Alright. So far I have lubricated the mechanism, changed the belt, replaced all electrolytic caps and adjusted the motor speed. Now it works great... so long as it's not on battery power. In this state, the speed is noticeably slower (436kHz on a 440kHz test tape) and it's annoying me quite a bit. Now I am using rechargeable batteries with a voltage of 1.2v, but It seems like there should be some sort of voltage regulation unless this is done by the motor controller? Anyway, just wondering whether this is some sort of fault, or that's just the way this player is? I should add that with a new set of alkaline AAAs the speed is as it should be, but if they are half used, same issue. Thanks!
First of all you should use a 3kHz test signal for adjusting/measuring tape speed, not a 440Hz (440Hz is used for level calibration). Note the frequency you are talking about is in Hz, not kHz. 1kHz = 1000Hz. Second, a 4Hz deviation is well within the acceptable limits and certainly not what I would consider a problem. Question is what it the tape speed when powered on a new set of alkalines or on mains adapter ? You did not provide that information. I am interested to know what is the difference in tape speed between a new set of alkalines and a half used set/recharghables. On the schematic, the motor governor board in marked as a black box, without any details of what's inside (you will have to provide picture to bring some light). There is no external regulation, the 3V rail is powering the motor PCB directly.
Right thanks for the clarification on tone frequency, I'll record the correct frequency tape once my deck has been repaired(it's direct drive dont worry). As for how I set the speed to begin with, that was done using the dc input as the control is covered by the batteries when installed. So here are the measurements I took: DC in(4.03v): 440Hz New alkaline(3.19v): 440Hz Old alkaline(2.65v): 436Hz New rechargeable(2.71v): 439Hz Old rechargeable(2.53v): 437Hz I didn't think to check the output on the dc supply before... looks a little high. I'll try to find a better supply, maybe that's the issue. Well I took the motor board out anyway, so here it is: Edit: I found a more suitable dc source of 2.95v. setting the speed at this voltage did improve things, but it is still not regulating and will now play fast with new batteries.
Ok, thanks for the detailed description of voltage --> speed correlation, that is what I was looking for. I'd say given the realtive simplicity of the circuit (it's a discrete design after all), I would say the regulation is pretty damn good within 4Hz and certainly not a reason of concern, at least in my opinion. Integrated circuits designed for this task usually contain a very precise voltage reference, which here is likely either the supply voltage itself or the Vbe drop of a transistor (which will have relatively significant variations with current). There are 3 SMD resistors on the top (green ones marked S, L, J). Those are used to adjust the motor torque. Note the initial configuration and play with different configurations if you really want and see if something improves in a different configuration. However, as said before, to me it seems pretty good as it is given how simple the circuit design is.
I always follow advice mentioned by @Valentin and do not mess with electronics if it works. Primarily because my understanding of it is very-extremely basic. When searching for ways to restore Sony BP-5 battery (used in D-350 Discmans) I discovered the existence of 1.5V Li-Ion rechargeables. Kratax 1100mWh, sold at Amazon and eBay. They were of no use for my BP-5 project so I use them in the remote for my Naim stereo, so far they work well. Another option (but ask @Valentin to be on the safe side): since controller can handle 4.03V off DC you can use one 3.7V AAA/10440 Li-Ion rechargeable plus one AAA 'dummy'. Or rewire battery holder for two 3.7V AAA to be set in parallel
I have a solution, if you're really concerned with the inconsistent tape speed: add a linear voltage regulator between the battery terminals and the motor board. 1. Measure what is the minimum voltage that the motor board will accept and still maintain a constant speed and choose the regulator according to that. 2. Measure what is the current drawn by motor board when unit is in FF/REW and table reels are completely stopped. Choose a low-dropout regulator with a current rating higher than what you measured at 2. Choose an output voltage according to 1., but make sure you have enough headroom when battery is almost depleted: Vbatt_min > Vout + Vdropout If you provide 1 and 2, I can look a part for you.
I reckon adding a voltage regulator is worth a shot. So I measured FF/RW current when stopped and got ~275mA. As for the lowest voltage the motor board will accept, I found that 2.1v was about the minimum. Then I went on Mouser and found these two that might work. Most of the other options were the type to use solder balls, but either way, they are all tiny. 2.1v 300mA output, 300mV dropout https://au.mouser.com/ProductDetail/Diodes-Incorporated/AP7343D-21W5-7?qs=EDJ299gKAZSBKz5VX6aw0g== 2.3v 300mA output, 270mV dropout https://au.mouser.com/ProductDetail/Torex-Semiconductor/XC6223H231MR-G?qs=AsjdqWjXhJ/q2xGSxXwv6Q== I measured the voltage across the batteries while the player was on. I couldn't exactly find what the lowest usable voltage of the battery is, but looking at a discharge graph, it seems to be about 2.3v. This is lower than the Vdropout + Vout by 100mV. Maybe it's still ok? I have the second option there as a backup just in case the first turns out to be too low. Are these acceptable or do you know of any better options?
You need some headroom in both current handling (especially cause you won't have any heatsink) and dropout. Output current should be 500mA at least, 1A capability would be even better, but it will also limit the regulators available a lot. In order the maximize battery usage, dropout voltage is preffered to be even lower than 300mV. This is still usable, but the maximum you will be able to use is 2.3V + 300mV = 2.6V. Below 2.6V battery voltage, regulator will stop regulating (won't have enough of a voltage drop between input and output). A brief look on mouser reveals there are not too many options available in fixed output voltage configuration (most are 2.5V output). TC1264 (Microchip) had 800mA current handling capability, only requires 1 x 1uF output capacitor for stability, droput is as low as 50mV @ 100mA current (it goes up to 150mV @ 300mA). Look also for external components needed, as some regulators will not operate stable without input and output caps, some will also have an enable pin. Adjustable output ones are a better option by far (as you can set the output very close to your minimum), but will require at least 2 output resistors for the feedback voltage divider and possibly some compensation capacitors as well.
Alright, I believe I have found quite the tasty regulator: 2A capability with a Vdropout of around 50mV at around 300mA (I think) https://au.mouser.com/ProductDetail/Semtech/SC4215ASTRT?qs=rBWM4%2BvDhIexMX9J8ZpUWA== Specifies a 4.7uf on Vin and 10uf on Vout so a 22uf ceramic for both will be fine? I have R1 = 32k ohms and R2 = 10k ohms
Seems good at first glance. On input and output, use tantalum/low ESR electrolytics instead of ceramic. You can add the recommended 100nF ceramic, but it's not necessary. The values are not critical in your application, you and use 1uF on both input and output as they recommend 10uF/A of output current, while you probably have 1/10 of that - 100mA or less when in play mode. Give it a try and let us know the results.
Alright, it's finally done! To install the regulator, I had to separate the supply from b+. I did this by cutting the trace leading from the dc jack after the internal switch. Then I soldered all the components onto the back of the pcb. Not the neatest thing, but it works . The regulator itself was positioned next to the headphone jacks, just above the door hinge, secured with some double-sided tape. Since the spring for the door detent comes pretty close I covered it with some insulating tape ( It's wiring harness tape). Now, this was all going fine and after connecting everything I tried some batteries and confirmed it worked, so went to set the speed. However, this model requires a dc input to be used to set the speed and of course, I made a grave mistake. I forgot to switch the voltage on the power supply and connected the player to 12v instead of 3v. This could have been bad - really bad, but that brave little regulator chip valiantly sacrificed itself to allow the player to live on. R.I.P first regulator. It was a good thing I had the foresight to buy a spare, just in case, and after swapping in a replacement and remembering to set the power supply voltage, I managed to set the speed. The next test was to see if it would remain stable on the rechargeables and I'm happy to say that it did! Thank you so much @Valentin! Your input was immensely helpful and I doubt I would have known what to do otherwise. Here are some photos if you wish to see my rubbish soldering: It even managed to close! Well almost... I'm not ready to use the dc input again anyway...
