help needed , restoration of the SB401 TX from Heathkit  - Page 1
Forum section: electronics / rf-microwave

author: Martin.M
author_rank: Frequent Contributor
timestamp: October 30, 2022, 08:44:00 am
content: hello friends,at first: I am not familiar with this transceiver, so I will ask sometimes for the correct setting of knobs at the front.The actually situation is this:The electrolytic from the power supply are changed to new.All rectifiers are tested to be OK.I have a good 760V, a good 250V and a good negative voltage for the bias.I have tested the transformer for primary isolation to ground, =best, using 1kVI have tested all PSU Voltages at cold radio by inserting 30V DC, there was no current = good, no shorts.I have removed the heat wires and insert from PSU external 6,3V to check the filaments, it`s all happy glowing, = good.I have extendet the isolation of the wires what feed the 730V doubler to be more safety.changes of values.The new electrolytics of the 730V doubler in the PSU are 150µF 500V typeThe new electrolytics in the 260V are 100/100 in the doubler, and a 100 from a camera and a 47µF around the choke.- done.Bias test following the manual.by hitting the PTT the bias on the instrument is adjusted to be 50mA, with removed driver tube 6CL6. = OKThe setting at frontpanel strongly following the procedure in the manual.Load.I use a japanese, Kuranishi Keisokuki, Powermeter with internal dummyload 50ohm for a maximum load of 150Wat the output,connected with RG58 cable.There are no other cables connected at the back.Input.I have buyed in the states a very cute crystal microphone, Turner +2,it delivers a nice signal and hit the PTT. Bec. the 401 have a glowing input the high impedance of the crystal may be welcome.At next I will test at the microphones gain pot if there is the microphone signal amplified present.Problems.I have NOT fout, any HF output.The instrument shows HV, OK, no Gate Current, not OK, no output, not OKthe driver tube 6CL6 is tested to have 24 from 30mA emmision (250/150/-3V on the Neuberger RPM 370/1 tester) = 5/6 from 100% , vacuum test = OKI have used a lot of time in this project and I still have the time and smile to continue, it will work.There is some nice side gears, the radio 313, the speaker 600 and the scope 610.the cinch plugs for the back wires are buyed and on the table.The 313 is done and full working.Now, 410 . any veterans here with the experience of SB410? I am pretty sure.greetings from germanyMartin- pictures will follow -

author: Martin.M
author_rank: Frequent Contributor
timestamp: October 30, 2022, 01:10:49 pm
content: Next step.I have now output, nice 110W.The sensitivity of the crystal microphone is to low (no amplifier in the mic)the VOX is always active ? why.it don`t work with the Key at .cwthere my be mistakes in wireing, the 301 stand allone actually, no radio connected.

author: A.Z.
author_rank: Super Contributor
timestamp: October 31, 2022, 12:03:28 pm
content: Quote from: Martin.M on October 30, 2022, 01:10:49 pmIt would be interesting, and useful to others facing your same issue, knowing what was causing the issue and how you solved itQuotestrange, the Turner +2 hada preamp, is the one you own a modified version ?

author: Martin.M
author_rank: Frequent Contributor
timestamp: November 07, 2022, 06:21:33 am
content: hello,still I have problems.There was a bad rotating switch contact plate, this is now allready.missing .cw : I am searching: there must be a "side tone" in the unit it ( don`t ! ) control RXTX by the VOXthe both power tubes seems to be bad, after short time the get overload of the g1 current, looks like a bad vacuum. The spare pair is much expensive but I will buy that, next month.The driver tube is also not the best, to be changed by a new one.The microphone sensitivity: The will be experiments with signal generator, maybe the input preamplidier tube is to buy also.greetingsMartin