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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 21st May 2017, 7:52 pm   #21
monaro0162
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Default Re: Converted to REVOX

The test sweep signal from the signal generator is coming from a standalone unit, and not linked to the analysis software, which is running on a separate PC, so I assume that there is no unwanted interaction going on between source and measurement.

The sweep I'm running, runs the sweep linear, in time, between 30 to 30khz, the portion between 50 to 200hz passes very quickly over the 5 second time period.

Quote:
Different parts of the scan will have different measurement bandwidths
Do I need to extend the time the sweep runs at lower frequencies, or run the sweep in a log fashion, to flatten out the dip I see at the start of the spectrum analysis? Need to get a totally flat response, on my test setup, before I take any proper measurements using live tape from both machines.
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Old 22nd May 2017, 1:37 am   #22
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Default Re: Converted to REVOX

If the PC is running a compound FFT then the apparent amplitude will be a function of what proportion of time the swept signal hangs around in eah bucket of the FFT and how much time it is elsewhere. You might get something sensible with a very slow sweep and a max hold function running on the FFT output.

Makes the old analogue way look almost simple!

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Old 24th May 2017, 6:07 pm   #23
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Default Re: Converted to REVOX

I tried increasing the time of the sweep, swapping between linear and log sweep, and increasing and reducing the sample rate and bit depth on Audacity ... none of which made any significant difference to the curves with the dip at the low end observed on the earlier runs.

So then I took a look on the hccc site ... and ... low and behold... the very first graph that I look at (recorded on some very nice HP kit) ... shows a very similar response curve to the one I'm getting via Audacity, which gives me confidence.

So I'm going to stick with what I've got, and the machine will be re-tested under the same parameters after the service and alignment. I'll be doing some 'industry standard' spot frequency tests too.

For signal injection as well as the FeelTech FY2200SP I've now got a Malden electronics 625 analogue generator which allegedly works down to a 1mv resolution. It seems to but my scope is picking up a lot of noise at this level. Once a Racal 9300 vrms meter arrives the fun work will start with the replacement of the pots and caps.

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Makes the old analogue way look almost simple!
The last company I worked for threw out an analogue spectrum analyser ... at the time I never thought I have a use for it
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Old 24th May 2017, 6:26 pm   #24
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Default Re: Converted to REVOX

At the high frequency end you will get a roll-off with sinc-shaped lobes due to the head gap being finite in size. This gets worse at slower tape speeds

At the low frequency end you get another roll-off due to the tape-polepiece contact length being finite. This gets worse at faster tape speeds.

So the ratio of polepiece length to gap length gives you a ratiometric bandwidth. Changing tape speed just moves it around. You gain some, you lose some.


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Old 24th May 2017, 6:58 pm   #25
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Default Re: Converted to REVOX

Thank you David ... You gain some, you loose some ... says it all!
During the sweep tests I carried out, I checked the input signal amplitude on the scope (which remained constant throughout) and at the same time watched the VU meters, where I did see some variation in the db levels (+/- 4?) Presumably this is something to do with the response curves of the record and playback amps?

With ref to the attached sweep posted earlier: I got this recording with the machine not recording, just in monitor/pass through mode, with the selector switch set to input. Got a strange curve but looking through the manual this seemed to match up with the response curves of the rec/playback amps. So I dismissed it at the time on the assumption that it was part of the design. But thinking about it, if that response curve was representative of what was coming out of the line out sockets, shouldn't music played through the machine and then out again whilst just in monitor mode sound incorrect?

The weird thing is, I re-ran the test several times a few days later, in monitor mode, switch set to input, and got a much flatter curves as expected. So how on earth did I get that trace? Only the line sockets were used, internal amps weren't accessed. Compared to the AKAI, these Revoxes seem hellishly complex, but fun nevertheless.
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Old 5th Jun 2017, 9:56 am   #26
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Default Re: Converted to REVOX

Made good progress on the A77 MK4.

The final pieces of test equipment have arrived, a Malden Electronics 625 analogue oscillator, and a Racal-Dana 9300 RMS voltmeter, which means I should be able to carry out the calibration process when I have fully understood the signal routing and the calibration process itself.

I started off by following the signal path through the various boards and checking to see what effect each potentiometer has.

I have replaced all of the trim pots on all the amplifiers, including the Dolby processors. Several of the pots were literally falling apart which would explain the wonky sound.
Even after the trim pot replacement the signal reading was still wrong on the right channel. This was eventually found to be caused by faulty Frako 47uF/40v capacitor C22 on the Dolby playback processor, which was going short circuit. So I replaced this and all the other 47uF caps with new electrolytics. This resulted in even meter readings at last.

I carried out basic signal balance and bias adjustments by ear and the machine now sounds very good indeed. There is very little difference when switching between input and tape, dare I say it, I think the tape sounds marginally better than the source (might be my rose tinted earphones!)

My basic signal balancing tests have been done with the Dolby switched out, when I switch Dolby in, I can hear a slight balance flutter between left and right, so the Dolby needs to be set up properly. When I replaced all the pots, I put the new ones in, in approximately the same position as the old ones, but I have done no calibration so far.

I have an English version of the service manual for the A77, I think that it is for the MK2, I also have the Dolby B version from the Studer site, BUT it is in German, can’t seem to find an English version anywhere. Is there an English version available?

What is the procedure for setting up the Dolby processors? Presumably the record processor provides level ‘lift’ (measured at spot frequencies?) and the playback processor provides level ‘drop’ on playback. No doubt that there is lots of other complicated stuff going on as well!

One pot I haven’t changed yet is P305 on the switch board – what is its purpose?

Also one thing I hadn’t noticed before was that the machine was not working at 3.75ips, it would only run at 7.5ips. That was a very easy fix – a squirt of cleaner and tweak of P201!

I’m still waiting for some parts to arrive from Nagravox in Australia, and then I should be able to get the machine finished. Any advice on how to set up the Dolby would be most appreciated.
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Old 30th Jul 2017, 2:03 pm   #27
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Default Re: Converted to REVOX

Made lots of progress on this lately. It had lots of problems (pots, caps, brakes etc) mostly all sorted, and sounding great. Dolby is calibrated and working fine too. Now using a Ferrograph RTS2 and some other new test kit which makes calibration so much easier. Just waiting for some reel bearings, then final calibration and it will be done. Next in the line up is the 15ips unit. Happy days
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Old 30th Jul 2017, 5:49 pm   #28
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Default Re: Converted to REVOX

Ah, that's good. I just looked up the thread a bit and those plots you'd found used an
HP 3325A as the source... back in about 1995 I had the job of redesigning the fractional-N synthesiser board in that instrument after the chipset it used had gone obsolete. Amazing what comes back! The 3325A is a function generator and its sine output isn't very low distortion, but it can do squares and triangles etc. For sines, the HP 8903B is a lot better.

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Old 1st Aug 2017, 1:23 pm   #29
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Default Re: Converted to REVOX

monaro0162,

You mention 15 ips in post no. 27. There were/are two 15 ips record/replay characteristics - NAB and IEC. Have you chosen which one to use?
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Old 1st Aug 2017, 5:18 pm   #30
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Default Re: Converted to REVOX

The 15ips mention referred to a 'HS' MK2 that I acquired recently (on another thread). But good point. I haven't yet. Which one did Revox apply on their 15ips machines? Did I read somewhere that factory 15ips only had ONE equaliser setting on the rotary switch?

Reel and Capstan bearings have arrived for the MK4 Dolby, so last thing to do before final calibration.
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Old 1st Aug 2017, 7:29 pm   #31
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Default Re: Converted to REVOX

High speed A77s were either NAB or IEC, and the two tape settings of the tape/input switch gave identical curves, unlike the standard versions.

The NAB standard is 50/3180uS, IEC is 35uS. The trade-off is slightly less hiss on IEC as opposed to more HF headroom on NAB. In practice, the difference is slight. Pushed to a choice, I'd say IEC, as with modern tape HF crushing isn't really a problem. Also, you can rig an external filter to convert to NAB if needs be, whilst going the other way involves undoing a rolloff, which is awkward. When these machines were current, in general broadcast people went for IEC, commercial studios used NAB.
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Old 6th Aug 2017, 4:52 pm   #32
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Default Re: Converted to REVOX

Which standard did the Dolby A77 use?

Made some more progress on this today. I started the final run with some new tape, but was interrupted by noticing slight level and slight balance differences between each channel. Only slight, but there, and have been there ever since I've had the machine, even after the repairs to the Dolby board. I just used to counter-act it with a slight adjustment to the balance control, assuming it was a pot somewhere that needed final setup. With a test tone and a scope on the output, I noticed a major phase shift between channels, and varying levels during a frequency sweep. Turns out Q401 had gone short on the input amplifier. Replaced it and all is good now. Just surprised that I hadn't noticed this, so late into the restoration, and that the machine was still able to work seemingly very well. It sounded absolutely fine with music, may be it's my ears!
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Old 6th Aug 2017, 6:34 pm   #33
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Default Re: Converted to REVOX

Dolby was not applied to high speed machines - Dolby were keen to avoid confusion between the A system, which was for professional use, and the B, which was domestic and fitted to the Revox. A was, not surprisingly, optimised for higher tape speeds, and domestic machines fitted with Dolby B were supposed to have the Dolby switched out automatically above 7 1/2 ips.

Most A77 Dolby machines were NAB, as far as I know - nearly all pre-recorded Dolby B tapes were NAB (Barclay Crocker and the like), so I suppose it makes sense - in any case, the hiss reduction makes the choice of curve less critical, provided you know which it is.
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