Cassette deck power supply droopy

[Quantumplate]

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

[wasedadoc]

I doubt that raising the rail voltage is going to fix your issue with the Dolby level.

[Quantumplate]

I doubt that raising the rail voltage is going to fix your issue with the Dolby level.

Well, bit of detail i left out: when i initially replaced the one cap i checked the Dolby cal and had a boost of 10mV.

[BeBuLamar]

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?

[Quantumplate]

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?

Yes, 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.

[BeBuLamar]

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.

[Quantumplate]

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.

Ok, 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).

[BeBuLamar]

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.

[Quantumplate]

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.

Sorry 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!

[BeBuLamar]

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.

[Quantumplate]

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.

I 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.

[wasedadoc]

Service manual available at https://elektrotanya.com/pioneer_ct-f700.pdf/download.html

[BeBuLamar]

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.

[MathWizard]

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.

[Quantumplate]

What kind of resistors are feeding the zener ?

Just 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.

[jwet]

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.

[BeBuLamar]

What kind of resistors are feeding the zener ?

Just 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.

I 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?

[Quantumplate]

I didn't think the slightly lower voltage is the problem but I couln't find the Service Manual with full instructions on how to calibrate the deck. Can you show me a link to one?

Sure https://www.hifiengine.com/manual_library/pioneer/ct-f700.shtml

[BeBuLamar]

thanks but I got banned from Hifi engine so I can't download from them any more.

[Quantumplate]

I would look elsewhere for being out of pot range.

Thanks 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.

[BeBuLamar]

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?

[jwet]

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...

[wasedadoc]

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.

[Quantumplate]

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?

Yes, 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.

[Quantumplate]

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.

Good 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.