29fb313
Freebander
Registered: 09-2009
Posts: 11
Karma: 0 (+0/-0)
|
Reply | Quote
|
|
cobra 148 not transmitting on am
hi guys, my 148gtl dx wont transmit on am, carrier but no mod, just a hum, perfect on fm and ssb, power meter shows 20w am, fm, this seems to be higher than i remember, thought it did about 10 w. anyone help please? Tom
|
21/Mar/2010, 10:03 am
|
Link to this post
Send Email to 29fb313
Send PM to 29fb313
Blog
|
madmark1
Accomplished DX-er
Registered: 10-2009
Location: Abertillery
Posts: 454
Karma: 9 (+9/-0)
|
Reply | Quote
|
|
Re: cobra 148 not transmitting on am
hi there, it sounds as though the regulator is blown, its located at the right handside of the radio near the back as the radio is upside down, mic socket on the right, its bolted to the sidepanel, it should be a 2sa1012 original part, but others will work, this regulator supplies around 8 volts on AM and FM and supplies 13.8 volts on SSB, when blown it supplies 13.8 volts to all modes, but AM modulation doesnt work. the regulator can be replaced with a 2sb754 or simulair.
mark 163fb20 / 163op20
Last edited by madmark1, 22/Mar/2010, 7:49 am
--- http://www.youtube.com/MrMadjones
http://gw0wvl.weebly.com/
|
22/Mar/2010, 7:47 am
|
Link to this post
Send Email to madmark1
Send PM to madmark1
Blog
|
29fb313
Freebander
Registered: 09-2009
Posts: 11
Karma: 0 (+0/-0)
|
Reply | Quote
|
|
Re: cobra 148 not transmitting on am
Thanks Mark, i will try that, love this old cobra and want to keep it running. 73's Tom
|
22/Mar/2010, 3:49 pm
|
Link to this post
Send Email to 29fb313
Send PM to 29fb313
Blog
|
madmark1
Accomplished DX-er
Registered: 10-2009
Location: Abertillery
Posts: 454
Karma: 9 (+9/-0)
|
Reply | Quote
|
|
Re: cobra 148 not transmitting on am
i know what you mean tom, nice ones are hard to find, hopefully the info helps you . although it is difficult trying to give information as t what is definatly wrong with the cobra without actually testing it . anyway test for supply voltage on the collector of the final transistor ' ie whatever your power suppl supplies, then test for 0.7 volts on the emmitter. the other pin grounded to the chassis.
as stated you should have supply voltage on the collector ' its the centre pin' this reading should be around 12 to 13 volts on SSB and should read 8 volts no more on AM or FM.
MARK 163FB20 / 163OP20
--- http://www.youtube.com/MrMadjones
http://gw0wvl.weebly.com/
|
22/Mar/2010, 8:26 pm
|
Link to this post
Send Email to madmark1
Send PM to madmark1
Blog
|
29fb313
Freebander
Registered: 09-2009
Posts: 11
Karma: 0 (+0/-0)
|
Reply | Quote
|
|
Re: cobra 148 not transmitting on am
thanks again mark, on further investigation this do seem to be the problem, just located a regulator at the cobraworxshopz on ebay, all the best, Tom
|
22/Mar/2010, 8:46 pm
|
Link to this post
Send Email to 29fb313
Send PM to 29fb313
Blog
|
29fb380
Freebander
Registered: 03-2010
Posts: 30
Karma: 1 (+1/-0)
|
Reply | Quote
|
|
Re: cobra 148 not transmitting on am
i have read that 7808 regulators work well if you can't source that part.
|
31/Mar/2010, 7:47 pm
|
Link to this post
Send Email to 29fb380
Send PM to 29fb380
|
29fb313
Freebander
Registered: 09-2009
Posts: 11
Karma: 0 (+0/-0)
|
Reply | Quote
|
|
Re: cobra 148 not transmitting on am
thanks robert, original part still available at the cobraworxshopz on ebay, tom
|
1/Apr/2010, 2:34 pm
|
Link to this post
Send Email to 29fb313
Send PM to 29fb313
Blog
|
madmark1
Accomplished DX-er
Registered: 10-2009
Location: Abertillery
Posts: 454
Karma: 9 (+9/-0)
|
Reply | Quote
|
|
Re: cobra 148 not transmitting on am
also you can use a 2SB754, this is what is used in the grant, its higher current. also some of the TIP transistors will work.
mark 163fb20
--- http://www.youtube.com/MrMadjones
http://gw0wvl.weebly.com/
|
2/Apr/2010, 10:28 am
|
Link to this post
Send Email to madmark1
Send PM to madmark1
Blog
|
29fb313
Freebander
Registered: 09-2009
Posts: 11
Karma: 0 (+0/-0)
|
Reply | Quote
|
|
Re: cobra 148 not transmitting on am
thanks mark, thats good to know, Tom
|
3/Apr/2010, 9:32 pm
|
Link to this post
Send Email to 29fb313
Send PM to 29fb313
Blog
|
madmark1
Accomplished DX-er
Registered: 10-2009
Location: Abertillery
Posts: 454
Karma: 9 (+9/-0)
|
Reply | Quote
|
|
Re: cobra 148 not transmitting on am
2sa1012 equivalents
2sb754
2sa1869
TIP42
2sb920
my preference is the 2sb754, its robust and high power, physically its a bigger transistor than the 2sa1012, its whats used in the grant and the high power cobra clones. make sure you order the mica washer as well, as the one of the 2sa1012 wont fit , its to small. pin for pin its the same so it solders in place easily.
knights cb sell this transistor for £2.95 but if you look around it can be found much cheaper.
mark 163fb20
--- http://www.youtube.com/MrMadjones
http://gw0wvl.weebly.com/
|
4/Apr/2010, 10:36 am
|
Link to this post
Send Email to madmark1
Send PM to madmark1
Blog
|