Forums :: Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives :: Books
Support This Site: Contributors :: Advertise


It is currently May Mon 20, 2019 6:02 pm


All times are UTC [ DST ]





Post New Topic Post Reply  [ 32 posts ]  Go to page Previous  1, 2
Author Message
 Post subject: Motorola 65T21 Question
PostPosted: Feb Tue 17, 2004 4:35 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 564
Location: Wisconsin
I don't agree that your tubes are necessarily THE problem. On multiple tubes, you have bias that is considerably more negative than your higher line voltage would suggest.<P>I think you need to start measuring those large resistors to make sure they're close to correct, first.<P>Tom<P>------------------<BR>


Top
 Profile  
 
 Post subject: Motorola 65T21 Question
PostPosted: Feb Tue 17, 2004 7:01 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 6249
Location: Oak Creek, WI USA
I'm confused by your new readings. The heater of 6SQ7GT is on pins 7 and 8 - you should read about 6.3VAC on one pin and 0 on the other, and the tube should light up. Does that happen?<P>If you're getting 7 volts on the plate (pin 6) of that tube, it is very definitely not going to work. As Norm said, maybe R14 (220,000 ohms, connects pin 6 to B+) is bad/open?<P>You might also have to see if mica cap C16 (500pf) is shorted - check its leads for continuity (it should be open). I assume you replaced paper caps C18 and C19.<P>(by the way, I'm getting the parts numbers off the Beitmans schematic at techpreservation.dyndns.org - I don't know if the Sams part numbers are the same.)<P>------------------<BR>Paul


Top
 Profile  
 
 Post subject: Motorola 65T21 Question
PostPosted: Feb Tue 17, 2004 7:16 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 4830
Location: Gainesville, Florida
Definately dont go sticking your bare fingers in there unless you have someone near with a resussitator. Any insulated probe should produce a click or buzz. Looks like youre narrowed down to the area. 6SQ7 pin 6 triode plate B+ is way off-too low. Something is causing that either filter section is bad, a resistor connected there is nearly open.


Top
 Profile  
 
 Post subject: Motorola 65T21 Question
PostPosted: Feb Tue 17, 2004 7:51 pm 
New Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 11
Location: Daleville, PA USA
Hi Paul...<P>>I'm confused by your new readings. The heater of 6SQ7GT is on pins 7 and 8 - you should read about 6.3VAC on one pin and 0 on the other, and the tube should light up. Does that happen?<P>I am getting 0V on both pins 7 and 8 although I thought the tube lit up (this might be a metal encassed one though. I can't remember which one it is.)<P><BR>> If you're getting 7 volts on the plate (pin 6) of that tube, it is very definitely not going to work. As Norm said, maybe R14 (220,000 ohms, connects pin 6 to B+) is bad/open?<P>I am going to stop at RS on the way home and pick up some new resistors. I believe they have all of them except the 3.3Mohm. I will check it with the meter before I replace it.<P><BR>> You might also have to see if mica cap C16 (500pf) is shorted - check its leads for continuity (it should be open). I assume you replaced paper caps C18 and C19.<P>I did replace all the paper caps.<P>(by the way, I'm getting the parts numbers off the Beitmans schematic)<P>Yes, the part numbers are the same. Thanks for your help, I really appreciate it.<P>------------------<BR>Tom M.


Top
 Profile  
 
 Post subject: Motorola 65T21 Question
PostPosted: Feb Tue 17, 2004 8:36 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 6249
Location: Oak Creek, WI USA
6SQ7GT is a glass tube; you ought to see the heater light up. If it does you have AC voltage on the filament, and that'll be fine. If you left your meter on DC when you read the voltage on those pins it would probably come up 0. Recheck it on AC, maybe.<BR>I do not believe you would have replaced C16 as it is a mica cap; you may just want to check it for a short if the resistors check out okay. Good luck!<P>------------------<BR>Paul


Top
 Profile  
 
 Post subject: Motorola 65T21 Question
PostPosted: Feb Wed 18, 2004 7:25 am 
New Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 11
Location: Daleville, PA USA
Ok, I just checked out R-14 which is supposed to be 220K. I kept getting strange flucuations with the meter so I removed it from the circuit and measured if to be 347k. I replaced it and now that static is much, much louder. I touched the multimeter lead to the antenna lead and all of a sudden I am picking up a shortwave station!!!<P>So I put the chassis back in the cabinet and I was able to tune in multiple stations on both AM and shortwave. I am still getting a lot of loud 'popping' static noise that I can't seem to isolate. (can't be good for the speaker either...) The volume and tone work nice though and have no effect on the static.<P>I did order replacement tubes from Radio Daze this morning because they were reasonable. I guess I should look at replacing all the resistors too since this one was so far out of range (and they are cheap).<P>Any thoughts on the loud popping noises?<P>Thanks...<BR><P>------------------<BR>Tom M.


Top
 Profile  
 
 Post subject: Motorola 65T21 Question
PostPosted: Feb Wed 18, 2004 5:33 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 1988
Location: Mt. Prospect, IL USA
Hi Tom,<BR> Just my hunch- "popping" could be due to arcing in a bad output transformer primary winding. Since volume can be turned down and noise persists, I believe it's somewhere in your audio section, or power supply. If you've replaced electrolytics, paper caps and bad resistors, the output transformer would be my next place to look. Of course, an intermittent tube pin/socket contact, or even a tube with an intermittent short could be present as well. But for me, it seems that 75% of the time, loud popping static was due to leakage in one of my sets' output transformers. Sets like this make me glad I have a signal tracer... <P>------------------<BR>


Top
 Profile  
 
 Post subject: Motorola 65T21 Question
PostPosted: Feb Wed 18, 2004 7:49 pm 
Moderator

Joined: Jan Thu 01, 1970 1:00 am
Posts: 37280
Location: Livermore, CA
Hi Tom<P> If pin #6 on your 6SQ7 has a small mica cap to ground cut one end loose for test. It can cause this static. A radio will play without this cap, can be replaced later.<P> <P>------------------<BR>Norm


Top
 Profile  
 
 Post subject: Motorola 65T21 Question
PostPosted: Feb Thu 19, 2004 12:11 am 
New Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 11
Location: Daleville, PA USA
oldspwr wrote:
<font>quote:</font><HR>Originally posted by Chuck Braun:<BR><B>Hi Tom,<BR> But for me, it seems that 75% of the time, loud popping static was due to leakage in one of my sets' output transformers. Sets like this make me glad I have a signal tracer... <P></B><HR>
<P>Hi Chuck...<P>If it was an output transformer, where would I be able to locate replacement?<P>Thanks...<BR><P>------------------<BR>Tom M.


Top
 Profile  
 
 Post subject: Motorola 65T21 Question
PostPosted: Feb Thu 19, 2004 1:21 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 6249
Location: Oak Creek, WI USA
Output transformers are $$$$$. RadioDaze sells Hammond transformers, I think the HX125CSE would do the job. Pricey, though, at $29.<P>If you want to pay less and buy used, try Gary at <A HREF="http://www.oldradioparts.com" TARGET=_blank>www.oldradioparts.com</A> (he has a $30 minimum order, though, so if you need anything else make sure you get it from him)<P>Do ensure you actually need it before buying, though - measure the DC resistance across both windings for a while first (power off). They should stay almost rock-steady. Neither side should be open. The primary is between pins 3 & 4 of the 6K6, the secondary is between ground and a green wire going to the speaker (I think - I'm checking very quickly. Review the schematic if it doesn't make sense.....)<P>Regards,<P>------------------<BR>Paul


Top
 Profile  
 
 Post subject: Motorola 65T21 Question
PostPosted: Feb Fri 20, 2004 1:50 am 
New Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 11
Location: Daleville, PA USA
Ok guys...<P>I plugged the radio in last night to listen to it and the same old problem was back. I was able to pick up maybe one channel but mostly static. <P>I ordered new tubes from Radio Daze and new resistors from Mouser. I will replace the resistors first to see if this is the problem. Then I'll move on to the tubes.<P>I'll keep you posted...<P>Thanks!<P>------------------<BR>Tom M.


Top
 Profile  
 
 Post subject: Motorola 65T21 Question
PostPosted: Feb Fri 20, 2004 2:43 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 6249
Location: Oak Creek, WI USA
Just out of curiosity, what's the condition of your chassis? Corroded/rusty? Sounds like you're having an intermittent of some sort, and they're the devil to troubleshoot. I have a radio that's like that because I don't have good grounds in certain places because the chassis isn't clean. Haven't gotten around to permanently fixing it, though.<P>Move some components and wires around while the radio plays (be careful for shocks!) See if you can get it to cut in and out. If so, see if you can find the problem component and make sure it has good connections to whatever it's supposed to be connected to.<P>Just a suggestion.....<P>Regards<BR><P>------------------<BR>Paul


Top
 Profile  
 
Post New Topic Post Reply  [ 32 posts ]  Moderators: Marcc, Norm Leal Go to page Previous  1, 2

All times are UTC [ DST ]


Who is online

Users browsing this forum: AevnsGrandpa, BillDiesch, lorenz200w, Notimetolooz, Raleighcopter, SHenion, voroush and 6 guests



Search for:
Jump to:  




























Privacy Policy :: Powered by phpBB