Why are transistors mounted like this? - Page 1
Forum section: electronics / repair

author: ballen
author_rank: Regular Contributor
timestamp: August 23, 2021, 11:03:15 pm
content: High end metrology gear from the late 1960s / early 70s UK. The transistors are germanium. Why are they mounted like this?? Is it because they could not resist the heat from soldering without long leads??PS: am tracking down noise (no schematic) in this bridge circuit. One possibility might be that the can from the fourth transistor (counting from the left) is touching/rubbing the leads of the 7uf/63V electrolytic capacitor to its left. Does anyone know, are the cans isolated? Transistor in question is a BCY34.

author: ataradov
author_rank: Super Contributor
timestamp: August 23, 2021, 11:16:47 pm
content: It was probably just easier that way because they don't use standard footprint, the holes are just randomly located where was convenient. So making short leads would mean more precise and individual bending would be required.I don't think those transistors were that sensitive to heat.

author: M4trix
author_rank: Frequent Contributor
timestamp: August 23, 2021, 11:22:30 pm
content: Waaaay obsolete is written all over that picture.

author: jpanhalt
author_rank: Super Contributor
timestamp: August 23, 2021, 11:31:12 pm
content: Also, that board has an obvious height limit, namely the mounting height of the pots. Putting the transistors on their side saves some height.

author: Manul
author_rank: Super Contributor
timestamp: August 23, 2021, 11:31:52 pm
content: I guess more freedom for the footprint on the PCB. As far as I remember, the can of this package is not isolated. You can check that with multimeter. Touching is definetly a problem, except if touching points belong to the same circuit node anyway.

author: sean0118
author_rank: Frequent Contributor
timestamp: August 24, 2021, 12:14:55 am
content: It's because it's a single sided PCB and they are trying to avoid using jumpers, that's also why some of the other components are placed diagonally.One pin would be connected to the case, pretty sure it's normally the collector for NPN.

author: SL4P
author_rank: Super Contributor
timestamp: August 24, 2021, 01:03:46 am
content: IIRC, some germaniums were particularly microphonic, so the leads may have atteniuated any mechanical noise from the PCB….?

author: vk6zgo
author_rank: Super Contributor
timestamp: August 24, 2021, 01:12:14 am
content: Quote from: ballen on August 23, 2021, 11:03:15 pmI just skimmed over your post & missed the UK reference.Nevertheless, my first impression was that it just screamed "1960s UK!"The Brits seemed to hang in with that sort of construction, way after it was discontinued in other "lesser realms without the law", such as Oz!PS: The BCY34 is not a Germanium transistor, it is a Silicon PNP.https://www.web-bcs.com/transistor/tc/bc/BCY34.php

author: TERRA Operative
author_rank: Super Contributor
timestamp: August 24, 2021, 01:17:49 am
content: FWIW, those electrolytics are worth checking, if they are the old Philips brand ones (they appear so) they are known for being bad by now.

author: ballen
author_rank: Regular Contributor
timestamp: August 24, 2021, 01:35:54 am
content: Thank you, this is very helpful. I checked the electrolytics in circuit, and they appear to be OK. But if I don't find the problem elsewhere I will pull off a lead and measure them with an ESR meter.There are two other parts that I can't identify.The one on the top of the board is marked LEM 510. Is that possibly a lamp? Some decades ago I remember fixing a signal generator (General Radio?) which had a lamp as part of the oscillator circuit. These were put into the feedback loop to stabilize the output amplitude.The one on the bottom of the board is marked 47 and 5. I thought it was an inductor or capacitor for trimming or bypass, but there is no continuity and the capacitance meter shows nought.

author: Andy Watson
author_rank: Super Contributor
timestamp: August 24, 2021, 01:43:43 am
content: Quote from: ballen on August 24, 2021, 01:35:54 amLooks like a poystyrene capacitor to me. Probably 510 pF. No idea what the LEM means.Ditto the component on the reverse side, except is a 47pF capacitor.

author: coppice
author_rank: Super Contributor
timestamp: August 24, 2021, 01:51:10 am
content: My guess is the board was laid out for a different package, and this was their way of fudging one footprint to another. I've seen similar things before.

author: sean0118
author_rank: Frequent Contributor
timestamp: August 24, 2021, 02:25:17 am
content: I think LEM might just be the brand of capacitor.

author: cvanc
author_rank: Frequent Contributor
timestamp: August 24, 2021, 02:45:27 am
content: Those are both polystyrene capacitors.For the electrolytics you should change these as a known risk: The three blue ones on the right and the three with red lettering on the left.

author: Circlotron
author_rank: Super Contributor
timestamp: August 24, 2021, 02:45:59 am
content: Quote from: Andy Watson on August 24, 2021, 01:43:43 amYep, me too.

author: rsjsouza
author_rank: Super Contributor
timestamp: August 24, 2021, 02:35:11 pm
content: Quote from: ballen on August 23, 2021, 11:03:15 pmGermanium transistors are much more susceptible to temperature than Silicon and certainly the longer leads help with both assembly and testing, but the only Ge transistor I see is in the bottom right of your photo. From that era, most of the products that have short leads on Ge transistors are the portable transistor radios, where space was at a premium.Quote from: ballen on August 23, 2021, 11:03:15 pmI reiterate others' suggestions and test/replace all electrolytic capacitors. I see a small Siemens 4.7µF capacitor near the Ge transistor that looks somewhat suspicious. That would be the first line of defense to reduce any type of instability and noise.Quote from: Circlotron on August 24, 2021, 02:45:59 amIndeed. They are also called Styroflex and have great stability. If you are soldering near them, beware that their enclosure melts easily, destroying the capacitor.

author: fcb
author_rank: Super Contributor
timestamp: August 24, 2021, 05:18:37 pm
content: LEM capacitors, made by LEMCO (London Electrical Manufacturing Company Ltd, defunct in the late 80's).Pretty ordinary caps, they used to private label for RS and a few others.

author: SeanB
author_rank: Super Contributor
timestamp: August 24, 2021, 07:56:20 pm
content: Will also add replace all those 160V waxed paper foil capacitors, they all tend to either get very leaky, or just go intermittent. Replace all those red label Siemens capacitors, after a half century past their warranty they will be well dried out, same for the blue Siemens ones. Insulate all the places where transistors touch cases with component leads, case is invariably connected to collector.Polystyrene capacitors are very temperature sensitive, and also very sensitive to common cleaning solvents, alcohol will destroy them, along with any other solvent. If replacing you can use a polypropylene film one, or a NPO ceramic capacitor, as both are stable enough in this application, and can go down to 47pf and 510pf in a sensible size case.

author: factory
author_rank: Super Contributor
timestamp: August 26, 2021, 04:33:17 pm
content: The blue capacitors are either Mullard or Philips, depending on the age of the unit.The 160V capacitors from WIMA are not necessarily paper, they also made plastic film types in this package, looking up the code should tell you what they are, or maybe it's listed in the manual?David

author: djidji
author_rank: Newbie
timestamp: August 28, 2021, 12:43:29 am
content: the yellow Siemens ones are the ones for immediately replacement, but consider to replace all electrolytic caps.