Tempo One problems

Discussion in 'Radio Circuits, Repair & Performance' started by N9ZGA, Feb 21, 2021.

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  1. HAMHOCK75

    HAMHOCK75 QRZ Member

    I suspect that where you mention V4 6EJ7, really is meant V3, 6CB6 the pre-mixer.

    Interesting that the solid state balanced modulator was the source of the problem. I have the same type. The 2SA695D pnp transistor acts as a current source. The "D" specifies an hfe range of 90-180. The current is controlled by the carrier potentiometer at the back. It sends a current to forward bias diodes D2 and D4 to unbalance the modulator.

    Fet Q1 is a source follower delivering the 8998.5 and 9001.5 kHz crystal frequencies to the diode modulator, fet Q2 is the output amplifier which goes to the crystal filter. VR1 and TC120 are adjusted to balance out the carrier. Pin 9 is where the audio comes in from the microphone amplifier.

    [​IMG]
     
    Last edited: Apr 29, 2021 at 7:35 AM
  2. W6NFU

    W6NFU Ham Member QRZ Page

    True, when I referenced V4 6EJ7, I really meant V3, 6CB6 the pre-mixer.

    I put a .01/2kv in series with C40 and C55 as a backup if less than known stellar capacitors are used in these two spots. I just ordered a bunch of 100 pf/2-3kv ceramics from Mouser to put in all my Tempo's. Will parallel two 100 pf's to get the needed 200 pf's for C40.

    I sure wish I had the knowledge you and N2EY possess regarding electronics. Your explanations of how the signal paths and SSBM circuits work has been a great help to me in resurrecting two Tempo Ones. Like I said before, I could measure voltages and resistances against the charts and track down discrepancies, but these recent problems did not involve wrong voltages, only "lost" signals. Thanks to the both of you for your help.

    The SSBM schematic shows R150, a 150K resistor in the main power line to drop the voltage from 150-V down to 8.5-V at Pin 2. When I got this radio, a previous owner had inserted a 14K instead of the 150K. Removing the 14K and replacing R150 with a 150K, the voltage at Pin 2 was way too high, I suspect this is what took out the SSBM FETS and BJT. Going back to 14K brought the voltage down to 8.5-V. Any comments from you guys?

    Three-Tempo Dave
     
    N2EY likes this.
  3. HAMHOCK75

    HAMHOCK75 QRZ Member

    The 9 volt zener diode, D6, is supposed to define the voltage at pin 2, not the resistor R150.
     
  4. G3YRO

    G3YRO Ham Member QRZ Page

    Yes but fitting a 14k resistor instead of a 150k one presumably blew the Zener Diode?

    Roger g3YRO
     
  5. W6NFU

    W6NFU Ham Member QRZ Page

    Good overlooked point by me, let me check, I'll get back to you all.
    Dave w6nfu
     
  6. G3YRO

    G3YRO Ham Member QRZ Page

    And I would also presume that Q1 Q2 & Q3 on that board have fried too !

    Roger G3YRO
     
  7. HAMHOCK75

    HAMHOCK75 QRZ Member

    Below is a schematic of the solid state balance mixer ( SSBM ) used in some later model Tempo One ( Yaesu FT-200 ) transceivers with actual measured voltages.

    The voltages were taken under the following conditions.
    1. mode switch to CW
    2. Receive
    3. Carrier level potentiometer set to maximim
    The -22 VDC on pin 3 is derived from the -100 VDC power supply. If the cw key is depressed, the -22 VDC drops to zero volts. The FET transistors Q1 and Q2 are a source follower to drive the low impedance of the diode mixer, and the output amplifier for the double sideband suppressed carrier signal, respectively. These are P channel junction FET’s that have a pinch off voltage of around -2 VDC maximum from gate to source. With the -5.7 VDC applied from the -100 VDC power supply, these FET’s are turned off hard resulting in the source voltages measured of zero volts.

    If the key is pressed, the gate voltage of these FET’s is zero volts. They turn on under this condition. In effect, this is a solid state version of “grid blocked keying” used in tube transmitters with the gate replacing the grid.

    Q3 acts as a current source. In CW mode, the mode switch connects the rear panel “Carrier” control to ground. The other end of the “Carrier” control connects to pin 8 where it can be adjusted between 0 ohms to 3 megohms. In the schematic below, the “Carrier” control is set for 0 ohms which results in the maximum current to the the balanced mixers diodes D2 and D4. This current unbalances the mixer so the 9001.5 kHz ( shifted down 200 Hz ) signal which also appears on pin 3 can pass through the SSBM to Q2. Pin 8 at 0 ohms results in Q3 saturating which is the reason the collector and emitter voltages of Q3 are both at +7.4 VDC. Under this condition, the current passing through Q3 to D2 and D4 is about 1.8 mA determined by the voltage across R11 ( 1 kohm ).

    There seems to be an error both in the manual and the pc board which identify R150 as a 150k resistor. This cannot possibly be true. If it were, then 170 VDC minus the 9 volt zener voltage leaves only about 1 mA. passing through R150 to the zener. That would make it impossible for the current source to send 1.8 mA to unbalance the balanced mixer. That would not be sufficient current for either Q1 or Q2 when they turn on. The actual measured value of R150 is 14.53 kohms that results in a current of 11 mA. which is far more reasonable.

    I do not see how pin 2 can rise above +9 VDC since the zener clamps the voltage at that pin unless the zener is bad.

    Also I wanted to mention although it is not shown in the schematic, T1 has a ferrite core so T1 needs to peaked at the carrier frequency.

    [​IMG]
     
  8. W6NFU

    W6NFU Ham Member QRZ Page

    This radio with the SSBM was working okay until I zapped the D6 Zener. So, 9.1-V 1-watt diodes were ordered, twenty each will be here in two days. The two FETs and transistor will be replaced along with the Zener. Which of the following FETs in stock here would you suggest as replacements for Q1 and Q2?

    2N5638
    MPF-102
    J-176
    J-310

    Is PN2222a okay to replace Q3, 2SA695D?

    Thank you for describing how this SSBM works, the clear schematic with voltages, and confirming the mistake in the Manual regarding R150. I will keep it at 14K.

    Three days ago I received a filthy Tempo One and last night 6883's were installed plus the rig aligned. It works very well. The 6883/6146 mod calls for C40 to be paralleled with another 200-pf capacitor (total of 400-pf) which I neglected to install initially. On Tune up, it seemed okay except power out on 40-m cw was only 20-watts compared to 100-watts out on 80-m cw. Installing another 200-pf across the existing C40 brought the power up to above 80-watts output on 40-m cw after another alignment.

    As soon as new 100pf/2kv Ceramic capacitors arrive this week, all my three Tempo One's will have C40 and C55 capacitors replaced with new. Then each can be operated without one hand on the OFF switch just in case! Was measuring 83-Vac between Tempo chassis and the chassis on other equipment, so 2-wire original power cords were replaced with 3-prong, eliminating that problem.
    Dave - w6nfu
     
  9. HAMHOCK75

    HAMHOCK75 QRZ Member

    These are direct replacements for the 2SK34. BFS70, 2N3369, 2N3821, 2SK381 but you can still buy new 2SK34's on eBay.

    https://www.ebay.com/itm/303882337759?hash=item46c0cc7ddf:g:N94AAOSwgAFgJVpd

    I can check tomorrow if any of the devices you mentioned will work too. This is a pretty low frequency application so I suspect there should not be a problem with a device from your list.

    The only specification unique to the 2SA695D is the "D" has a guaranteed Hfe of 90-180 ( Hfe is current gain ). A device with lower Hfe may not be able to source as much current so the maximum carrier level set by the carrier control might be lower.
     

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