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Old 29th May 2024, 10:33 pm   #21
Phil G4jhs
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Default Re: Codar Multiband 6 info

The new coil holder turned up today and was installed so no more bad connections there. I replaced the FET firstly with a 2N3819 and the with a MPF 102 with only a small difference being made. If i inject a signal I can tune it but the reaction note is very rough and at times sounds more like a rapid pulse. I also managed to tune in a carrier but no audio on it.
With the MPF102 the voltages to ground are as follows. Drain to ground 6.19, Gate to ground 0.6mV and source to ground 2.6v. The voltage on pin 5 of the audio module is 6.19. Comparing the audio module connections on the circuit diagram to the actual radio there appears to be some differences. The other thing about the circuit diagram is the connections of the reaction cap and coil connections, surely the person who drew the diagram would know that the coil connections are pins 3 and 4? or is there another reason that 4 has been substituted with a ? Mmmm
All I had time for tonight!!!
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Old 30th May 2024, 11:54 am   #22
frsimen
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Default Re: Codar Multiband 6 info

What you are describing could be "motorboating". If it is, have a look at the power supply decoupling capacitor before you go any further. On the circuit diagram posted above, it's the 220uF capacitor.

The voltages on the FET look reasonable now. As you can get the detector to oscillate I think you have the wiring to the valve holder as it should be.

Paula
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Old 30th May 2024, 5:53 pm   #23
David G4EBT
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Default Re: Codar Multiband 6 info

Quote:
Originally Posted by Phil G4jhs View Post
The other thing about the circuit diagram is the connections of the reaction cap and coil connections, surely the person who drew the diagram would know that the coil connections are pins 3 and 4? or is there another reason that 4 has been substituted with a ? Mmmm
All I had time for tonight!!!
If you look at Fig 5 on page 30 of the Denco Data Bulletin 'DTB4', you will see the connections of the Green coil, (albeit in that instance a suggested circuit for a 1 valve RX). It shows that the pin in question is pin 4 - the other end of that winding being pin 3. A check with an Ohmmeter will confirm that:

https://g4dmp.co.uk/dencocoil.pdf
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Old 30th May 2024, 9:47 pm   #24
Phil G4jhs
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Default Re: Codar Multiband 6 info

Hi David, thanks for that, it is just the way it was done on the circuit diagram, a ? instead of a 4. Just made me think, maybe too deeply HI HI
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Old 3rd Jun 2024, 2:43 pm   #25
Phil G4jhs
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Default Re: Codar Multiband 6 info

I changed the 220uf cap re motorboating and no difference was noted. I also made a closer inspection of the audio module and noticed that the VA1040 thermistor has lost all its paint so I have to assume that it has been really hot at some time. Slow progress but will get there.
Phil
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Old 3rd Jun 2024, 9:23 pm   #26
frsimen
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Default Re: Codar Multiband 6 info

With a better look at the circuit diagram, especially the internal arrangement of the Mullard module, I think I pointed you to the wrong capacitor. The 220uF is connected across the supply rails. The detector takes its supply from the module. There is a 150uF capacitor connected between pins 1 and 5 of the module. That is the one which decouples the detector's supply. If you have access to it, try replacing it - use a 220uF one. Just connecting it externally between pins 1 and 5 will prove the point.

Paula
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Old 4th Jun 2024, 12:38 pm   #27
Phil G4jhs
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Default Re: Codar Multiband 6 info

Ok, thanks for the info, i might remove the AF module anyway as there appears to be some corrosion within. Access looks to be awkward but i can get across pins 1 and 5.

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Old 5th Jun 2024, 4:28 pm   #28
Phil G4jhs
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Default Re: Codar Multiband 6 info

Hi Paula, i did as you suggested with the 220uF cap and things did improve. The audio output is still low but i managed to tune in a broadcast station on about 15mhz but nothing on coil 2 which tunes .515 to 1.515 approximately so i checked the inductance of coils 1 to 5 and according to the info i have available coils 3/4/5 checked out but 1 and 2 did not. Now i am not sure if the info i have is correct.
On coil 1 between pins 5/2...... 224uH and between 8/9......25uH and coil 2 between pins 5/2......3.48uH and between 8/9...... 259uH which does not agree with my info. Does anyone know the correct values or are able to check?? So i am making progress, i need to work on the audio module to see if i can improve the volume.
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Old 5th Jun 2024, 4:47 pm   #29
Phil G4jhs
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Default Re: Codar Multiband 6 info

I Have just had a senior moment!!! the inductance of both coils is correct. To start with the coils were in the wrong tins when purchased and i thought i saw uH on my meter instead of mH
ah well.

Phil
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Old 5th Jun 2024, 5:19 pm   #30
frsimen
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Default Re: Codar Multiband 6 info

That sounds like you are making progress. Is the regeneration now smooth and no longer pulsing?

Paula
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Old 5th Jun 2024, 9:21 pm   #31
Phil G4jhs
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Default Re: Codar Multiband 6 info

There is some progress thankfully, the pulsing only happens now on a very weak signal which cannot be tuned or resolved but i am still baffled by the Denco coils. My info says that coil 4 tunes from approx 5 to 15MHz and i can resolve a few stations in the 11.6 to 13.4MHz bands yet on the Codar dial those frequencies are on range 2!! and on the dial range 4 covers from .5 to 1.5MHz so i have to assume Green coil 4 does not correspond to Range 4 on the dial. I can hear the heterodyne from the Codar on my FT-2000 but not very strong. I still think the voltage from the audio module to the rf stage is low i have 6.4v and i think it should be about 7.4v and the volume control only seems effective when turned fully clockwise. So more rainy day work to come

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Old 5th Jun 2024, 11:03 pm   #32
frsimen
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Default Re: Codar Multiband 6 info

Hello Phil,

The spread in the characteristics of FETs is so great that I wouldn't worry unduly about the low voltage that you are measuring. The circuit will still work at the reduced voltage, although its gain might be a little lower than expected.

For the coils, it really doesn't matter what they say they are. Sort them so that they match the markings on the dial and all will be well. As a guide, (and stating the obvious, perhaps!) the range 1 coil will have the fewest turns and the range 4 coil will have the highest number of turns.

If you are are opening up the AF amplifier module, check the resistors and change the 200uF capacitor that connects to the emitter of the input transistor. If that has a high ESR, the gain of the amplifier will be reduced.

Paula
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Old 7th Jun 2024, 10:24 pm   #33
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Default Re: Codar Multiband 6 info

Hi ( the range 1 coil will have the fewest turns and the range 4 coil will have the highest number of turns.) the other way round range 1 coil is the long wave coil. Ajusting the brass screw in the top changes the inductance (moves the slug) so changes the range in MHZ covered with a given tuning cap The coil covering .5 to 1.5 is coil 2 Mick
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Old 7th Jun 2024, 10:36 pm   #34
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Default Re: Codar Multiband 6 info

Hi this taken from Denco DTB4, yes the coil numbers on mb6 are wrong Mick
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Old 11th Jun 2024, 12:19 pm   #35
Phil G4jhs
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Default Re: Codar Multiband 6 info

Thanks for all the info, much appreciated. Finally found my audio signal/injector and tracer, lots of intermittent on the audio module so i have removed it thus morning!! it was quite a challenge for my poor eye and hands
Now for testing and replacing faulty components.
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Old 11th Jun 2024, 12:24 pm   #36
Phil G4jhs
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Default Re: Codar Multiband 6 info

Regarding the coils i could tune stations using coil 4 on or about 11 to 13 mHz but no stations on coil 2. I only got a click when rotating the regen but the audio module was so intermittent i decided to repair it before going any further.
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