How to repair Collins KWM2 Negative bias Intermittent power supply operation

Part 3 of the Collins KWM2 restoration. Moving forward on this project. Turns out that a few of the symptoms were caused by a worn power supply 11 pin connector. These connectors are difficult to locate and very pricey. Lucky, I had one (original Amphenol) on hand, so was able to swap it out. The negative bias issue turned out to be a matter of re scaling the voltage divider. Now I have enough bias to set the plate idle current. Lots of problems with this radio, however tackling them one at a time, soon it will be air worthy. This has been a real learning experience for me. Part 4 on the way soon. Hope this info is of value to you. TD

[@rciancia]

It is proven over and over again, that looking at the simple stuff is often overlooked and often the culprit :) Great detective work Terry. - Ron

[@coldfinger459sub0]

As always I learned something new again from one of your videos thank you very much. That little trick about resistors in the change in voltage from when the unit was manufactured and then today’s voltage having to go back and swap out resistance to bring it back in line. 👍

[@TheRadioShop]

Thanks for another repair job on this classic radio. Well done Terry

[@MikeY-hl5pi]

Looks like it's coming along nicely. I have a KWM-2A along with a 312B-5 312B-3 and a 30S-1. I inherited this gear from my Uncle several years ago and have not had it on the air yet.

[@superhet7281]

Hi Terry, happy new year! I just got a KWM-2 and I’m really enjoying this series. Great job!

[@robertcalkjr.8325]

Thanks Terry. Sometimes I also forget to check the most simple things first.

[@RobertKohut]

Looking at the circuits I think they might have been the inspiration for "circus boards"....LOL Man that's jam packed!!

[@raymondheath7668]

That's good to know. I have several units with seperate power supplies that I haven't dug into yet

[@dfpolitowski2]

What happen to part 4??? I was looking forward to the final fixes on this thing.

[@SwanseaTitanFan]

Hey Terry, another great job, on a side note, Brad the Guitologist gave up on that Messa Boogie after like 4 attempts. It's your turn to have a go.

[@daveogarf]

TERRY! I LOVE your opening soundscape! And all from that little hand-held synthesizer, yes? Speaking of music, that 11-pin power connector reminds me of the connector from my combo organ to my Leslie speaker! KEWEL! (Would that monster socket be called a "sockeramus"?)

[@umajunkcollector]

Just got done watching an Ampeg SVT ""Corvette"" amp repair, now the 57 Cadillac of ham radio, a nice Collins. I'd have Collins taste on a Drake budget. But um very happy with Drake, kinda like a Lincoln, naaa, make that an Imperial, um not a Ferd guy.

[@kevingray707]

Excellent video!!

[@johnsimms3957]

Happy new year Terry!

[@RobertKohut]

Nice!!

[@electronixTech]

Nice repair job.

[@Not-Only-Reaper-Tutorials]

Great Collins. I owned one ... super!

[@johnstrunck3659]

Doing all the good!

[@sincerelyyours7538]

Interesting video of a nice repair. To test your theory that the change in source voltage since the P/S's manufacture date caused the low negative bias, could you have proved it by putting the transmitter on a variac and lowering your voltage to 117VAC? Would it have then worked as normal? I'm always leery about changing resistor values from a trusted schematic because I think something else may burn out sometime later if I don't thoroughly test the circuit first. Always enjoy these videos, thanks.

[@msf60khz]

I presume that if the ground wire in the power supply cable failed, the rig case would be at HT voltage.

[@willrobbinson1]

Would be a good to check the grid 1 coupling cap from driver for leakage? also watch the bias at the 6146's for drift while keyed (no rf) for 5mins see if stable? good vid thanx lov working on tube gear also.

[@mikes4408]

The key on my 75s-1 was worn and put in on wrong. Blew all the tube filaments. Took some work but got it working. Question, what is that DX engineering box inside?? Thanks!

[@andarthome142]

Always check the cabels first :-)

[@buildstoys]

And the cables too :)

[@DeadKoby]

One step at a time...

[@robertjonasson2527]

You can source the 11 pin Amphenol female connector at BB Organ. www.bborgan.com/products/amphenol-female-socket-5-6-7-9-or-11-pin-5 Bob WB6JRY.

[@dhpbear2]

-80V bias? I thought they were around -10 to -15V (?)

[@d-labelectronics]

516 power supply provides approx -70 to -80 vdc

[@bebo5558]

Collins, not quite as roomy as a Fender eyelet board, is it? It figures, the problem is always in the last place you look! Ha Ha

[@russellhltn1396]

Shouldn't a higher line voltage cause a higher bias voltage? It seems to me that having to go to a lower resistance suggests something is drawing more than originally designed.

[@d-labelectronics]

Remember this is a negative voltage, current draw in the radio is normal, just needed to be adapted to the rig. This same issue arises with vintage guitar amplifiers. I will be posting a follow up video showing a full tune up/grid,plate current, etc.

[@russellhltn1396]

@@d-labelectronics Yes, a higher line voltage would give you a bigger negative voltage number (a mathematician would argue that a higher negative number is actually a smaller value).

[@russellhltn1396]

@@d-labelectronics Adapting is fine, but unless I misunderstand something, you'd adapted the power supply to confirm to "as designed" when paired with this transmitter but it wasn't working right. That raises a red flag. (And I'm not buying into the higher line voltage argument.) If this wasn't the right power supply, then perhaps the transformer was different and that would explain things. Likewise, if you're running different transmitter tubes then designed, that could explain things. But from this armchair, it appears something isn't working as designed.

[@d-labelectronics]

Hello Russell. Its hard to say. However the radio is working fine now. I run across the same issue with negative bias in amplifiers. May be different characteristics of tubes. The negative bias is simply used to throttle down the output tubes for proper idle current, which works fine & smooth now. This is a 50+ years old radio, not expected to perform perfect

[@russellhltn1396]

@@d-labelectronics Ok, if it's not an isolated incident. It's just when things don't go back right, it doesn't give me a "warm fuzzy". There's a number of situations where changing the value on components might fix the problem, but it's only covering up the problem, not fixing it.

[@jp040759]

As usual. 95% of gear trouble is bad or corroded connectivity.