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Posts: 0 Member Since: 26/03/2017


Nov 9 12 1:50 PM

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Hello All,

After 5 years now, of trying to improve the Stephens 821 series transport, we've finally got enough empirically born data to share w/ current owners. All of these ideas have been tested on various machines and we have a very high level of confidence that they will improve any Stephens deck that is "otherwise" healthy. SO many of them have been monkeyed with over the years that we see many that need to be taken back to a more original state before considering these mods.

The biggest buggaboo with John's 821 servo system was/is flutter. He tried numerous band-aids over the years, but never really quantified what it takes, more globally, machine to machine, to fix things and make them reliable at 15/30/60. We are still furthering the design of NEW servo/converter cards that give more useful improvements, but the following will go a long ways to making the original systems work a LOT better.

We went through all of the various modifications proposed on all of the schematics
regarding stability and came up with a 'fix' that involved some minor
modifications to the original boards. Here they are:

 - Change 4uF capacitor C713 to a 2.2uF capacitor in parallel with a 22uF
capacitor and a 10k resistor in series with it. This slows down the
long-term output of the 60 Hz phase-locked loop but still allows fast
corrections to pass. It's sort of like the dual time constant of the

 - Change the 430k resistor R2004 in the Q2 remote control down to 180k. This
allows the PLL to adjust the tape speed over a wider range. Stability would
be impaired except that we fixed that by changing the time constant above.
This allows the speed pots to not need to be tweaked every other day. It
seems to work very well, except that when it looses lock the speed wanders
around more than it would otherwise. But if the pots are adjusted right it
seems to not matter much.

 - Add a 10uF capacitor and 10k resistor between the collector of Q711. This
is a 'tweak' that is along the lines of the 68k resistor added to some of
the later converter boards, except this goes a bit further. One could
use a trim pot here and perhaps a few different capacitor values that
would be selected by a jumper or switch for tuning to that particular
machine, if it was required.

 - Use the 'modded' values of 220k and 0.047uF for the velocity lead network
C701/R701. This 'mod' is already present on some converter cards. This
should probably be a trimpot. Note that many of the 'revised' boards do not
have the diode network D704,D705,C725 installed.

 - Change C716 and C718 to .22uF from .47uF. This allows more of the speed
pickup signal through to the differential amplifier, but since we have
reduced the high-frequency gain of that amplifier this tends to speed the
feedback loop and stabilizes the speed control.

- In addition, one mod done to the machines is to remove the damping capacitor
from the tape lifter solenoid and replace it with a diode, cathode to
chassis ground. This prevents the tape from wobbling back-and-forth when
stop is pressed. And the servo boards need to have Polyswitch circuit
breakers added so they don't burn up.

With these mods, the speed response is very acceptable, even with a
somewhat sticky roll of tape. As well, it seems to exhibit much less of the
instability when going from rewind to play. We believe the source of that
instability is actually mechanical - maybe the tilt of the guides is off
and it takes about 10 seconds of normal playback to get it back to normal.
It could also be the brushes in the servomotors being a bit cranky.
As stated earlier, there are other issues that should be addressed on
a Stephens machine. The guides need to be NOT too smooth...which causes stiction
(800 grit sandpaper, properly used to roughen up the guides a bit), the speed pots may
need to be replaced (often bad) and power supply cap upgrades are all worth looking

Hoping this help the lucky few that own these machines.

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Platinum Blonde

Posts: 2,150 Member Since:02/02/2011

#1 [url]

Nov 9 12 2:15 PM

HI David,

I certainly don't own a Stephens, but if I did, I would be so so grateful for this. It is very very generous of you to share such hard earned information.

    kindest regards,


Ruh Roh . . . . .

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New Forum Friend

Posts: 0 Member Since:26/03/2017

#2 [url]

Nov 9 12 2:20 PM

Generosity is the hallmark of this forum. Glad to contribute when possible.

The Stephens machines are worth every effort to keep them in the world a little longer.



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dr funk

Platinum Blonde

Posts: 1,606 Member Since:24/12/2011

#3 [url]

Nov 9 12 4:12 PM

Hi David.  I don't own a Stephens either, but I just wanted to also thank you for your generosity in sharing this valuable information.


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New Forum Friend

Posts: 0 Member Since:26/03/2017

#5 [url]

Nov 10 12 2:42 PM

Incredible David, thanks.   I'd buy one just to do these mods...!


You're welcome.

There are 3 guys in your town that really SHOULD do these mods. One of them, that lives on LI, is probably just trying to get his dried out right now.

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New Forum Friend

Posts: 0 Member Since:26/03/2017

#7 [url]

Nov 12 12 7:32 PM

Any decent tech, armed with some test equipment and a Stephens 821b schematic could follow the theory of operation and implementation of it in any example and find a lot of improvement in these mods. Simple fact. And, I DID say any "otherwise healthy machine. SO many of them have been monkeyed with over the years that we see many that need to be taken back to a more original state before considering these mods." I guess you missed that part. Yes, many Stephens machines have some obvious attempts at evolving the design, but these changes from the original 821b are minor, track-able and reverse-able in just about every instance. Some of the ideas offered earlier started from John's own notebooks and he just never got around to implementing them.

The tach wheel is not just "plastic", it's Kodalux FILM. That's important. And if someone wants to go to the trouble and expense of replacing the original tach system with a more modern, metal one, by ALL means...do so, but if the original is not damaged, and the state of the caps in the opto circuit are healthy, it will work fine. It's a simple chopper circuit, a scope will tell you if you've got a nice symmetrical square wave pulse or not. If the trims don't get them in line, look at the caps first, then the condition of the transistors. The above suggestions are the easy version. If you want to rebuild the machine from the ground up, bon appetit!

We've spent years working with and on these machines. 8 of them, so far. We're building the amps. We can re-brush the motors. New servo cards. Parts from the original stock. Schematics (not just some guy’s collection, ALL of them, notebooks, napkin drawings, computer files)...on and on. I offered these suggestions in the spirit of sharing our hard won victories and celebrating the continued relevance and usefulness of John's machines. If you have any less general suggestions for improvements that are known to work, I'm sure we would ALL love to see them. The biggest problem is getting them TO a good tech. Our goal in supplying this information is to help them help others with MORE familiarity of the Stephens designs. It's entirely too much trouble to go to for just a pissing contest.

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Diamond Forever

Posts: 21,106 Member Since:08/01/2011

#9 [url]

Nov 13 12 2:02 AM

Ryland, your attitude is not appreciated here.

Your posts have been deleted.

We will not have any sort of cat fight which denigrates good information.

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New Forum Friend

Posts: 0 Member Since:26/03/2017

#10 [url]

Nov 13 12 8:11 AM

To be even more specific on these 'modifications', most do not monkey around with the
operation of the servo or speed control cards *much*.

Modding John's boards - they are fragile. Just be careful. We would replace the card altogether,
but it seems less self interested to offer a fix for what you have.
However we *do* know that the servo cards 'burn up' if the output transistor
fails or goes open-circuit. A 'simple' modification is a polyswitch fuse in series
with the parts on the servo card that blow up.
Although a careful competent tech could modify this on an existing card, it
would be better to simply replace the card with one made with modern
laminates and plated through-holes for the robustness...yes, like the ones we are
making now.

Regarding the speed controls, there are only a few issues with John's
original design. The first being that he *did* do some modifications to
stabilize machines as required. But then you swap motors because the brushes
wore out, or whatever, and you then can't get the machine to stay stable. So
what we ended up doing was putting a pot where John had a 'selected'
resistor. In fact, these 'mods' really consist of grouping all of John's mods
that we've seen and putting them into copper - and making the main one a
potentiometer rather than a fixed resistor.

To sum up-

There really are only four 'problems' with the Stephens design.

1. The F-V converter is not that precise, and you have to adjust the
speed control pots once in a while. No other deck that I am aware of has
this problem of pots that need to be fiddled as often. Now, one could either
increase the control authority of the PLL circuit or make the F-V converter
more precise so that you don't have to fiddle. The former is easier (and
also allows easier integration with external synchronizers), the second has
less overshoot on acceleration to speed. MOST F-V converters use more gain
than one transistor and they are more precise.

2. The feedback mechanism via just the tape reel voltage is a bit
suspect and John also realized this - some of his converter cards had a
modification to address this issue; our version of the card has a
potentiometer so you can choose to use this compensation or not - and how

3. The PLL issue. If you decide to give the PLL circuit more authority
than it currently has - or if you are interested in using a frequency-based
synchronizer like the Lynx - then it is a pretty much a must to use a
phase-frequency detector. This is because if the PLL is on its margin of
control the speed will slowly ramp up (or down) then 'jump' in the opposite
direction. A phase-frequency detector will slowly ramp up (or down) and stay
at its control limit if it can't quite get there. This is far preferable on
a speed control application.

4. The optical card. One of the pots pulls a lot of current through its wiper
which is a big no-no and pot manufacturers all warn about long-term pot
degradation when you do this.

IF a Stephens deck is working properly and does not ever get upset at the
odd reel of tape, then maybe leave well enough alone. If the machine is
being cranky or you've replaced the motor brushes or something and need the ability
to get the machine working and stable, the addition of a gain potentiometer
to the converter card really is not that big of a modification and simply is
a copy of what John seemed to do from machine-to-machine.

Well, there is one final issue if you have a Q2.  The autolocator chips do degrade with time
(as do all chips, but especially NMOS.) and some of the chips located
on the autolocator card are either unobtanium OR have all degraded by the
same amount. So the autolocator needs attention in some way but that's not
today's project. We have access to a Z-80 workstation, but have not tackled this one yet.

But, we WILL.

All that is required, as usual, is competence and an understanding of the circuits. John is no longer with us and sending the machine to him is off the menu. WHAT he would do and what he did is essentially outlined here...the only exception or new modification is the
phase-frequency detector, but that part of the circuit is not rocket science. Also, we are here.
If someone wants to send their machine to us, we'll make it right. Happens everyday in the
most arcane service markets. We would NEVER suggest that some rookie solder iron jockey attempt
these changes...OR even understand the ideas in the first place. But then, I didn't post this stuff
just anyhwere. I posted it here because I know this audience and know that the information can be used with competence, caution and understanding. Yes, all Stephens 821b machines (who said anything about the A?) are different, but not deeply...and certainly not beyond the understanding of this crowd.

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hank alrich

Platinum Blonde

Posts: 1,732 Member Since:28/01/2011

#11 [url]

Nov 13 12 8:46 PM

David, two thumbs up on this work and sharing of info. Thank you.

hank alrich

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Aqua Marine

Posts: 4,034 Member Since:22/01/2011

#13 [url]

Nov 22 12 5:34 PM

This is rich stuff... Thanks David!

Definitely needs to be stickied!

-Keith Andrews -If I can't fix it, I can fix it so [i]NOBODY[/i] can fix it!

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New Forum Friend

Posts: 0 Member Since:26/03/2017

#15 [url]

Dec 30 12 12:48 AM

A note for any of you that have a Stephens Electronics tape recorder, or know someone that does. We are sending in a batch of worn out motor torque rings for re-brushing in early February. To get a single, 10 contact, ring done costs 750.00. Getting 6 or more= 650.00 each. Getting 10 done at once gets the price down to 550.00 US. We have 6 to send in now, IF you know someone that needs this done, have them contact me and we'll include them. 403-225-2166. This brush material has a higher silver content and double the life of the ring, compared to the stock carbon/silver alloy that John used. Thanks.

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