10-02-2020, 07:50 PM
Great work Jeffrey.
It is coming along nicely.
Frank
It is coming along nicely.
Frank
Converting a monitor from 625 to 405
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10-02-2020, 07:50 PM
Great work Jeffrey.
It is coming along nicely. Frank
11-02-2020, 09:30 AM
Main HT is now up to 27V where it should be. Just had to put 10n across C4 to make the unusual HT stabiliser happy at 405. Now things are definitely improving. I checked the distortion at the left and right edges using crosshatch. It's severe cramping. Probably need to play with the S correction cap.
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
11-02-2020, 10:28 AM
Cramping and or RH foldover seems to be the norm when you go down in frequency. It could be S correction related but I've found its a mismatch between the scan coils and line transformer. Using a Thorn jellypots 625 scan coil drive connections to feed on 405 gives this severe non linearity, reverting to the 405 connections gives perfect results.
I'm watching this with interest. Good work so far.
Cheers.
Trevor MM0KJJ. Member of, RSGB, GQRP, WACRAL, K&LARC.
11-02-2020, 12:04 PM
(This post was last modified: 11-02-2020, 12:05 PM by ppppenguin.)
The S correction cap is 2x 2u2 in parallel. Added another 2u2 and nowhere near enough. Even adding 4u7 isn't getting there. Looks quite nice on test card but errors are obvious on grille.
Also now getting too much EHT if I increase the width to fill the screen. Will need to change the flyback tuning. Looking quite good now, so I'm confident i'll get the right answer. After trimming the LOP components I'll need to put the LOP and CRT heater back on their internal supplies. I've found some TIP121 darlingtons which should be ideal for dropping the main HT to feed the LOP. PS: moire is JPEG compression artefact. Original hi res photo is fine but a bit big to post here.
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
11-02-2020, 04:10 PM
(This post was last modified: 11-02-2020, 04:17 PM by ppppenguin.)
Had to go up to about 16uF total for S correction. I put an extra 10n from the LOP collector to 0V to slow down the flyback. This reduces EHT for a given width.
Need to order some 10u film caps from RS or Farnell. Anything higher gets expensive and/or too big. Can then easily pad it out to required value. Needs to be a decent plastic film cap but doesn't actually have more than about 20V AC across it. This might do as a 15uF part: https://uk.rs-online.com/web/p/polyester...s/8961275/
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
11-02-2020, 05:32 PM
Great result Jeffrey.
I'm watching this with great interest. I've a cheap 625 line monitor in the loft I'm sure, you are spurring me on to try this.
Cheers.
Trevor MM0KJJ. Member of, RSGB, GQRP, WACRAL, K&LARC.
11-02-2020, 06:06 PM
(This post was last modified: 11-02-2020, 06:07 PM by ppppenguin.)
Going down in frequency is easier than going up.
These Melfords are high quality monitors, used widely by broadcasters. Black level, EHT and picture size are all well stabilised. Other parameters such as geometry are very good. 10u caps on order. Got these: https://uk.rs-online.com/web/p/polyester...s/8462695/
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
12-02-2020, 01:04 PM
I've lashed in the darlington voltage dropper for the line output stage. I fitted the TIP121 darlington to the back panel as far as possible from the main regulator transistor to give it the best possible heatsink. Collector to main 27V HT, emitter to LOP HT, base to 470R pot with 330R to 27V and 1K8 to 0V. Gives a sensible range of adjustment.
The back panel now gets quite hot but probably no hotter than it did in the original 625 version. The lashup could be replaced with a bit of verobaord but it's unlikely to happen on this monitor. Next one perhaps. The red wire going past in the foreground is part of the temporary CRT heater supply. Next job is to get that back to internal power. I'll float the monitor with an isolating transformer and measure the RMS heater voltage with my Rigol scope. C15 is AOT to give correct voltage. After that it's time to put the lid on and give it a good long soak test.
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
12-02-2020, 01:56 PM
10uF caps arrived. Fit just nicely in place of one of the original 2u2 parts. I used a 6u8 in parallel which may be giving a bit too much S correction. So I might go down to 4u7. Or even replace the pair of 2u2 seeing as there are a total of 3 sites for S correction caps on the PCB.
I hooked the CRT heater back to the internal supply. It works but it's clearly not getting enough voltage. With the Fluke 87 it read about 5.5V on 625 (which may or may not have been actual 6.3V RMS) and it's about 4.5V now. I'll drag out the isolating TX and trim it to about 6V true RMS usiing the scope. There's a 330n capacitor dropper fitted as standard. 470n may do the job. I also have 680n to hand.
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
12-02-2020, 04:14 PM
I took the S correction down to 10+ 2.2+ 2.2 = 14.4uF and that seems about right.
I floated the monitor and played with the CRT heater supply. Ended up with 1uF and 1R in series. That gave me 6V RMS on the heater as measured with the Rigol scope. Read about 5.5V on the Fluke 87. Subject to the whole thing staying working on soak I reckon that's done. One for me, one for Dulwich and one for Frank Cuffe. Is anyone else interested? This isn't a casual offer, it's only for those who have a genuine need for a good 405 line monitor. I could probably spare one or two more. I would epxect a sensible contribution to BECG funds: www.becg.org.uk I may also consider selling my Prowest PM14/1A. This is a very high quality dual standard monitor but it's not entirely reliable.
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
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