Cassette deck power supply droopy - Page 1
Forum section: electronics / beginners

author: Quantumplate
author_rank: Regular Contributor
timestamp: October 24, 2023, 11:47:00 am
content: Hello,I could have put this in the repair section maybe, but i plumped for here. I have this Pioneer ct-f700 deck which has a slightly low output on the main rail which should be 24v.(spec are figures in black pen) i'm getting 22.7v (figures in blue ink) on the o/p. Downstream, when i try to calibrate the playback level on the Dolby board, i'm 30mV short - the trimpot is at the max it can go.So my thinking is that if i get back to spec on the power supply board, then i should be in with a chance back on the Dolby board.I replaced all the electrolytics in the attached circuit after initially only replacing the 33uF one under the base of Q1. Just replacing that one gave me 1/3v more on the output. However, replacing all the caps brought me back where i started at 22.7v o/p.Ok, so my brain finally twigged the Zener wz-250 was measuring 24v across. It should be a nominal 25v according to the little info i can find on it. With a bit more current it might be up to 25.2v in circuit, which, with the two base/emitter transistor drops would output 24v hooray.So i'm wondering whether to go looking for a replacement Zener, or to add two diodes in series with it to bring the base of Q1 up.And a general question about the notion of old Zeners having a drooping voltage in old age (1978 deck)?Thanks

author: wasedadoc
author_rank: Super Contributor
timestamp: October 24, 2023, 12:09:45 pm
content: I doubt that raising the rail voltage is going to fix your issue with the Dolby level.

author: Quantumplate
author_rank: Regular Contributor
timestamp: October 24, 2023, 12:18:14 pm
content: Quote from: wasedadoc on October 24, 2023, 12:09:45 pmWell, bit of detail i left out: when i initially replaced the one cap i checked the Dolby cal and had a boost of 10mV.

author: BeBuLamar
author_rank: Super Contributor
timestamp: October 24, 2023, 12:27:43 pm
content: I think the real circuit if more complex than what you draw but how about simply supply the rail with exactly 24V with an external power supply and see if you can get the Dolby circuit to work correctly?

author: Quantumplate
author_rank: Regular Contributor
timestamp: October 24, 2023, 12:36:03 pm
content: Quote from: BeBuLamar on October 24, 2023, 12:27:43 pmYes, that's an idea thanks. If the relationship between the rail and Dolby voltage is linear then 24v should get me within the zone without maxing out the trimpots.The circuit i've drawn is comp!ete for that part of the power supply.

author: BeBuLamar
author_rank: Super Contributor
timestamp: October 24, 2023, 01:22:59 pm
content: What I meant that I believe the power supply would supply more voltages than just the 24V. So without seeing the other parts of the power supply I am afraid connecting an external power supply may cause problem with the other power rails.

author: Quantumplate
author_rank: Regular Contributor
timestamp: October 24, 2023, 02:13:07 pm
content: Quote from: BeBuLamar on October 24, 2023, 01:22:59 pmOk, got you. Yes, you're right of course, there's another rectifier circuit that provides for the motor and lamps. I did just try connecting 24v to see what did and didn't function. All i got was one dolby indicator light. So i'm a bit of a novice at connecting external supplies into circuitry like this. If i power the deck with mains as normal, can i still plug in the 24v using my external PSU? Like having two supplies in parallel at that point in the circuit (the r.h.s. of my hand schematic).

author: BeBuLamar
author_rank: Super Contributor
timestamp: October 24, 2023, 02:35:56 pm
content: I would have to disconnect the positive of the deck power supply and substitute with the positive of the external power supply. The negative would be connected together. However, I have to be sure not to ground something that is not supposed to. The common connection may be floating and not at ground potential.

author: Quantumplate
author_rank: Regular Contributor
timestamp: October 24, 2023, 03:53:06 pm
content: Quote from: BeBuLamar on October 24, 2023, 02:35:56 pmSorry for the out of focus pic. Best i can do on this device, but hopefully you can recognise the main elements. My drawing is from the upper part of the schematic. Ac lamps being powered from the lower winding of the transformer, and the middle winding goes off to the motor. The lower wire on the third (bottom) winding is at -7v, so does that mean the commons (bold lines) are floating or not?Getting back to my original question - would you source a better Zener for this, ie on spec 25v, or add two diodes to the circuit?Many thanks!

author: BeBuLamar
author_rank: Super Contributor
timestamp: October 24, 2023, 03:59:41 pm
content: I think if the zener voltage is 25V then the output would be a little higher. I want to try to supply the 24V to the power rail only because I don't think having 1.3V lower than specs would be the problem.

author: Quantumplate
author_rank: Regular Contributor
timestamp: October 24, 2023, 04:14:58 pm
content: Quote from: BeBuLamar on October 24, 2023, 03:59:41 pmI may well be losing more voltage due to old caps throughout the deck, but my idea was to learn and tackle stage by stage starting with the all-important power rails.The Zener voltage is SUPPOSED to be 25v. I hope that was clear.So if i disconnected the 24v rail internally and plugged in my external psu whilst powering the other windings as normal, do you see a problem? You didn't really answer my floating ground question just now. I get confused/go blank on some of this stuff. Really hungry to learn.

author: wasedadoc
author_rank: Super Contributor
timestamp: October 24, 2023, 04:18:36 pm
content: Service manual available athttps://elektrotanya.com/pioneer_ct-f700.pdf/download.html

author: BeBuLamar
author_rank: Super Contributor
timestamp: October 24, 2023, 08:08:05 pm
content: I think it's OK to do what you said. Also when you disconnect the 24V I would want to see if the voltage goes up.

author: MathWizard
author_rank: Super Contributor
timestamp: October 25, 2023, 05:28:02 am
content: What kind of resistors are feeding the zener ? Maybe they drifted too, or the BJT resistors.Yeah if there's nothing else a bit off, I'd replace zener, maybe even use 2 or 3 smaller ones in series if heat/power dissipation is a problem.But you could try adding a diode or 2 as well, if you have a program like LTSpice, iyou could try different diode/resistor combos. Although standard LTSPice doesn't have a big selection of zeners, but there's models out there.You could even use a 3 pin adjustable zener reference, like a TL431, and more resistors, and a variable resistor if you really need more voltage to that board.

author: Quantumplate
author_rank: Regular Contributor
timestamp: October 25, 2023, 01:01:41 pm
content: Quote from: MathWizard on October 25, 2023, 05:28:02 amJust carbon film type. They measured bang on. I put in two 1n4007 diodes as i had some lying around. The output is now 24.1v so that's good. I did the Dolby calibration, but i'm still short by about the same amount - i must have been hallucinating when i saw the voltage creep up before. So, many variables, could be the cal tape even though it's new. But probably the whole deck needs a recap - i see kits available at a price.Thanks for your and everyone's input.

author: jwet
author_rank: Frequent Contributor
timestamp: October 25, 2023, 02:34:03 pm
content: I got your PM-I'm not surprised that the rail voltage being a bit low did nothing to fix things. Zeners are about a 5% tolerance item and this varies with current etc and the you have two Vbe's with their own spread. This isn't really a regulated power supply- its just a zener shunt followed by a voltage followers. Putting two diodes in series with the zener was a good test- you can remove them. If this rail was critical to to calibration, it would be regulated.Also there is no "aging" mechanism for zeners or other power semis if they're operated within specs. Likewise for the transistors that follow. This power supply has made this somewhat low voltage since birth.I would look elsewhere for being out of pot range. Dolby is a record and playback system, high frequencies are enhanced on record and filtered on playback. Since most tape hiss is HF, this tends to reduce hiss. Look at the filter circuits to see if they produce the kind of boost and cut that is required. Tape heads do wear with use (not age) and they do lose some of their high frequency mojo. Heads also get magnetized and this effects playback and record bias levels and frequency response. Thoroughly clean and demag the head and then look at signal chain stuff.

author: BeBuLamar
author_rank: Super Contributor
timestamp: October 25, 2023, 03:43:28 pm
content: Quote from: Quantumplate on October 25, 2023, 01:01:41 pmI didn't think the slightly lower voltage is the problem but I couldn't find the Service Manual with full instructions on how to calibrate the deck. Can you show me a link to one?

author: Quantumplate
author_rank: Regular Contributor
timestamp: October 25, 2023, 03:49:25 pm
content: Quote from: BeBuLamar on October 25, 2023, 03:43:28 pmSurehttps://www.hifiengine.com/manual_library/pioneer/ct-f700.shtml

author: BeBuLamar
author_rank: Super Contributor
timestamp: October 25, 2023, 03:50:23 pm
content: thanks but I got banned from Hifi engine so I can't download from them any more.

author: Quantumplate
author_rank: Regular Contributor
timestamp: October 25, 2023, 04:06:09 pm
content: Quote from: jwet on October 25, 2023, 02:34:03 pmThanks John. They don't droop like humans then. That's what i thought.So the head looks very new and i've demagnetised it previously. Perhaps appearances are deceptive though. I did the playback equalizer adjustment before the Dolby calibration as per the book. Got on spec with the left channel but fell 1.5dB short on the right. Again, ran out of pot range.

author: BeBuLamar
author_rank: Super Contributor
timestamp: October 25, 2023, 04:28:10 pm
content: I downloaded the manual from manuallib but I didn't see the dolby level calibration procedure? Are you supposed to adjust the level while playing a test tape? Could the level on the test tape be wrong?

author: jwet
author_rank: Frequent Contributor
timestamp: October 25, 2023, 04:58:38 pm
content: A test that you could try would be tape record and playback some sweeps or just high audio tones with dolby on and off on this machine. See what the frequency response looks like. It might also be good to play this tape on a known good recorder to see if the problem is in record or playback. You can buy test tapes but they're likely getting a bit rare by now...

author: wasedadoc
author_rank: Super Contributor
timestamp: October 25, 2023, 05:07:54 pm
content: I had a Dolby calibration tape back in the first half of the 1970s when I built my own external Dolby B record/replay box. Schematic and licence could be purchased from Dolby Labs for a very modest fee. Totally discrete design using about 5 bipolar transistors and 1 FET (the variable gain element).The tone on the tape is 400Hz so replay level is not much affected by head azimuth or equalisation errors. If one channel is adjustable to the correct level with still some range left on its pot but the other channel is low even turned fully up I would suspect a problem with its replay amp. Try bridging each of the audio coupling caps (not power rail decouplers) with another capacitor in turn to see if one has gone very low capacitance or very high ESR.

author: Quantumplate
author_rank: Regular Contributor
timestamp: October 25, 2023, 05:46:35 pm
content: Quote from: BeBuLamar on October 25, 2023, 04:28:10 pmYes, you play a 333Hz tape recorded at 0dB 160nWb/m and adjust for 650mV at the test point (upper right of the Dolby board). The tape is a GennLab, supposed to be an accurate reproduction of the original Pioneer specific tape, which i'm sure it is.

author: Quantumplate
author_rank: Regular Contributor
timestamp: October 25, 2023, 06:24:52 pm
content: Quote from: wasedadoc on October 25, 2023, 05:07:54 pmGood tip thanks. I wonder if you still have your homemade Dolby unit....Am i right in thinking that if i can only reach under voltage at the Dolby test point, then when i play a prerecorded Dolbyfied album, more of the sound will be 'swallowed up' so sounding not right? That is certainly the experience i'm hearing.

author: BeBuLamar
author_rank: Super Contributor
timestamp: October 25, 2023, 06:42:37 pm
content: Tape decks may have the 0 dB calibrated at different flux level 160nWb/m, 200, even 250. However, the Dolby level is always 200 nWb/m. So using the 160 nWb/m tape the level is about 2 dB low.

author: Quantumplate
author_rank: Regular Contributor
timestamp: October 25, 2023, 06:47:56 pm
content: Quote from: BeBuLamar on October 25, 2023, 06:42:37 pmWell i agree, but Pioneer seem to do things their own way. See page 25 of the service manual. It states their STD-341a tape, 333Hz, 0dB, 160nWb/m is to be used, and emphasises the importance of this adjustment for determining the Dolby level.

author: wasedadoc
author_rank: Super Contributor
timestamp: October 25, 2023, 07:09:47 pm
content: Quote from: Quantumplate on October 25, 2023, 06:24:52 pmYes still have it. Been in the loft for years. Used it with an Akai GXC-40D cassette deck. That has been boxed up for years but I did fire it up about 2 years ago to check its belt. It was all fine unlike the belts in a much, much younger Sony 3 head Dolby B/C/S deck that I sorted out for a colleague. Belts had turned to a gooey black sticky mess.Yes, the sytem works by attenuating low level, high frequency components. So if the playback level of that channel is too low it will sound less "bright" than the other one.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: October 25, 2023, 07:17:44 pm
content: Quote from: BeBuLamar on October 25, 2023, 06:42:37 pmThat is not quite correct. The Dolby level for Compact Cassette is 200nWb/m ANSI = 218 nWb/m DIN (according to Dolby Lab information), so it is roughly +2.8dB from 160nWb/m DIN.The CR860 Dolby chip used in Pioneer is an equivalent of NE545 and the Dolby reference level is 580mV on pin 3 (playing back a true Dolby level cassette, like Teac MTT-150 or my L04) .CheersAlex

author: Alex Nikitin
author_rank: Super Contributor
timestamp: October 25, 2023, 07:41:11 pm
content: I've looked at the service manual, the tape used was 250nWb/m, not 160nWb/m, no surprise you've run out of the adjustment range! 650mV on pin 7 is roughly correct for 250nWb/m DIN level (the accurate figure is 665mV). Use a proper level cassette and it should adjust just fine.CheersAlexP.S. - you can see from the manual that for 160nWb/m the voltage is 410mV (the accurate figure is 425mV) - it is clear from the level diagram.

author: Quantumplate
author_rank: Regular Contributor
timestamp: October 25, 2023, 08:35:23 pm
content: Quote from: Alex Nikitin on October 25, 2023, 07:41:11 pmHi Alex and thanks!That's altered things a bit.Before i read your post i just checked pin 3 using your dolby tape and got 860mV. Oops.But what happened to the 580mV you quoted originally? out of interest..I've scoured the manual but i can't see where they ever mention a 160 or 250nWb/m. What page was it on? I found the level diagram ok.Head's spinning a bit as this deck is 1978 - i thought that was pre the 250 cal level established in 1981? It also means Gennlab have the wrong information on their page. But i'm sure you're right. It makes sense now, the numbers - it seemed crazy to run out of pot adjustment.Btw what do you mean by 'the accurate figure'?CheersTim (thinkchronicity)

author: Alex Nikitin
author_rank: Super Contributor
timestamp: October 25, 2023, 09:23:50 pm
content: As I’ve said, the voltage should be 580mV for the Dolby level tape, 665mV for 250nWb/m DIN, 425mV for 160nWb/m DIN. By “accurate “ figures I mean derived from the Dolby level (580mV) and 160-218-250 ratios.CheersAlex

author: Quantumplate
author_rank: Regular Contributor
timestamp: October 25, 2023, 09:37:29 pm
content: Quote from: Alex Nikitin on October 25, 2023, 09:23:50 pmYep, my mistake. Brilliant thanks.

author: Quantumplate
author_rank: Regular Contributor
timestamp: October 27, 2023, 11:40:26 am
content: Quote from: Alex Nikitin on October 25, 2023, 07:41:11 pmActually, 650mV on pin 7 (the test point) is right as this pin is ~0.3dB lower than pin 3.Just for the record!