Is this a 4-pin transistor? - Page 1
Forum section: electronics / repair

author: Deckert
author_rank: Regular Contributor
timestamp: September 20, 2020, 01:49:41 pm
content: Hi,I am restoring this old FM transistor radio for my father. It looks like the RF input amp stage got damaged due to a static discharge on the antenna (that's the description I got). I'm guessing it dates from the early 1970's or late 60's. The audio-amp section seems to be working fine (can inject a tone and it's loud and clear on the speaker).The tuner itself also seems to work (can measure the tuning frequency on the back of the variable cap-set), but it's just not picking up anything at all.The antenna feeds directly into this four-pin device via a cap - I suspect that's the culprit.It's labeled "121", but there are other devices - most are labeled "123" and some are labeled "125". Any idea what these devices are? (Four-pin transistors, perhaps?)--deckert

author: PA0PBZ
author_rank: Super Contributor
timestamp: September 20, 2020, 02:53:52 pm
content: Did you check the bottom side? Maybe 2 pins are connected.Edit: I think I found it:https://www.donberg.ie/catalogue/bf_121.html

author: coromonadalix
author_rank: Super Contributor
timestamp: September 20, 2020, 02:58:22 pm
content: seems an rf transistor ?? or somekind of an amplifier cant recall their name maybe mar something ??EDIT loll beaten while writting this

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: September 20, 2020, 03:30:33 pm
content: Two pins could be connected. Either internally or visibly, PAOPBZ is probably correct.Incredible that he found documentation.But the "S" pin might be the case (maybe there is one inside the plastic) and not connected to the transistor itself. Check if it is connected to ground maybe. Maybe it is just there for mechanical stability in mounting?This is really old and probably from the 1950s.Newer pill type transistors like some GaAs (microwave stuff) have the two side pins common, emitter leads, I believe.They look a lot like your transistor.This MIGHT be similar in it's connections. Maybe top and bottom are connected.

author: bdunham7
author_rank: Super Contributor
timestamp: September 20, 2020, 03:56:14 pm
content: A schematic of a different radio using these shows a fourth terminal connected to ground. I don't know what an 'S' means in German in this context.https://www.radiomuseum.org/tubes/tube_bf121.html

author: schmitt trigger
author_rank: Super Contributor
timestamp: September 20, 2020, 04:04:43 pm
content: Several of the early RF/IF transistors had a shield terminal in addition to the CBE terminals.The shield was commonly grounded.An example I am familiar with is the AF126:https://m.electronic-spare-parts.com/descript/a/af_126.htm

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: September 20, 2020, 04:08:16 pm
content: MaybeShield = Schild (Ger)Posted before i read the previous post

author: Twoflower
author_rank: Frequent Contributor
timestamp: September 20, 2020, 04:09:41 pm
content: The S pin is probably the substrate. Some transistor designs don't have the collector 'on the backside' of the crystal.

author: drussell
author_rank: Super Contributor
timestamp: September 20, 2020, 05:33:13 pm
content: Many early transistors had a connection to the substrate brought out to a pin.

author: eurgenca
author_rank: Regular Contributor
timestamp: September 21, 2020, 03:16:57 pm
content: Normally replaceable-slightly different:BF153 ; BF198 ; BF310Regards.

author: amyk
author_rank: Super Contributor
timestamp: September 22, 2020, 12:07:04 am
content: Datecode on speaker says 13th week of 1971.Later electronics would be higher density, earlier would not have that style of electrolytics.

author: vk6zgo
author_rank: Super Contributor
timestamp: September 22, 2020, 04:20:01 am
content: Quote from: amyk on September 22, 2020, 12:07:04 amCertainly not 1950s as someone said!That style of transistor package was around in the late '60s, but not before.From the spec sheet, I would think the "S" pin is probably "screen".This terminology was used for the pin connected to the metal case on a lot of transistors.The plastic package may have an internal equivalent.Of course, the exra pin might just be used for more secure mounting.

author: james_s
author_rank: Super Contributor
timestamp: September 22, 2020, 05:06:49 am
content: I have seen dual gate FETs with 4 pins in radio gear but it doesn't look like that's what this is.

author: paschulke2
author_rank: Contributor
timestamp: September 22, 2020, 06:05:07 am
content: BF121 is a late 60ies silicon RF-Transistor, typically used in IF amplifiers. And: Yes, the device shown in the photo is a BF121.S = "Schirm" = Screen.This was taken over from the germanium RF transistors, e.g. AF124, in the metallic TO-72 case, where "s" is simply connected to the case."S" is typically connected to ground.

author: thinkfat
author_rank: Unknown
timestamp: September 22, 2020, 11:40:20 am
content: The datasheet mentions for the RF parameters "Abschirmung auf Emitterpotential", so I suspect that "S" is commonly connected to "E", whatever that is connected to. Frequently you'll find a "common base" amplifier in RF or IF stages, so E is not necessarily GND.

author: paschulke2
author_rank: Contributor
timestamp: September 22, 2020, 01:30:30 pm
content: Quote from: thinkfat on September 22, 2020, 11:40:20 amThat's why S is not internally connected to E, B or C.Here's an example: Bang & Olufsen Beolit 500TR1 and TR2 are BF115 in a 4-pin TO-72 case. "S" is drawn as a screen in the schematic and is connected to ground. RF amplifier (TR1) and oscillator/mixer are both common base.TR3, 4 and 5 are BF121. "S" is drawn as a screen in the schematic and is connected to ground through 47nF capacitors. These IF amplifers are common emitter amplifiers.Edit: Quote from the Intermetall datasheet for the BF125: Die Schirmelektrode ist hochfrequenzmäßig zu erden (The screen electrode must be grounded for RF).

author: PA0PBZ
author_rank: Super Contributor
timestamp: September 22, 2020, 01:45:59 pm
content: Quote from: paschulke2 on September 22, 2020, 01:30:30 pmI see a connection between the emitter and the shield on all three.

author: paschulke2
author_rank: Contributor
timestamp: September 22, 2020, 01:56:06 pm
content: Quote from: PA0PBZ on September 22, 2020, 01:45:59 pm…outsidethe transistor, which is normal for a common emitter RF amplifier.But I give up. Oh … wait … what do you see for TR1 and TR2? Is the screen connected to the emitter? Would a common base RF amplifier work with a grounded emitter?