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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 12th May 2022, 9:21 am   #21
TIMTAPE
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Default Re: A77 to revert or not?

As far as I know the motors are the same. Just the mounting plate holes are displaced to compensate for the shaft diameter differences between small, medium and large diameter shafts. The super slow speed model also slows the motor further with a different capacitor or two on the PCB.
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Old 12th May 2022, 9:25 am   #22
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Default Re: A77 to revert or not?

Motor problem solved. Faulty connection on the tacho unit (the unit was open circuit) plus a loss of 21V on the card. The feed wire from PS to Speed Control board had been cut/shortened in a past life and new connectors fitted. One was loose and was not making contact. I probably disturbed it when swapping the capstan motor.

1600/800Hz present and correct.
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Old 12th May 2022, 9:27 am   #23
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Default Re: A77 to revert or not?

True. Standard and high speed use the same tacho frequencies, only the shaft size and mounting hole position being changed. Halving the standard capstan size would result in a panel pin for a shaft, so the low speed option is achieved by reducing the size by a third, with a corresponding change in tacho frequency.
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Old 12th May 2022, 12:12 pm   #24
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I am not qualified to comment but this conversion does not seem straightforward. There is some odd and 'bodgy' things going on with it. I cannot argue with the resultant recordings because they are very good. Perhaps the skill was in the theory, not the practical.
Right now though I just want to get it back to a standard machine.

The fun begins here, I just hope will end well.
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Old 12th May 2022, 7:34 pm   #25
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Default Re: A77 to revert or not?

Thank you all for your suggestions and information. All seems to be working ok now, I've spent some time this afternoon changing trimmers on the audio board. The ones on there were small (8mm?) soldered onto pieces of wire so they fitted the 15mm holes on the board! Not ideal. I've decided not to recap now though, rather to start calibrating and setting up and see what problems arise as I go. When its all done I can then calibrate one card at a time and check, just in case I cock something up I'll know which card it is.

I do have a question though. Setting playback level from test tape. The manual specifies a DIN 45513/19H tape for setting the playback level. I have a DIN Bezugsband 19S which I believe is a variant of the one specified. Trouble is it's seen better days. Made in 1965 its had some use and is worn. I also have a 185nWb/m, 250nWb/m and 320nWb/m range of calibration tapes. Would anyone care to suggest which one would be the most appropriate for my A77. They are much newer and in good condition and give a much better signal. I know its right at the start of the calibration process but I don't want to guess and find myself having problems down the line.
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Old 12th May 2022, 9:00 pm   #26
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Default Re: A77 to revert or not?

Magnetic recording tape is, by its nature, eraseable and re-recordable. Don't be too trusting of what it's supposed to be. It would not take long to erase it and record something at say another level. Just because it's not very old is no guarantee of the accuracy of the flux density.

OK, you probably don't have malicious cat burglars popping in to re-do your level reference tapes, but there are other risks with similar outcomes.

Just playing one on a machine with some residual magnetisation can spoil a reference tape. You really have to treat these things like crown jewels.

Never play one on a machine unless you've carefully demagnetised it first. You can also while the tape is new make a 'user' version recorded so that the replay level matches the original.

So how do yougrow some confidence in what you now have?

Compare them. Provided you have ref level sections of the same frequencies, you can compare outputs on a trusted machine. Measure the differences in dB. You should have 20*LOG(320/250) and 20*LOG(250/185) and 20*LOG(320/185) dB

Having three tapes is good. If one pair seem correct and one is oddball, it's a fair chance that one has been damaged.

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Old 13th May 2022, 12:14 am   #27
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Default Re: A77 to revert or not?

According to the Canford Audio test tape page, the DIN reference level was supposed to be 320nWb/m but due to an inconsistency in the measurement method, the actual level was more like 290nWb/m.

https://www.canford.co.uk/MRL-TEST-TAPES
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Old 13th May 2022, 8:19 am   #28
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Ahh, thank you James, I didn't know whether that was suitable for my A77 (in reality though I use that on my PR99 so it should be ok, don't know why I didn't think of that in the first place . . .).

And thank you David for a full explanation. I have in fact dubbed my level tapes via my Studer and use the copies for the initial set ups. They are too expensive to take chances with. I do confess though, I left my Log tables, along with my slide rule, somewhere in the sachel a long long time ago.
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Old 13th May 2022, 10:01 am   #29
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Quote:
Originally Posted by Linnovice View Post
I do confess though, I left my Log tables, along with my slide rule, somewhere in the sachel a long long time ago.
A grave loss.

Grandad to a young junior school kid: "David, you know those sums you do at school? You know, it it takes seven men two weeks to dig a trench two feet wide, three feet deep and a hundred yards long, how long would it take if two men had a week off?"

"Yes...." wondering where this is going.

"Well, I'm going along to the shop for some woodbines. When I get back, you tell me whether it's easier to do the sum, or to dig the trench to find out."

He also pointed out later that if you learned to do the sums, you could choose whether or not to dig the trench.

Maths is one of the best cheats ever. Join the lazy side of the force!

Maths is a toolkit, shinier and more powerful than anything you can't afford in a snap-on catalogue. It's also a language.. And a short-cut through other dimensions.

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Old 13th May 2022, 11:12 am   #30
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I preferred Players Weights personally (or was it Cadets?)

Yes, many a true word said in jest. I’m off to the golf course now. Back to the calibrations tomorrow (if I can still stand up!).
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Old 15th May 2022, 10:45 am   #31
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Right, confusion now reigns . . .
I've now started to calibrate the machine, all good until I get to 6.4 in the SM.

Oscillator and RF Adjustments
RF-Voltages and Frequency Checking

It states, load tape and Record (stereo). Plus set the controls as indicated in manual. Which I've done.

A. Measure erase head voltage against ground with VTVM at VU meter board, Org HB3 (Chan2) and Blue (CH1), 2 track machine = 21Vac each.

When I do this I get 44Vac on Org, 0V on Blue and, if I move to the Ocillator schematic which indicates the erase head, I get 22V on the Brown.
If I release the Record button to CH1 I get 22V on the Blue.

Does anyone have an idea what is going on?
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Old 15th May 2022, 12:59 pm   #32
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Default Re: A77 to revert or not?

A quick look at the documentation and I have the same confusion with no easy answer.

Looking at the Oscillator schematic it shows 44V at the orange (point B) and 22V at the brown (point A) sounds the same as your results.

Very unsure if it is a documentation mix up or just misunderstanding the procedure, needs a lot more studying to get a better handle on it.

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Old 15th May 2022, 2:26 pm   #33
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Default Re: A77 to revert or not?

Looking at the Oscillator schematic more closely, I see that the blue wire connects to 0V when the channel 1 Record switch is switched down (Record on) so to me it sounds like all your results are correct when comparing to the voltages shown on the Oscillator schematic.

So if so then it would suggest that the Service Manual instruction at 6.4.1. A which states "Measure the erase head voltage against ground with VTVM at VU-meter board (fig 6.4-60) orange HB3 (CH 2) and blue HB6 (CH 1)." should read "Measure the erase head voltage against ground with VTVM at VU meter board (fig 6.4-60) orange HB3 (CH 2) and brown HB1 (CH 1)."

Seems strange but it would not be the first time for there to be errors in manuals.

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Old 16th May 2022, 9:31 am   #34
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Yes, I found that very confusing to say that he least. I think in fairness though. The manual was written with qualified and experienced engineers in mind. Not rank amateur wally’s like wot I am.

I’ve learnt something new. I don’t know what it as I’ve forgotten but hey, I know I e learnt something.

I’m very lucky in having a very knowledgable friend and he pointed out the ‘error’ in the manual as well so I’m onto the next stage later. Watch this space as I’m bound to be back!
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Old 18th May 2022, 8:48 am   #35
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I’m gradually getting somewhere now but there is much more work than I originally anticipated. A few unexpected problems have turned up. For instance, when trying to set up the bias traps I could not get channel one on the record card to function properly. It turned out the bias coil had been ‘stabbed’ by something sharp and the coil broken. A minute break which took some finding. I’ve had to find a good coil on a spare board and substitute. Not easy soldering wires the thickness of a human hair!
I said I would not recap until it was all up and running but it seems at each stage I’m finding another unexpected problem. I am now waiting a delivery from Cricklewood Electronics so that I can have spare caps/trimmers etc on hand as I go along. I’m sure it will ultimately be a good machine but at the moment it is presenting quite a challenge.
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Last edited by Linnovice; 18th May 2022 at 9:04 am.
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