The Defpom Forum, for RadioMods, S&M Software and Music by The Defpom

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 usual suspects have already been replaced by someone else (pcb screws, Tip42c) so i'm at a loss.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

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.

Thanks Rick, i'll try that tomorrow. by the way, the diodes attached to the output transistors, are their values critical? should they be the same and can you recommend a replacement?

cheers, Alexthegasman

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.

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

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.

TR45 need to be a 2SC2166. It will sound pretty bad on SSB with 2SC2086.

cheers rick, i'll swap it out and let you know results.

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

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).

cheers rick, the tranny was hot enough for me to pull my finger away a bit sharpish got the bias set up though but didn't want to do any more till i heard back from you. will check those parts tomorrow and post results asap.

cheers rick, all new trannies in the radio now and biased up no probs, worked on ssb for 10 minutes with no distortion at all. problem was probably a combination of faults. cheers for the info and the use of the site!!!