Zetagi b132 technical help

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Ur5by9
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Re: Zetagi b132 technical help

Post by Ur5by9 » 16 Apr 2017, 18:54

Perfect, I will leave out the diodes, thanks for clearing that up. I noticed on the last test the relay was chattering a little so I will take a look at this again, The issues I can see is that to get to the relay requires a de-soldering the SO239's and a switch to release the board. A fiddly job sums it up.

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Re: Zetagi b132 technical help

Post by ghost123uk » 17 Apr 2017, 14:27

What mode and what input power were you using when the relay was chattering?

It is unlikely (but not impossible) that it is the relay itself. I would check the 180 Ohm 1W resistor first. Also the 10μF cap by the 2 x 1N4148 diodes would be next..
John, on the S. Cheshire / N. Shropshire border. Muppet handle "Grey ghost" and on UK C25 most working days.
Don't just monitor, key up and talk, otherwise everyone thinks no one else is out there !!!

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Re: Zetagi b132 technical help

Post by Ur5by9 » 18 Apr 2017, 14:58

The Mode was AM, I will get round to testing this again later tonight and come back with the findings. Is the Transistor small working as a switch to put the Amplifier into transmit? powering up the relay?
This unit although pristine is quite old and may not have worked for many years 15+

For the amount of components on the board If I have to go to the trouble of removing it out then I could just replace the other few caps for piece of mind.

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Re: Zetagi b132 technical help

Post by ghost123uk » 18 Apr 2017, 16:29

Ur5by9 wrote:Is the Transistor small working as a switch to put the Amplifier into transmit? powering up the relay?
Yes it is :thumbup:
Ur5by9 wrote:This unit although pristine is quite old and may not have worked for many years 15+
Caps, especially electrolytics, and especially if left unused for long periods, can deteriorate.

You could key up with 4 or 5 watts going in (FM preferably = a steady carrier) and check the voltage on the relay end of that 180 Ohm resistor. I'm guessing (and it's usually written on it) that the relay will be a 12V one, so you would expect somewhere around 12V to be at that point.
Last edited by ghost123uk on 18 Apr 2017, 16:45, edited 6 times in total.
John, on the S. Cheshire / N. Shropshire border. Muppet handle "Grey ghost" and on UK C25 most working days.
Don't just monitor, key up and talk, otherwise everyone thinks no one else is out there !!!

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Re: Zetagi b132 technical help

Post by ghost123uk » 18 Apr 2017, 16:30

'
Last edited by ghost123uk on 18 Apr 2017, 16:40, edited 1 time in total.
John, on the S. Cheshire / N. Shropshire border. Muppet handle "Grey ghost" and on UK C25 most working days.
Don't just monitor, key up and talk, otherwise everyone thinks no one else is out there !!!

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Re: Zetagi b132 technical help

Post by ghost123uk » 18 Apr 2017, 16:32

Sorry, summat odd going on with my posts for some unknown reason {bnghd}
Last edited by ghost123uk on 18 Apr 2017, 16:33, edited 1 time in total.
John, on the S. Cheshire / N. Shropshire border. Muppet handle "Grey ghost" and on UK C25 most working days.
Don't just monitor, key up and talk, otherwise everyone thinks no one else is out there !!!

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Re: Zetagi b132 technical help

Post by ghost123uk » 18 Apr 2017, 16:32

Why don't we have a post delete button {bnghd}
John, on the S. Cheshire / N. Shropshire border. Muppet handle "Grey ghost" and on UK C25 most working days.
Don't just monitor, key up and talk, otherwise everyone thinks no one else is out there !!!

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Re: Zetagi b132 technical help

Post by stanogs68 » 18 Apr 2017, 17:36

ghost123uk wrote:Why don't we have a post delete button {bnghd}
i wondered that you cant just remove your post lol :?
computers mess with my brain !

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Re: Zetagi b132 technical help

Post by ghost123uk » 18 Apr 2017, 17:47

stanogs68 wrote:
ghost123uk wrote:Why don't we have a post delete button {bnghd}
i wondered that you cant just remove your post lol :?
Exactly, on many forums you can delete your last post, in case of double posting errors etc.

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Ur5by9 wrote:Put it all back together and it works into a dummy load, 5 watts in returned around 40 out and 10 watts gives 100 watts
Hey, Ur5by9 = when doing your tests, are you using a suitably rated (100W) dummy load ;)
John, on the S. Cheshire / N. Shropshire border. Muppet handle "Grey ghost" and on UK C25 most working days.
Don't just monitor, key up and talk, otherwise everyone thinks no one else is out there !!!

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Re: Zetagi b132 technical help

Post by Ur5by9 » 18 Apr 2017, 20:34

The dummy load I have will take 250 w , I have some time to spare so I'm heading off to the work bench and take a look at this amplifier.
I will run it up at 4 w and test it out :)

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Re: Zetagi b132 technical help

Post by Ur5by9 » 18 Apr 2017, 20:53

Well that did not take me long to find a new fault, I thought Id post my findings while I strip it down in case a reply comes in while i'm working on it.

I set this up using a radio capable of variable 4-10 watts output.
Setting this to 4 watts I keyed up and get a nice almost 45 watts out, Switching to around 8 watts It creeps up to around 80 watts.
As said in previous posts this would most probably be the limit for this circuitry and cooling arrangement.


I thought Id run it for a few minutes and watch out for any relay chatter or un-towards burning smell's
I un keyed however the relay stays shut and the amplifier still continues to produce RF out despite no input even with the radio switched off.

When I remove the power from the amplifier I can see the relay unlatching until RF is fed back in again where this continues.

So my thoughts now turn to the small transistor on the board that must sense RF and switch the unit.

I will now get out the de soldering pump and start to strip out this board and try to identify the components.

The saga continues.

Thoughts and comments are always appreciated.

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Re: Zetagi b132 technical help

Post by Ur5by9 » 18 Apr 2017, 22:26

Okay so a quick update,

firstly the 180 ohm resistor is quite burned up looking and has been dissipating a fair amount of heat, this is going to need changed, also the 100 ohm one looks to be in better shape however again has had a rough time of it.

The switching transistor is a BC237B

I then looked at the electrolytics which on the board are 2x 47uf 25v 8 w and 1x 22uf 25v
However the schematic shows different values?
Could this have had a repair before should I switch them out and for what would be recommended?
For quickness I have popped back up the schematic to save going back to page 1.

Image

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Re: Zetagi b132 technical help

Post by ghost123uk » 19 Apr 2017, 07:38

Ur5by9 wrote:firstly the 180 ohm resistor is quite burned up looking and has been dissipating a fair amount of heat
I'm guessing you're looking at the 180 Ohm in the amps input circuit. The one I was talking about, re the relay, is the other one (by the relay in the schematic). Mind you there is no harm in swapping out anything that might need swapping out. Use carbon resistors, not wire-wound ones.
Ur5by9 wrote:I then looked at the electrolytics which on the board are 2x 47uf 25v 8 w and 1x 22uf 25v. However the schematic shows different values?
Could this have had a repair before should I switch them out and for what would be recommended?
Where are these 2 x 47μF caps?
You should be able to tell if they have been bodged in, or if they are standard, by looking at the PCB and the associated tracks. The two electrolytics shown near the SSB switch on the schematic are there to control the relay's switching delay when on SSB.

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Ur5by9 wrote:I un keyed however the relay stays shut and the amplifier still continues to produce RF out despite no input even with the radio switched off
This obviously needs sorting. Try and prevent it from doing this by killing the power asap as soon as it locks up. You can't be certain what frequencies and power levels are happening and summat might be getting well stressed.

It's highly likely RF feedback. Pretty much the same as feedback you can get on a stage, from a singers microphone picking up the sound from the speakers and around and around it goes, often very loud and sometimes something in the PA amp pops because of it.

Firstly, try temporarily undoing that mains earth from the case. It might have an influence on this. Take care. I would use my mains testing screwdriver on the case during this test, to look for any mains present. Don't use the mains screwdriver when TXing though is it might then give a false positive due to RF fields.

Next, is your coax and dummy load of good quality. If it is RF feedback (as I suspect) then any concentrations of RF emanating locally, from the output end of the amp, could be finding there way into the input = feedback. Perhaps try it into an aerial (that has a good SWR) and see if it still happens.

Oh, just thought, is it doing this with the casing firmly attached? Or perhaps only with the "lid off" = check that ;)
John, on the S. Cheshire / N. Shropshire border. Muppet handle "Grey ghost" and on UK C25 most working days.
Don't just monitor, key up and talk, otherwise everyone thinks no one else is out there !!!

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Re: Zetagi b132 technical help

Post by Ur5by9 » 19 Apr 2017, 18:52

Done more testing and it seems to be fine from cold however after a few minutes it continues to make RF with no input. tried on 2 different dummy loads and swapped around the patch leads which are good quality cables. let it cool down and its fine again until the temps rise.
Lid on or off make no difference to its behaviour.

The caps look standard and I have replaced 2 of them however it is odd that the values on the unit are different from the schematic.
I have popped up a picture of the board, you can see the 180 ohms resistor has been getting fairly warm, on the last test burning was in the air and this resistor was giving of a faint bit of smoke :(

Image

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Re: Zetagi b132 technical help

Post by ghost123uk » 20 Apr 2017, 08:00

Anyone else got any thoughts on this, I'm running out of ideas.
John, on the S. Cheshire / N. Shropshire border. Muppet handle "Grey ghost" and on UK C25 most working days.
Don't just monitor, key up and talk, otherwise everyone thinks no one else is out there !!!

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