Old Component Identification Please - Page 1
Forum section: electronics / beginners

author: ButchJames
author_rank: Newbie
timestamp: October 25, 2024, 10:14:00 am
content: Working on an old DC brushed motor, and I'm not sure what this is?According to the schematic of the said motor - it's two diodes in series!? My meter confirms it has diode-like properties - open circuit one way, and a high resistance (> 500KΩ) the other way.At first I thought the schematic was wrong, I thought it might just be a micro-thermal switch in case the motor jammed internally?It is the green and yellow component.Okay, so calling on all those old enough to identify this ...BJ.

author: CaptDon
author_rank: Super Contributor
timestamp: October 25, 2024, 12:25:20 pm
content: I see three diode devices in the schematic and I see three physical glass axial lead diodes on the board. It could be that the dual diode is the one with the blue stripe on the cathode end. Dual diodes are very common for temperature compensation and basically look like any normal diode. They are very common in comp-sym audio amplifiers for thermal stabilization. I have no clue about the green axial lead blob although it may be an MOV or transorb to suppress motor noise. Look at the strange symbol that looks like a capacitor tied directly across the motor. Trace the PCB wiring, where is the blob in the schematic? You can answer your own question.

author: ButchJames
author_rank: Newbie
timestamp: October 25, 2024, 01:43:32 pm
content: Thanks for the reply.The more I think about it, the more it looks like double diode temperature compensation, although the green/yellow device is located at the outer point on the pcb, this threw me off at first. I have seen one of these before, but I cannot recall where and when!?Drawing a diode-resistorload linefor the said diodes and that resistor above would give a nominal voltage across the two diodes, and if and when they do heat up a little, moreIdcurrent would flow, and thus lower (slightly) the biasing of the middle transistor.The capacitor across the DC motor bi-passes back EMF noise.The diode 'across' the motor is incorrect - the actual circuit contains two.Thanks again for you input.

author: CaptDon
author_rank: Super Contributor
timestamp: October 26, 2024, 12:49:54 am
content: According to the schematic there is no capaciitor across the motor. Look real close at the symbol, it is not a capacitor symbol. So did you trace out where the green blob is in the circuit by looking at what it is connected to on the PCB?

author: Analog Kid
author_rank: Super Contributor
timestamp: October 26, 2024, 01:02:50 am
content: You might be correct, although I have seen that symbol (a regular cap symbol with crosshatches between the plates) used in schematics for capacitors. What else could it be?Since there's another obvious cap there using the standard symbol, I'd say this one would be an electrolytic cap. Would that make sense to have it across the motor?

author: coromonadalix
author_rank: Super Contributor
timestamp: October 26, 2024, 02:45:48 am
content: you have 2 caps on the board an black lytic and a blue tantalumthe green was the inductor, still confused by this special diode / dual diode package, i thought it could have been some kind of a diacbut you do have another winded inductance, and the schema still show 3 diodes with this special part, unless this part is the 4th diode with some special specbecause thoses 2 in serial must form an pseudo "zenner" voltage ??the biggest resistor must be the "3 ones in parallel", since the resistor count is not the same as the schematicthe schematic could be a crude one to show working principlesbut what intrigues me, normally the motor must be regulated up if the speed goes down, and the opposite if it speed up, you do have the 3x white shrinked parts on the rotor, and i was thinking this special part could do somekind of feedback

author: amyk
author_rank: Super Contributor
timestamp: October 26, 2024, 03:18:13 am
content: Constant-speed motor used for something like a tape capstan or turntable?I recommend to trace the PCB to see if it actually matches the schematic.

author: Analog Kid
author_rank: Super Contributor
timestamp: October 26, 2024, 04:02:59 am
content: My guess™ is that it's from a cassette tape deck.

author: ButchJames
author_rank: Newbie
timestamp: October 26, 2024, 09:22:44 am
content: Yes, it's a motor controller for a cassette deck.Typical default PLAY DC current into the circuit (inc motor) are around 50mA ... 75mA.I have indeed traced out the circuit, it is a little different -There are two diodes across the DC motor '+' terminal and T2 Emitter ... it seems to be as a temp compensation injecting slight variable current into 270 ohm and hence vary the series-voltage feedback to T2.There's an electrolytic 10uF cap across the DC motor, and a 0.1uF tantalum between T2 base and +Vcc.The revised circuit is shown with the values I have determined, but not (of course 100%) certain.Can't be certain until I de-solder that 'L1' is an inductor - it looks like one, if it's a 'coiled' resistance then it'll have inductance too of course.Be aware that this circuit is old, circa 1976, hence the reason why I was intrigued regarding the yellow-green component.I've partially analysed this circuit, and would like to use it again elsewhere for other cassette decks, if and when the time arises. It has good speed stability characteristics.If there is one thing I don't like about it - it is the sensitivity of motor speed to the micro movement of the potentiometer wiper - I need to revise the design a little to tweak this.All constructive comments are welcome, thanks for all so far.

author: ButchJames
author_rank: Newbie
timestamp: October 26, 2024, 09:26:12 am
content: Forget to mention - there's no tacho on this, so norevs/secfeedback etc.(Hope I've typed it all out correctly)

author: CaptDon
author_rank: Super Contributor
timestamp: October 26, 2024, 01:29:40 pm
content: I modified a very similar cassette deck motor by removing the variable resistor and replacing it with a front panel mounted 10 turn pot of the same resistance. The reason I did this was to bring tapes recorded slightly off speed into perfect pitch alignment to our band for rehearsal. This was extremely useful!! The Tom Petty song 'Mary Janes last dance' is recorded weird, it's about one half of a semi tone low so we speeded it up. Also the Extreme song 'Whole hearted' I believe was recorded one full semitone up from normal guitar tuning or perhaps was capo'd in the first fret. We slowed that one down to standard tuning which also made learning the song a touch easier. The ten turn pot gave a very fine smooth speed control.

author: ButchJames
author_rank: Newbie
timestamp: October 26, 2024, 02:25:05 pm
content: @Captdon: I was thinking similar thoughts.Currently I believe one source of motor drift is the potentiometer as its effective resistance at the head of the wiper is probably not stable enough? This may/must give rise to an overly-variable driving voltage for T2, something that's not wanted.The motor in its most stable configuration had a very stable speed score: after 20 minutes testing, a full track ABEX reference tape would drift wellwithin±3Hz from a 3150Hz reference - that's pretty darn good. I'll let you work out that as a percentage.However, on replacing this motor for another little fix, the drift later would (more often than not) move usually to about +20Hz over a much shorter period.So, as the potentiometer was so sensitive, I'd like to eliminate that from my list of potential fixes/modifications.I understand there may also be temperature considerations to consider, but the 'big' drift happens too quickly.The motor is question is here for this Technics 630TUS ...https://cassettedeckman.blogspot.com/2024/10/technics-630tus-cassette-deck.html

author: Jeff eelcr
author_rank: Frequent Contributor
timestamp: October 28, 2024, 12:29:36 am
content: Captdon the green thing is the inductor.Jeff