Cobra 29 early 23 ch info

Hello guys,

I acquired a Cobra 29 early 23 channel version. The receiver is sounding weird, sounds raspy and this problem goes away when I try to align the IF stage. Transmit works nicely. I'm hoping to find proper alignment information for this radio. I found the schematic and you can tell the receive and transmit stages apart but some proper alignment information would be nice.

The PC board is marked as PC-121AB. Perhaps there are other radios out there with this same chassis for which you can find service info for.

Any help would be appreciated.

Hey guys,

The problem with the receive seems to mostly be caused by the adjustment of T7 and T8. The receive chain had me a bit confused but I think I got it. Turns out this is dual conversion.

Tell me if I have this right:

TR 3 is the 23Mhz oscillator. I peaked T3 for maximum output of 23Mhz as read off TP1. TR4 is the 14Mhz oscillator. TR5 is a mixer circuit which mixes the 23 and 14Mhz oscillators resulting in 38Mhz coming out of T6. Also peaked T4, T5, and T6 for maximum output of 38Mhz. as read from TP2.

Here's how I have the receiver down. T1,TR1, and T2 look like the front end RF amp. TR2, T7, and T8 look like a mixer/IF stage since it's taking the output from T6. So let's say we're on channel 1. Crystals 23.290Mhz and 14.950Mhz are selected. This is mixed at TR5 it produces 38.240Mhz at the output of T6 which is then injected into TR2 which is a mixer/amplifier. The difference between 26.965Mhz and 38.240Mhz is 11.275Mhz. This resulting 11.275Mhz is passed on through to the next stage via T7 and T8. At TR6 the 11.275Mhz is mixed with the 11.730Mhz oscillator. The difference between those two is .455Mhz which is the second IF stage starting from T9 and onwards until it's detected.

So here I would adjust T1 and T2 for maximum RF sensitivity mid band (channel 11). Then I would adjust T7 and T8 with a frequency generator to allow maximum 11.275Mhz through. From T9 and onwards I generate 455khz and adjust T9, T10, T11, and T12 for peak 455khz signal.

TR7 is the 11.730 oscillator. VR2 adjusts the delta tune range. Could not get a proper frequency reading off this oscillator with my frequency counter so it is still not been adjusted.

TR16 is the 11.275 oscillator and this one appears to have no frequency adjustment. TR17 is another mixer circuit, this time for the transmitter chain. It takes the 11.275 from TR16 and mixes it with the output from the synth circuit, 38.240 for when channel 1 is selected. 38.240-11.275 is 26.965 (Channel 1). This 26.965 is then passed on through T15, T16, and T17 on through TR18 the pre driver, TR19 the driver, and TR20 the output.

Here I would adjust T15, T16, and T17 for maximum output at TP4, or probably the reading at the watt meter would suffice as well. The RF amp stage can be peaked using L7, L8, and L11 for maximum RF output at the watt meter. I think L13 is the TVI trap so that should be set for minimum 54Mhz harmonic.

T18, IC1, and T19 make up the noise blanker circuit. I do not have the equipment (or know how) to set this adjustment so I will leave as is. If anyone wants to write a little procedure for that it should help anyone who is going through this same alignment.

Still having a hard time with this. If I adjust one way, it receives with very strong sensitivity but it will only receive the carrier and no modulation. If i adjust another way it will receive but once the other person stops transmitting instead of static I have a ton of squealing in the receiver. A proper alignment procedure for this would be great. Anyone have a sams procedure out there?

Issue fixed. The problem was the AGC voltage. D4 was leaking and was injecting voltage into this line which caused an uncontrollable amount of gain which overloaded the final IF stage.

What I need to figure out now is how to calibrate the AGC adjustment (VR6) to its factory setting. Other than radio is working again and very nicely!