Marconi 2017 signal generator motor drive is not working - Page 1
Forum section: electronics / repair

author: zibadun
author_rank: Regular Contributor
timestamp: October 14, 2016, 12:20:13 am
content: 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

author: singapol
author_rank: Frequent Contributor
timestamp: October 14, 2016, 07:45:30 am
content: QuoteI 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.

author: zibadun
author_rank: Regular Contributor
timestamp: October 14, 2016, 07:44:15 pm
content: Quote from: singapol on October 14, 2016, 07:45:30 amthank 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.

author: singapol
author_rank: Frequent Contributor
timestamp: October 15, 2016, 04:34:43 am
content: Quote from: zibadun on October 14, 2016, 07:44:15 pmAre we on the same page?QuoteAnyway I think This is what happened...Quote

author: zibadun
author_rank: Regular Contributor
timestamp: October 17, 2016, 12:41:56 am
content: the ECL input at SP8647www.elektronik.ropla.eu/pdf/stock/gps/sp8647.pdfdivider 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 blockdoes 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

author: singapol
author_rank: Frequent Contributor
timestamp: October 20, 2016, 10:10:35 am
content: I think you are troubleshooting in the wrong way. Please read service manual carefully. It states that :QuoteDo you see the above conditions and if not first measure the power supply voltages as per manual and thenlook at the trouble shootingt chart.

author: zibadun
author_rank: Regular Contributor
timestamp: October 20, 2016, 01:13:43 pm
content: singapolI'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.

author: singapol
author_rank: Frequent Contributor
timestamp: October 20, 2016, 06:20:20 pm
content: Quote from: zibadun on October 20, 2016, 01:13:43 pmRight, 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?

author: zibadun
author_rank: Regular Contributor
timestamp: October 20, 2016, 06:37:05 pm
content: 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 $17It does look dead to me. I just want to be absolutely sure before replacing.

author: zibadun
author_rank: Regular Contributor
timestamp: October 25, 2016, 03:11:35 am
content: that was it!after replacing IC1/AL5 the motor works againhttps://youtu.be/ZqkJqpE0_EE

author: zibadun
author_rank: Regular Contributor
timestamp: November 01, 2016, 10:08:17 pm
content: fixed the broken knob. 3d printed at the local library for $1.5