Help needed to run cassette deck from bench psu

[Janaaoi]

Hello,

I want to run a Tascam 112MkII pro audio cassette deck from a precision linear bench psu, but don't know how to connect one. Can anyone help?

I attach the power supply schematic.

 

[D.A.(Tony)Stewart]

locate drawing C-2 coord. +12V, Gnd,-26V and lowV ac on P4.

 

[Janaaoi]

locate drawing C-2 coord. +12V, Gnd,-26V and lowV ac on P4.

I do not understand your instruction. I cannot find such a reference in the Service Manual, which I attach.

 

[zuisti]

There's some bad news: I think this is not possible because the original power supply is quite complex, it gives multiple AC and DC voltages.
The solution is the repairing if the transformer is good yet. It would not so hard because only the other items be tested and replaced.

 

[Janaaoi]

There's some bad news: I think this is not possible because the original power supply is quite complex, it gives multiple AC and DC voltages.
The solution is the repairing if the transformer is good yet. It would not so hard because only the other items be tested and replaced.

No, please let me explain.

The machine is brand new and unused, and I wish to modify it for forensic science usage. I am trying to copy some of the specification found in the JBR Technology Standard Tape Deck, which is currently unavailable for purchase.

The purpose of replacing the standard transformer with a high quality linear external psu is to move the 50Hz magnetic field away from the playback head and electronics.

 

[D.A.(Tony)Stewart]

If no current is drawn, there may be no need to move it. Locate the row and column address outside the schematic and see location C-1 for the location of DC connections and ac signals.

 

[Janaaoi]

If no current is drawn, there may be no need to move it. Locate the row and column address outside the schematic and see location C-1 for the location of DC connections and ac signals.

Once again, I cannot locate your references within the schematic, or the Service Manual.

 

[zuisti]

....The machine is brand new and unused....
The purpose of replacing the standard transformer with a high quality linear external psu is to move the 50Hz magnetic field away from the playback head and electronics.

Well, that is a different situation! What I would do (if you cannot reduce the magnetic disturbances (to the minimum as you want) with newly inbuilt permalloy shielding plates):

- Should be removed the whole transformer and an appropriate length wires should be connected to all pins.
- If this is not possible or problematic, should be removed the whole PSU PCB (CONT M PCB 2/2).
It has two sockets (P4 and P12) and five hard-wired cables (1 to 5 numbered) to the "PCB CONT M 1/2" (if I interpret the diagram). They should be extended accordingly.

Otherwise we do not really understand the diagram: the two 'F' transformer terminal (15 and 17), both are going to the same point (1) on the PCB 1/2. Short circuit or a diagram error?

It can be seen that a simple external DC power supply is not a solution ...

 

[Janaaoi]

Well, that is a different situation! What I would do (if you cannot reduce the magnetic disturbances (to the minimum as you want) with newly inbuilt permalloy shielding plates):

- Should be removed the whole transformer and an appropriate length wires should be connected to all pins.
- If this is not possible or problematic, should be removed the whole PSU PCB (CONT M PCB 2/2).
It has two sockets (P4 and P12) and five hard-wired cables (1 to 5 numbered) to the "PCB CONT M 1/2" (if I interpret the diagram). They should be extended accordingly.

Otherwise we do not really understand the diagram: the two 'F' transformer terminal (15 and 17), both are going to the same point (1) on the PCB 1/2. Short circuit or a diagram error?

It can be seen that a simple external DC power supply is not a solution ...

I attach a copy of the JBR forensic audio cassette recorder brochure which describes the provision of an external DC power source, in the way as I am trying to replicate.

The JBR is based on the Tascam 122MkIII, whereas I have the slightly less specified 112MkII. The differences only relate to recording, which has no relevance in forensics - where only playback is of interest, and a further direct drive motor in the transport mechanism, plus balanced outputs which can be purchased as an optional extra.

 

[zuisti]

Well, good luck, I cannot help anymore.
zuisti

 

[D.A.(Tony)Stewart]

Janaaoi. Your post #1 with attached schematic has spreadsheet like numbers and letters on outline to help locate intersection on a page. C-1 is between U.S.A. and Canada on left side which shows the common exit DC voltage pins for the external power supply, which is what you asked for.

Now I'd like to ask, where do you find a brand new $6k tape recorder with no external power supply and no after sales support? ( or did someone find it and ran a 120V unit off 240Vac?

 

[Janaaoi]

Janaaoi. Your post #1 with attached schematic has spreadsheet like numbers and letters on outline to help locate intersection on a page. C-1 is between U.S.A. and Canada on left side which shows the common exit DC voltage pins for the external power supply, which is what you asked for.

Now I'd like to ask, where do you find a brand new $6k tape recorder with no external power supply and no after sales support? ( or did someone find it and ran a 120V unit off 240Vac?

I see how you were referencing the schematic now. I am not an electrical engineer used to schematics.

What I asked for was for someone to walk me through the process ie how to physically connect everthing up.

I did not find a $6k tape deck with no external supply... etc. The Tascam MKII was around $1000 new back in the 2000's, and I found an unsold "old-stock" example for a good price. Tascam still have most of the parts for it and the 122MkIII, as far as I am aware. These stock models obviously have normal internal transformers.

The JBR unit is an extensively modified 122MkIII, including the external transformer, which is why it costs around $6000. However it is not being manufactured at present and may not be again.

As I do not need most of the JBR extras, other than the external transformer, it seemed a reasonable idea to simply attempt to add one to the 112MkII that I could obtain, giving me a useable machine now, and at a much reduced cost as a bonus.

 

[D.A.(Tony)Stewart]

Measure voltage at D21, D23, D25 ( right side of diode on schema. ) and inject 3 external psu voltages using diodes at slightly higher voltage so as to supply all the current. Ensure anode is same direction in each case (depends on V polarity).

With no significant AC current there should be no significant hum

 

[Janaaoi]

Measure voltage at D21, D23, D25 ( right side of diode on schema. ) and inject 3 external psu voltages using diodes at slightly higher voltage so as to supply all the current. Ensure anode is same direction in each case (depends on V polarity).

With no significant AC current there should be no significant hum

Sounds good. I will measure the voltages and get back (I have yet to choose a PSU).

 

[Janaaoi]

D21 = 09.26 VDC
D23 = 17.25 VDC (16.31 under load)
D25 = -28.78 VDC (measured on the anode side)

I have a number of questions:

1. Are these the only three relevant diode bypass points, what about D20?
2. As I will be using a quality low ripple linear PSU, won't the feeds at these points be put unecessarily through two sets of similar components?
3. How would I physically attach the different voltage rails to such small diodes?
4. How would I introduce the negative polarity VDC at D25?
5. What about the PSU ground feed - I don't know where that would be attached to common ground?

I attach jpgs showing the components.

 

[D.A.(Tony)Stewart]

1. Are these the only three relevant diode bypass points, what about D20?
2. As I will be using a quality low ripple linear PSU, won't the feeds at these points be put unecessarily through two sets of similar components?
3. How would I physically attach the different voltage rails to such small diodes?
4. How would I introduce the negative polarity VDC at D25?
5. What about the PSU ground feed - I don't know where that would be attached to common ground?

1. D20 and D21 are wired OR full wave Ac rectifier diodes, therefore common cathode point so D20 is irrelevant, however the AC is fed to the same connector for some other timing purpose so that is still be required.
2. No if you supply a diode drop higher external power source, the internal regulator will be idle with no output current.
3. Solder to the underside pads if you cannot solder tiny wire to leads.
4. A floating external 24Vdc can be positive grounded
5. P4-3 looks like a convenient access point for ground or locate a suitable ground pad underside.

 

[Janaaoi]

A few further questions:

1. Is there anything I can damage by attempting this set up? Bearing in mind this is a new deck that I cannot afford to spoil. For example, if I use crocodile clips to try it, and one comes off, will that loss of voltage upset anything else permanently?
2. Can you explain what you mean by positive floating external Vdc? The psu will be earthed to a common ground on the board. How is it "floating"?
3. And why have you quoted 24V when I found 28.78 (29)V?

 

[Janaaoi]

A few further questions:

1. Is there anything I can damage by attempting this set up? Bearing in mind this is a new deck that I cannot afford to spoil. For example, if I use crocodile clips to try it, and one comes off, will that loss of voltage upset anything else permanently?
2. Can you explain what you mean by positive floating external Vdc? The psu will be earthed to a common ground on the board. How is it "floating"?
3. And why have you quoted 24V when I found 28.78 (29)V?

A further question:

If I feed only DC into the circuit that ignores the F AC voltage. What would be the consequences of that?