Dynatron Tourist TP14 problem - UK Vintage Radio Repair and Restoration Discussion Forum

Vintage_Man2012 · 25th May 2020, 12:07 pm

Hi everyone.

This bank holiday I’ve decided to try and resurrect an old Dynatron Tourist.

The set is essentially dead, apart from a pop from the speaker when switched on, I’ve 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

I’ve checked the battery and it’s a little over 9V so the supply is fine - can anyone suggest any reasons for the low voltages?

Thanks
Paul

Station X · 25th May 2020, 1:04 pm

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?

Vintage_Man2012 · 25th May 2020, 1:38 pm

Graham,
Thanks for your reply - I’ve 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. I’ve fitted another OC78 and the set is working!

Thanks again
Paul

Station X · 25th May 2020, 1:51 pm

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

25th May 2020, 12:07 pm	#1
Vintage_Man2012 Tetrode Join Date: Apr 2015 Location: Chester, Cheshire, UK. Posts: 62	Dynatron Tourist TP14 problem Hi everyone. This bank holiday I’ve decided to try and resurrect an old Dynatron Tourist. The set is essentially dead, apart from a pop from the speaker when switched on, I’ve 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 I’ve checked the battery and it’s a little over 9V so the supply is fine - can anyone suggest any reasons for the low voltages? Thanks Paul

25th May 2020, 1:04 pm	#2
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,289	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? __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

25th May 2020, 1:38 pm	#3
Vintage_Man2012 Tetrode Join Date: Apr 2015 Location: Chester, Cheshire, UK. Posts: 62	Re: Dynatron Tourist TP14 problem Graham, Thanks for your reply - I’ve 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. I’ve fitted another OC78 and the set is working! Thanks again Paul

25th May 2020, 1:51 pm	#4
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,289	Re: Dynatron Tourist TP14 problem Wow! Only three posts to fix the fault. That must be a record. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.