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Old 25th May 2020, 12:07 pm   #1
Vintage_Man2012
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Location: Chester, Cheshire, UK.
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Default Dynatron Tourist TP14 problem

Hi everyone.

This bank holiday Iíve decided to try and resurrect an old Dynatron Tourist.

The set is essentially dead, apart from a pop from the speaker when switched on, Iíve replaced the capacitors to no avail but when I probed the transistor voltages they are way off the values given in the service sheet.

The voltages are (in the order of Emitter, Collector, Base respectively):
VT1: 0.5, 4.3, 0.6
VT2: 0.3, 4.1, 0.4
VT3: 0.5, 4.0, 0.7
VT4: 0.7, 4.7, 0.8
VT5: 1.3, 1.8, 1.3
VT6: 1.3, 4.7, 0.7

Iíve checked the battery and itís a little over 9V so the supply is fine - can anyone suggest any reasons for the low voltages?

Thanks
Paul
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Old 25th May 2020, 1:04 pm   #2
Station X
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Default Re: Dynatron Tourist problem

Given that VT5 and VT6 collectors are connected to the negative rail via the output transformer primary winding something would appear to be seriously wrong. Only about 0.1V should be dropped in the windings.

Are you measuring voltages wrt the positive rail?
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Old 25th May 2020, 1:38 pm   #3
Vintage_Man2012
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Default Re: Dynatron Tourist TP14 problem

Graham,
Thanks for your reply - Iíve just diagnosed the problem:

One of the output transistors was reading as a depletion mode N-channel MOSFET so was obviously causing a catastrophic problem with the circuit. Iíve fitted another OC78 and the set is working!

Thanks again
Paul
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Old 25th May 2020, 1:51 pm   #4
Station X
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Default Re: Dynatron Tourist TP14 problem

Wow! Only three posts to fix the fault. That must be a record.
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