Batboard

I have acquired a Low Band Maxtrac which currently RX's/TX's around 31 - 33 MHz....so should be good for 29MHz.

The RX side works OK.

There is a problem with the TX. When I press the PTT I get 50W for a fraction of a second and then the power rapidly falls back to mW's over a 2 second period. If I press the PTT again, I get the same effect, 50W and then rapid fall back to mW's.

Its almost like a high SWR protection circuit is switching in BUT I am running the radio into a dummy load and a power meter so theoretically the radio is seeing a 1:1 SWR (or thereabouts).

The radio was received with the corner heat sink fin broken off (the broken off bit was in the box), so it was obviously dropped in the post so maybe the PA circuit has taken some damage?

When I release the PTT the radio TX's a data signal (2 tone??).

Could the radio have previously been a "data radio" and was programmed to only TX for a fraction of a second??

The accessory socket plug is still in place.

Any ideas?

Thanks

If you have the skills, disassemble the radio and check the PA board and components for damage. Likely a shoved out of place component if the heatsink was busted. Or, file a claim with the shipper for damage.

The data signal when you release the PTT could be "post-MDC". It sounds like you need to completely reprogram the radio.

Check the "SL" steering-line test point on the RF board and verify that it's between 2 and 7.6VDC on both RX and TX. You may need to adjust the TX VCO coil if the radio was dropped parallel to the bottom.

Verify that the RF board and PA are correct for that radio. Someone may have thrown something together out of random parts and you've actually got a FrankenTrac.

Make sure your power supply can adequately put out 12-13A to the radio.

I've seen 42-50 MHz radios do that "power burst" when modified to operate in the 50-54 MHz range if the power is set too high. Could someone have tried to modify the PA?

Bob M.

Thanks for the feedback.

I lowered the TX power (from 100 to 50) but still get the "power burst"....and I assume its still Txing "in band".

The radio was from an SK sale so nobody to ask about its history.

Maybe its a "36 - 42 MHz" radio...but the radio still had 31 - 33 MHz frequencies programmed rather than 29 MHz although as you say it could be could be a FrankenTrac.

Yes, data signal is post MDC

I will check the SL test point. Is the TX VCO coil easy to access ie not under a soldered screened can?

Thanks

The VCO coil is on the RF brd under a cover that can be pried with a screwdriver.

Low-band MaxTracs are only rated for 60 watts RF output. If you crank it all the way up, the power control circuit DC output will be sensed by the cpu as being too high and it will do exactly what you are seeing: TX briefly then cut off. If the radio has been modified or is operating out of band, the PA may be very inefficient, requiring more DC control voltage to get a desired output power. If this is too high, the cpu will shut it off.

Once you remove the top plastic cover, there should be a shield over the majority of the right half of the RF board. This can be pried off. The two VCO coils are in the right rear section. There's an article about aligning MaxTrac radios that has photos identifying several adjustable components. The service manual, section 3 of 4, has all the RF board information. I'll let you figure out where to find these items.

Consult the service manual to find the board numbers and what bands they correspond to. These numbers will have three letters followed by four numbers, followed by "A" or "B". The first letter is usually "H"; the second letter is "L" for the logic board and "R" for the RF board; the third letter tells you the band, not the sub-band, so "B" would be low-band, "D" would be high-band, "E" would be UHF, and so on. The digits are board-specific. The radio's model number contains NO information about the sub-band, so a D51... could be 29-36, 36-42, or 42-50 MHz.

Bob M.

Correction to the above post: The first letter is "H"; the second letter is "L".

Bob M.

How robust is the power supply you are using to run it. If the power supply is bad or is a low current supply and can't handle the current draw the radio needs, the radio TX power will drop off as the power supply goes into fold back or current limiting. I believe those radio draw between 15 and 20 amps. If your power supply has an amp meter on it see what it's drawing for current.

The other thing, since you mentioned a tone chirp on unkey, is maybe the time-out-timer is set to only a few seconds but I believe when the TOT engages you get a steady tone until you unkey the PTT.

My PSU is rated at 25A so should be OK.

The TOT was originally set to 60 secs.

The radio was also originally running around 50W with a TX output setting of "100". I lowered this to "50" but still got the power burst effect...but maybe I need to realign the whole TX stage...but the original frequencies (31/33 MHz) are still programmed...rather than 29 MHz...although maybe 31/33 MHz are not the original operating frequencies and somebody had just put in those frequencies to do some monitoring?

Open the radio and check the HLN numbers on the RF and PA boards. They're either on small white stickers or stamped in ink on the board. Then you won't have to "guess" any more. Once you determine what band the radio really is supposed to be on, blank it and reinitialize it. TX Power normally resides around "80" on most clean MaxTracs.

Bob M.