CRT 3900 Resurrection....

A forum for the discussion of matters of a technical nature. All such activities are undertaken at the readers discretion and own risk. If you don't know what you are doing, don't blame us if it all goes wrong!
User avatar
Snake Plissken
Regular
Regular
Posts: 45
Joined: 01 Aug 2017, 13:47
Location: Escaped NY to Berkshire UK

CRT 3900 Resurrection....

Postby Snake Plissken » 18 Aug 2017, 18:47

Evening All,

I love minty radios, me.....but I also seem to love radios that are as rough as a badger's 4rse, I guess it's the challenge.....
This CRT 3900 (EPT360014B, 1996) falls into the latter category..... :lol:

It was in a scrap parts lot I bought a while back, but kind of looked saveable to me, although fairly robbed-out...so far I have:

Replaced missing signal meter
Rebuilt usual knackered vol/sq control
Refitted clarifier (properly)
Replaced missing 2SA473
Replaced missing driver and final (I THINK they're Eleflow, not sure)
Replaced about 9 missing horseshoe trimmers
Replaced missing Idiot Diode
Replaced blown-out D77 (probably why it was scrapped)
Biased the driver and final
Fitted another front and cases I had lying around (happened to be the correct one for a 6-way band switch :thumbup: )
....and the speaker.

So this is where I'm at, but there's a problem with TX in SSB modes (Mic Gain at zero) when I try to fit the upper case....radio starts to transmit a dead carrier, this can be between about 1/4W and 4 or 5W....

Happens when I lower the rear end of the case the last couple centimetres or so, with the front edge already down....
Audio seems distorted too, until I lift the case back off....

With the case removed it can still sometimes be maybe quarter of a Watt...

Anyone seen this problem before, or know what might be causing it?

I think I've sworn at it 3900 times this afternoon :wtf:

Cheers

Dan
Unidens (one or two)..
Cybernets (a few more)..
Stalker ST-9F DX Colonel FR360 Stag 357 Midland 79-900 Zodiac Tokyo Pegasus III Transceiver MkV
Cheiza CB702 workshop monitor :lol:
A few more, I forget...

Snake Plissken - Rescuing Presidents since a 1981 version of 1997 :D

User avatar
Black Spirit
Veteran
Veteran
Posts: 3941
Joined: 16 Mar 2009, 19:30
Location: S.Wales

Re: CRT 3900 Resurrection....

Postby Black Spirit » 18 Aug 2017, 19:08

Snake Plissken wrote:
when I try to fit the upper case....radio starts to transmit a dead carrier, this can be between about 1/4W and 4 or 5W....


Dan


Is that TX on its own or carrier on SSB when TXing ?

If it is carrier on SSB when TXing

Rubbish finals, Make sure board to chassis screws are tight, If its the same replace driver/final for 2166/2312 [Or 1969] genuine mitsibushi parts, Other than that the 331pf cap that comes off the pre driver to driver you can play about with that value to get rid of the carrier, I have used in the past a 151pf then added 47pf across the 151pf on the solder side, Increasing value didnt seem to work but adding two separate caps did :wtf:

You will lose some output though but not a great deal.

User avatar
Snake Plissken
Regular
Regular
Posts: 45
Joined: 01 Aug 2017, 13:47
Location: Escaped NY to Berkshire UK

Re: CRT 3900 Resurrection....

Postby Snake Plissken » 18 Aug 2017, 19:30

Hi Nick, just the person I need to happen along! :thumbup:

Your suggestions confirm what I was beginning to believe....output stage of dubious origin.
I know they are not Mitsubishi parts, and who knows who really made them....

And this also ties in with a few other symptoms - I've checked through the regulator area and it checks out OK, but this chassis is struggling to manage 4W in AM and FM modes with the AM carrier trimmer fully cranked, and maybe 7 or 8W MAX in SSB modes, ALC wide open...and I've (roughly) checked the TX strip alignment...

Will have to invest in a genuine driver and final - I was lucky to have all the missing stuff lying around, the chassis owes me £13 minus the parts :lol:

Non-genuine parts are so hit-and-miss....I took a chance on a couple of 8719PLLs a couple weeks ago from China, only £3 each....

One was for a dead Mk1 148 GTL-DX - It worked a treat, but who knows how long it will last...and transistors are a different story of course.

Do you have any suggestions regarding where to find some originals?
There's a little shop in Alabama I buy from sometimes, they have them but quite pricey...

Thanks Nick!
Unidens (one or two)..
Cybernets (a few more)..
Stalker ST-9F DX Colonel FR360 Stag 357 Midland 79-900 Zodiac Tokyo Pegasus III Transceiver MkV
Cheiza CB702 workshop monitor :lol:
A few more, I forget...

Snake Plissken - Rescuing Presidents since a 1981 version of 1997 :D

User avatar
Snake Plissken
Regular
Regular
Posts: 45
Joined: 01 Aug 2017, 13:47
Location: Escaped NY to Berkshire UK

Re: CRT 3900 Resurrection....

Postby Snake Plissken » 18 Aug 2017, 19:32

Carrier on SSB when TXing mate.....

TX and RX switching is all good on it now (it wasn't...)
Unidens (one or two)..
Cybernets (a few more)..
Stalker ST-9F DX Colonel FR360 Stag 357 Midland 79-900 Zodiac Tokyo Pegasus III Transceiver MkV
Cheiza CB702 workshop monitor :lol:
A few more, I forget...

Snake Plissken - Rescuing Presidents since a 1981 version of 1997 :D

User avatar
Snake Plissken
Regular
Regular
Posts: 45
Joined: 01 Aug 2017, 13:47
Location: Escaped NY to Berkshire UK

Re: CRT 3900 Resurrection....

Postby Snake Plissken » 18 Aug 2017, 19:39

Snake Plissken wrote:
TX and RX switching is all good on it now (it wasn't...)


I say that, TR51 regulator checked out OK, but TR52 seems to be getting pretty warm...and I was seeing 12 or 13v on the collector in AM/FM (the spec says around 5.6v?)
Unidens (one or two)..
Cybernets (a few more)..
Stalker ST-9F DX Colonel FR360 Stag 357 Midland 79-900 Zodiac Tokyo Pegasus III Transceiver MkV
Cheiza CB702 workshop monitor :lol:
A few more, I forget...

Snake Plissken - Rescuing Presidents since a 1981 version of 1997 :D

User avatar
cb4ever104
Veteran
Veteran
Posts: 4114
Joined: 28 Jul 2011, 21:26
Location: Alicante , España

Re: CRT 3900 Resurrection....

Postby cb4ever104 » 18 Aug 2017, 20:22

The 14B is a pretty crappy board. I've had the same carrier issue as you Snake a few times. The last couple of occasions I had to do the capacitor work that Nick mentioned. That cured it for me. Personally I much prefer the old 10 and 11 Ranger EPT boards.

Sammy
30TM060
Paul aka "Sammy". (108) PY60 Old Paisley DX Group / ex GM0 , now EA5
Handle "Fortune Hunter" back in the "day"
Radios ..... anything that says "TAIWAN R.O.C" on the back.

User avatar
Snake Plissken
Regular
Regular
Posts: 45
Joined: 01 Aug 2017, 13:47
Location: Escaped NY to Berkshire UK

Re: CRT 3900 Resurrection....

Postby Snake Plissken » 18 Aug 2017, 20:29

Yeah Sammy, I knew they had a pretty bad rep...!

Never owned anything CRT before, I'd heard the ranger radios were much better....

I'll have a go at playing around with the caps Nick suggested...and I just came across a genuine used 1969 in a drawer that seems to test okay.At least I can try swapping out just the final if I have no luck cap toonin.... :thumbup:
Unidens (one or two)..
Cybernets (a few more)..
Stalker ST-9F DX Colonel FR360 Stag 357 Midland 79-900 Zodiac Tokyo Pegasus III Transceiver MkV
Cheiza CB702 workshop monitor :lol:
A few more, I forget...

Snake Plissken - Rescuing Presidents since a 1981 version of 1997 :D

User avatar
Snake Plissken
Regular
Regular
Posts: 45
Joined: 01 Aug 2017, 13:47
Location: Escaped NY to Berkshire UK

Re: CRT 3900 Resurrection....

Postby Snake Plissken » 19 Aug 2017, 13:41

Update....

Radio seems to have 'calmed down' concerning the oscillation on SSB modes since last night...after fitting the cases and new speaker, I've put about 6 hours runtime on it and the fault hasn't re-appeared...no sign of a carrier at all on SSB! :thumbup:

Didn't even have a chance to tinker with the coupling cap within the pre-driver and driver stage...
Maybe the fault will re-appear, who knows....time will tell.

I will still remove the dubious driver and final I fitted when I source some genuine ones, power is definitely below spec still, but other than that all seems well! Also will fit an echo board I have lying around - this radio has the 'RB-EC' switch, but the echo board was missing / never fitted...

Not the biggest fan of echos but I like switches to do as they say :lol:

The echo board I have came from an online seller in Bulgaria - based on a PT2399 IC, the board is about the size of a postage stamp, has adjustable echo volume and repeat rate - and is about a third of the price of CRT ones.....

I can highly recommend these - I ghetto rigged it to an old York 861 chassis the other week, and the audio quality is excellent - something more like a Sadelta rather than an old Altai 315e :lol:

Thanks BS, Sammy - I'm now armed with the knowledge to tackle this problem if it returns, or if I see it on another radio in future.. :clap:
Unidens (one or two)..
Cybernets (a few more)..
Stalker ST-9F DX Colonel FR360 Stag 357 Midland 79-900 Zodiac Tokyo Pegasus III Transceiver MkV
Cheiza CB702 workshop monitor :lol:
A few more, I forget...

Snake Plissken - Rescuing Presidents since a 1981 version of 1997 :D

User avatar
Black Spirit
Veteran
Veteran
Posts: 3941
Joined: 16 Mar 2009, 19:30
Location: S.Wales

Re: CRT 3900 Resurrection....

Postby Black Spirit » 23 Aug 2017, 17:30

Yet another, A 3900 with carrier on SSB [no modulation] and this time i only had to replace C175 [331PF] with a 221pf and carrier gone.

DSC01876.JPG
You do not have the required permissions to view the files attached to this post.

User avatar
Snake Plissken
Regular
Regular
Posts: 45
Joined: 01 Aug 2017, 13:47
Location: Escaped NY to Berkshire UK

Re: CRT 3900 Resurrection....

Postby Snake Plissken » 24 Aug 2017, 13:36

I will go ahead and do the same mod on my radio Nick....

It's there and gone still on mine - stays away once you fit cases, but more often than not it will re-appear with cases removed...

...nothing more annoying when you're trying to set up the SSB tx :lol:

Still got to swap out the suspect 2312 I used for a genuine used 1969 I found lying around too...
Unidens (one or two)..
Cybernets (a few more)..
Stalker ST-9F DX Colonel FR360 Stag 357 Midland 79-900 Zodiac Tokyo Pegasus III Transceiver MkV
Cheiza CB702 workshop monitor :lol:
A few more, I forget...

Snake Plissken - Rescuing Presidents since a 1981 version of 1997 :D

User avatar
Snake Plissken
Regular
Regular
Posts: 45
Joined: 01 Aug 2017, 13:47
Location: Escaped NY to Berkshire UK

Re: CRT 3900 Resurrection....

Postby Snake Plissken » 24 Aug 2017, 15:55

Update...

Swapping out the 330pF for a 220pF solved the issue entirely for me too it seems...no sign of a carrier, cases on or off - how it should be!

No chance of setting up the ALC until I do the final tr swap; the regulator voltages check out OK, but still seems a little low on power (also waiting on a pair of MV1Ys...).

But audio is great, all modes...the echo unit I added was over-limiting it's audio input making the radio quiet with the echo on, a little fudging around with the echo unit's audio input series resistance matched it to the radio's usual level...sounds like an expensive base mic but only cost 8 quid :lol:

That's this puppy almost finished.
Was a total dog...but no longer :thumbup:
Unidens (one or two)..
Cybernets (a few more)..
Stalker ST-9F DX Colonel FR360 Stag 357 Midland 79-900 Zodiac Tokyo Pegasus III Transceiver MkV
Cheiza CB702 workshop monitor :lol:
A few more, I forget...

Snake Plissken - Rescuing Presidents since a 1981 version of 1997 :D


Return to “CB & 11m Multi-Mode Modification & Repair”

Who is online

Users browsing this forum: No registered users and 2 guests