old record player to fix - Page 1
Forum section: electronics / beginners

author: Daxxin
author_rank: Regular Contributor
timestamp: November 06, 2022, 06:24:20 pm
content: hi , got 1963 record player that missing the speaker , wich has 3 wires ..for what pourpouse?how modify it to use 2 wires speaker?thankshere the schematic

author: radiolistener
author_rank: Super Contributor
timestamp: November 06, 2022, 06:50:53 pm
content: may be it uses custom speaker with a tap from the middle of the winding.

author: Ian.M
author_rank: Super Contributor
timestamp: November 06, 2022, 06:54:36 pm
content: That's a positive chassis design form the Germanium transistor era, and NPN power transistors were very hard to make and not very reliable (seehttps://www.vintage-radio.net/forum/showthread.php?t=165153), so the only practical way of implementing a high power*push-pull output stage with only PNPs was with a center-tapped output transformer or center-tapped speaker, negative supply to the center tap, so the standing DC current in the two output transistors through it cancelled out. This design chose to use a center-tapped speaker to save the cost of an output transformer.Without a center-tapped speaker you'd need a center-tapped output transformer, probably custom wound to match the impedance of a modern speaker to whatever its designed to drive, which you'd have to determine.Two separate speakers wont be usable as their voice coils will be biased (offset) in the gap by the DC current distorting their response, and the physical separation between them will result in extremely unpleasant frequency dependent crossover distortion.* Lower power push-pull output stages could use high and low side PNPs, each with its base controlled by its own independent secondary on the driver transformer, and with sizable emitter degeneration resistors to stabilize the operating point. Either a large speaker coupling capacitor, or a center-tapped supply was also required, which at higher power levels negated any savings from omitting the output transformer.

author: james_s
author_rank: Super Contributor
timestamp: November 06, 2022, 07:13:45 pm
content: That's bizarre, I've never heard of a center tapped speaker before. An output transformer is probably the only practical way to make that work.

author: TimFox
author_rank: Super Contributor
timestamp: November 06, 2022, 07:33:03 pm
content: I've never seen a center-tapped voice coil before, either, but it's a clever way to avoid an output transformer.

author: Ian.M
author_rank: Super Contributor
timestamp: November 06, 2022, 08:01:14 pm
content: Tapped speakers were fairly rare. A common circuit of the era that saved on the size of the 'output' transformer required was to supply the output stage via the center tap of an autotransformer acting as a balanced feed choke, with the speaker across the ends of its winding.I was somewhat surprised to note the O.P's schematic was from an Italian manufacturer, not from "Madman" Muntz!

author: andy3055
author_rank: Super Contributor
timestamp: November 06, 2022, 08:29:49 pm
content: May be a dual voice coil speaker like what is used in subwoofers might work. The frequency response will be very poor.The other alternative will be to use a separate driver and output stage and feed it from that last transistor base point and do away with the rest of the circuit.

author: Ed.Kloonk
author_rank: Super Contributor
timestamp: November 06, 2022, 08:39:13 pm
content: ?https://www.soundlabsgroup.com.au/c/RetroSound-Technical-Dual/Dual+Voice+Coil+Speakers.html

author: radiolistener
author_rank: Super Contributor
timestamp: November 06, 2022, 08:49:29 pm
content: yes, it looks that adding transformer with center-taped primary coil and speaker on the secondary coil is the best solution to use usual speaker in this circuit

author: james_s
author_rank: Super Contributor
timestamp: November 07, 2022, 03:48:06 am
content: It might not be too hard to find a transformer that will work, I remember there being a lot of center tapped audio transformers back in the day, a record player like that is unlikely to be hi-fi in the first place.

author: BeBuLamar
author_rank: Super Contributor
timestamp: November 07, 2022, 02:08:01 pm
content: I was thinking of replacing the entire amplifier but I found that amplifer circuits for ceramic cartridge are not widely available.

author: james_s
author_rank: Super Contributor
timestamp: November 07, 2022, 06:25:31 pm
content: If you're not concerned about originality you could build a replacement amplifier easily enough.

author: TimFox
author_rank: Super Contributor
timestamp: November 07, 2022, 07:59:53 pm
content: Quote from: BeBuLamar on November 07, 2022, 02:08:01 pmCheck the old audio literature: the preamplifiers for ceramic cartridges (capacitive and displacement responding) are much simpler than for magnetic cartridges (inductive and velocity responding).

author: Jeff eelcr
author_rank: Frequent Contributor
timestamp: November 10, 2022, 06:01:30 pm
content: Ed's dual voice coil speaker might work using three contact pointsoff of the 2nd drawing, problem being that 4 ohm speaker.Might try with an 8 ohm dual VC?Missing speaker impedance of part number Dm 22039?Jeff

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: November 10, 2022, 06:16:37 pm
content: Use two speakers with center tap to both.But what ohms are they?

author: TimFox
author_rank: Super Contributor
timestamp: November 10, 2022, 07:45:30 pm
content: Two speakers with independent voice coils: there will be DC current through each coil, which might not be a good idea.A center-tapped coil (or transformer primary) driven from push-pull collectors will avoid net DC if properly balanced.

author: james_s
author_rank: Super Contributor
timestamp: November 10, 2022, 08:25:41 pm
content: I would try a junkbox power transformer with multiple secondaries, connect a center tapped secondary to the amp and use one of the others to drive a speaker. Or take the original primary and step it down with another transformer. The impedance might be close enough to work, it's probably not critical.Otherwise just replace the amp, it's pretty trivial to amplify the output of a crystal stylus.

author: Terry Bites
author_rank: Super Contributor
timestamp: November 11, 2022, 08:07:07 pm
content: If either of the transitors fails short on the output side say good bye to your speaker and transformer. The 10R is for current limiting.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: November 11, 2022, 10:19:01 pm
content: Just a thought...does the original speaker have a permanent magnet??

author: vk6zgo
author_rank: Super Contributor
timestamp: November 11, 2022, 11:57:27 pm
content: Quote from: Wallace Gasiewicz on November 11, 2022, 10:19:01 pmIt will have!"Dynamic" speakers with electromagnets were but a distant memory when that was made.Centre tapped speakers were rare as hen's teeth, even back in the 1960s.We learnt them as a "clever idea" that we were unlikely to encounter in practice.Solid state stuff either used push-pull PNPs with real output transformers, or what was increasingly popular at the time, the "new fangled" PNP/NPN Complementary symmetry circuit.It may be possible to substitute the latter type of output circuit without too much butchery, or even just replace the whole output section with an appropriate IC.

author: james_s
author_rank: Super Contributor
timestamp: November 12, 2022, 02:21:53 am
content: It occurred to me that replacing the output stage could be fairly simple. It's already driven by a transformer so it's fairly isolated.

author: gbaddeley
author_rank: Regular Contributor
timestamp: November 12, 2022, 02:33:17 am
content: Quote from: TimFox on November 07, 2022, 07:59:53 pmCeramic and crystal cartridges generally need a high resistance load to function, eg 1 Meg. No equalisation required. Ceramic output ca. 100mv, Crystal about double that. Been there, done that, about 50 years ago, with salvaged 1960’s record players.