Snake Doctor Cobra Mods

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Cobra Mods

Radio Modulation Power

HH-70	RV-4
7 PLUS	VR4 or CUT D8	L11, L12, L13
10 PLUS	VR4 or CUT D8	L11, L12, L13
18 LTD	CUT D13 or CUT R84	L7, L6, L5
18 PLUS	CUT D12	L 7
18 RV	RV501 or CUT D502	L304, L305, L306
19	CUT D11	L5, L8, L9
19 GTL	VR6 or CUT D9	L15, L16, L17
19 LTD	VR6 or CUT D9	L15, L16, L17
19 LTD CLASSIC	VR4 or CUT D7	L7, L8
19 M	REMOVE C73	L8, L5, L3
19 PLUS	RV501 or CUT C511	L305, L306
19 PLUS (Plastic Case)	RV4 or CUT D8	L11, L12, L13
19 XS	CUT D105
20 B&K	CUT CD20 & CD21	T11, T10, C60
20 LTD	CUT D13
20 PLUS	RV501 or CUT C511	L305, L306
20 PLUS (Plastic Case)	RV4 or CUT D8	L11, L12, L13
21	REMOVE C105 & C106	T11, T10, C60
21A (Old)	CUT D24	L9, L8
21 GTL	VR5 or CUT D9	L13, L10, L9, L8
21 LTD	VR5 or CUT D9	L13, L10, L9, L8
21 LTD CLASSIC	VR4 or CUT D7	L7, L8
21 PLUS	CUT 1.5K res. next to TR34	L12, L10
21 X	VR207 or CUT D212	L212, L214
21 XLR	RT4 or CUT D13	L15, L12, L11
23 PLUS	RV501 or CUT D502	L304, L306, L307
25 GTL	VR5 or CUT D9	L13, L10, L9, L8
25 LTD	VR5 or CUT D9	L13, L10, L9, L8
25 LTD CLASSIC	VR5 or CUT D9	L10, L9, L8
25 PLUS	CUT 1.5K res. next to TR34	L12, L10
26	CUT D24	L7, L8
28 A	CUT CD28 & CD29	T12, L2, L5
29 A (OLD)	CUT D24	L7, L8
29XLR	VR5 or CUT D14	L15, L12, L11
29 GTL	VR4 or CUT D11	L14, L13, L12
29 LTD	VR4 or CUT D11	L14, L13, L12
29 LTD CLASSIC	VR4 or CUT D11	L14, L13, L12
29 LTD GOLD	VR4 or CUT D11	L14, L13, L12
29 PLUS	CUT D20	L12
31 PLUS	CUT D19	L13, L12, L11
32 XLR	CUT CD11 & CD12	L5, L3
33 PLUS	VR4 or CUT D17	L11, L9
39 LTD (S.O.S.)	RV1
40 PLUS	RV104 or CUT D203	L305, L306
40 X	RV201 or CUT D203 or C215	L304, L305, L306
41 PLUS	VR4 or CUT D21	L19, L20, L21
45 XLR	VR105 or CUT D119	L113, L109
46 XLR	VR105 or CUT D119	L113, L109
47 XLR	VR105 or CUT D119	L113, L109
50 XLR	VR5 or CUT D119	L109

55 XLR	VR5 or CUT D119	L109
62 XLR	VR3 or REMOVE C67	L16, L15, L12
63 GTL	VR3 or CUT D11	L11, L10, L9
66 GTL	VR2 or CUT D4	L14, L15, L17
66 LTD	CUT R54
67 LTD	RV4 or CUT D212	L306, L307, L308
77 X	VR207 or CUT D212	L211, L212, L214
78 X	VR6 or CUT D9	L11, L12, L15
85	REMOVE Q17	L3, T4
86 XLR	CUT CD9	L3
CAM 89	ADD 1K res. in series with C110	L13, L12, L9
87 GTL	VR6 or CUT D16	L17, L16, L13
89 GTL	VR6 or CUT D16	L17, L16, L13
89 XLR	VR5 or CUT D14	L15, L12, L11
90 LTD	VR201 or CUT D203	L304, L305, L306
93LTD WX	VR-3	L16, L17, L18
132 (OLD)	CUT Q25	T14, T15, L3, C116, (ssb) R136
132 A	CUT CD39 & CD40	R79 (ssb) R87
132 XLR	R134 or CUT R125	L3, (ssb)R130
134	CUT D40 & D41	L8, L10 (ssb)VR15
135 (OLD)	CUT Q25	T14, T15, L3, C116 (ssb)R136
135 XLR	R134 or CUT R125	L3, (ssb)R130
138 A (OLD)	CUT D40 & D41	L8, L10 (ssb) VR15
138 XLR	VR7 or VR6	VR8, (ssb)CT7
139	VR12 or CUT D63	L12, L10, (ssb)VR15
139 XLR	VR7 & VR6	VR8, (ssb)CT7
140 GTL	CUT R104	VR6, (ssb)VR7
142 GTL	CUT R104	VR6, (ssb)VR7
146 GTL	VR5 or CUT TR29	VR10, (ssb)VR6
148 GTL	CUT R131	VR10, (ssb)VR11
148 GTL-DX (early prod. model)	VR5	VR11, (ssb)VR7
148 GTL-DX (late prod. model)	REMOVE VR14 or CUT TR32	VR13, (ssb)VR12
148 GTL-B	VR14	RV15, (ssb)RV13
148 GTL-DX (copy ver.)	RV12	RV11, (ssb)RV6,RV3
1000 GTL	VR6 or CUT D16	L17, L16, L13, (ssb)
2000 GTL	CUT R131	VR10, (ssb)VR11
GTL 150	RV14	RV13, (ssb)RV12,RV4

Cobra Power Mods

Radio Try This

2000 GTL,2010	CUT TR24 Change R126 to a 2.2k resistor Change R124 to a 4.7k resistor Tune L47,L48,L46,L45,L38 for max RF output AM Carrier Power VR10 Mod Meter VR7 AM Signal meter VR1
142GTL	CUT TR32 Change R99 to a 2.2k resistor Change R94 to a 2.2k resistor Adjust L36,L37 for max RF output AM Power VR6 SSB Power VR7 VR1 "S"Meter VR2 Squelch VR3 TX Freq VR12 Mod Meter VR10 TX Power Meter
148GTL+F	CUT TR24 Change R126 to a 2.2k resistor Change R124 to a 2.2k resistor Tune L37,L38 for max RF output
29GTL/LTD/CLASSIC	CUT D11 Adjust L17,,L20,L21,L16{Some Models},L14 for max RF output Modification: Change stock final to a 2SC1969 Change stock driver to a 2SC1957,{some model use a 2SC2028 rf driver}Remove it Change R123 to a 0.47 ohm resistor Change R58 to a 2.7 ohm resistor Put a jumper across D8{by audio transformer} Change C71 to a 33uf 16volt polarized electrolytic capacitor Put a 47pf disc cap across C57 Put a 47pf disc cap across R55 For a higher dead key place a 100pf disc cap across C62{by rf driver} Key radio and say HEEELLLOOOOOO while adjusting L17,L20,L121,L16,L14 Some models dead key 8-10 watts swing 40-50PEP VR1 rx gain VR2 rx signal meter VR3 squelch VR4 AMC VR5 tx power meter VR6 antenna warning light
25GTL/LTD/CLASSIC	CUT D9 Change R76 to a 1k resistor Change R43 to a 2.7 ohm resistor Change R108 to a .47 ohm resistor Adjust L10,L9,L8 for max RF output Modification: Change stock final to a 2SC1969 Put a jumper across the D10 diode that connects from VR5 to audio transformer{pc trace side} Adjust L10,L9,L8 again Change C91 to a 33uf 16volt polarized electrolytic capacitor
Cobra 18RV, 23 Plus	Change R505 (3.3k) to a 1k, Change R310 (2.7Ohm) to a .47Ohm, Retune L305, L306
Cobra 90LTD	Cut D203, Change R320 (5.6Ohm) to a 1 Ohm, Change C335 (100pf-NPO) to a 150pf-NPO, retune L304,L305,L306

Receive Boost

Cobra 29
1. Locate pin 14 of the PLL chip, and the Positive (+) side of C-12. 2. Install a 22k-220k ohm resistor between these two points. The smaller the value, the more the receive is boosted. 3. If desired, this mod may be wired through a switch.

How to make Cobra 29LTD,Classics slide on TX & RX

1)Remove D-18 , R-110 , C131 and R-112. 2)Put jumper were R-110 was. 3)Take orange wire from delta tune and unsolder it from board then resolder it to Tr-23 (Solder to the pin closest to front of board. It should have around 8 VDC on this pin.) 4)Put radio back together and if all is done right you should slide transmit 5-6 KCs up and down. For more slide change D-16 to a 1N-4007

Cobra 2000 No Audio on AM Transmit with good Carrier The Fix: This problem is more common in the "older" 2000's but is relatively easy to fix. Look for the collector of TR-23, you should read about 4.5 V with a dead key. If you find that this voltage is not present, check on the opposite side of R-120 for 6.5V. Most likely this voltage will not be present; if this is the case, C18 has shorted and needs replaced. This is a 330uf, 10V electrolytic capacitor beside L31. To prevent "future" problems, replace with a 16V.