Portastudio Weak Signal | Read Schematics & Replace Capacitors | Tascam 488

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Here is some footage I shot while i was refurbishing a Tascam 488.
Having carried out basic refurbishment taks such as deep cleaning, lubrication, replacement of rubber belts etc, on attempting to calibrate the playback amplifiers I found I could not get track 8 of my test tape as loud as the others.
In my experience weak, distorted or dead signals in one or more tracks of a portastudio or other ' multitrack cassette recorder are often resolved by replacing the coupling capacitors surrounding the operational amplifiers which provide amplification in these circuits.
So in this video I show how I interpreted the schematic, colour coding in photoshop to identify the audio path, power rails, and major components through which the audio signal passes from the input to the output of the audio circuit and which might therefore account for weak signal, or bad impedance matching between gain stages resulting in gain issues:
I then go on to make a list of these parts' reference designators(c101, r202, are reference designators, sometimes I just call them "parts numbers" but sometimes that gets mixed up with serial numbers, which is somethingdifferent...I digress....), identify them on the board using sharpie then absent-mindedly replace them while I watch TV (sic).
I find that at least 4 times out of 5 this works, and it did this time. Even where further diagnostics are required, I figure touching up sodler and replacing capacitors which are 20-40 years old is good due-diligence/future-proofing anyway? You mileage may vary....
Her eis another video about colour coding schematics
• Using Photoshop to Und...
And this one shows my approach to soldering in more detail
• Tascam Porta 03 | Repa...
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1 сен 2022

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Комментарии : 24

@no_wrong_notes Год назад

I just found this channel and I'm self taught with very little electronics experience. I have a 424 mkiii that I've owned since the nineties and recently re-soldered the power supply jack (it was loose and would kill power in the middle of recording). That little exercise boosted my confidence, and now I'm using my four track more often! I am very shaky on schematics but this is VERY HELPFUL. I know channel 3 on mine is not as strong in level and I always thought there was nothing I could do about it. You've inspired me to dive into this deeper and dust off my electronics bench!

@Tetrakan Год назад

Thank you. If you look through my recent videos there are a few with schematics in the thumbnails which might be worth you watching. Also consider whether the playback or record amp trimpots for your track 3 simply need turned up? See this video ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-cYoXWq3N5js.html

@nodrog567 Год назад

Very nice!

@markbilyeu1129 Год назад

Another extremely helpful video - thanks for sharing your procedures!

@BadChizzle Год назад

Thanks

@abalaster9085 5 месяцев назад

Thanks for this video. I've got a similar issue with my 488 mkII on channel three, where it records alright (flipping the cassette, I can hear channel 6 having the audio there sounding alright), but there's no playback (can faintly hear what I've recorded). Going to go in and see what needs replacing, fingers crossed, taking my chill pill

@Tetrakan 5 месяцев назад

Likelihood is the electrolytic capacitors around the playback amp opamp need replacing, you could just replacecthose and cross ypur fingers or use an audio probe or scope to establish for certain where the signal from tape cuts out, good luck!

@abalaster9085 5 месяцев назад

@@Tetrakan Thanks a lot! Turns out it's the low pass filter. Have you ever had to buy one? The service manual simply says "Coil LPF 85KHZ", and the component itself says "085181 Tk-", but I'm struggling to find a suitable replacement online with just that info. Any pointers would be appreciated! Thanks again.

@francescahanson2128 Год назад

Thanks so much for your videos! They have been extremely helpful! I was wondering if you ever have had to totally replace op amp chips? I have a porta two I'm working on where all channels see no signal at all when monitoring or recording line in. Playback is fine and VU meters respond appropriately to tape audio. On a whim I pulled out one of the NJM2068SD op amps to test it and it is definitely fried. Idk why all of them would fail, but it seems like a possible culprit? Are those chips serving as preamplifier circuit for the four mic/line inputs?

@Tetrakan Год назад

Yeah occasionally I do have to replace opamps, sometimes they are visibly singed or have actually blown up, osmetimes they just give me funny behaviour and replacing them fixes the issue. It comes up less often than bad electrolytics and bad solder though. How did you establish it is definitely fried? I don't have the schematic for the porta two, closest i ahve is the porta one, but I don't see the chip you mentioned in the parts list. Can I double check it is a porta two, not a porta 02 you are working on? (easy to mix those two up becasue the names are so similar but one is late 80s black and massive the other is miod nieties tiny and blue))

@francescahanson2128 Год назад

@@Tetrakan Good to know ! It is the 80's black shell one. I haven't totally ruled out electrolytics, though I did mostly recap at least one channel and it hasn't improved anything. I know I should just do the whole board. I removed one NJM2068SD chip and tested it as a comparator (0v to both inputs and reading the output which was always V+). It's possible it was damaged when I desoldered it, but I was pretty careful? The datasheet says those chips are optimal for audio preamplifier circuits and there are exactly 4 near each trim pot so I just assumed. I wish I could check the schematic somehow though. I may be misunderstanding the signal chain but it seems if the deck can playback audio and bounce to other tracks then it must be earlier on.

@Tetrakan Год назад

Ah ok. For whatever reason I haven’t had to fix the preamp stage of portastudios very often. I seem to remember from scanning over schematics that it’s common for a pair of transistors to provide most of the gain but there’s an opamp stage immediately after the gain trim variable resistor, I assume that’s a buffer isolating that stage of the circuit, but whether that’s the topology of your porta two, I can’t say without the schematic. Out of interest how did you set up the comparator test? Breadboard and a bench power supply? I really need to get myself set up for that sort of testing! If you hadn’t confirmed the chip was faulty, my first thought would be something is preventing power from reaching that stage of the circuit, on the principle it seems unlikely that all inputs were damaged get the same way but likely that they share a power rail. If you find a schematic for that model for sale I’d be willing to chip in if I could get a scan or digital copy, let me know how you get on.

@francescahanson2128 Год назад

@@Tetrakan Hey thanks so much! This is all really helpful. Yes I just used breadboard, power supply, and multimeter. I am very lucky because a friend of mine is an electrician and he has given me a lot of old equipment being thrown out by his work. I think disruption of power distribution is also very likely! Especially because the vu lamps also dont come on even though I tested all four bulbs and they work! I've cleaned all pin headers and sockets etc. with deoxit so it isn't that either. The chips just came today and I swapped one out. Just gotta connect it all together and power it up again, I will also test the voltages on the chips V+ V- legs. Thanks so much re:schematic! I was definitely thinking about it and I would be more than happy to make a scan for you. I would love for it to be accessible to people and Im grateful you share them all on your blog. I will let you know. Do you want to exchange email?

@Tetrakan Год назад

Great, I'm still wary about putting my email in the public comments, would you be able to message me on Facebook?

@heffe8056 Год назад

you are a wizard haha. How does someone learn all of this (how to read diagrams mostly i guess)? My repair guy just moved across the country and my record button just went caput on my 488 mk ii. figure its time I start repairing it myself.

@Tetrakan Год назад

In a word.. aspergers! I just obsessively sat and took the buggers apart and reassembled them until their secrets revealed themselves to me, interspersed with a lot of youtube videos on amp and radio repair etc, looking for clues that I could apply to cassette tech.

@jeetout660 Год назад

@@Tetrakan Do you have any recommendations on books as far as learning electronics?

@Tetrakan Год назад

@@jeetout660 I didn't really learn that way becasue I'm dyslexic and read slowly ...but...I have "How to diagnose and fix everything electronic" by michael Jay Geier, and its pretty good. Only thing ids he loves loves loves the asocilloscope, and never mentions an audio probe, and for portastudios at least the audio rpobe is essential. much cheaper whereas the oscilloscope is nice ot have but not essential in my opinion. Make: elecrtonics by Charles Platt seems like a good introductory book. I recently got into an excellent YT channel/ site called RSD academy, he strats at the beginning, but also goes heavy into the detail with the theory, up to a pro/college level, the videos are accompanied by articles you could read or print out.

@jeetout660 Год назад

@@Tetrakan Thanks for the reply! I’ve been fascinated with electronics for sometime but I’ve only made a few guitar pedals. I’m about to get a Yamaha MT8x and I’m hoping all it needs is a belt swap. The seller says it only plays for 10 seconds but the reverse and fast forward work.

@Tetrakan Год назад

That's a known issue on the MT8x it's to do with the shortest motor belt needing replaced. It can be a bit hit or miss getting g the right size though...3rd island made a video about it, here ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-PhEQbCKqBMc.html