Replacement of transistor in VHF section by different type

[paulfifties]

I've got a ITT Schaub Lorenz GOLF Europa 103 transistor radio.
One of the failures was FM no longer working, while other bands were okay.
So I replaced the T301, which was a BF125 according to the schematics, but a BF224 was fitted in the factory instead.
As I had none of those types available, I replaced it with a BF199.
In order to obtain the same emitter voltage shown in the schematics, I had to increase the base resistor from 2K2 original to 18K.
I include a detail of the original schematics below.

The BF199 does a good job, but the reception is improving considerably if I just hold the probe of my scope against the base of the BF199.
At first I thought I had to modify the base-to-ground capacitor value, but playing with that did not improve anything.
So I wonder if it has to do with some feedback that is induced by holding the probe against the base ? I noticed the same effect while holding 1 leg of a metal drum-shaped trimmer against the base, and holding the other leg (actually the outer, grounded part) with my fingers.

It looks like this has an effect of inducing an additional oscillation, or maybe contrarily: removing some spurious oscillation.

Does anyone have a hint on how I can create this improved reception permanently ? Or maybe point me in the direction of what might be going on here ?

[crackle]

BF125 transistors are available on Ebay from the Netherlands.

Mike

[GeoffK]

Might be cheaper and better from here even after adding postage and euro conversion.
https://www.cricklewoodelectronics.com/BF125.html

[paulsherwin]

Domestic designs like these aren't usually fussy about transistor types, so I would look elsewhere before chasing replacement transistors.

[crackle]

The OP is in Belgium

[paulfifties]

Actually, the originally mounted transistor is a BF224, so I am looking for data on a BF224B which is available for hardly any money.

But I would rather like to understand what is really going on in the circuit.
Perhaps it is not really the transistor after all, and something might be causing some spurious oscillations at weak input signals ? Or just preventing the correct oscillations.

As I would like to understand the UHF circuit used in this ITT Schaub-Lorenz radio, I would appreciate also if someone could point me to a link or forum that explains all the details.
I, for example, noticed a curious feedback system in the LO using a varicap.

On the other hand, the title of a handbook explaining practical UHF circuits in (transistor and tube) radio receivers would also be helpful.

Paul

[Skywave]

Originally Posted by paulfifties

The BF199 does a good job, but the reception is improving considerably if I just hold the probe of my scope against the base of the BF199.
At first I thought I had to modify the base-to-ground capacitor value, but playing with that did not improve anything.

Your 'scope probe will add about 15 pF, base to ground (plus some inductance if the earth lead from the probe was connected to chassis.) That, of course, will modify the behaviour of the set. The "improvement" in reception may not be an evenly-distributed improvement over the whole tuning range. I just wonder what range of capacitances you experimented with. However, in summary, I strongly suspect that varying the value of that capacitance is, in point of fact, a red herring.

Having said all that, it sounds as though the stage associated with T301 (the R.F. amplifier) could be off-tune. Look for changed value low-value capacitors in that stage. L301 / L302 or C305, C306, L303 might be out of adjustment or faulty; possible dry-joints or fractured pcb tracks. R302 looks like it is connected to the AGC line: there may be a problem there. Checking the sensitivity and tuning range with a signal generator could help if you really get stuck.

This fault does not look like it'll be an easy one to crack - so patience and perseverance will be required! Good luck with it.

Al.

[tri-comp]

Originally Posted by paulfifties

I, for example, noticed a curious feedback system in the LO using a varicap.

Hello Paul,

Not so curious after all.
This is a quite ordinary AFC-circuit (Automatic Frequency Control)
https://en.wikipedia.org/wiki/Automa...quency_control

I've repaired numerous FM-tuners in receivers after thunderstorms.
The RF-amplifier transistor would go dead after the statics especially of course if connected to an outdoor rooftop antenna.
I remember using the BF115 as an almost universal replacement type if the input transistor was a silicon NPN type.
Bang & Olufsen used this type extensively in FM-tuners mounted in their various models portable transistor-radios.
It compaires fairly well with the BF125.
Perhaps a tad lower gain, about the same noise figure and a little lower Ptot and fT.
Probably not something that will upset your FM-tuner much if used as a replacement.
My guess is that if this doesn't cure the sensitivity-issue you may well experience an entirely different problem than that of a defective input transistor.
Considering the age of the radio I would suspect every electrolytic capacitor of being defective.
Here C391/1000uF would probably be a good place to start along with C389/10uF
I also seem to remember some problems with D307/ZF1,5 in this model.