Thorn 1591 line output transistor issue

[high_vacuum_house]

Good evening,
I am looking at the second Thorn 1591 television which looks in better condition than the first.

At the moment the line output stage is currently not working. The line oscillator is running at the correct speed and the line driver is working. The line output transistor was drawing quite a lot of current and getting warm at a low HT voltage so I removed it before it damaged the LOPT.

Someone has been here before as the line output transistor was an RS badged AU113 which is a germanium TO3 PNP transistor.

The links on the PCB indicate that it is set for a silicon output transistor not germanium for a start. No mention in the service manual on these links or their configuration. No information either of the device part numbers other then its a TVT19 which isn't very helpful!!.

There is more confusion which is that the secondary of the line driver transformer according to the manual is connected between the base and the emitter of the output transistor then to pin 2 of the LOPT.

In the configuration in my set the secondary of the line driver transformer goes between the base of the line output transformer and the 0V line along with the collector of the output transistor. There is also a resistor strapped in parallel with the secondary winding but not on the circuit diagram or printed on the PCB. When the HT is running at 11.5V I get 4+bit volts peak to peak on the secondary with relation to the 0V line. Obviously as one end of the secondary is tied to 0V there is no DC bias on the output transistor base connection.

Does anyone know what is going on here. With the current configuration of the line driver transformer, should a silicon NPN be put in its place. What would be a good transistor to fit in this position.

I was trying to download the line output stage circuit diagram to prevent any confusion but this is playing up tonight.
Many thanks,
Christopher Capener

[FERNSEH]

Final production 1590 series sets made about 1976/77 were fitted with a silicon BD160 line output transistor.
Info about the BD160: https://www.radiomuseum.org/tubes/tube_bd160.html
The later production PCBs have provision for PNP or NPN line output transistors.
Initial production sets used the germanium AU113 with no provision for an NPN transistor.
The connection of the secondary of the driver transformer will have to be rewired in order that a silicon transistor can be used.
The PNP AU113 collector is connected to 0V and the emitter goes to the line output primary, the secondary of the driver transformer is connected to the base of the AU113 and LOPTx primary.
When the silicon NPN transistor is fitted the emitter goes to 0V and the collector goes to the LOPTx. The secondary of the driver transformer goes to 0V and the base of the BD160.

DFWB.

[Al (astral highway)]

Hello, Christopher,

I've got a pulled Phillips BUX52 which is epitaxial silicon planar.

Here's the data sheet...http://pdf.datasheetcatalog.com/data...-LAB/BUX82.pdf

Yours for postage if you like.

[high_vacuum_house]

Many thanks for your help. The PCB is set up for an NPN silicon transistor so I have just bought 2 BD160 transistors from Langrex for £1.69 for 2 including postage which is very reasonable!!

Hopefully this should get the LOP stage working and get some life out of it!!

Quite why someone fitted a PNP germanium transistor in its place never know why. Hopefully the LOPT hasn't been damaged by high DC current as the primary winding resistance is only 0.1 ohms

Christopher Capener

[FERNSEH]

I hope the attached sketch clarifies the situation. It has to be remembered that the line output transistor functions a as switch and is driven by a square waveform. Conduction takes place on the latter part of the scan so the PNP (AU113) transistor is turned on by the negative part of the waveform and the positive part when a NPN (BD160) transistor is used. This means that the secondary winding connection of the driver transformer must be reversed when an NPN transistor is fitted.

DFWB.