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Vintage Tape (Audio), Cassette, Wire and Magnetic Disc Recorders and Players Open-reel tape recorders, cassette recorders, 8-track players etc.

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Old 19th Aug 2022, 11:47 am   #1
DMcMahon
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Default Tesla B700 Magnetofon Reel to Reel Check Out.

Late 1970's Czech manufactured solid state, 2 speed, 4 track mono recorder.

It was sold as "untested" so was not hoping for too much. It came with a User manual and schematic but only in Czech language.

Reading up on the B700, reviews imply not great for build quality or performance.

Overall looks quite reasonable condition, it has an inner clear Perspex lid and an outer black Perspex lid both in quite good condition and luckily not damaged during shipping.

Looking inside by removing top panel and base (a couple of minor attachment pieces broke off during removal but all glueable), all looks reasonable (but does require some cleaning and lubrication), some white rust type corrosion on some screws and metalwork.

Mechanically it all looks rather lightweight and delicate, glad to see all drive belts intact/good condition plus the 3 small rubber tyred idler/drive wheels look good.

First job was to replace the continental 2 pin mains plug with UK 3 pin fitted with 1A fuse. I may later replace the twin flex mains cable with UK 3 core and earth the metalwork/chassis.

Slowly powered up from Variac all OK, the single motor starts rotating at around 120V, at 240 volts the recorder is drawing 290mA

Fitted a tape and the tape transport all works well (only tried 5" tape so far).

Playback sounds OK, the VU meter illuminates and shows the playback signal, not attempted recording yet.
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Old 20th Aug 2022, 2:29 pm   #2
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Default Re: Tesla B700 Magnetofon Reel to Reel Check Out.

On its original guarantee paperwork it is dated 1979.

David
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Old 21st Aug 2022, 12:48 am   #3
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Default Re: Tesla B700 Magnetofon Reel to Reel Check Out.

I like the neat look of that!
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Old 21st Aug 2022, 12:20 pm   #4
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Default Re: Tesla B700 Magnetofon Reel to Reel Check Out.

Even though it is lightweight plasticky I also think it looks neat.

David
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Old 21st Aug 2022, 8:00 pm   #5
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Default Re: Tesla B700 Magnetofon Reel to Reel Check Out.

Just tried recording but hit a couple of issues.

First had no record signal on the VU meter, looking at the schematic see that the gramophone DIN uses pins 3 & 5 for its inputs whereas pins 1 & 4 are more commonly used in most of my other recorders.

Using a cable wired to pins 3 and 5 get very strong record signal so need to fit some attenuating resistors.

There are 2 red push buttons "AUT 1" and "AUT 2" which I think are for Automatic recording level control. Depending upon whether AUT 1 or AUT 2 or both are selected/pushed down, am getting different record level shown on the VU meter and maybe also different levels depending upon which track is selected.

Now trying to translate relevant section of the Czech User manual to better understand the operation of these controls. Am scanning the relevant sections to produce OCR text file (s) and then use Google translate to get English translation.

But the OCR text results are quite corrupted probably due to the many special/accented characters in the Czech text and also possibly due to the embedded graphics/images.

So am having to manually edit the OCR text which is tediously slow.

David

Last edited by DMcMahon; 21st Aug 2022 at 8:11 pm. Reason: Typo correction
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Old 22nd Aug 2022, 4:41 pm   #6
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Default Re: Tesla B700 Magnetofon Reel to Reel Check Out.

After some translation now know that:-

AUT 1 = Automatic voice microphone recording.
AUT 2 = Automatic music recording.
AUT 1 + AUT 2 = Automatic recording with manual record level also active (did not fully understand this function from the translation).

Retested manual recording of line music signal into the gramophone DIN, as before it looked like the input signal was too strong, having to keep the record level slider at almost its zero setting to avoid overloading the VU meter.

But found by lightly touching the record slider control could get the VU meter level to change dramatically. After cleaning the record slider with contact cleaner now get good VU meter indication at sensible record level setting. So looks like do not need resistor attenuators in the cable, which is good as really difficult to fit the resistors into the tight space in the DIN plug.

Even though now getting a good record level signal + monitored sound, on playback of new recordings no sound is heard and I think that erase may also not be working, very disappointing.

During recording tests had big problems with the Record piano key either not pressing down or when it is pressed down often not releasing, not visually easy to see what is occurring.

The tape counter has stopped working and I found that the metal coiled drive belt had got jammed in the counter drive gearing, kinking the belt, so will try a rubber belt replacement.

David
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Old 28th Aug 2022, 9:58 am   #7
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Default Re: Tesla B700 Magnetofon Reel to Reel Check Out.

After quite a big strip down, clean and lubricate of the various connected mechanisms the Record key now works well mechanically.

Another test recording confirmed that erase/record are not working, scoping the Erase & PB/Record heads wiring shows no erase or record bias signals during Record.

By removing 2 screws the main PCB hinges down to allow access to the components so now need to trouble shoot the bias oscillator circuitry. Will be difficult to identify the relevant components, I do have a component layout drawing but poor quality and very difficult to see many of the component designations.

David
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Old 31st Aug 2022, 3:42 pm   #8
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Default Re: Tesla B700 Magnetofon Reel to Reel Check Out.

After identifying the Bias Oscillator components, measured that the oscillator was definitely not running in Record mode. Measured that the +15V supply was being switched through the Record/Play switch to power/enable the oscillator, also checked the various other switched connections of the Record/Play switch are working OK.

All heads measured OK and connecting back OK though the various switches to the Bias Oscillator. The oscillator is fairly simple having one transistor, 3 resistors, 4 capacitors and the oscillator coil.

All capacitors (apart from tiny ones) and all the transistors are identified on their bodies as "Tesla", the oscillator transistor (T7) being a GC 512, this transistor looks to be a proprietary device.

Tried to measure the oscillator components in-situ but for most got silly readings, so removed some of the components and lifted one end of others.

All resistors checked ok, as well as the coil and 2 of the capacitors.

The 2 other capacitors read somewhat high so will replace for good measure, namely C42 (10uF) which read around 15uF and C40 (220nF) which read around 450nF.

Initially was not sure about the C40 correct value, on the schematic and also on its body it was identified as M22. On the schematic there is a table of capacitor codes, one example being "M1 =0.1uF" so from that thought that M22 must be 0.22uF (220nF) but subsequently wondered if M22 could be 22 x 0.1uF (2.2uF) but after checking similar capacitor on the B730 schematic (stereo version of the B700) saw that it was identified on that schematic as 220nF.

When I lifted one end of C41 found that other end was loose due to broken track it was soldered to, so repaired that track. impossible to know if the track was originally broken or got broken when I disturbed the other end of C41.

When I unsoldered the first leg (emitter) of transistor T7 the transistor fell out and I found that the other 2 legs were broken near the body of the transistor. The legs were sleeved so could not see if the legs were broken before unsoldering the emitter. Seems very strange especially as the legs appear to be quite strong.

The transistor measures OK so I am tying to refit it by soldering the broken legs to the leg stubs that are still soldered to the PCB, apart from being difficult do space wise, am struggling to get the solder to flow across the broken legs, maybe because the legs are gold plated.

Before removing the components I measured the voltage at C42 which was around 14.8V, on a voltage table on the schematic it shows it to be 8.7V, this via the oscillator coil is the supply voltage to the emitter of T7, so possibly indicates that T7 was not conducting, I was a bit slow at the time and did not then measure T7 emitter and base (collector is grounded).

David
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Last edited by DMcMahon; 31st Aug 2022 at 4:02 pm. Reason: changed lead to leg
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Old 9th Sep 2022, 9:34 pm   #9
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Default Re: Tesla B700 Magnetofon Reel to Reel Check Out.

Refitted the T7 transistor by soldering on some new legs and replaced C40 & C42.

Rechecked all connections but oscillator still will not run in Record Mode.

C42 voltage slightly lower than before at 13.5V but still no where near the 8.7V shown on the schematic.

So back to the drawing board.

David
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Old 13th Sep 2022, 4:10 pm   #10
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Default Re: Tesla B700 Magnetofon Reel to Reel Check Out.

Went to do another test recording and found that Play is no longer mechanically working.

Currently cannot see what drives the Take Up spool table in Playback mode, there is a small belt driving a small wheel that may drive the spool table but need to strip the reel table down to ascertain what the drive wheel should contact

Due to imminent family visitors (visiting for a few days) have had to move out many machines from my main work area (spare bedroom) to my secondary work area the garage, the garage is absolutely overflowing with machines, making it impossible to do any more practical work currently.

David
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