ReVox B710 MK1 Tape Deck Reproduce Amplifier - A Successful Repair

[John Tulett]

Introduction
* As documented in report, ReVox B710 Tape Deck – Successful Wow & Flutter Restoration, section "What did this restoration accomplish? While it took almost 2 years to complete this restoration, partly due to my first experience with a tape deck, my breaking the solenoid plunger (ouch), some delayed parts deliveries, and a random failure in the Reproduce Amplifier board (which I repaired and will document separately), pleased to report my B710 Cassette Tape Deck now looks, operates, and performs like it was new!"

ReVox B710 Tape Deck - Successful Wow & Flutter Restoration

ReVox B710 Tape Deck - Successful Wow & Flutter Restoration

www.audiocircle.com

Purpose
* The purpose of this report is to document the troubleshooting and successful repair of an intermittent power-up failure in my B710 MK1 series Reproduce Amplifier PCB.

Failure Description
* This B710 cassette tape deck MK1 serial number 003055, which I purchased new in February 1982 was fitted at factory with the early series Reproduce Amplifier PCB, Layout 1.710.490-12. The circuit diagram published in early service manual section 7/25 displays the date 26.06.81 in the lower left corner (Fig. 1). And indeed, this is the circuit diagram that corresponds to my unit serial number 003055.


Figure 1. Circuit diagram of MK1 early series Reproduce Amplifier PCB

* Suddenly, one day upon power-up the Left channel would intermittently fail, with symptoms no audio sound or activity on the Peak Reading Meter. The Right channel was fine with both audio sound and Peak Reading Meter operating normally.
* Strangely, the Left channel could sometimes be recovered by touching J1/1, the input of the Right channel - how to explain that?

Troubleshooting
* To facilitate the troubleshooting, I purchased a B710 Distance Card (Fig. 2) which when inserted into the B710 Interconnection PCB enabled accessing both front and back sides of the Reproduce Amplifier PCB. This proved invaluable, without this extension card it would have been extremely difficult to carry out fault finding and troubleshooting.


Figure 2. B710 Distance Card - for troubleshooting PCBs in-circuit

* Something was misleading though with labeling in the Reproduce Amplifier circuit diagram, where Right channel J1/1 and J1/2 should have been labelled J1/3 and J1/5. Likewise for Left channel J1/3 and J1/5 should have been labelled J1/1 and J1/2. I'm not complaining as I'm very grateful Studer ReVox made their circuit diagrams available, but it explains why the Left channel could sometimes be recovered by touching the input of the (presumed) Right channel.
* Probing the PCB voltages showed serious imbalances throughout the Left channel. A few examples:
- Capacitor C23 positive terminal, expected ~ +9V. Actual +12.9V.
- Capacitor C7 minus terminal, expected ~ -9V. Actual -1.2V.
- Quad Switch IC2 pin 10, expected ~ +2V. Actual -0.6V.
- Quad Switch IC2 pin 11, expected ~ +2V. Actual -0.7V.
* Carefully inspecting every PCB's solder joint showed nothing untoward.
* Thinking the cause may have been a weak or troublesome transistor, replaced all 4 transistors in the Left channel. NPN transistors Q2, Q3 and Q5 of obsolete BC550C were all replaced with new BC547C. PNP transistor Q4 of obsolete BC560C was replaced with new BC557C. (The "C" designation denotes the higher gain versions). Alas, these transistor changes made no difference, the circuitry was still imbalanced.
* Next, thinking there was a weak or troublesome quad switch IC2 in the Left channel, replaced original MC14066BCP with a new TC4066BP and mounted this IC on new 14-pin DIP socket. As the original MC14066BCP was soldered directly to the PCB, desoldered it carefully using a vacuum desoldering gun set to 300 Deg C. This worked well with no trace delamination or board damage.
* With this refresh of IC2 quad switch the Left channel immediately started working! Feeling elated the fault was now fixed reassembled everything back together, and guess what? The Left channel no longer worked. So, took it apart again and using the extender card the Left channel worked again, put it all back together and the Left channel stopped working. Hmmm.
* Double checking all solder joints once more, and measuring all electrolytic capacitors showed everything in good order.
* Now, just by chance, looking through later version service manual section 7/27 for Reproduce Amplifier PCB "ESE" (labelled with same Layout as before 1.710.490-12), noticed a pair of diodes identified only in handwriting as 1N4448. There is no mention of these diodes in the parts list, nor could I find a reference or documentation what their purpose is or when they were introduced. This does not appear to be an official or released upgrade, although suspect the diode's purpose could have been to clamp voltage swings during powerup. As all applicable factory upgrades were implemented by Studer ReVox when my B710 was sent for repair in 1992, presume the installation of these new diodes came later.
* Armed with this knowledge added a 1N4148 signal diode from my parts supply, cathode to junction of D5/R20 and anode to analog ground. Pleased to report this fixed the Left channel powerup fault, checking with the extender card all in-circuit voltages now measured good!
* However, after putting everything back together the previously working Right channel now failed at powerup, groan.. So, for the Right channel added another 1N4148 signal diode, cathode to junction of D7/R52 and anode to analog ground (Fig. 3). Also refreshed IC4 quad switch with a new TC4066BP and 14-pin DIP socket.


Figure 3. Reproduce Amplifier, Right channel - added diode from D7/R52 to analog ground (left), new TC4066BP on socket (right)

* Finally, by adding these two signal diodes the Left and Right audio channels now power up reliably. I feel confident this has fixed it, as part of the Wow and Flutter investigations the unit was switched on, switched off to cool down, then switched on again multiple times with the Left and Right audio channels powering up reliably every time.
* Figure 4 summarizes what was wrong and what was changed. As part of the initial restoration and circled in Yellow are the tantalum and electrolytic capacitors that were all changed to new, circled in Green are the good components changed but no fault found, and in Red are the critical items that needed to be addressed during troubleshooting.


Figure 4. Reproduce Amplifier component investigations - Capacitors (Yellow), No fault found (Green), Critical items (Red).

Final comments
* As mentioned, I'm very grateful that Studer ReVox made all their circuit diagrams available. Without access to and being able to compare both circuit diagrams, early Reproduce Amplifier PCB and later Reproduce Amplifier PCB "ESE", it would have been impossible for me to know what was changed.

References
* ReVox B710 Tape Deck - Successful Wow & Flutter Restoration

ReVox B710 Tape Deck - Successful Wow & Flutter Restoration

ReVox B710 Tape Deck - Successful Wow & Flutter Restoration

www.audiocircle.com

* Supplier: revox.name for B710 Distance Card
https://www.revox.name/sell/b710distancePCB/
* ReVox B710 service manuals, both early series and later series
https://www.reeltoreel.nl/studer/Public/Products/Revox/Revox_B710

 

[borchee]

WOW, without flutter, another excellent report!...although I just had a quick glance at it.

The flywheel clips make a difference in W&F. You maintained the original position. Studer Revox states W&F below 0,1%, so it is very likely, that the first clips position resulting in it, was kept.

There are six dimples on the flywheel and the clip can be pushed on the shaft two ways, so 12 different clip-flywheel-shaft positions are possible. With two flywheels, the number of combiations goes up to 144.

After some random tests, I concluded that going through all of them is of no real benefit, so I decided on:

- marking the dimples on flywheels, ones with A-F and the others 1-6
- one clip-shaft position, which reduces the number of combinations to 36
- five repetitions/recordings with one position
- setting the locators to a 40 second difference in about the middle of the tape
- first going through the six positions of one flywheel (A1-A6)
- discarding the lowest and highest result
- of the three results left for each combination, the one with the best was set (A1 in my case) as reference
- the same was repeated with the other flywheel (first flywheel set to 1), resulting in position D1.

I was messing with it solely to satisfy my curiousity about W&F changing with clips positions. Deck is Revox B215, test tape is 3.000Hz by Alex Nikitin, W&F meter is WFGUI 8.







I have also gone a bit silly and did it with reels only, no shell: Revox B215: Pancake time!

 

[John Tulett]

WOW, without flutter, another excellent report!...although I just had a quick glance at it.

The flywheel clips make a difference in W&F. You maintained the original position. Studer Revox states W&F below 0,1%, so it is very likely, that the first clips position resulting in it, was kept.

There are six dimples on the flywheel and the clip can be pushed on the shaft two ways, so 12 different clip-flywheel-shaft positions are possible. With two flywheels, the number of combiations goes up to 144.

After some random tests, I concluded that going through all of them is of no real benefit, so I decided on:

- marking the dimples on flywheels, ones with A-F and the others 1-6
- one clip-shaft position, which reduces the number of combinations to 36
- five repetitions/recordings with one position
- setting the locators to a 40 second difference in about the middle of the tape
- first going through the six positions of one flywheel (A1-A6)
- discarding the lowest and highest result
- of the three results left for each combination, the one with the best was set (A1 in my case) as reference
- the same was repeated with the other flywheel (first flywheel set to 1), resulting in position D1.

I was messing with it solely to satisfy my curiousity about W&F changing with clips positions. Deck is Revox B215, test tape is 3.000Hz by Alex Nikitin, W&F meter is WFGUI 8.

View attachment 3303147

View attachment 3303148

View attachment 3303150

I have also gone a bit silly and did it with reels only, no shell: Revox B215: Pancake time!

Hello Borut,

As always, thank you for your kind review and helpful comments.

Please allow me to reply to your remarks regarding your own tape deck investigation, "Deck is Revox B215, test tape is 3.000Hz by Alex Nikitin, W&F meter is WFGUI 8".
Thank you for sharing this information.
As mentioned in my report "ReVox B710 Tape Deck - Successful Wow & Flutter Restoration" where the AnalogMagik's turntable software proved effective in measuring the progressive reductions in my B710 tape deck's wow and flutter, and wrote "It would be interesting to see how these results compare using a professional W&F meter, although it would be difficult to justify the purchase of a W&F meter for a single experiment."
https://www.audiocircle.com/index.php?topic=190279.0

I didn't know about the software WFGUI (version 8), so again thanks for sharing this information.
As the final W&F measurements of my B710 tape deck succeeded in bringing down the W&F value to below ReVox's specification I'm not expecting further improvements, but I will try the WFGUI software and see how it compares.

Thank you,
John

 

[borchee]

Hi John,

I'm pretty sure, although I might be wrong...

"According to the GennLab site, "The residual W&F of the tape itself is 0.022 %", from this statement it is reasonable to say the actual W&F of this B710 unit is now a respectable 0.069% (0.091 - 0.022 = 0.069)."

You can not just simply subtract, or add, W&F figures.

Example:

If you would measure 0,022%, going with it, the actual W&F would be 0%...or in the negative, if you measure less. As moving the tape relies on friction, there is W&F present by default - it can't be null, tape always slips to some extent. Capstans in Revox drive rotate with the same speed/rpms, yet diameters of capstan shafts are different. Revox drives are sensible about pinch rollers, simmilar to Nakamichis about belts.

Kind regards,
Borut

 

[John Tulett]

Hi John,

I'm pretty sure, although I might be wrong...

"According to the GennLab site, "The residual W&F of the tape itself is 0.022 %", from this statement it is reasonable to say the actual W&F of this B710 unit is now a respectable 0.069% (0.091 - 0.022 = 0.069)."

You can not just simply subtract, or add, W&F figures.

Example:

If you would measure 0,022%, going with it, the actual W&F would be 0%...or in the negative, if you measure less. As moving the tape relies on friction, there is W&F present by default - it can't be null, tape always slips to some extent. Capstans in Revox drive rotate with the same speed/rpms, yet diameters of capstan shafts are different. Revox drives are sensible about pinch rollers, simmilar to Nakamichis about belts.

Kind regards,
Borut

Hello Borut,

Thank you for this reply and agree, one can't simply subtract or mix W&F measurements.

As mentioned in the report, this was my first experience of working on a tape deck of any kind and being concerned about the wording I checked it with a tape expert for review before publishing, oh well.

Moving forward, I tidied up the wording regarding the residual W&F of the master calibration tape vs. the AnalogMagik W&F measurement.
The text changes you can see in the wording following "Figure 15. Example of a final W&F measurement".
ReVox B710 Tape Deck - Successful Wow & Flutter Restoration, https://www.audiocircle.com/index.php?topic=190279.0

Again thank you, I valued and appreciated your reply.
John

P.S. As mentioned in my earlier reply, I will try the WFGUI program and report on how the W&F measurements compare.

 

[John Tulett]

Hello Borut,

As promised from my earlier reply, I will try the WFGUI program and report on how the W&F measurements compare.

Thank you for your patience, pleased to report I completed the W&F results achieved with my ReVox B710 tape deck together with the AnalogMagik turntable/cartridge test program and the WFGUI tape player test program.

This report is now posted to the AudioCircle Vintage site, “ReVox B710 MK1 Tape Deck - A Wow & Flutter Software Comparison Test”.

ReVox B710 MK1 Tape Deck - A Wow & Flutter Software Comparison Test

ReVox B710 MK1 Tape Deck - A Wow & Flutter Software Comparison Test

www.audiocircle.com

Thank you,
John

 

[txturbo]

It would be very interesting to see how the AnalogMagik and WFGUI compare to the NAK-T software. The NAK-T software seems very popular.

NAK T-100 Audio Analyzer for Mac/Windows

 

[Dj831]

Hi John, many thanks for such an inevaluable explanations

I've just purchased a B710 as-is from same era than yours few days ago for cheap. It was stored for a while in an attic and is in correct condition apart top cover. But curiously, it misses erase head - previous owner broke it, I don't know how this could be possible - and head was stored inside deck with packing tape, with a (bad) repair attempt. Sadly, it misses left roller and all parts related to it (spring, special scew ...). It seems owner used it that way, since left capstan is in pristine condition unlike right one.

With help of your topic from Audiocircle, I contacted german seller you mentioned (premium-hifi.com, formerly revox-online), but also to reeltoreel.nl, and eBay Belgian seller, Voxparts.

Let's cross fingers that I can find missing parts

When I'm able to complete mechanism, I will recap it fist and remove processor when I first power it on - hopefully, seller had no power cord, hence unit had not been powered on since it was stored. Then, I will do exactly what you explain, but I will use a B&K wow & flutter to test, a 6203.

 

[John Tulett]

Hello Dj831,

Thank you for your kind words, welcome to the B710 Club!

In reply to your report of a broken Erase Head and missing Left Pinch Roller Assembly, while it's difficult to know the history when purchasing used, to offer one possible explanation as described in my report "ReVox B710 Tape Deck - Successful Wow & Flutter Restoration".
https://www.audiocircle.com/index.php?topic=190279.0
The early series of the B710 MK1 were factory fitted with 8.7mm pinch rollers (both left and right) and erase head part no. 1.116.711.01.
For later series of the B710 MK1, factory revision changed to a larger erase head, which necessitated a smaller left pinch roller to maintain clearance.
Reference Studer ReVox Service Instructions section "3.4.3 Adjusting the Erase Head". "Version B710 MKI: Erase head 1.116.711.01 has been installed in the MKI versions. If replacement is required, erase head 1.116.711.02 is also used in these recorders. In this case, however, the left-hand pinch roller (diameter 8.7 mm) must be replaced by a smaller roller (diameter 8.0 mm, 1.710.201.00)."
https://www.reeltoreel.nl/studer/Public/Products/Revox/Revox_B710/

Although the history of your machine is unknown, possibly a previous owner tried to fit the newer (larger) erase head without changing to the smaller left pinch roller, or not understanding the physical differences between the earlier and later series of erase head and left pinch roller assemblies and gave up.

Just a caution about your remark "I will.... remove processor when I first power it on".
Not sure why the processor's removal is needed for a standard refurbishing, the processor is a 40-pin chip that requires extreme caution when de-soldering it.
That said, if there's a valid reason to remove the processor chip, I can share my experience documented in "Successful MK3 kit installation with ReVox B710 MK1 Cassette Deck".
https://www.audiocircle.com/index.php?topic=192508.0

I have ordered parts to repair my ReVox vintage equipment from Premium-Hifi.com in Germany, and Nagravox.com in Australia including the Full Monty kits to refurbish my B710, B739, and B790.
Hopefully you will be able to find the parts you need and carry out a successful restoration, good luck!

John

 

[Dj831]

Hi John, many thanks for your remarks I had a thorough reading of your post about Wow & Flutter on Audiocircle, and I perfectly understood differences between old and new erase head. Note that I understood it well, since you put emphasis on differences of roller sizes and about erase heads versions. And yes, your explanation about roller exchange of previous owner seems to be correct, since head was broken and very badly reglued, and it misses all parts related to left lever

I contacted several sellers, including Premium-hifi, and they do not have parts. Though, they advised me to contact Revox.de, and ask them for the parts I need, since they could have. Believe it or not, they have almost everything I need, apart erase head and damper. They also have rollers (both diameters) though they dot not cost the price you paid (circa 125€ each, but this is a revox part ).

Next step is to find an erase head, the most difficult part of the job. And depending of the version I find, order the right roller. If you have any clues about finding a refurbished erase head, this is very welcome! There are two sets of heads on eBay, but seller is not answering to questions (and his feedback tells that this is the issue with that seller). Do you have a picture of both differents heads that were used, in order to differentiate them?

About processor: no worries to test deck without it since it is socketed. And I want to avoid by all means to blow it if there is an issue somewhere - prevention is better than cure. I think that I will 'Deoxit' it, since pins have tarnished a lot (they are now black), and replace socket while I'm in.

So, any version of B710 erase head is welcome

 

[eiraved]

Hi John, many thanks for such an inevaluable explanations

Welcome to AK!!

 

[borchee]

Check the mounting hole - it is either elongated or, I prosume, round. There also are two different head carriers, so not just any erase head. What's the serial number?

Get service manuals (John already linked a few):

- Revox B710 MKI-MKII_manual_service_10.18.1931 ed. 0484_de en fr_reeltoreel.nl_BORCHEE
- Revox B710 MKI-MKII_manual_service_10.18.1932 ed. 0985_de en fr_reeltoreel.nl_BORCHEE
- Revox B710 MKI-MKII_manual_service_18.193.0183_de en fr_audioservicemanuals.com_BORCHEE
- Revox B710 MKI-MKII_manual_service_xyz_de en fr_revox.name_BORCHEE
- Studer A710_manual_service_10.23.5033 ed. 0385_de en fr_archive.org_BORCHEE
- Studer A710_manual_service_xyz_de en fr_reeltoreel.nl_BORCHEE
- Studer A710 modifications_manual_service info_SI 106-87 10.85.5910 ed. 0487_de en_reeltoreel.nl_BORCHEE

Follow URL in file name - you won't find files named like above, because I named them.

Good luck.

 

[Dj831]

Hi everyone, I was eventually able to find a whole B710 mechanism And I now understand what John said earlier: I have apparently a V2 head on that mechanism, since roller is a bit under 8mm. There's no much room at all! If previous owner replaced roller with a 8.7mm diameter, head has no other choice than breaking (and it did ...). Bonus: I needed a damper too, this one is okay. Great! Though, mechanism isn't in the best condition, since there is rust between rollers and their axles. However, since most parts are available at Revox (apart heads and dumper) and since they are cheap, I will order them new.

For those who are interested in: Revox now sells again rollers for B710. They cost c.a. 125€ each, whatever diameter (8.0 or 8.7mm), references ends by .81 instead of .01.

Now time to order electronic parts (capacitors and potentiometers), and mechanical parts, and resurrect that 45 years old deck.

 

[John Tulett]

Hello Dj831,

Thank you for your status update, looks like you are making very good progress.
Some quick comments/advice for ordering the new pinch rollers.
- Recommend ordering both left and right pinch rollers together (roller material degrades over time; they don't last forever).
- The price you mentioned, 125€ each doesn't look right. I would expect the price to about half that amount for a set of left and right pinch rollers, including all Teflon washers and circlips.
- There are different pinch rollers depending on the ReVox model, make sure you specify it's for a B710, and that you require 8.0mm left and 8.7mm right.
- The small circlips can be tricky to fit. If using circlips pliers, check it fits the small size circlips.

Although not mentioned in your post, recommend ordering PDP65 lubricant (a small bottle will last a long time). While isopropyl alcohol works well for cleaning the mechanical parts, PDP65 is the only lubricant prescribed by ReVox for the sintered bearings in the capstan motors.
In the report, "ReVox B710 Tape Deck - Successful Wow & Flutter Restoration", are some PDP65 application notes I included in "Table 1. Summary of W&F investigations and improvements". ReVox B710 Tape Deck - Successful Wow & Flutter Restoration

Hope all goes well with your B710 restoration!

Thanks,
John