Cassette deck: uncooperative scratchy potentiometer & adjustment - Page 1
Forum section: electronics / repair

author: tooki
author_rank: Super Contributor
timestamp: November 09, 2017, 10:45:05 am
content: Earlier this year, I bought a (supposedly) new old stock Sony TC-KB920S cassette deck (one of the very last standalone decks Sony ever made), date codes place it at late 2003.Anyway, the record level pot is scratchy, and several cleanings have brought no improvement: while the spray is still wet, the crackling goes away, but once it's dried out, it becomes scratchy again. (FWIW, none of the other pots are scratchy.)The first cleaning I did, a few months ago, was just with Kontakt 60 (before learning that it's bad to leave it on without rinsing*), the second cleaning, done yesterday, was Kontakt 61 to clean, rinsed liberally with Kontakt WL, then applied Kontakt 61. Again, great at first, but a few hours later, scratchy again. Repeating it later in the evening, same thing: smooth at first, scratchy when dry.Is there something else to try? Or should I just replace the pot?Theservice manualof course lists only a Sony part number, but I got lucky on my first guess that it's an ALPS, and quickly found the part, theRK14K12C0A0T. Well, almost: the original part has the threaded bushing and the replacement doesn't, but since the bushing isn't actually used on this device, it doesn't matter.The other thing is calibration/adjustment: despite being supposedly NOS (and looking pristine), it doesn't perform as well as I remember the cassette deck in my old stereo as being. Moreover, this deck has Dolby S, but when I use S, it sounds more muffled than I expect. Anything I should try? Is it just that all the tapes are also NOS? If anyone has any tips, or knows of some really good guides, lemme know. (Or if anyone nearby wants to help me, lemme know!)FWIW, the belts look absolutely pristine.Thanks!P.S. If anyone has a similar-vintage QS-series Sony CD player for sale, ideally in silver, I'd be interested. (CDP-XB630/720/730/740/820/920/930)* The can I have has instructions in German and French, and only the French instructions include the "rinse with Kontakt WL, then apply Kontakt 61" steps. It's a German company...

author: tooki
author_rank: Super Contributor
timestamp: November 09, 2017, 11:03:09 am
content: Mouser also has the same pot with the bushing,RK14K12D0A18, but with a shaft that's 2.5mm longer, so I'd have to trim that down. I'm not sure if the bushing is used as a locator within the front panel. (No nut used.)

author: capt bullshot
author_rank: Super Contributor
timestamp: November 09, 2017, 11:05:21 am
content: Replace the pot, IMO the only way out.Regarding sound quality: Back in the times one was used to the sound of the tapes, so a high end one indeed was believed to sound some nuances better than an el cheapo. Today we're used to CD or MP3 sound, that's a completely different thing, so IMO all cassette tapes sound bad, no matter high end or not. Even high end back then is quite lousy in todays standards. Did you ever look at a classic TV screen after getting used to modern HD LCD 40" and up flat screen? Same for modern digital cinema and older high end analog film.AFAIR, using whatever Dolby always had worse sound than without.

author: Le_Bassiste
author_rank: Frequent Contributor
timestamp: November 09, 2017, 11:38:49 am
content: basically concur w/ bullshot here, but one more thing to try:if scratching happens w/o input signal, then chances are, that coupling capacitor(s) at the pot's wiper (or end) have dried up over time, and the pot now gets some DC leakage current from the downstream (or upstream) amplifier bias voltage. exchanging those caps may bring down scratch noises to a, well, "acceptable" level.as for the muffled sound of NOS tapes: after long storage, these tapes may have picked up some "DC" biasing from the earth's magnetic field. tapes w/ such biasing are prone to loss of high frequency response. try to give them a full erase run in your deck (which uses HF erase) to restore high frequency performance.on DOLBY S performance, i can't give any comments. however, my REVOX H1 has 20 years of use as master tape recorder on the clock, and it's still sounding superbly with a good metal tape and activated DOLBY C.if freshly recorded material on a high quality tape sounds muffled, you may want to reduce HF bias settings on your recorder. reducing HF biasing will boost high frequency audio signals ( cymbals)considerably, but will increase low- and midrange THD.hth!edit: about DOLBY performance (sound quality) on tape decks in general: DOLBY NR totally relies on absolutely accurate magnetizing levels of the tape. if the soundlevels of the recording do not exactly match the playback expander's threshold levels, all sorts of "pumping" and overtone loss will occur on the playback. in order to properly adjust rec/playback levels on the head amplifiers, you "normally" need a special adjustment- or calibration- tape. good luck in finding one these days...

author: DimitriP
author_rank: Super Contributor
timestamp: November 09, 2017, 12:07:22 pm
content: QuoteClean the heads.Use the cal button that "should" figure out the correct bias for the tape you are using.If the calibration doesnt' give you the resultsyou expect,listen to the playback while you are recording (push the Monitor button) and adjust the Bias manually

author: tooki
author_rank: Super Contributor
timestamp: November 09, 2017, 12:49:16 pm
content: Thanks for the answers so far!Quote from: capt bullshot on November 09, 2017, 11:05:21 amOK, I will. They're not that expensive.Quote from: capt bullshot on November 09, 2017, 11:05:21 amWell, though that may be the case for a lot of people, but I pretty much grew up with CD, so that was always the benchmark for me. I have a discerning ear, and the deck I used in my teens produced very satisfactory sound. Moreover, the Dolby S (which obviously my 1992-model deck didn't have) on this doesn't sound as good as it should, based on demos of it I've seen elsewhere, e.g. Techmoan. His video about cassettes is titled "better than you remember", which was probably true for most people, but not me, because I was used to high-quality tape.I need to grab my old tapes from my mom's house, to see how those recordings play now.Quote from: Le_Bassiste on November 09, 2017, 11:38:49 amWith nothing connected to the input jacks, there's no scratching. With a source connected and turned on, but with nothing playing, a little bit of scratching. With music playing, lots of scratching. So I guess that means the caps are OK?Quote from: Le_Bassiste on November 09, 2017, 11:38:49 amOh, interesting! I'll have to give that a try.Quote from: Le_Bassiste on November 09, 2017, 11:38:49 amQuote from: DimitriP on November 09, 2017, 12:07:22 pmRight, gotcha. My tests so far have been by using the deck's built-in bias calibration function. (If I understand it correctly, it records an alternating high/low frequency test signal, and uses the two VU meters to show the signal level of low and high, so you can balance them. Right?)Quote from: Le_Bassiste on November 09, 2017, 11:38:49 amYeah, indeed. I'll need to see if the old man who has a TV repair shop around the corner has them and will let me borrow them.Quote from: DimitriP on November 09, 2017, 12:07:22 pmI thought I did when I got it. But I'll do it again.Could the heads need a demagnetizing? As I said, it's supposedly a NOS machine. (When I got it, it had no box, but still had the packing materials in the cassette compartment. It was from the liquidation of a Sony dealer.)

author: capt bullshot
author_rank: Super Contributor
timestamp: November 09, 2017, 01:08:04 pm
content: So heaving read all the suggestions, I'd give most of them a try (regarding the sound quality) - nothing to loose. Don't know the properties of this deck, but it seems to have a "listen while recording" (Hinterbandkontrolle auf Deutsch) function, this makes it easier to try the adjustments. Maybe I'd also try to adjust the record / playback heads alignment, first try while playing a known good cassette tape. Though I wouldn't expect anything de-adjusted with a NOS and obviously never used before device.

author: tooki
author_rank: Super Contributor
timestamp: November 09, 2017, 01:22:46 pm
content: Quote from: capt bullshot on November 09, 2017, 01:08:04 pmYes, it does have tape monitoring (the German word "Hinterbandkontrolle" is indeedfarmore precise and descriptive), and I have used it.Yeah, I'm very leery to touch the head alignment.I'm pretty sure I did do some recordings with the bias tweaked by ear instead of by calibration, and listening to it, it's far more pleasing.For what it's worth, it's not that I even need a tape deck. But I like it, and this machine is a beauty. (And the matching MDS-JB930 MiniDisc deck even more so.) There's just something special about a mechanical gadget producing lovely music, it's tactile and time consuming. Not just music as background music like so many people now default to.

author: capt bullshot
author_rank: Super Contributor
timestamp: November 09, 2017, 01:27:42 pm
content: Quote from: tooki on November 09, 2017, 01:22:46 pmDone that back then just by listening, didn't have any suitable equipment. Quite an easy job.

author: tooki
author_rank: Super Contributor
timestamp: November 09, 2017, 01:35:46 pm
content: Quote from: capt bullshot on November 09, 2017, 01:27:42 pmBut only if you have known-good tapes, right? I wouldn't want to re-align to a, um, custom alignment.Quote from: capt bullshot on November 09, 2017, 01:08:04 pmFYI, some pages with specs:https://www.hifiengine.com/manual_library/sony/tc-kb920s.shtml(EN)http://www.hifi-wiki.de/index.php/Sony_TC-KB_920_S(DE)User's manual EN:https://www.sony.co.uk/electronics/support/res/manuals/3862/38623011M.pdfOh yeah, one weird little aside: this thing has an IR sensor, but never shipped with a remote! But it works with the remote from my old stereo (also a Sony) that I kept as a souvenir when disposing of the stereo after it released the magic smoke around age 18.

author: capt bullshot
author_rank: Super Contributor
timestamp: November 09, 2017, 02:06:27 pm
content: In case you've got old tapes you want to play back, your best alignment would be against these ...That thing looks nice and cool, Sony style. I had two Sony DAT recorders in the 90s, when I checked them last year, the drive mechanism were totally borked due to long term storage. Gave them to the local electronic waste disposal.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: November 09, 2017, 02:09:11 pm
content: Quote from: tooki on November 09, 2017, 01:35:46 pmWell, I listen to music only from tape and vinyl, as for me these are better sounding sources than the CD. Only a high resolution digital is OK, but it is not as convenient and requires a lot of attention to sound right.1) You can get an alignment cassette from me, if you would like to do a proper job on the deck. Have a look at my website and PM me if interested.2) You may have a look at Tapeheads.net, it is a friendly community dedicated to all things tape related. You'll get lots of good advice and support there.3) There are ways to treat the pot with special DeoxIt Fader Lube, unless the track is broken (Kontact wouldn't be any good, as you've discovered), however if a replacement pot is available, it will be a cheaper and better solution.4) This Sony is not the best choice of a deck, it is by design not so good sounding unit. There are some much better options - even for not much money.CheersAlex

author: floobydust
author_rank: Super Contributor
timestamp: November 09, 2017, 02:13:28 pm
content: Noisy potentiometers can also be caused by the presence of DC. This can be due to shorted coupling capacitors, missing coupling capacitors (sometimes a bad design), or direct coupling to IC's with high input bias currents.The schematic I think shows Line In direct-coupled to the REC level pots. So any DC offset coming into the deck will make the pots scratchy. I would also check the coupling capacitors C101, C201 4.7uF electrolytics.I have seen small DC offset on potentiometers cause them to fail noisy, due to ion migration, in applications like recording studio consoles, which are powered on a lot. No amount of cleaning seems to fix it other than replacing the pot and fixing the DC problem. Some op-amp's offset changes with age (including polarity) which can also cause these problems.

author: tooki
author_rank: Super Contributor
timestamp: November 09, 2017, 02:45:02 pm
content: Quote from: capt bullshot on November 09, 2017, 02:06:27 pmNooooo! They probably just needed $10 of belts!

author: capt bullshot
author_rank: Super Contributor
timestamp: November 09, 2017, 02:55:16 pm
content: Quote from: tooki on November 09, 2017, 02:45:02 pmThat may be true, but hadn't a use for them anyway and needed storage room. Due to a "accident" while they were in storage (shelves crashed), they also had some broken knobs. If I knew back then, I'd have happily given them to you. Still have a box of recorded DAT tapes around, don't know if they are useable anymore.

author: tooki
author_rank: Super Contributor
timestamp: November 13, 2017, 03:13:38 pm
content: And deeper the rabbit hole goes...So while the thing was open, a little mishap caused two transformer taps to get shorted for a split second (I am not 100% sure which, but I think it was the ones highlighted in orange in the image attached, pins 7 and 8 on the main PCB). The deck stillworksworked, but I noticed the VFD being brighter than expected, with some crosstalk between segments I'd never noticed before. A bit of probing showed that the -24V rail (highlighted in green) was at about -31V, roughly the voltage of the input to the -24V regulator (the red box). Looks like the pass transistor for -24V (Q708) went short. I put in a BD140 (the only PNP I have on hand with sufficient wattage and the correct ECB pinout), and while it's now regulating again, it's about 4V low. But the input to the base of Q708 is low, too, so I don't think the different transistor specs are to blame. I replaced Q707 (which drives the base of Q708) with another BD140, no difference other than a very slight additional drop in the voltage of the -24V rail. (I kept it, in case it's advisable to put it back.)Both sides of zener D716 and its parallel C715 measure at -19.5V, so probably one of them has failed short? (Powered off, 16 ohms across them.) This node, which also has a 47K resistor up to ground, connects to the center tap (highlighted in blue) of the VFD filaments' transformer winding. I don't understand this part of the circuit.The +7.5, -7.5, +12, and +5.5V rails are all fine. (5.5V is actually at about 5.38V.)So I went to measure voltages today to give you more info, and lo and behold, in between last night and now, the thing won't come alive. All I did was unplug it, put the disconnected parts inside, and put the cover back on.- The VFD filaments (which are 4.4V AC powered from their own transformer winding) do glow, and the capstan motor does its power-on thing, but no display, no reaction to buttons.- All voltages check out, save for the -24V rail.- The control CPU/ASIC has 4MHz oscillation on the corresponding pin (manual says it should be 4.1Vpp, but it's about 5.4V peak to peak.)- CPU supply voltages are OK.- Testing more voltages shows that the POWER.OUT output (pin 34) is high, indicating the CPU is not detecting power on the AC detect signal input (POWER.IN, pin 25), and checking that pin measures about 0.17V, whereas it should be high.- Furthermore, the reset pin (pin 27, active low) is high, and scoping that pin on powerup shows no "several 100msec" low as is supposed to happen according to the schematic.- The schematic says the input and output of the reset generator (IC804) should be at 5.5V and 5.3V, resp., but they're both at 5.38V, so no 0.2V voltage drop on the output.- If I short the output of the reset generator to ground (short pins 2 and 3), the capstan motor turns on, and letting go, the motor turns back off. This indicates to me that the capstan control output of the CPU (pin 13) is working, since that pin actively pulls down to turn the motor off.So as best I can guess, the CPU is not getting a reset, and it's not getting the high on the AC detect signal. Tracing that pin out, it's got a 22K pull-up resistor and a 0.1uF cap to ground, and then goes off through diode D711 to the transformer tap that powers the 5V rail. (Powered off, D711 measures about 1.5V forward and 0.6V in reverse, as measured in-circuit.) Measuring the voltage on D711, it's the 0.17V on the anode side, on the cathode it's 8.6V DC, 1.5V AC.(The rectified but unregulated voltage to the 5V supply is supposed to be 8.5V, but it measures 9.9V. But since the regulated 5V rail is within 2% of where it's supposed to be, I guess this isn't a problem?)I'm obviously in way over my head now. I'd be appreciative of any help you can give. (No rush as such, I'm about to leave for a trip for 2 weeks, so won't be able to look at it.)

author: CJay
author_rank: Super Contributor
timestamp: November 13, 2017, 03:27:30 pm
content: Quote from: Le_Bassiste on November 09, 2017, 11:38:49 amConcur on the accumulation of magnetic field but not necessarily on the tapes, degauss the heads.Would also be worth giving them a polish with a gentle metal cleaner (seem to remember Duraglit was the favoured metal polish wadding) to remove any accumulated 'crud', it certainly won't hurt them.

author: tooki
author_rank: Super Contributor
timestamp: November 17, 2017, 07:35:49 pm
content: Anyone got any ideas on the more serious problems described in my previous reply? Thanks!

author: floobydust
author_rank: Super Contributor
timestamp: November 18, 2017, 04:35:58 am
content: The VFD is basically a triode, but the filaments are the cathode, it is the directly-heated type.D716 biases the VFD filaments at (my estimate) -17V which is 7.5 volts (Vz) above the -24V rail. You aren't seeing the 7.5V drop on D716 so it's probably shorted. Check D717 too.If Q708 shorted it might have taken out Q707/D715. I think that and D714 ensures the rails come up in the right sequence, first +5.5V then -24V so IC801 does not latch up.This might be why the MCU cannot come up. But with the -24V rail going high, it might have damaged IC801 as these parts are always run near their max. - it normally sees 30V and got 37V. I just keep that in mind.If POWER IN, and RESET are stuck, check it on ohms to see if the MCU pin is damaged (shorted). I go from the MCU pin to GND.If the -24V rail is loaded then Q708 and IC801 would get hot; otherwise I would see what else could be wrong with the -24V regulator.Possibly you could disable the -24V rail and see if the MCU can start up.The pin numbers on CN901 look flipped between main section and panel board.

author: AndyC_772
author_rank: Super Contributor
timestamp: November 18, 2017, 09:52:20 am
content: Quote from: capt bullshot on November 09, 2017, 02:06:27 pmThat's a real shame, Sony's consumer DAT recorders have some well documented failure modes, but they're easily fixed and a pleasure to work on.: I've picked up a few DAT machines over the last year or so; it's a format I wanted to get into when it came out, but it was way too expensive. Now, of course, even professional machines are relatively cheap, and if you do get one with a fault, chances are there's a full service manual with schematics.I use my PCM-7040 (Sony's top-of-the-line studio DAT machine) as a DAC every day.

author: tooki
author_rank: Super Contributor
timestamp: November 21, 2017, 06:11:06 pm
content: Quote from: floobydust on November 18, 2017, 04:35:58 amThanks, I’ll check these things when I get home. Hopefully my butterfingers won’t fry anything else in the process...