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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only. |
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23rd Jul 2017, 4:55 pm | #1 |
Triode
Join Date: Apr 2017
Location: East Flanders, Belgium.
Posts: 17
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Replacement of transistor in VHF section by different type
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 ? |
23rd Jul 2017, 5:12 pm | #2 |
Rest in Peace
Join Date: Mar 2011
Location: Basildon, Essex, UK.
Posts: 4,100
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Re: Replacement of transistor in UHF section by different type
BF125 transistors are available on Ebay from the Netherlands.
Mike |
23rd Jul 2017, 5:41 pm | #3 |
Heptode
Join Date: Mar 2016
Location: Essex, UK.
Posts: 602
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Re: Replacement of transistor in UHF section by different type
Might be cheaper and better from here even after adding postage and euro conversion.
https://www.cricklewoodelectronics.com/BF125.html
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Geoff |
23rd Jul 2017, 7:37 pm | #4 |
Moderator
Join Date: Jun 2003
Location: Oxford, UK
Posts: 27,967
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Re: Replacement of transistor in UHF section by different type
Domestic designs like these aren't usually fussy about transistor types, so I would look elsewhere before chasing replacement transistors.
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23rd Jul 2017, 7:38 pm | #5 |
Rest in Peace
Join Date: Mar 2011
Location: Basildon, Essex, UK.
Posts: 4,100
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Re: Replacement of transistor in UHF section by different type
The OP is in Belgium
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24th Jul 2017, 11:53 am | #6 |
Triode
Join Date: Apr 2017
Location: East Flanders, Belgium.
Posts: 17
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Re: Replacement of transistor in UHF section by different type
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 |
24th Jul 2017, 12:12 pm | #7 | |
Rest in Peace
Join Date: Jun 2006
Location: Chard, South Somerset, UK.
Posts: 7,457
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Re: Replacement of transistor in UHF section by different type
Quote:
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. Last edited by Skywave; 24th Jul 2017 at 12:38 pm. Reason: General re-write, etc. |
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25th Jul 2017, 9:22 pm | #8 | |
Heptode
Join Date: Oct 2008
Location: Aalborg, Denmark
Posts: 903
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Re: Replacement of transistor in UHF section by different type
Quote:
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. Last edited by tri-comp; 25th Jul 2017 at 9:31 pm. Reason: Added info on D307 |
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