Thorn 1591 IF stages transistor issue

[high_vacuum_house]

Good evening,
I am currently trying to breathe some life into one of these old portable televisions.
So far the 4 presets on the rear have been changed. A transistor in the line oscillator was C-E short and one in the frame oscillator was o/c.
I now have a raster of sorts. The timebase frequencies are wrong but I will come back to that later.

All 4 transistors in the IF amplifier stages were found to be O/C (VT2-VT5) and not drawing any current. As a quick test I have replaced them all with standard BC549 NPN transistors and although they are drawing current and the voltages are all a bit out as would be expected, there is no signal getting through to the detector diode. These are all different type numbers though all small signal NPN type through the IF stages. Should I collect the correct transistors for these stages or should BC549 transistors work.

The set is in quite poor condition with plenty of corrosion around and I don't really want to spend too much on it if it proves not worth it.

Many thanks,
Christopher Capener

[paulsherwin]

I suspect there is another problem somewhere. Bipolar silicon transistors are pretty interchangeable so long as the ratings are adequate.

[KeithsTV]

VT2 & 3 are the AGC controlled stages with their gain controlled by VT1 so check the voltages around VT1.

Keith

[pastyboy]

If you are coming to Mikes do I can bring a spares set - I sort of want to hang on to the chassis to rob but it might be a good opportunity to swap a few bits, Keith is having its speaker and the case is bleached and cracked

[Nuvistor]

Have you signal generator to feed a signal into each IF stage working back from the last IF amp, confirm where the signal is lost?

[Studio263]

Originally Posted by high_vacuum_house

Should I collect the correct transistors for these stages or should BC549 transistors work.

BC549 = BC109 in a plastic housing, this won't work at TV IF frequencies. Penny-pinching BRC wouldn't have used fancy RF transistors if boggo general purpose ones would do! The originals are matched to the design of the AGC system so you have to get them right if you want it to work properly. As a quick test, BF240s would make more sense and would at least stand some chance of giving results.

[paulsherwin]

They do have adequate gain at IF frequencies so should work, though there are better choices.

[Studio263]

Originally Posted by paulsherwin

They do have adequate gain at IF frequencies so should work...

Perhaps for AM radio, not for UHF TV.

'Television' magazine published some interesting articles on transistor IF strip design in the late 60s and early 70s, it is an interesting and exacting field. In particular, the differences between using increasing collector current to either increase or reduce the gain. Different transistors are optimised depending on which method is chosen, which is why the right ones must be used.

The IF strip of the BRC 1500 was designed along similar lines to that of the 1591, here's plenty to read about on those.

[Andrew2]

I think BC548's should work sufficiently well at 35 MHz. I've used them an all manner of RF stuff and they always do something. Maybe the AGC would act a bit strangely, but there should be enough gain to put a picture of some description on the screen.
Obviously you wouldn't attempt to build a low noise UHF pre-amp with one, but for a TV IF strip they should perform well enough to prove a point.
I suspect there's something else amiss.

[saxmaniac]

I do have some Thorn transistors NOS if you can come up with their type no.s I might find the right ones out, pm me if this helps