Anybody know what this is? - Page 1
Forum section: electronics / beginners

author: StuUK
author_rank: Frequent Contributor
timestamp: January 17, 2016, 06:05:20 pm
content: Hi thereAnybody have a clue what this is? i took the can off so you can see the internals...Picture attached and thanks in advance.Stu

author: Simon
author_rank: Unknown
timestamp: January 17, 2016, 06:07:37 pm
content: some sort of filter, not sure what the diodes are for.

author: bktemp
author_rank: Super Contributor
timestamp: January 17, 2016, 06:19:56 pm
content: Could it be a Foster-Seeley discriminator (fm demodulator)?

author: edpalmer42
author_rank: Super Contributor
timestamp: January 17, 2016, 07:28:58 pm
content: Is that a coil in the center (the long white cylinder). Is there a tuning slug inside it?Ed

author: StuUK
author_rank: Frequent Contributor
timestamp: January 17, 2016, 10:00:40 pm
content: yes definitely a coil

author: Unknown
author_rank: Unknown
timestamp: January 17, 2016, 10:28:41 pm
content: Quote from: bktemp on January 17, 2016, 06:19:56 pmCertainly looks like it.

author: Paul Price
author_rank: Super Contributor
timestamp: January 17, 2016, 11:21:39 pm
content: It is either a Forster-Seely Discriminator or Ratio Detector demodulator for an FM radio.Because the two germanium diodes are mounted in reverse to each other, it more likely a Ratio Detector.https://www.google.dk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0ahUKEwid07uf_rHKAhUF6g4KHYTiA9cQFggqMAA&url=https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FFoster%25E2%2580%2593Seeley_discriminator&usg=AFQjCNGjsyMxFG_glXz9VCQD-ZsaK1A5uwRatio detectorA ratio detector using solid-state diodesTheratio detector is a variant of the Foster-Seeley discriminator, but one diode conducts in an opposite direction. The output in this case is taken between the sum of the diode voltages and the center tap. The output across the diodes is connected to a large value capacitor, which eliminates AM noise in the ratio detector output. The ratio detector has the advantage over the Foster-Seeley discriminator that it will not respond to AM signals, thus potentially saving a limiter stage; however the output is only 50% of the output of a discriminator for the same input signal. The ratio detector has wider bandwidth but more distortion than the Foster-Seeley discriminator.But, if this device is from a premium Hi-Fi FM tuner, it is more likely to be a Foster-Seely discriminator because of lower recovered audio distortion, but if it is from a cheaper FM or oldie boombox it would be the choice for cutting cost, so it would use a Ratio Detector.

author: bills
author_rank: Frequent Contributor
timestamp: January 18, 2016, 02:34:58 am
content: RF coils and I don't believe those glass devices are diodes, They are low value caps.I have some in my junk box and they test as caps. (Let the debate begin)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 18, 2016, 04:27:57 am
content: 1N542 = Ge point contact diode, 45V 10mA.I'm going to dissent from popular opinion, because the coils on top have far too many turns to be an FM detector stage (typically done at 10.7MHz). And there are far too many coils and diodes for this to be just an FM IF/detector module.The lower coils might be consistent with a ~45MHz TV IF stage, in which case the one diode (unmarked) should be detecting that, and the 4.5MHz audio carrier will be present in that detector output, which is tapped off by the top pair, and immediately detected.So I think it's from a classic TV, probably 50s-60s era.Tim

author: AF6LJ
author_rank: Unknown
timestamp: January 18, 2016, 04:33:52 am
content: Quote from: T3sl4co1l on January 18, 2016, 04:27:57 amI think you nailed it.

author: StuUK
author_rank: Frequent Contributor
timestamp: January 18, 2016, 09:07:57 am
content: Must admit my first thought was that it was likely from a really old TV.I guess these days it's no use for anything and I've inherited about 30 of themThanks for all the help though.

author: MrSlack
author_rank: Frequent Contributor
timestamp: January 18, 2016, 11:06:02 am
content: Stick the diodes on ebay. Some idiot will pay £20 for them for a fuzz pedal or something

author: Simon
author_rank: Unknown
timestamp: January 18, 2016, 12:37:35 pm
content: Quote from: MrSlack on January 18, 2016, 11:06:02 amYou mean valued customer don't you ?or is it valuable customer

author: MrSlack
author_rank: Frequent Contributor
timestamp: January 18, 2016, 02:14:04 pm
content: Valuable

author: StuUK
author_rank: Frequent Contributor
timestamp: January 20, 2016, 01:35:03 pm
content: Quote from: Simon on January 18, 2016, 12:37:35 pmWorth a punt I guess..... What are the diode types, can you tell from the picture?

author: MrSlack
author_rank: Frequent Contributor
timestamp: January 20, 2016, 01:48:33 pm
content: The red/yellow/green ones are likely to be 1N542 germanium point contact detector diodes (1N,then read the colours like resistors; same code!). Seem to be popularish on ebay.The other one with the orange band; no idea. If you've got a multimeter with a diode tester, check it with that after desoldering or clipping it out and post the forward voltage drop.

author: StuUK
author_rank: Frequent Contributor
timestamp: January 20, 2016, 03:26:44 pm
content: Clipped one (the orange band) out and tested it with an Atlas DCA which read a Vf of 0.64v and an If of 4.77mA

author: MrSlack
author_rank: Frequent Contributor
timestamp: January 20, 2016, 05:20:28 pm
content: Sounds like a boring old silicon one then. I'd ignore it. The Vf is too high for a nice germanium one.