I have a 110w UHF Spectra w/ an A9 head. When I TX, I have to scream in the mic to get the faintest whisper to come out.
I have tried the following:
different head
different cable
different power supply
different antenna (also a dummy load)
3 different mics (1 hand and 2 desk)
I pulled the command board out and checked it. It had leaky caps around the receiver audio amp (caps were promptly removed, cleaned the board and caps replaced). Still same problem.
I found the mic audio pins in the DTMF decoder plug on the command board and checked them with an oscilloscope. Waveform amplitude matched that of a working Spectra (so mic audio is getting to the command board just fine).
Is this something that can be fixed in the RSS w/ a setting? I looked all through it and found nothing on mic gain settings. Any other ideas to try?
Also, anybody got a schematic they are willing to part with reasonably (or borrow for short time period)? This would really take out a lot of the guesswork. Trace-following through a 4 layer board is not fun.
Thanks,
Jared
Spectra low TX audio
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jackhackett
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ke4zdg
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- What radios do you own?: Spectras, Sabers, MCS/MTS2000
Didn't replace the caps on the PA or RF board. As mentioned before, I replaced the entire control head.
I'll check out those caps on the PA and RF boards now. However, by the time the signal gets to the PA board, it's too late to make a difference in the audio level. In the mean time, any other ideas?
I'll check out those caps on the PA and RF boards now. However, by the time the signal gets to the PA board, it's too late to make a difference in the audio level. In the mean time, any other ideas?
You can put the radio into test mode from the control head and have it send audio through the transmitter. Then you can measure the deviation that way, and even set it via the control head. The Basic Service Manual has the procedures. It's possible that the deviation is just set real low.
The RF board provides modulation audio to the VCO which feeds the PA. There are leakable caps on the RF board. The REF MOD and VCO MOD signals would also be good things to look at. How does the radio do on PL/DPL deviation?
I could scan the command board schematic and layout and send it to you if that would help. The files will be big. Send me (via PM) the board number so I can make sure the one I have matches, otherwise it won't matter. Also, provide an e-mail address that I can use to send the scanned images. There's some troubleshooting info in the manuals that I'll include as well.
Bob M.
The RF board provides modulation audio to the VCO which feeds the PA. There are leakable caps on the RF board. The REF MOD and VCO MOD signals would also be good things to look at. How does the radio do on PL/DPL deviation?
I could scan the command board schematic and layout and send it to you if that would help. The files will be big. Send me (via PM) the board number so I can make sure the one I have matches, otherwise it won't matter. Also, provide an e-mail address that I can use to send the scanned images. There's some troubleshooting info in the manuals that I'll include as well.
Bob M.
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ke4zdg
- Posts: 85
- Joined: Fri Jun 30, 2006 6:05 am
- What radios do you own?: Spectras, Sabers, MCS/MTS2000
I played with the deviation yesterday and it didn't seem to have any effect on it. Besides, I thought you used deviation to fine-tune the VCO for the output frequency, not audio deviation. If I had a manual, I wouldn't have to make these assumptions 
Anyway, I'll PM you my address, etc.
BTW, which one is the RF board? Is it the one next to the command board (under the screwed down shield), or on the other side next to the VCO (with the pop-off shield)?
Anyway, I'll PM you my address, etc.
BTW, which one is the RF board? Is it the one next to the command board (under the screwed down shield), or on the other side next to the VCO (with the pop-off shield)?
Whoa ! Time for school: "Spectra 101".
The command board is the larger board on top. It probably has a small shield over one module, the MLM, which contains the firmware and other memory. The shielded assembly to the right is the receiver pre-selector, otherwise known as the front end.
On the bottom, the larger board is the RF board. The VCO plugs into that and has a full metal shield over it.
The PA is obviously in the rear of the radio.
The deviation settings are for the audio modulation of the transmitter. There are multiple adjustments that are normally taken care of by a technician with the radio on the bench using a program. There are also some compensation adjustments that are used to balance the low and high frequency audio levels.
There's a frequency WARP that fine-tunes the carrier frequency to within 100 Hz of the exact value. This is also settable with software.
Output power is also settable and tunable so the radio meets specifications over its entire frequency range. You don't need special equipment to do most of the common adjustments, but you do need the right equipment: a way to measure output power, a dummy load, a way to measure transmit frequency and audio deviation, and a signal generator to check the receiver. Service monitors would have all of these in one box, but some people prefer separate pieces of test equipment.
The basic and detailed service manuals are often for sale on eBay. The detailed one is still available from Motorola and it covers all (VHF, UHF, 800, 900) low/medium power models. There are supplements for the high-power radios. The basic manual might also be available new; I got mine on eBay. Each band has its own manual, although most of the info is the same.
Spectras can be finicky. I'd almost suggest that you find someone local who knows a lot about them, and take the radio to that person. Maybe you can sit in while it's being worked on and pick up a lot of good information that way. Most radio shops won't let you do that, but a lot employ one or more hams who should be glad to help you out on their own time.
Bob M.
The command board is the larger board on top. It probably has a small shield over one module, the MLM, which contains the firmware and other memory. The shielded assembly to the right is the receiver pre-selector, otherwise known as the front end.
On the bottom, the larger board is the RF board. The VCO plugs into that and has a full metal shield over it.
The PA is obviously in the rear of the radio.
The deviation settings are for the audio modulation of the transmitter. There are multiple adjustments that are normally taken care of by a technician with the radio on the bench using a program. There are also some compensation adjustments that are used to balance the low and high frequency audio levels.
There's a frequency WARP that fine-tunes the carrier frequency to within 100 Hz of the exact value. This is also settable with software.
Output power is also settable and tunable so the radio meets specifications over its entire frequency range. You don't need special equipment to do most of the common adjustments, but you do need the right equipment: a way to measure output power, a dummy load, a way to measure transmit frequency and audio deviation, and a signal generator to check the receiver. Service monitors would have all of these in one box, but some people prefer separate pieces of test equipment.
The basic and detailed service manuals are often for sale on eBay. The detailed one is still available from Motorola and it covers all (VHF, UHF, 800, 900) low/medium power models. There are supplements for the high-power radios. The basic manual might also be available new; I got mine on eBay. Each band has its own manual, although most of the info is the same.
Spectras can be finicky. I'd almost suggest that you find someone local who knows a lot about them, and take the radio to that person. Maybe you can sit in while it's being worked on and pick up a lot of good information that way. Most radio shops won't let you do that, but a lot employ one or more hams who should be glad to help you out on their own time.
Bob M.
As a follow-up, I ran my UHF Spectra through the various test modes and looked at the REF MOD and VCO MOD signals on the command board at J500. 100 Hz PL tone produces about 100mV on the VCO MOD pin, about 225mV on the REF MOD pin, and 710 Hz deviation. 1500 Hz tone produces about 450mV on the VCO MOD pin, about 1.1V on the REF MOD pin, and 3.42 kHz deviation. Loud whistling into the microphone produces 4.4 kHz deviation. This is how I received the radio; I haven't done a thing with it.
Bob M.
Bob M.
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ke4zdg
- Posts: 85
- Joined: Fri Jun 30, 2006 6:05 am
- What radios do you own?: Spectras, Sabers, MCS/MTS2000
Just to update on this topic.
I broke the RF board (FAIL 001) trying to replace bad caps. Turns out I replaced a bad cap w/ another bad cap (RRRR!!!!).
Tracing the audio around the command board I found a 120 kHz signal on the mic input that was drowning out my mic audio. I figured easiest way to fix this is to get a parts radio and scavenge the command board (radio should be here next week).
I think U400 (audio IC) was the culprit. I'm trying to find a replacement right now. I have emailed a few "discontinued parts" dealers to see if they have any stock.
Any objections or suggestions?
Jared
I broke the RF board (FAIL 001) trying to replace bad caps. Turns out I replaced a bad cap w/ another bad cap (RRRR!!!!).
Tracing the audio around the command board I found a 120 kHz signal on the mic input that was drowning out my mic audio. I figured easiest way to fix this is to get a parts radio and scavenge the command board (radio should be here next week).
I think U400 (audio IC) was the culprit. I'm trying to find a replacement right now. I have emailed a few "discontinued parts" dealers to see if they have any stock.
Any objections or suggestions?
Jared