Important info for Revox B215 alignment

[mitajohn]

This is an important info for those who want to restore a Studer A721 or a Revox B215 cassette deck.

Here's a short story:
During the pandemic lock-downs I had time to spent restoring a B215 cassette deck.
A friend of mine wanted me to restore his favorite deck, a Revox B215 more than 30 years old.
The machine looked good but just turn it on, nothing was working!
For starting I decided to perform a full recap. I ordered a bunch of caps to replace all of them.
I also ordered a set of pinch rollers.
Waiting for my orders arrival, it took about three months, I cleaned the deck and lubricated the
capstan shafts with a special oil "Isoflex PDP65" ordered from REVOX online.
After 48hours of waiting the capstan motors finally came alive!
After the full recap the machine came alive and I started the calibration procedure according to the SM that was unsuccessful.
The result from a test cassette was not good at all. I discovered that someone had unfixed the mounting screws and misaligned the soundheads... That was bad, I had now to use the REVOX alignment
tool kit, but first I had to found out it, more than 20 years had passed since its last use...
Finally I managed to perform the alignment but I found a contradiction between the instructions in the SM and the guidance in the kit concerning the mechanical soundhead alignment.
The problem was that in the SM it is mentioned that gauge or slide (2), the Zenith gauge (FIG1 pic), must be used to adjust the soundhead height in order that the gauge moves free without contact between the tape guides on the right of the soundhead (ERROR_From B215_SM pic).
In the alignment tool kit instructions it is mentioned that gauge (3)(Tape Guide Gauge FIG1 pic) must be used to adjust the head height in the way to allow free movement of gauge (3) within the tape guides (FIG2_Head_Height pic). That is the CORRECT way!!!

PS:
The initial gauge (2) was thinner than the tape guides space and so it led to soundhead misalignment.
Studer/Revox, I assume, realized the misleading instructions and changed the gauge (2) in the alignment tool kit with a thicker one so it cannot be used for the head height adjustment simply because it cannot pass within the soundhead tape guides, but I have never found any correction
instructions or technical info in the SMs of A721 or B215 during the production of the decks.

Look at the attached pics of the face and side of the two gauges (2).

[DMcMahon]

Good information.

David

[TIMTAPE]

I dont understand the need for the thinner gauge, ie: thinner than cassette tape, or than the guides. The one gauge can normally be used in conjunction with the plate, to adjust head height, guide height, head zenith and head protusion on a wide range of cassette decks.

[mitajohn]

Originally Posted by TIMTAPE

I dont understand the need for the thinner gauge

As I recall the length of gauge (2) (initial or new) defines the exact position of the sound head as it accomplished by the adjustment of the head carrier. This setting can be seen by the horizontal line on the alignment base plate putting there correctly the edge of gauge (2) to the front of the head. See FIG1 on my previous post.

[TIMTAPE]

Originally Posted by mitajohn

As I recall the length of gauge (2) (initial or new) defines the exact position of the sound head as it accomplished by the adjustment of the head carrier. This setting can be seen by the horizontal line on the alignment base plate putting there correctly the edge of gauge (2) to the front of the head. See FIG1 on my previous post.

Yes, the head protrusion. But that relies on the length of the gauge part, not its width or thickness. I don't understand the reason for a gauge part substantially thinner than the 3.81mm width of cassette tape, for checking guides. 3.81 mm width seems ideal. At least I've found it so.

[mitajohn]

Originally Posted by TIMTAPE

Yes, the head protrusion. But that relies on the length of the gauge part, not its width or thickness. I don't understand the reason for a gauge part substantially thinner than the 3.81mm width of cassette tape, for checking guides. 3.81 mm width seems ideal. At least I've found it so.

Me too. May be, that's why Studer/Revox was mistaken suggesting the use of gauge (2) and not gauge (3) for the head height adjustment and finally it replaced the gauge (2) with a thicker one instead of correcting the wrong suggestion...

[mitajohn]

Originally Posted by mitajohn

...Studer/Revox was mistaken suggesting the use of gauge (2) and not gauge (3)...

May be the mistake was simply a typo(?) in the SMs of Studer A721 and Revox B215...

[mitajohn]

This an update to this info.
An audio engineer who had been worked at a radio station (retired now) informed me that the station was equipped with a number of STUDER A721 decks. STUDER had also supplied them, for technical reasons, their alignment tool kit which has a different gauge(3), as seen in the attached pic. The thickness is the same as the original gauge(3) but it has a peculiar cut. Unfortunately the alignment tool kit instructions have been lost. Only some guy from STUDER can explain to us what the cut was for.

[TIMTAPE]

Its hard to believe they'd make a tool which was unnecessary. A typo sounds more likely. I've never worked on one of these so can't comment from experience. But since all cassette machines have to play the one standard Philips cassette, the gauge must relate to something specific about the machine's design in that area. Maybe something related to the dual capstans and the three heads which can make things crowded around the cassette windows. I've always found adjusting head zenith the most difficult to judge, even just to see, compared to height and protusion. Same problem with reel to reel heads. Zenith can't be checked or adjusted using tones on a test tape. But height, azimuth and wrap can. Maybe wrong zenith on a standard cassette deck isnt as serious but on a wide tape multi track deck, it can greatly shorten the life of an expensive multi track head due to keystoning. But re the narrower piece, surely its use would be obvious in either making a particular adjustment easier or even possible.

[mitajohn]

Opto sensor adjustment on REVOX B215 / STUDER A721.

This is another important info for those who want to restore a Studer A721 or a Revox B215 cassette deck.

Restoring a B215 and during the adjustment of the opto sensor, on the SYSTEM CONTROL PCB, I found that the relevant circuitry of the IC7 and the trimmer R63, PIC:"REVOX B215_OPTO SENSOR ADJUST", was different than that on the PCB. The layout of the SYSTEM CONTROL PCB of B215 was as the one of STUDER A721 at least in this part of the circuitry.


As can be seen on PICS: "STUDER A721_OPTO SENSOR ADJUST" and "STUDER A721_OPTO_SENSOR ADJUST_PCB" the circuit is different and have to be careful during the adjustment. As TP2 for B215 can be used the upper tag of the R64 just below of J8. Then the adjustment of the opto sensor can follow the guidance in the SM of A721.


I believe that this was a factory modification implemented on the SYSTEM CONTROL BOARD PCB but not included in the SM of B215 schematics.

[mitajohn]

Play mode counter torque modification.

While searching my archives I found a Technical protocol written by Studer concerning an interesting and may be important modification for those who want to apply it to their Studer A721 or Revox B215 cassette deck.

The recommendation is as follows:

Studer Technical protocol reports that in PLAY mode the counter torque in several cassette decks (Studer A721 and Revox B215) is too low which may result in a chewing up of the tape.

To remedy that effect the counter torque of the left hand spooling motor will be increased from 10cmg to 17cmg measuring that by using a reliable cassette torque meter.

To accomplish that, Studer recommends to add a diode 1N4448 (D27) as well as a resistor of 47kOhm (R115) which have to be soldered on the System Control PCB on the components side of the board in accordance to the layout drawing "PLAY COUNTER TORQUE_mod_pcb" and circuit diagram "PLAY COUNTER TORQUE_mod_cir" which are attached.

From my experience it is better to be used a 100kOhm multi-turn pot in place of the fixed resistor of 47kOhm (R115) because it makes the adjustment easier to the recommended setting of the counter torque value for that deck due to the different tolerances in the motors and the electronics of each deck.

I attached a relevant pic.


NB: Check your deck if it has already the mod.

[mitajohn]

The thread since August 2022, when opened, seems to have some popularity to the people who technically interested to these two legendary machines by STUDER/REVOX.
I will keep searching in my "paper-kingdom" for finding some more mods, if they exist.
So stay tuned!

[mitajohn]

An Important info for STUDER A721 deck, BAL LINE OUT OSCILLATION Problem.

Searching my paper archives I found (not For REVOX B215) a modification referring to STUDER A721 balanced line output oscillation.

This mod maybe have interest to those who owned an A721 deck which has that problem.

Studer reports that under certain unbalanced load conditions oscillation of the line output stage may occur.

To remedy the problem STUDER proposed a modification as follows:

1-Replace capacitors C54 & C56 with 470pF capacitors.
2-Remove capacitors C57 & C64.
3-Replace capacitors C61 & C72 with 47uF capacitors.

Check your deck if it has already the mod applied.

Attached is the relevant part of the schematic.

NB: Most of the mods I have found in my papers refer to both B215 and A721 decks. This one is referred only to A721.

[mitajohn]

Treble meter indication equalization in recording high frequencies.

Treble equalization for IEC I and II tapes is necessary to prevent increasing of distortion when recording high frequencies.
Therefore a higher indication of the meter display will automatically is taking place in order the user to keep the recording level lower to prevent tape from high frequency distortion.
So during recording when you switch between "source" and "tape" you will notice a higher display indication with rich high-frequency content.
If you want to cancel this function remove R162 and R165 to be found on B215 1.721.270.00 PCB, seen on the attached image.

[mitajohn]

REVOX B215 Opto Sensor (EOT) update.

This is an update concerning some different circuitry versions of the "End Of Tape" function of the deck as seen in the pics.

I have found 4 versions.

The adjustment procedure follows the instructions as per STUDER A721 SM using an ordinary cassette or the REVOX Opto Measuring-Cassette 37% Nr:46038.

After a correct adjustment the deck will stop at FRW of the tape at every leader just at the magnetic tape, goes to play, resets the counter at 0:00:00 and stops.

When FFW at full speed to the end of the tape the deck safely stops the tape at the leader and never hits the tape's mechanical stop.

[mitajohn]

REVOX B215 Opto Sensor (EOT) update.

Something went wrong with the pics.

Here are the missing 2 of them.

[mitajohn]

Another REVOX B215 Opto Sensor (EOT) update.

Hi to all,

This is an info for those who run into problems with the EOT function, except of component failure.

After the repair of a B215 deck followed by the several adjustments I run into a weird behavior of the EOT function.

After the EOT adjustment according to A721 SM the function worked OK as
described in a previous post.

I played a cassette for about 30 mins and then the EOT function failed! The voltage at the upper tag of the R64 (TP2)(IC7 pin 6) was almost 0 volts.

I left the deck to cool and then turn it on.

The EOT function was working. I played a cassette as before and the previous mentioned scenario repeated...I did this many times with the same result.

I run an extensive investigation to find out the fault.

From the circuit diagram I noticed that the signal output of IC7 goes to the IC MAB 8440 as well as the tacho signals for the spooling motors.

I checked the 4 tacho signals with the scope and found that they need adjustment because the shape of the square wave signals must be symmetrical as possible, according to the SM, but they were not!.
I adjusted all for best shape symmetry using the relevant 4 trim-pots.

The result after these adjustments is that the EOT function never failed again and worked OK.

For your info the following measured figures may be a useful guide.

The Tp2 (IC7 pin 6) voltage at different EOT states is:

1-No cassette: ~4.95VDC,
2-Led of EOT covered by a black plastic card: ~0.025VDC,
3-A Type I tape cassette in the deck, at the magnetic coating: ~0.33VDC,
4-At the cassette tape leader: ~1.36VDC.

(Cassette used: TDK D90)

[mitajohn]

CONCERNING RF INTERFERENCE TO REVOX B215 deck.

Owners of B215 reported RF Interference during playback in some cases.

The following modification may help:

Install on FET transistors Q6 and Q5, to be found on input/output PCB,

between Gate (R98 and R51) and Source (R97 and R49), a capacitor of 330pF.

(A capacitor of 100pF is already installed on FET transistors of STUDER A721 deck.)

[mitajohn]

CONCERNING EXCESSIVE AUDIO LEVEL LOSS IN THE LOW FREQUENCY RANGE.

This mod maybe is interesting to Audiophiles...

In a STUDER Technical protocol is reported that Excessive level loss
in the low audio frequency range noticed in the record control.

The modification refers to the replacement of C32 and C60 68nF capacitors
by 47nF capacitors on the Record Control board.

This mod applies to both decks.

The use of good quality film caps is recommended.

(Check if the mod is already applied).

See the attached pic.

[mitajohn]

REVOX B215/STUDER A721 cassette deck restoration info.

The thread since August 2022, have popularity to the people who technically interested to restore these two legendary decks.

This info is concerning to the findings during the restoration of a STUDER A721 cassette deck built at 1989 but also may apply to the consumer version, REVOX B215, built at the same time period or later.

Here it is:

After removing the BAL-IN/OUT pcb from the deck I found, glued on the bottom plate of the deck, three rotten rubber "stand-offs", as seen in the attached pic.

These "stand-offs" had left a sticky material, where they abut against the pcb, which I cleared using IPA.

These "things" look like self adhesive rubber feet bumpons dome shaped but they are not solid, it is like they had a rubber bubble on the top, now there is a hole.

I assume that these "things" work as absorbers(?) where they touch
the pcb because the pcb has also two through-hole solid plastic stand-offs installed at its back as seen in the relevant pcb pic.

Has anybody any idea what these "things" are or any other suggestion for replacement?

Other findings may follow if any.