Marconi 2017 signal generator motor drive is not working

[zibadun]

I just received a new 2017 generator and everything seems to work fine with the exception of the motor drive. 
Is anybody familiar with this instrument and can assist with troubleshooting?

Here is what I found so far

- the service manual is here: https://dl.dropboxusercontent.com/u/28207218/marconi%202017/prodpdf_784_1.pdf
- The motor control circuit is on page 301 and the description is on p.81

- I have checked that the motor is not burned out (12 ohms)

- After selecting the frequency on the keyboard the generator switches to the correct range, however the Motor Drive Enable(-) signal which is supposed to start the motor does not go to Low.
The correct frequency can be set by manually adjusting the tuning knob.

It seems that the next circuit to trace is somewhere on the filter/processor board but I'm now a bit lost 

[singapol]

It seems that the next circuit to trace is somewhere on the filter/processor board but I'm now a bit lost 

I would be too. It's a complex system. Since you say motor does not activate then you should be looking at AL4 pcb and schematic. Read chapter 4-2 pg.67 for a description of motor drive control. See fig.34 .The signal you said is at ic.18 (74LS00). Check this ic18. Read the logic control too. AL4 schematic is Fig.11 chapter 7 pg.23. 
AL4 test points is in chapter5 pg. 39.
                                                                 Good luck.

[zibadun]

motor drive control. See fig.34 .The signal you said is at ic.18 (74LS00).

thank you.
i think there is a typo on fig 34, IC18 is actually IC8 looking at the schematic..  TP11 (IC-8 pin9) never goes  high, from out-of-lock control . that must be the next circuit to check.

[singapol]

motor drive control. See fig.34 .The signal you said is at ic.18 (74LS00).

thank you.
i think there is a typo on fig 34, IC18 is actually IC8 looking at the schematic..  TP11 (IC-8 pin9) never goes  high, from out-of-lock control . that must be the next circuit to check.

Are we on the same page?

   

Motor drive circuit description


143. In the SWEEP mode, SWEEP{ON'signal at IC8 pin 8 is asserted low, TR7 is
turned off and TR8 on. This_connects a potentiometer chain consisting of
R35, R35 and the variable front panel SWEEP RATE control AOR2 so that Vi
(sweep) is a variable proportion of Vi (fast drive), see Fig. 19(c 

Anyway I think This is what happened...

 

                                                              H 52017-900K
                                                               Vol. 2

SYNCHRONIZER/COUNTER 'A' (A|I5)- see Fig. 20
Circuit diagram : Chap. 7, Fig. 12


147. Synchronizer/counter 'A' board AL5 operates in conjunction with
synchronizer/counter 'B' board AL4 to provide two main functions:-

(1) In the Frequency Lock mode AL4 and AL5 provide frequency synchronization.
Electrical (fine) tuning is derived from the variable ratio
divider (VRD) together with phase detector, and motor (coarse) tuning is
derived from a frequency (or period) comparator.

 (2) In the Manual or Unlocked Frequency mode the VRD circuit is reconfigured
as a standard decade counter providing the frequency display
data(motor tuning is disabled).

(3) A subsidiary function enables AL4 and AL5 to act as a counter to
externally applied signals.

148. The Counter mode of operation gives continuous monitoring of the carrier
frequency in the Manual (UNLOCK) mode. It provides for a 7% decade display
of carrier frequencies from I0 kHz to 1024 MHz with a resolution of I0 Hz
below 128 MHz, and 100 Hz above 128 MHZ. The counting process in AL5 varies
according to the selected range as shown in Table 3.

                                                                     Apr. 81 Chap. 4-2
                                                                               Page 43 
                                                                                                   

[zibadun]

the ECL input at SP8647 www.elektronik.ropla.eu/pdf/stock/gps/sp8647.pdf  divider has Vpp of 600 mv , and there is nothing at the TTL output.   If I lift TP11/TP12 jumper the signal goes up to 1.3 Vpp (which I think would have been sufficient to trigger the input).   the ECL signal sags somewhere from C8 to the input of IC1 . But I'm not sure if this is normal Here is the signal path.



This signal is the input to the VRD block

does this look normal or should i try to replace IC1?

edit: it looks like ECL input is within spec (400-800 mV with 3.8 v bias) . but there is nothing  on pin 11 TTL output

[singapol]

I think you are troubleshooting in the wrong way. Please read service manual carefully. It states that :

PREPARATION FOR USE

Switching on

7. With the instrument connected to a suitable a.c. supply proceed as
follows.

(1) Switch ON and check that the SUPPLY switch is lit and the fan is
operating.

(2) Check that the instrument has taken up the LOCAL operating mode
condition, that is CARRIER FREQ tuned to 400 MHZ, no FM or AM MODULATION
and minimum RP LEVEL setting.

(3) Check that the rear STANDARD INT/EXT switch is set to INT, unless
an external standard is being used as described in para. 22.

(4) When using the internal reference standard, allow a warm-up period
of 5 minutes.


                                                              Chap.3 pg.5 

Do you see the above conditions and if not first measure the power supply voltages as per manual and then
look at the trouble shootingt chart.

[zibadun]

singapol

I've done basics checks, yes.  The fault I'm experiencing is described on line 4 in Table 10 (Ch.5 p36) "Motor drive inoperative. Lock indicator operative".   I have narrowed it down to Out of Lock control and from there to the VRD IC1 as on the schematic above.   There are no pulses on TP7/AL5, but there is a pulse at the input of AL5 IC1.  That is a divide by 10/11 ECL to TTL converter. 

[singapol]

singapol

I've done basics checks, yes.  The fault I'm experiencing is described on line 4 in Table 10 (Ch.5 p36) "Motor drive inoperative. Lock indicator operative".   I have narrowed it down to Out of Lock control and from there to the VRD IC1 as on the schematic above.   There are no pulses on TP7/AL5, but there is a pulse at the input of AL5 IC1.  That is a divide by 10/11 ECL to TTL converter.

Right, if there is no waveform on TP7 then it means IC1 is bad? But first is there 5V for IC1 and input from TP11.. is it a pulse or waveform as stated in chart? IC1 is SP8646 when your datasheet is SP8647? Obsolete part ...can you still find it? Still check the components around IC1?

[zibadun]

signal on TP11:


5v present. also DC bias 3.8V present on pin 16 for TTL mode.
SP8647 is the same as 8646 only higher frequency part. it's what there now.

I've ordered one for $17 

It does look dead to me. I just want to be absolutely sure before replacing.

[zibadun]

that was it!
after replacing IC1/AL5 the motor works again

https://youtu.be/ZqkJqpE0_EE

[zibadun]

fixed the broken knob. 3d printed at the local library for $1.5