Harvard H-407 homebase, with PCMA002F mainboard (aka Amstrad 901, mustang 1000 and many more)
When the volume is turned up loud whilst recieving a strong signal, a feedback howl accompanies the voices.
on weak and medium strength signals this does not happen, therefor i ruled out it being an audio feedback problem and it must be RF feedback
I recall having the same problem on a mustang many years ago, but cant remember what i did to fix it... anyone know?
amstrad 901 schematic:
PCMA002F RF feedback through speaker on RX??
Re: PCMA002F RF feedback through speaker on RX??
Not familiar with that radio, but it its over 15 years old it's a classic symptom of dried out electrolytics.
Usually they look OK visually, but have high ESR when measured. An ESR meter will find these quickly, otherwise it's lots of substitution and guesswork
Sometimes, really bad ones are visible (Google "badcaps" if you don't know what these look like)
Usually they look OK visually, but have high ESR when measured. An ESR meter will find these quickly, otherwise it's lots of substitution and guesswork
Sometimes, really bad ones are visible (Google "badcaps" if you don't know what these look like)
Re: PCMA002F RF feedback through speaker on RX??
The radio is a UK only "CB 27/81" spec, from around the mid-late 80's i'd say. so i'm not surprised you dont know it 
I had suspected bad caps, but not had chance to change any yet. all look visually ok, none have bulged or leaked.. Had to rewire the CB/PA switch as this was faulty.
The previous owner had turned the power supply up to maximum 16.8v to make it handle the "power mods" that had been done to it... I now notice that most of the electrolytics in the radio are 16v ones, so i'm not surprised if some are blown
all mods have been removed and the radio is now standard.
I'll replace them with 25v ones... starting around the audio amp.
UPDATE: All elect. caps around audio amp and dc input replaced... feedback still there, but recieved audio now a lot cleaner. All the caps i removed have brown scorch marks on them indicating that they have probably over heated at some stage. Not surprising, seeing that the power supply was set to 16.8 volts and most of these are 16 volt caps!!
Anyway I'll continue changing caps and post again when i find the culprit!
I had suspected bad caps, but not had chance to change any yet. all look visually ok, none have bulged or leaked.. Had to rewire the CB/PA switch as this was faulty.
The previous owner had turned the power supply up to maximum 16.8v to make it handle the "power mods" that had been done to it... I now notice that most of the electrolytics in the radio are 16v ones, so i'm not surprised if some are blown
all mods have been removed and the radio is now standard.
I'll replace them with 25v ones... starting around the audio amp.
UPDATE: All elect. caps around audio amp and dc input replaced... feedback still there, but recieved audio now a lot cleaner. All the caps i removed have brown scorch marks on them indicating that they have probably over heated at some stage. Not surprising, seeing that the power supply was set to 16.8 volts and most of these are 16 volt caps!!
Anyway I'll continue changing caps and post again when i find the culprit!
Norm
Re: PCMA002F RF feedback through speaker on RX??
Radio Sorted! 
The fault was C95 (1uf 16v) which had gone bad. At least, I think it is C95 - My schematic is not very clear. Its located at the right side of the board, just above and to the right of U3 (LA1230)
I really must invest in an ESR meter, then i will find faults like this easily!
The fault was C95 (1uf 16v) which had gone bad. At least, I think it is C95 - My schematic is not very clear. Its located at the right side of the board, just above and to the right of U3 (LA1230)I really must invest in an ESR meter, then i will find faults like this easily!
Norm
Re: PCMA002F RF feedback through speaker on RX??
If you want it to be reliable, then "recap" the radio. In other words, replace ALL the electrolytics, every one of them.
If we get any pre-1995 Uniden/Cybernet/Similar based CB radio in here for repair, we refuse to do them unless the owner agrees to this (and agrees to pay for this). If any are left in, it's a guaranteed "bouncer" (repair warranty return that is nothing to do with the original fault we fixed).
Good to see you got it fixed!
Cheers;
Sue
If we get any pre-1995 Uniden/Cybernet/Similar based CB radio in here for repair, we refuse to do them unless the owner agrees to this (and agrees to pay for this). If any are left in, it's a guaranteed "bouncer" (repair warranty return that is nothing to do with the original fault we fixed).
Good to see you got it fixed!
Cheers;
Sue