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Reel to reel tape recorder adjustment
Reel to reel tape recorder adjustment
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Old 19th July 2019, 02:16 PM   #1
lcsaszar is offline lcsaszar  Hungary
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Default Reel to reel tape recorder adjustment

I have an old Sony TC-645 tape recoder that I took out of the basement where it has been until now, and I started paying with it. The kickoff was given by kevinkr's thread
So you think you want to play with tape: An Otari Story

This machine is not in the same league, but it is still capable of good sound. It has 3 motors and 3 "glass" heads, but mediocre tape handling.

I purchased an IEC calibration tape with reference level 250 nWb/m and various test tones for setting the head azimuth etc. After I adjusted everything what the cal tape offered, I was still unsatisfied. Especially the S/N was still poor: some 53 dB (A-weighted, rec to playback).

Then I vaguely remembered that the head should be demagnetized, so I created a quick tool from a soldering gun and a couple of turns of copper wire. It has done the trick, now I could reach the factory specs (56 dBA).
The next step was the playback EQ. I could set it roughly with the help of the calibration tape (700Hz/10kHz -10dB played back at equal level).

Now came the tricky part: the recording bias. I fed a low distortion 1kHz sine to the line input, and monitored the output with a distortion analyzer. I fed the residual (distortion+noise, i.e. the 1kHz filtered out with a notch filter) o the oscilloscope. Then I adjusted the bias until the distortion (mainly 3rd harmonics) and noise were the lowest. The VU meter showed increasing level when I increased the bias, and then dropped by about -0.5dB. This setting gave the lowest distortion.

I checked the frequency response with a pink noise generator that I originally built for loudspeaker testing. It is basically a PRBS (pseudo random binary sequence) generator in a MM5837N chip, and a subsequent -3dB/slope R-C filter. The first picture shows the frequency response after having done the above adjustments. I did not like the 3dB rise in the high frequency region, so I adjusted the playback EQ a little bit back. The second picture shows the result. The recording level was around -20dB.
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File Type: jpg Sony_1.jpg (140.2 KB, 160 views)
File Type: jpg Sony_2.jpg (144.3 KB, 157 views)
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Old 19th July 2019, 09:33 PM   #2
PRR is online now PRR  United States
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> I adjusted the playback EQ a little bit back.

If the Play EQ is per IEC Calibration tape, recording using maker-specified tape, level is not-high (-20dB may be good, though I would prefer sine-sweep to pink-noise), and bias is good, any residual error should be fixed in Record EQ. (Yes, you can't buy Sony 150 or SLH tape anymore, and modern tape may be a hair hotter than what we had in 1974.)

Sony TC-645 - Manual - Stereo Tapecorder - HiFi Engine

Ah, the Record EQ, all that stuff in Q101 emitter leg, is switchable/fixed. And the network and switching is complicated enough to foil intuitive tinkering. You could pick a speed/tape, then SPICE that configuration, tinker until you find your 2-3dB drop @ 6KHz.
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Old 19th July 2019, 10:58 PM   #3
wiseoldtech is offline wiseoldtech  United States
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Depending on how magnetized the heads were, you may have compromised the calibration tape by using it before de-magnetizing the heads. If that happens, you cannot now depend on the tape for calibration.
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Old 20th July 2019, 03:22 AM   #4
Bohuweno is offline Bohuweno
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If it's been sitting unused for years, don't you also have to consider mechanical aspects? The electronic adjustment is the next area I want to go into with my own decks - it's a little outside my experience and understanding at this point - but it seems to me that one of the first things people do with an old machine is replace the belt - some will do it whether it "needs it" or not - and make sure that the pinch roller in good condition. If not, that gets replaced too. It can be expensive, but it's crucial.

There's some variation between brands in what's likely to need attention - with TEACs, it's often the pinch roller bearing and arm, which tends to seize up, so needs to be cleaned out and relubricated. Capstan motor can be noisy too, and that can need lubrication. Technics decks can have a problem with squealing during fast winding: it's caused by the pulley for the tape counter, and again is fixed with a little lubrication of the pulley bearing. You get the picture - lot of things that involve lubricating, and a lot of other things related to rubber parts. I'm just a little surprised that you didn't mention any of this but did some pretty technical adjustment on the electronic side of things.

Also a lot of people recap the boards, especially with decks of this age - it's 1970s, isn't it? I have seen a lone voice or two saying you don't need to, but the general opinion seems to be that if you're working in the back of the deck anyway, this is something worth doing. Reading about how capacitors perform after 30 years or so, it's hard to disagree. I haven't done it on my decks - yet - but it doesn't mean I don't need to.

In my case, for peace of mind, I wouldn't attempt demagging with a homemade device either. But this site is DIY Audio, so I'd better not get into that....
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Old 20th July 2019, 03:11 PM   #5
lcsaszar is offline lcsaszar  Hungary
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Quote:
Originally Posted by Bohuweno View Post
If it's been sitting unused for years, don't you also have to consider mechanical aspects? ... I'm just a little surprised that you didn't mention any of this but did some pretty technical adjustment on the electronic side of things.
I did not mention it, but the first thing was to disassembe, lubricate, and check the mechanical parts. Fortunately this is an idler wheel transport machine, no belts at all (except the counter). The pinch roller and all bearings are in relatively good condition.
Quote:
Originally Posted by Bohuweno View Post
Also a lot of people recap the boards, especially with decks of this age - it's 1970s, isn't it? I have seen a lone voice or two saying you don't need to, but the general opinion seems to be that if you're working in the back of the deck anyway, this is something worth doing. Reading about how capacitors perform after 30 years or so, it's hard to disagree. I haven't done it on my decks - yet - but it doesn't mean I don't need to.
The PCB of this machine is rather difficult to service. Cables are soldered in and are too short to access the component side easily. I had one bad transistor and it was a pain to replace it (it measured good, though). I am still considering recapping, only 5 or 6 different types are needed. Just collecting the courage.
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Old 20th July 2019, 03:20 PM   #6
kevinkr is offline kevinkr  United States
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Reel to reel tape recorder adjustment
I have recapped 3 of the 4 Otari machines I own, and in all cases this has resulted in a rather large subjective improvement in performance. The Otaris are 3 motor, 3 - 4 head (maybe 5 in some with time code recording) machines of considerable complexity, but were designed with service in mind.

The 4th machine is a very minty MX-50N II which urgently needs a recap (things stop working when the caps start to go in this model).

In short recap is worthwhile regardless of what the "experts" will claim to the contrary.
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Old 20th July 2019, 03:28 PM   #7
Bohuweno is offline Bohuweno
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Quote:
I did not mention it, but the first thing was to disassembe, lubricate, and check the mechanical parts. Fortunately this is an idler wheel transport machine, no belts at all (except the counter). The pinch roller and all bearings are in relatively good condition.
Sounds like you're well ahead of me. Great to hear about another machine being brought back to use.

I'm looking forward to learning how to do the electronic adjustments for my machines - got 4 of them that'll have to be sorted out eventually.
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Old 20th July 2019, 04:36 PM   #8
Max Headroom is offline Max Headroom  Australia
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Standard Tape Manual


Dan.

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Old 23rd July 2019, 04:08 PM   #9
lcsaszar is offline lcsaszar  Hungary
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Default Some more spectra

Playback noise (in playback mode, tape stopped)
1kHz 0dBu playback
Attached Images
File Type: jpg Play tape stopped.jpg (155.4 KB, 94 views)
File Type: jpg Playback 1 kHz 0 dB.jpg (145.6 KB, 88 views)
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Old 23rd July 2019, 04:16 PM   #10
lcsaszar is offline lcsaszar  Hungary
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Pink noise with monitor swith in Source and in Tape position
Attached Images
File Type: jpg Pink rec 8.jpg (129.5 KB, 86 views)
File Type: jpg Pink play 5.jpg (135.7 KB, 15 views)
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