I just picked up a President Washington, not to be mistaken by the Uniden but a President. It's from the first year they were manufactured in 77. Anyways, the problem I'm having with it is, it will transmitt and modulate on lower channels like 2, but once I go higher to 12 or 17 it does nothing. Anyone with any ideas on what that means or how to solve that problem? Also how do I or can I turn the carrier up or down on it and if so, what am I looking for inside to turn it up or down? Also is it possible to turn the mod up or down internally and if so, what do I look for inside to do that? Any advice would be welcome. Thank you
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President Washington
- Thread starter Jediwtr
- Start date
That 1977 radio is typically identified by the PLL integrated circuit (chip) that it used. The uPD858 made by NEC. The FCC outlawed the use of this type PLL device in 1978 or so, and all later radios that have the name "Washington" used a newer chip called the MB8719. The difference between chips is not so much the big deal, but the printed circuit board and the block diagram both changed immensely from the "858" to the "8719" radios made by Uniden for a double handful of name brands and models. The 8719 design was made in a variety of brands and models for decades.
The "858" chip was outlawed for new radio designs because it was too easy to expand channel coverage beyond the legal 40 channels. Just took some toggle switches. FCC told manufacturers they had to make it harder than that.
The problem you describe sounds as if the Phase Locked Loop (PLL) section of the radio is losing its grip (lock) on the channel frequency. There is a shutdown circuit that disables the radio when this happens, to keep it from transmitting on some random frequency that's not legal. And yes, the shutdown circuit is a legal requirement.
The symptom you describe sounds for all the world as if someone has either
1) expanded the channel coverage, or fumbled the attempt.
or
2) set about twisting the tuning-adjustment slugs in random RF coils in the radio's PLL section.
If you have a DC voltmeter on hand, clip the negative probe to the metal shield around the PLL section at the corner of the pc board where the fat uPD858 chip is located. Find the test point "TP7" and probe the positive probe there. This chip runs from 5 Volts DC so you shouldn't see any readings higher than that. This test point is the tuning voltage that sets the radio's channel frequency. A slug-tuned coil L17 is used to keep this tuning voltage from hitting the floor (zero Volts) or the ceiling (5 Volts). The radio's channel frequency is supposed to be tuned by a range of voltage between these limits. Set the radio to channel 1 and click upwards, watching this tuning voltage as you go. If it hits the upper limit before reaching channel 40, L17 is probably out of adjustment.
If you don't have a non-metallic tool with a 1.5 mm flat tip to adjust L17, you'll need to obtain one. A metal tool will disrupt the magnetic field inside the can. Can't set the adjustment that way. Aim for 2.5 Volts at channel 1.
But that's my educated guess, that someone carelessly twisted the tuning slug in L17 so that the tuning voltage hits its upper limit before reaching all the way up to channel 40.
73
The "858" chip was outlawed for new radio designs because it was too easy to expand channel coverage beyond the legal 40 channels. Just took some toggle switches. FCC told manufacturers they had to make it harder than that.
The problem you describe sounds as if the Phase Locked Loop (PLL) section of the radio is losing its grip (lock) on the channel frequency. There is a shutdown circuit that disables the radio when this happens, to keep it from transmitting on some random frequency that's not legal. And yes, the shutdown circuit is a legal requirement.
The symptom you describe sounds for all the world as if someone has either
1) expanded the channel coverage, or fumbled the attempt.
or
2) set about twisting the tuning-adjustment slugs in random RF coils in the radio's PLL section.
If you have a DC voltmeter on hand, clip the negative probe to the metal shield around the PLL section at the corner of the pc board where the fat uPD858 chip is located. Find the test point "TP7" and probe the positive probe there. This chip runs from 5 Volts DC so you shouldn't see any readings higher than that. This test point is the tuning voltage that sets the radio's channel frequency. A slug-tuned coil L17 is used to keep this tuning voltage from hitting the floor (zero Volts) or the ceiling (5 Volts). The radio's channel frequency is supposed to be tuned by a range of voltage between these limits. Set the radio to channel 1 and click upwards, watching this tuning voltage as you go. If it hits the upper limit before reaching channel 40, L17 is probably out of adjustment.
If you don't have a non-metallic tool with a 1.5 mm flat tip to adjust L17, you'll need to obtain one. A metal tool will disrupt the magnetic field inside the can. Can't set the adjustment that way. Aim for 2.5 Volts at channel 1.
But that's my educated guess, that someone carelessly twisted the tuning slug in L17 so that the tuning voltage hits its upper limit before reaching all the way up to channel 40.
73
I would look at the capacitors first, 1977, bet it has been shelved for years, plenty of drying time.