GenRad RCL-Digibridge  1687B  active probe replication - Page 1
Forum section: electronics / repair

author: madao
author_rank: Frequent Contributor
timestamp: November 17, 2019, 12:32:30 pm
content: Hello people.I have buying two GenRad 1687 (B) RCL Digibridge, but without active probe.Schematic tell me, it contains a transformer.It must transfer 1MHz signal without phase shifting. On my works is TC4-1T RF minatur transformer using for TTL-ECL Clock shiftingI have tried with him (in backwards) and 12 Ohm at secondary winding: a bit about 1V RMS, it is very good. A big luck, that it fit with 12resistor very well (a bit much, 70mVrms to much). Obiviously , zero phase-shifting condition with TC4-1T is satisfied.https://www.minicircuits.com/pdfs/TC4-1T+.pdfactive probe with manhattan style. After few trial & error with capacitor at input for impedance amplifier runs it with pausible value on display. Impedance amplifier is a MAX4477 , of course is it too slow (to much phase shifting)I'll trying with video-buffer. Sziklai-pair is just a ordinary emitter-follower, typical early ciruit for using in fast signal acquisition.But, my result with MAX4477 tells me: This is why is using transistor with fT of gigacycles.Cable is high quality VGA cable, it was accessory of BARCO monitor.

author: madao
author_rank: Frequent Contributor
timestamp: November 17, 2019, 05:38:59 pm
content: Next trying with BUF634 as buffer is a success. This amplifier is fast enough for zero phase shifting.I am needed very good capacitor as GenRad 1404 for verifity of result.Luck, i have a GenRad 1404-A, it is out of service (Reason: whisker) I must searching him in storage.Pic shows C measurement in series and parallel mode.Capacitor is a styroflex-typ, 301pf 1%.seriesparallelMeasurement with 33µH choke tells me same: pausible resultIt looks very good with TC4-1t transformer and BUF634 as buffer.regardsmatt

author: madao
author_rank: Frequent Contributor
timestamp: December 12, 2019, 07:35:18 pm
content: PCB from china is arrivied.But, i must search correct value of input capacitor-Divider. (It doesn't run with orginal value, too high amplitude)now is value 10pf /100pf.RegardsmattBTW: a little teardown while washing of case

author: madao
author_rank: Frequent Contributor
timestamp: February 20, 2020, 04:15:05 pm
content: good news:I am finished with probe-replica.I have needed much time, because GenRad 1687B hasn't high priority and i haven't workplace for mechanic stuff.Friends from me have turning a pieces brass for me.Turned brassIt fits well.Finished :-) (check with 39pf NPO ceramic capacitor )Check with capacitor with extreme low loss D = 0.0003early concept and schematic ( only input capative divider is other, acutally 10/100pf)Transistor-type is BC817-25Buffer/amplifier, of course: BUF634 (or high speed op-amp )If you need PCB and TC4-1T, pleas wrote me a PN. You got it for donation and shipping cost (from germany to other country (EU 8€, outside EU 16€)

author: madao
author_rank: Frequent Contributor
timestamp: February 23, 2020, 01:46:41 pm
content: GenRad 1404A Capcitor is found by digging in pile :-) (I have only part of GR 1404A, it is written of, Whisker)After whisker removing with russian style ( 30V 4A PSU) is GR 1404A useable.