Maxtrac Bad Audio Problem, Please Help

[ANB_Medic]

Hi all. I have a 16 pin, 800 Mhx C6 Maxtrac that is misbehaving. When first turned on (cold), the audio is decent (for an old Maxtrac). When it heats up (15-20 mins) the audio becomes distorted. It gets worse and worse until it becomes unreadable. I have replaced the control head and then put in a new speaker (I had both lying around), but nothing has changed. Obviously something is failing. Anyone had similar problems, and been able to fix it?

[BrienD]

In the past I have traced problems like this by using component freeze spray.
I run the device until the trouble appears and freeze parts until the problem clears up.
I would start wear the speaker leads go to the pc board.

[ANB_Medic]

Never thought of that... I'll pick some up tomorrow and give it a shot. Thx!

[Wowbagger]

Could the unit's frequency reference be drifting, thus causing the signal to "slam into" the side of the IF filter?

If you have a freq counter, look at the TX freq when the unit is cold, then again when it is hot, and see if there is a difference.

Alternatively, if you have a signal generator, try moving the siggen's frequency around, and see if the signal clears up.

[CTAMontrose]

i had a UHF that was sent to me that did the same thing, apparantly whoever had it before me blanked it and reinitialized it with lab and never bothered to go thru alignment. I spent some time in RSS properly realigning the unit, and is worked fine ever since.

[Will]

There has been some problems with a couple of caps in the audio low level filter stages.

Look for a posting titled Slow Maxtrac warm-up

http://batboard.batlabs.com/viewtopic.p ... =slow+warm

[ANB_Medic]

Yeah, I read that on yesterday... Unfortunately, I don't have a service manual. Can someone describe to me which is the C551 Cap? Shouldn't be too hard to describe, as there are only 16 caps on the board and they're spread out. I tried cooling various parts of the radio last night, and that doesn't seem to help.

[kcbooboo]

The C551 problem usually manifests itself as distorted/low or NO audio when the radio is first turned on, due mainly to leakage in the cap that upsets the DC bias throughout the audio section (almost all of the radio's transmit and receive audio sections are DC-coupled). After perhaps 10 - 15 seconds, or less if you key the transmitter, the audio will clear up. There's another cap that is used when the radio has a PA function built into it, that tries to mute the audio when you press the PTT button, and when this cap is leaky (this one is on the RF board) it will present symptoms exactly like the C551 problem, but it will clear itself up when you press the PTT button the first time.

Since your radio gets worse as it warms up, I'd suggest using the freeze spray to locate the suspect component(s). Also if you have the proper test equipment, you could inject a modulated signal into the IF stream to attempt to figure out if you have an RF/IF/oscillator problem or an audio problem. Otherwise you won't even know which board to start working on.

Regarding the location of these components, we'd need the board assembly numbers to give you any further help, as there are at least four different logic board schematics in the manual. I've never looked at the RF board schematics for 800 MHz radios, but there could be multiple layouts for that one as well. Someone could then scan a piece of the manual for you and point out some of the components.

Bob M.