President Washington (PC-176AB) Transmit Power Decays

[Jon_001]

Greetings, I have been working on a two button President Washington (PC-176AB) with a D858 PLL. It dead keys 2 watts on AM transmit and steadily decreases power to under a half watt. Wait 2 seconds and you can dead key 2 watts and repeat the slow decay of power. Adjusted VR8 to try and boost power. Max I could get was 2 watts. Scoped the output voltage at the final very low 2.19 volts and drops from there. Radio does successfully transmit and recieve, but transmit is very low power and gets lower with time on key. Anyone had this problem before? Ideas? Solutions?

[MDYoungblood]

Hello Jon_001,
Welcome to the forum.
What is the power supply read when transmitting? The radio is 35 years old it may need recapping.

If you have some time it would be good to go to the "Welcome to the Forum" topic and post an intro to the forum, just so the members can say"Hello".

3's

Greg

[Jon_001]

I have 13.8 V from the radio power supply measured across the DC input pins from inside the radio. For a sanity check I have removed the fuse and also used an external 13.8 V supply connected to the DC input on the back of the radio and get the same results.

I tested the final with a continuity meter outside the radio and it allows current from base to collector and base to emitter (as it should) and it does not allow reverse current (as it should) and it doesn't allow Collector emitter current (as it should). The voltage at the base is very low (in the 305 MV range) it is difficult to distinguish between actual voltage or noise on the scope. A similar radio (robyn SB-510D also with a PC-176AB board and D858 PLL) is able to reach 1.25 volts at the transistor base when transmitting. So I deduce that I have something that is affecting the driver cicuit. Unfortunately I do not have a circuit diagram yet to further explore the problem. I bought a service manual on ebay last week and I hope it will have the information I'm looking for (whenever it arrives).

Any suggestions would help.

Jon

[sonoma]

I have 3 of this same radio that had some of the same problems. had to change all the 10 volt caps in the radio to get them to start working so I could fix any thing else that was wrong. even some of the VR's were bad from age.C118 or C119 are at the right rear of the radio and one of them goes bad fairly easy.change them out and see if it starts coming back if you have those 2. forget which of the 2 but one feeds the transmit for the audio to the finals. all 3 of them were free since the guy could not get them working. recap took car of a lot. also at the adjustment for the bias there is a 2.2uf titanium cap that can cause the same symtom. it is bad to go out. put a 2.2 electro there and it could be causing it. this is a common problem in that area. make sure you look for the positive leg when you pull it.
that cap can also work one time then actually blwn the fuse when on ssb and drive you crazy trying to find what is blowing the fuse.

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February 20 2013

c179 is the cap at the finals. here is a link to radionuts sight that has a lot of info for this 858 pll board. shows all radios that have the same board so take a look.

[Please login or register to view this link]

[MDYoungblood]

Could be the AM regulator 2SC1419 is bad. Replace it with a GE-66, ECG-152 or a NTE-152. C179, it is a 2.2uF 25v tantalum cap located near finals replace it with a standard 2.2uF electrolytic capacitor. Just a couple more things to try.

3's

Greg

[Jon_001]

Greetings Again,

Here is the progress on the washington:

I got my VTVM in the mail recently and I quickly put it to work. At TP6 on the radio I struggle to get half the maximum voltage reading that I would on a healthy pc-176ab board (.1volts when I would expect .2 these values are only relative because at my VTVM is not going to be too precise at this resolution). Then when I check TP7 I only have 0.5 Volts when I should have 2.0 volts (the TP7 value is specified in the technical manual and my meter is calibrated to the 5 volt scale so I can measure this value accurately). Naturally I think "I'll adjust L14 to get to 2.0 volts" I go to L14 and start to adjust and happily I am able to make the voltage increase... then I get to 1.5 volts and the radio speaker goes quiet and I can no longer transmit at all (that is the radio keys, the relay clicks but no power hits the antenna. When I stop just short of 1.5 volts the radio still transmits a deadkey at about 2 watts and then steadily decays.

I am continuing to chase the voltage problem but if any of you have further details/experience feel free.

Jon

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Wednesday, 06 March 2013, 15:37 PM

Update on the Washington synthesizer voltage:

Somehow I now have the proper voltage (2.0 volts dc) at TP7. I switched probe leads on my VTVM and rotated the radio on its other side to match the schematic in the manual and suddenly I was able to get the values at TP6 and TP7 that I was trying to get. Since things were going well I continued to set the frequencies within 5 hz at TP8 for AM, USB, and LSB. They are stable I can switch between bands and come back to them and everything is just fine. The radio still only dead keys 2 watts and decays. There is one EXCEPTION when I leave the VTVM connected to TP8 measuring AC voltage while I key the radio I can dead key 4 watts and then the power slowly decays. I am now studying the schematic diagram to see what is around TP8.

Also, I attempted to measure the amperage between TP3 and TP4 (Manual says adjust VR15 to bias 50 mA when keying upper sideband. I put my meter in series with aligator clips, set the amperage scale to 500 mA to be safe, and keyed the mike... Pegged the needle and blew the fuse. I don't have anymore fuses or a bigger scale on my meter so I don't know how much it is flowing, and I will have to try something else.

Till next time...

[sonoma]

did you ever check any of the 10 volt caps I mentioned. c118 or c119 can be bad and cause the same problems.