cobra 148 gtl-dx help!!
- alexthegasman
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cobra 148 gtl-dx help!!
Hi, could anyone tell me why this 148 gtl-dx (pb010) wont let me bias the output?? Passed on to me by a friend with ssb garbled, thought it would just need biasing but it wont let me! usual suspects have already been replaced by someone else (pcb screws, Tip42c) so i'm at a loss.
Fault, low mic gain, Audio readable on near-by radio.
Mic gain full, very distorted audio with possibly a carrier noise and unreadable.
pre-set vr's checked and ok and c150 replaced with new one. Only other thing is bolt on diodes across outputs have been replaced with 1n4007 below board.
Just to add, when i connect to tp 8/9 vr11, and measure current, it rises up and beyond 50mA if you turn it down a little, it keeps going. Cant keep it steady. when i move test point to measure, can only get 24/25 mA max and 0 mA lowest on vr10
any help would be geatly appreciated.
Cheers, Alexthegasman
Fault, low mic gain, Audio readable on near-by radio.
Mic gain full, very distorted audio with possibly a carrier noise and unreadable.
pre-set vr's checked and ok and c150 replaced with new one. Only other thing is bolt on diodes across outputs have been replaced with 1n4007 below board.
Just to add, when i connect to tp 8/9 vr11, and measure current, it rises up and beyond 50mA if you turn it down a little, it keeps going. Cant keep it steady. when i move test point to measure, can only get 24/25 mA max and 0 mA lowest on vr10
any help would be geatly appreciated.
Cheers, Alexthegasman
Re: Cobra 148 GTL-DX Bias fault.
Check C205 maybe leaking, just replace it.
Check R263 maybe o/c.
Check R267 maybe o/c.
has for bed TX audio it could be a number of faults, but try replacing C144 and make sure that C150 is not a fault one, when soldering in C150 be quick as they do not like getting to hot.
Check R263 maybe o/c.
Check R267 maybe o/c.
has for bed TX audio it could be a number of faults, but try replacing C144 and make sure that C150 is not a fault one, when soldering in C150 be quick as they do not like getting to hot.
Rick.
- alexthegasman
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Re: Cobra 148 GTL-DX Bias fault.
I use 1N4148 but 4007 will be ok. But I mount the diodes so they are just touching front of the Driver and Final just like the originals.
Rick.
- alexthegasman
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should i use a little bit of heat-sink compound between diodes and transistors?? I will replace all of those componants to be on the safe side, thanks again rick, i will post results a.s.a.p
found r267 dead short up to yet, more results to follow. only got 1n4001's in the spares box so gonna get some 1n4148's like you suggest Rick.
cheers, alexthegasman
found r267 dead short up to yet, more results to follow. only got 1n4001's in the spares box so gonna get some 1n4148's like you suggest Rick.
cheers, alexthegasman
- alexthegasman
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Hi All, update on that troublesome cobra. Replaced all components that rick advised. replaced 1n4007 with old stock (after checking them) thermal diodes from junked out CRT 3900. Finally managed to bias up the outputs as per the service manual but now tr 45 (c2086) is getting very hot to the touch. Any ideas as to why?
cheers, alexthegasman
p.s resistor 267, 2.2 ohms was open circuit, everything else checked out ok but all electrolytics were replaced, but not ceramic type.
cheers, alexthegasman
p.s resistor 267, 2.2 ohms was open circuit, everything else checked out ok but all electrolytics were replaced, but not ceramic type.
Re: Cobra 148 GTL-DX Bias fault.
TR45 need to be a 2SC2166. It will sound pretty bad on SSB with 2SC2086.
Rick.
- alexthegasman
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- alexthegasman
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- Posts: 14
- Joined: Wednesday 28th May 2008, 0:47
- Location: United kingdom
sorry rick, it was tr46 (c2086) that was getting hot, not tr45, i'm an idiot!! do you know of a replacement for this transistor?? or could something else be wrong causing this tranny to heat up?? Also noticed that tr's 48,53 have been replaced with c710's but not put on the board as the markings say to, i guess the pin-out is different?
cheers, alexthegasman
cheers, alexthegasman
All the big numbers, Alexthegasman
Re: Cobra 148 GTL-DX Bias fault.
Your TR46 is the right one and it will get hot, if it is getting very hot check the components between Tr45 and TR46.
You should put the right transistors in TR48 (2SC1675) and TR53 (2SC945).
You should put the right transistors in TR48 (2SC1675) and TR53 (2SC945).
Rick.
- alexthegasman
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- Posts: 14
- Joined: Wednesday 28th May 2008, 0:47
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- alexthegasman
- Dead Key
- Posts: 14
- Joined: Wednesday 28th May 2008, 0:47
- Location: United kingdom