blaupunkt: 4W77 (4 W 77); - Problem with AM2 functions

Hello colleagues,

While reading about some early problems with the C/EM2 and the AM2 tuning indicators . . . . .the problems ALSO developed within the USA early tuning eye tubes 6E5 and 6G5 according to the Sylvania Technical publications from their tube engineers in the late 1930s.

Is it possible that an minor error may have been overlooked within this early Blaupunkt model and still appears in the schematic? The specific circuitry and components feeding into the Abstimmanzeiger and NF stop does not agree with some Funke tube test data.

Respectfully,
Robert

NOTE: I have reviewed most of the forum threads and have considered the postings by JR as they relate to  a possible "fix"

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Hello Mr. Sarbell

 

There is a minor print-error in the schematic: the common  resistor  for the negative return of the AF3, AB2 and AM2 should read 1.5 KOhm instead of 1.5 MOhm.

 

But more importantly, The FUNKE test-configuration for the AM2 (Card 194 of my W16) has nothing to do with the actual circuitry designed by Blaupunkt in those days. The engineers simply followed the recommended factory applications for this type of indicator tube.

 

Function:

The right hand triode is solely used as lf-amplifier.

The left hand indicator system gets its variable voltage from the screen-grids of the ACH1 and AF3, which is fed via voltage dividers to the indicator grid GL.

 

Peculiarity:

With this very early type of “magic eye”, the more positive the indicator grid becomes (= higher negative AVC voltages), the wider the green pattern of the AM2 will illuminate. This is opposite to what we know from the later tuning indicators with the triode used as an amplifier for the AVC voltage and the anode(s) tied to the deflection bars.

 

There is an important disadvantage to be mentioned: In case of reception of weak stations, the green pattern becomes darker and at the same time somewhat blurred!

For more details click here:

http://www.radiomuseum.org/forum/am2_am2_peculiarities.html

Kind regards

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Hello Herr Holtmann,

Thank you so much for the explanation. The lengthy article in the Sylvania technical notes identified this deficiency of the 6E5 tuning indicator - its performance was noticeably impaired as you mention. With grid voltage of approx -8 volts the the shadow had normally disappeared.

Less than 1 year had elapsed before the engineers introduced the 6G5 in which the triode grid had been changed so that the plate cutoff would occur at nearly -22volts instead of -8 volts.

In this instance, all of the AVC developed would allow for correct indication on the tuning eye - regardless of the signal strength. The replacement of the 6G5 for the 6E5 would be somewhat simple to solve this dilemma in the radios which used them.

However, for the deficiency of the AM2 in the BP 4W77 configuration, there should normally be a "circuit improvement" made if the operator wishes the radio to function with a more accurate tuning indicator (as we normally expect). 

I believe that is what Herr Jacob Roschy had in mind when his posted thread contained the illustrations of the circuit modification.

Respectfully,
Robert


 
 

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Hello Jacob,

Thank you so very much for confirming my suspicions that the AM2 installation in the Blaupunkt has the design deficiency. Presently the AM2 functions in a manner similar to the 6E5 tube as opposed to the 6G5 or the 6U5.

After reviewing the schematic of the Philips model 765M (which also applies to the 766RF), which uses the AM2 indicator it appears as though the added AF pre-amp tube ABC1 may help to alleviate that problem.

But, those changes will significantly alter the originality pf the radio. . . . .

I must admit that the dilemma is not of my Blaupunkt on the workbench; but it belongs to a very good friend - and he was hoping to "solve the strange behavior" of the AM2 indicator.

I should scan the few pages from my Sylvania Technical News volumes and submit to the Papers forum - would that have any value for other members?

Respectfully,
Robert

  

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Dear Members,

I am restoring this nice model and after replace many items and repair some previous wrong modifications, the radio are playing with surprising sensibility and good sound.

I agree with Herr Holtmann regarding the error in the schematics with the negative AF3 return 1.5 M resistor, nevertheless I have notified another discrepancy among the "normal" AM2 voltage suggested in 4W77 Spannungen und Ströme and the normal voltages operation for the magical eye according to the original Telefunken Technical data sheet in the 15.06.37. 

The TFK suggested voltage for an AM2 normal function is - 6 to +3 (max) volt in grid (GL) pin 5.

The Blaupunkt Factory suggested voltage for the same AM2 pin5 (Schirmgitter) is 110 volt!!

My model is feeding with normal voltages all the tubes with +7,4 volt in AM2 pin 5 GL.  This voltages is achieved with the power rheostat in full clock wise position.

The replaced TFK AM2 NOS have a discrete behavior.  Probably this corresponds to  "the worst indication pattern of any eye tubes" as commented Herr Roschy.

I will be very grateful to receive some comments regarding the voltage discrepancy and the disappointing AM2 behavior. 

Best regards, Bruno.

 

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