Zetagi B132 help please

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3pinplug
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Zetagi B132 help please

Post by 3pinplug »

Hi all

My linear has just started playing up, it s a Zetagi B132 but I believe there are 2 versions? I have enclosed a pic of mine as found a schematic on line but mine appears to have different value resistors in it, but they do look factory.

The fault is that when I transmit when i dekey the radio sig meter is pegged back to zero, i can still receive but the stations signals are dramatically reduced, it will either kick back to life itself randomly or if i move the patch cable or areial cable at the back of linear which move the rear plate of the linear, the signal meter on the radio goes back to normal level.

I checked for dodgey solder joints and found a couple on the linears pcb, replaced a 47Uf cap that looked a bit distressed on the top but then noticed a blackened resistor - see pic below. Removing some bits to get at the board it has "15" stamped on it for the value so assuming 15 ohm, the others have 180 and are shown on the schematic I found as 180 ohm ,but both the 15 ohm and the 68 ohm shown in the pics are not on the schematic, unless waving at me and I have missed them which can happen.

I need to replace this resistor as backened but can anyone tell me whether this brown type of resistor is a 'carbon' or 'metal' film type and what "watt rating" i would need, others shown on that schematic are 1watt so would that be ok? The 15 ohm resistor has one track going to the relay so hoping once replaced this sorts the issue. radio on its own works fine it only when linear connected have also tried another patch lead - no difference.

Hope someone can help

Thanks In advance.

Pics show the model of my linear and also schematic I found online and the buned resistor.
IMG_8156.PNG
IMG_8158.JPG
IMG_8157.JPG
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cb4ever104
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Re: Zetagi B132 help please

Post by cb4ever104 »

The main issue/s I've had with zetagi amps are the patch leads (lengths) . I have a 135 and it'll work quite happily with most radios . However , I changed it for an old Grant a while ago , and it started playing up . Changed the patch cord and it now works OK on that radio . The resistors look like metal film , but I may be wrong . They're definitely bigger that 1W . Maybe 4 or 5W ?
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Re: Zetagi B132 help please

Post by 3pinplug »

Hi Sammy
Interesting about the patch lead, did you just try another patch lead or change the length.
4/5watt resistors don't look to be that common, mainly in states on fleabay. I will see if i can make out anymore info on it as i am going to desolder and remove it. Pain my amp not covered by the schematic that seems to be doing the rounds, this is where the old ‘CB Tricks’ site was useful but you cant get on anymore as they had loads of schematics. Cheers👍
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Re: Zetagi B132 help please

Post by cb4ever104 »

I've got about a dozen heavy duty patch cords here of different lengths. If I have any issues then I just tend to try one until I get the result I need. I bought some 4W resistors for another 135 that I had. Only cost my a couple of quid from China. They might be a bit less , but looking at the other components it looks around 3W.
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Re: Zetagi B132 help please

Post by 3pinplug »

Had a look and found some 5w 15 ohm in china quite cheap so will try one of ghose when they arrive. Thanks for the pointer hopefully this will sort it.
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Re: Zetagi B132 help please

Post by cb4ever104 »

3pinplug wrote: 29 May 2022, 17:49 Had a look and found some 5w 15 ohm in china quite cheap so will try one of ghose when they arrive. Thanks for the pointer hopefully this will sort it.
I used 3W resistors in mine . There's some info on this thread that might be of help .

https://transmission1.net/viewtopic.php?t=59535

Your resistors are for the HI/LO circuit as well , so you might want to watch the amount of power you're putting into the amp .
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Re: Zetagi B132 help please

Post by Otter »

3pinplug wrote: 28 May 2022, 19:09 This is where the old ‘CB Tricks’ site was useful but you cant get on anymore as they had loads of schematics.
Why can't you get on?
I just had a look and they don't have a schematic for a B132. http://www.cbtricks.com/Amp/zetagi/index.htm
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Re: Zetagi B132 help please

Post by 3pinplug »

Hi cb4ever,
thanks for the link,can see the 68
And 180 ohm resistors in that link for the hi/lo power but can’t see the 15ohm, do u have that value on yours.

Hi Otter
I had a link to cb tricks but it stopped working a whileback, someone posted it in here ,the link works great, many thanks i will save this to my desktop👍
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Re: Zetagi B132 help please

Post by cb4ever104 »

3pinplug wrote: 29 May 2022, 20:25 but can’t see the 15ohm, do u have that value on yours.
Your 15 is probably the 22 in the diagram. I don't have that resistor as you have an MRF in there. Mine is valve , so that part of the circuit is different.
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Handle "Fortune Hunter" back in "the day"
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Re: Zetagi B132 help please

Post by 3pinplug »

Ok many thanks.
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Re: Zetagi B132 help please

Post by ANDREWB »

Hi, could you do me a couple of close up fotos of the ferrite windings please? I might make a copy of one, thanks
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Re: Zetagi B132 help please

Post by 3pinplug »

Here you go!
ferr2.jpg
ferr1.jpg
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Re: Zetagi B132 help please

Post by ANDREWB »

By the way, it's sounds like your amplifier might be "taking off", that is self oscillating, try a few ferrite beads on the power rails close up to the amp, There does seem to be a lack of parasitic protection in the whole thing.
I knew Zetagi and Bremi stuff in the early 80's and it wasn't built with good practice's or quality parts, everything pared to the cheapest.

The 15ohm resistor is the bias resistor of the base of the MRF output transistor, too much input power will cook it, toast it, best replace it with a quality carbon film of 3W, better still 2 of 33ohm in parallel, the other resistor is part of the collector load in series with the relay, so as not to overload the NPN switching transistor. You could put a small computer fan inside to keep things cool.

Building one from scratch would be easy, the MRF power transistor is over £70 though............... :crazy:
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Re: Zetagi B132 help please

Post by ANDREWB »

https://www.macom.com/products/product-detail/MRF422

Hope this helps a bit.
I have also been cheeky and requested 2 free samples. :lol:
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Re: Zetagi B132 help please

Post by 3pinplug »

Cheers AndrewB , hope u get your freebies let me know if they send them. Thanks for the resistor info i will also watch the power. Would the 15 ohm resistor
be causing the loss of signal issue or dry joint would you think.
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