President Washington modulation dives as mike gain rises

[Plumber-G]

In SSB when I raise the mike gain the modulation and power goes down, on AM solid 4 watts 100 modulation. radio does not swing, what am I nor doing or need to do??

Thanks
Plumber-G

 

[sonoma]

a couple things can cause this. internal adjustments are off or your voltage to feed the radio is going down. does it do the same hooked to an external power supply and bypassing the internal supply. I have 3 of the 858 PLL units and the power switch was getting bad on all 3. the radio would be on and working and the power would drop way down as far as the 12 volts. had to put a relay on each of them to bypass the on off switch when turned on. what PLL is in your radio?
if a 858 PLL unit check a tantalum cap at C179 position. it can go bad and cause problems like this also.

 

[Plumber-G]

I installed a on/off switch to bypass the original switch, so looking at the cap is the next step

I will reply with results

Thanks Plumber-G

 

[JAF0]

Have you checked to make certain the ALC and AMC variable resistors aren't set too tight for too much limiting?

 

[Robb]

What needs to be done is going to be determined by which chassis it is.
The 858 and 8719 chassis aren't even close.

 

[sonoma]

did you at least make sure your power supply maintains the 13.8 volts as it should?
you need to try an external power supply also.
you need to state if it has a 858PLL or the 8719 PLL so you can at least get some help. check the other items 1st before you even think about caps. this is not a bad idea since caps go bad with age but the oter items need to be verified 1st.

 

[Plumber-G]

858 Chassis,
voltage stable 14.8v,
looks like the cap was already replaced,
according to the service manual ALC and AMC do not affect SSB

Re aligned SSB and got up to 80% Modulation at 10 watts by lowering output I got higher modulation.
Looks like someone has made a few mods on this radio?? just not sure what they are <GRIN>

Thanks for all the help

 

[sonoma]

since this is a 858 PLL unit the caps can cause all kinds of problems when they start going bad. look and see if the radio is full of a bunch of 10 volt caps, if so it is a sign they need to be changed do to age. I have 3 of these I recapped a couple years ago and it brought the radios back to life. I just got another 858 radio and it needs caps changed in it. you need to buy a kit with all new caps to change them out before you will really be able to do much to the radio to get it back to how it was intended to work. do not pull used ones out of old radios as a few people try to do. this radio is about 40 years old now. the 10 volt caps should be changed out with 16 to 25 volt caps. you say the radio is at 14.8 volts and since the 10 volt caps are rated at 20% you are over driving the caps with that much voltage. more than likely the 8 volt rail is at about 9 plus volts on the caps at some areas.the caps are weak and will need to be changed out. the plus side is the radio is trying to work so new caps should help it out more than you would think.
C179 is bad about going out and it will even blow the fuse at radio power up when it goes bad.you can use a electro cap in that location in place of the tantalum cap. it will work just fine as long as you use the proper uf rating. at this ago you may even find a few VR's that are bad and have dead spots in them.
good luck with your repair.

 

[Plumber-G]

Sonoma
Were can I buy the cap kits?? looked on ebay but the boards do not match, my board is a PC-353BB

Thanks

 

[sonoma]

Klondike mike sells them and should have the kit. he is a member here. see if you can locate him. he can get you a kit. he sells them on ebay from Canada.
I sent him a PM to look for your post.

 

[Plumber-G]

After I posted here I found Klondike Mike, He replied almost immediately!!
So I ordered the caps and they are on the way. Looking forward to the project