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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 15th Sep 2021, 11:50 am   #1
kehall1
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Unhappy Tandberg 64x bias issue

Hi

I acquired a Tandberg 64x in pretty good shape last year, clean and mechanically sound, all operates nicely (apart from a botch on the motor 'tape presence' lever), all original components so it was embarked on a long term restoration project considering the Rifa "crackpacitor" issues.

All of the offending Rifa have been swapped for new PET film caps of same form factor so doesn't look too much out of place - electrolytics are all in great shape, main issues have been poor contact on valve bases so pins tightened up and cleaned, everything is much nicer now. Playback is great, recording not so though (hardly any signal), so checking bias now.

Already checked Osc board output and readjusted to 85.5kHz (was around 88), but can't adjust bias level until get the next issue sorted.

Someone has clearly been 'in there' before as the green wire on the bias head had been snipped at the terminal block, wire end stripped back floating in thin air.

Connecting that green wire back to its home on the terminal results in good level of signal now on one channel but the other is dead (red/yellow wires) (oddly those have not been touched).

I have continuity between green/black on bias head but red/yellow appears open when checked on top of head (to rule out the wiring).

Not sure what would have caused that to fail (there's a momentary spike when HV comes on for the EAM86 indicators - yet to get to C15/16 to check them), but looks like I may need to try to either repair or source a replacement bias head type 42H or alternative of some kind..

Any help/pointers gratefully received.

Kind regards,

Keith.
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Old 15th Sep 2021, 12:28 pm   #2
TIMTAPE
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Default Re: Tandberg 64x bias issue

Is it half track or quarter track? I'm not familiar with Tandberg X Field models, only some of the Akai types.

Have you measured for continuity in the two coils in the bias head? It seems unlikely they would go OC. The head is basically an erase design and of course there is no physical contact with the tape but distances are quite sensitive.

Have you tested for signal at the record/play head in both tracks? It's easier as there is no bias signal mixed in with the audio signal
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Old 15th Sep 2021, 1:14 pm   #3
kehall1
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Default Re: Tandberg 64x bias issue

It is indeed 4-track

Quote:
Have you measured for continuity in the two coils in the bias head? It seems unlikely they would go OC. The head is basically an erase design and of course there is no physical contact with the tape but distances are quite sensitive.
Measured ~35ohm green-black, O/C red-yellow (red and black are tied to ground)

Quote:
Have you tested for signal at the record/play head in both tracks? It's easier as there is no bias signal mixed in with the audio signal
Yes there is signal from record amp to record head...

You got me thinking - I wonder if we could find and re-purpose another 4 track erase head of similar properties, although mounting it could prove to be the challenge.

I may get this one under the microscope yet, but a known good replacement would be best if anyone has any sources
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Old 15th Sep 2021, 2:05 pm   #4
TIMTAPE
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Default Re: Tandberg 64x bias issue

Quote:
Originally Posted by kehall1 View Post

I may get this one under the microscope...
That's what I'd do. Probably the break is where the wire is connected to the terminal. Sometimes access is possible.
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Old 15th Sep 2021, 5:04 pm   #5
kehall1
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Default Re: Tandberg 64x bias issue

I hope you're right in that the break could be near the end!

Photo attached - doesn't look pretty but you can just make out the coils through the hole and see attachment wire on the top right (yellow) connection point..

I think I may be able to desolder the other two green and black, free up the wires and pluck that board off (the resin/glue they've uses is brittle but goes rubbery and pliable when heated), and at least then see what's going on with the red/yellow side. Nothing came alive when heating up during desoldering so it's a pretty substantial break if there is one near the terminals.

Will have a play later
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Old 15th Sep 2021, 9:39 pm   #6
kehall1
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Talking Re: Tandberg 64x bias issue

Some photos attached...

I could just make out the enamelled wire underneath the epoxy at the top of the head, so I went on an archaeology mission and gently scraped away the layer to eventually reveal some bare copper... The epoxy here is different to the rubbery stuff that held the wires in place - this stuff didn't soften with heat and was quite powdery...

See green highlighted circles on one of the photos..

Unfortunately I discovered they both had continuity to the pads on the top so the break was elsewhere...

I double checked schematic and the yellow/red wires ought to have corresponded with the lower head coil so flipped the head over... couldn't see much at first but then a little bit of red on the left made itself known to me... another archaeology mission, feeling like Time Team...

Got to the bare copper (this time fairly deep), and checked continuity between that point and the 'yellow wire' pad (top right on top surface).

BINGO! 35 ohms....

I have some very very fine Roadrunner 0.19mm solderable enamelled wire so a dab of flux and ginger soldering with steady hands just to get contact with the head wire was made, joined up to the 'red wire' pad on the top and we have 35 ohms now on both coils!

Very strange the break was clearly round the back somewhere, not exactly vulnerable position but hey ho!

I think it was very lucky it was the lower head coil too - clearly the upper coil is completely internally wired and short of finding something to safely dissolve the epoxy (and not the enamel) it would have been practically impossible I think!

Anyway reinstalled back in machine, 10 ohm resistors in series with coils and measuring roughly equal voltages/current across the resistors when recording engaged, per scope photo

Now the question is the service manual suggests adjusting oscillator to get the maximum reading (between 100 and 400mV) on a VTVM - I'm presuming that's RMS as the scope p-p measurement is about 600mV, RMS 210mV

Later on the bias head adjustment again is to get the maximum reading based on using Tandberg test tapes (which I don't have), nothing specific for adjustments without only 'adjust the bias slightly if the treble response is too high', presumably trying to best match the total response curves at the back of guide.

Anyway this is just as an update, thanks for the encouragement to go digging with the microscope!

I'll probably add some more later on this project if anyone's interested.
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Old 16th Sep 2021, 2:50 am   #7
TIMTAPE
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Default Re: Tandberg 64x bias issue

Hard to tell from photo but it looks like the wire is not there at the terminal/ eyelet. Perhaps in unsoldering , the coil wire was still soldered to the coloured wire, and when the coloured lead was removed the coil wire end broke away from the coil. If so this could be a very tricky repair.
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Old 16th Sep 2021, 9:27 am   #8
kehall1
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Default Re: Tandberg 64x bias issue

The wire was there, it's all in an update that's been held for moderation I think as I posted it yesterday evening but not appeared?
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Old 16th Sep 2021, 11:32 am   #9
TIMTAPE
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Default Re: Tandberg 64x bias issue

Persistence paid off. Well done.
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Old 16th Sep 2021, 12:47 pm   #10
kehall1
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Default Re: Tandberg 64x bias issue

Thanks TIMTAPE

Now I'm trying to get my head around the wording (poor translation perhaps) of some parts of the manual:

The wording on the bias adjustment feels ambiguous.

"Insert one 10 ohm resistor in series with each bias head half to ground and connect a VTVM across each resistor"

To me, it makes sense if we're setting the bias current -- the text isn't actually explicit in that, just 'adjust X to max reading on meter 100-400mV' -- only if the 10ohm resistor is in circuit with the bias head, but to me the wording actually suggests either:

+v ------------ head ------ [10R] ----- gnd

or

+v ------+----- head ----------- gnd
|
+-----------[10R]----- gnd

The latter doesn't really make much sense, the key being 'in series', but the wording irks me I think 'temporarily insert a 10R resistor in to the ground wire circuit of each head half, connect meter across the resistor' is clearer, but only if that's REALLY what is meant! I do have some sympathy with the writers - hard to put some stuff into words!


Later on we can check playback (and record amplifiers) with similar text:

Insert a 100 ohm resistor in series with the playback head for the upper track (CH1) to ground (red wire). Connect a signal generator across the resistor. (400c/s @ 2mV)

A little more specific in mentioning the red wire but this is still nasty!

I presume this is to load down the sig gen to avoid blowing up the head coils.



+ ---------- playback head -------- [100R] ------ gnd

Got to love old terminology



I've managed to utterly confuse myself too over dB and signal levels... text: 'tune the signal generator to 400[Hz] and 30dB below maximum recording level', in the section dealing with control of total frequency response. The readings on VTVM connected to output are 0dB reference points.

Now the max recording level is specified under input impedance section as "Minimum input signal for maximum recording level at 1000c/s: 50mV" (high level input). Minimum for maximum ahrghrhgrhg

so if I output 50mV (RMS!) at 400Hz that would be maximum level, but what is 30dB below that - I don't have dB levels on my siggen and that's another minefield, so many different dB units...

I'm not of the right mind at the moment, clearly
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Old 16th Sep 2021, 3:28 pm   #11
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Default Re: Tandberg 64x bias issue

Quote:
Originally Posted by kehall1 View Post
so if I output 50mV (RMS!) at 400Hz that would be maximum level, but what is 30dB below that - I don't have dB levels on my siggen and that's another minefield, so many different dB units...
http://www.sengpielaudio.com/calculator-gainloss.htm

Scroll down to the 3rd calculator.

In the reference voltage box enter .05 (that's for 50mV)

In the gain box enter -30 (that's for -30dB)

Then hit calculate.

Then backspace 'till you get three whole numbers displayed in the measured voltage box to get a reasonable number to handle.

That should show 0.00158, that's the voltage (which is 1.58mV)

Lawrence.
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Old 16th Sep 2021, 9:49 pm   #12
kehall1
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Default Re: Tandberg 64x bias issue

Quote:
Originally Posted by ms660 View Post
http://www.sengpielaudio.com/calculator-gainloss.htm

Scroll down to the 3rd calculator.
That's wonderful, many thanks, been scratching my head on that one.

It does seem rather low as an input level but it says this equals 0dB reference on outputs (with all controls maxed I would hope!)

I'll give it a try and see if I get any meaningful output - my scope can drive the signal generator and do Bode plots so could be fairly interesting to see what comes of it.

I'm after a distortion meter next as it's one thing I'm missing (Keithley 2015 seems to be quite popular), although I think at a push the scope's FFT capabilities can also be used with some effort
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Old 16th Sep 2021, 9:53 pm   #13
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Default Re: Tandberg 64x bias issue

I used a Ferrograph RTS2 for set ups plus the usual test tapes etc when I worked for Sony, long time ago now

Lawrence.
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