Ts4d Motorola
 

Just a question about the ts4d I have it has been recapped but what I was left with is what sounds like a 60 hz hum or buzz in the audio when low but as its raised up the noise disappears . Every cap was replaced and as for the electrolytics I parallel each one, one by one with a high uf to rule out a possible bad cap and I have since been left with the hum or buzz. I am trying to figure out where or what would give me this hum but have been unsuccessful . I thought maybe the verticle but how it would get into the audio I don't know as there are no other caps to change. Other then that noise the set works perfect but would like to know where it's coming from.

Turn the vertical hold so the picture rolls. If the 'buzz' changes pitch, that will tell you it's coming from the vertical.

Another possibility is a heater-cathode short, or heater to signal element coupling of the heater AC.

Heater to cathode short within a tube or heater short between one leg of the heater to a coupling to a signal source or ground on an IF tube ?

Not to put words in to Tom C.'s post, I think he meant leakage between the heater and cathode of a tube. most likely are the 12SN7 vertical osc. 1st audio amp and maybe the 25L6 audio output. Swap each tube one at a time to see if it helps. Heater cathode leakage was fairly common in the days before controled warm up tubes and still fairly common after but gut better as the technology got better. All so check lead dress (placement) of any leads that carry line voltage keep them away from the high gain audio stages. All the best, Tom

That and other forms of coupling to the heater such as capacitive and inductive coupling. Some tubes test fine but have a tendency to hum....I have a 12AX7 that tests fine , but induces several times more hum in to any given circuit it is installed in than all the others I have.

Well I ruled out the verticle as the ac noise in the audio by causing the pic to role and no change in the hum. So next I'll try different tubes starting with the audio amp, 25l6 and the output 6sq7.

well still got the hum in the audio after changing the audio amp and output tubes no change also the 6al5 . would an IF tube possibly do this if it has a heater cathode short in one of the IF strip tubes? would it follow all the way to the output stage. kind of stumped at this point as i even paralled caps to rule out a bad one.

To answer the question from #8 yes it can. The ts4d has a stacked B+ setup so H-K leakage in v1,v4,v6,v11a could put the buzz in the sound those before mentioned tubes are between B++ and B+so anything in the B+ line will be affected. Best to check for hum on B+ line with a scope. Also failing selenium rectifiers can sometimes do that as well. Un bypassed silicon diodes also can add noise on B+ lines. If you don't have the Motorola factory manual you can get it at the ETF site. All the best,Tom

Is it a Hum or more of a buzz? Might be the sound discriminator coil needs a slight touch up, seems like very set I do needs that done. Some sets just seem to have a low level buzz no matter what you do.

The discriminator coils I already tried and no it's more a hum like 60hz ac and there are no seleniums I put in 1n007 diodes so maybe I'll look into those other tubes for an HK short in them. I don't know where else the ac would come from . It almost seems like a bad cap like powering an old set after it sat for 50 years only the hum is not loud.

I have another set with diodes and don't have the hum so there has to be something to blame here so I could change the diodes and see after I check the tubes in question. What are you referring to when you say unbypassed diodes? I am certain about a bad cap in the B+ since paralleling other caps with really high UF so if that was the case the noise would have stopped doing that.

Ok, got one more thing I can try and that's about it, change the diodes otherwise tubes , caps, ect, ruled out at this point, and discriminator coils, pro and sec. I have nos ge-509 universal tv silicon rectifiers , I guess these are the old version of the 1n4007 of today, these ge rectifiers are rated for 1000piv 1amp.

1N4007s' and like diodes generate a lot of noise, they are bypassed by 0.001ufd disc caps. min. of 500 volts wired across the diode . You have another set try subbing V1,V4,V6, and V11 one at a time to see if any thing changes. If no change then start checking layout of A.C. power wiring if to near audio stages move the leads away from the audio wiring. All the best, Tom

I subbed all those one at a time and if I remember correctly I already tried several different disc caps when I first finished this set and it took very little of the hum away.