of the manual, as this will show you what is running and what isn't.
In the later scopes (400 series, TM500 series, 7000 series), the Tantalum caps are a common source of trouble, though they more often fail short than open. Jim. |
12th Nov 2006, 12:36 am | #7 |
Dekatron
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 5,997
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Re: Tektronix 454a expert needed!
Hi Jim, PSU voltages came out as -11.98/11.98 and 74.9 which is more accurate than my DVM is supposed to be! I have posted on the Tek forum and it appears I am probably doing the right thing adjusting the resistance values to compensate for drift so I have done similar to the B sweep and now have the scope fully working.
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