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 Post subject: Motorola 65T21 Question
PostPosted: Feb Mon 16, 2004 2:42 am 
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Location: Daleville, PA USA
Hi...<P>I just acquried a Motorola 65T21 that had had been sitting in an unheated garage for quite some time. I plugged in it when I got it home and all I could get was some light static (both in SW and AM) and no channels.<P>I did little reseach on the web and found out that paper caps are typically the problem. So I ordered a cap kit and re-capped it this morning. I replaced all the .05 mfd and .005 mfd. I did not replace the three filter caps. The Sams schematic I received is a bit unclear on this. <P>I tried it out again with no luck. Same thing, just light static. It does appear that all the tubes are working althought the mixer tube (6SG7) is fairly dim. Any help would be greatly appreciated.<P>Thanks!<P>------------------<BR>Tom M.


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 Post subject: Motorola 65T21 Question
PostPosted: Feb Mon 16, 2004 2:49 am 
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Replace the filter caps as a starter.<P>------------------<BR>Rodney


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 Post subject: Motorola 65T21 Question
PostPosted: Feb Mon 16, 2004 2:51 am 
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Ok... I am on my way out to the garage to do it now. I'll keep you posted.<P>Thanks...<P>Tom<P>------------------<BR>Tom M.


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 Post subject: Motorola 65T21 Question
PostPosted: Feb Mon 16, 2004 4:04 am 
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Ok...<P>I replaced the filter caps with no luck. I still get static and cannot pick up any channels. Any other thoughts?<P>Thanks for your help!<P>Tom<P>------------------<BR>Tom M.


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 Post subject: Motorola 65T21 Question
PostPosted: Feb Mon 16, 2004 4:58 am 
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Well, lucky you - you get to start troubleshooting. Sometimes just simple cap replacement isn't enough to get a radio going.<P>If you don't already own a multimeter, it's time to go get one. They're available all over, in places where electrical supplies are sold. Then visit this web site, IMO one of the best for learning how radios work:<BR> <A HREF="http://www.gbronline.com/radioguy/trouble.htm" TARGET=_blank>http://www.gbronline.com/radioguy/trouble.htm</A> <P>You may find the schematic helpful, too, if you don't already have it. For some reason, this is not as useful as some (the audio output stages are missing); maybe somebody with an actual paper copy of Riders could get you something better. <BR> <A HREF="http://www.nostalgiaair.org/PagesByModel/762/M0006762.pdf" TARGET=_blank>http://www.nostalgiaair.org/PagesByModel/762/M0006762.pdf</A> <P>One place to start is by touching the center lug of the volume control pot with your multimeter probe (or your finger, but watch for shocks). If you get a hum in the speaker, your audio section is fine. Chances are that since you're getting static, it probably is. Then you have to troubleshoot the radio section. Or, if your radio has a phono jack, you can connect up a CD player or something for the same effect.<P>You have an antenna attached?<P>Not to ask stupid questions, but you don't have the bandswitch in the shortwave position, do you? It can be difficult to pick up SW stations sometimes.<P>------------------<BR>Paul


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 Post subject: Motorola 65T21 Question
PostPosted: Feb Mon 16, 2004 6:11 am 
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Hi Paul...<P>I already have a multi-meter so I should be in good shape there. <P>The back of the volume control has the on off switch so there is only line voltage there. I touched the other leads on the on the inside volume pot with an insulated screw driver and I didn't hear any change in the static.<P>As for the AM/SW switch, I tried both positions several times and adjusted the tuner each time.<P>I am still getting lots of static. Where do I go from here?<P>Thanks...<BR><P>------------------<BR>Tom M.


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 Post subject: Motorola 65T21 Question
PostPosted: Feb Mon 16, 2004 6:24 am 
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Before you go to a lot of trouble if you have flouresant lights in your shop turn them off and see if the static lessons in your radio.<P>------------------<BR>Rodney


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 Post subject: Motorola 65T21 Question
PostPosted: Feb Mon 16, 2004 6:47 am 
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Try measuring the voltage on pin 2 of your 6SQ7 with the multimeter. That should make a buzz in your speaker. <P>I found a schematic for you here: <A HREF="http://techpreservation.dyndns.org/schematics/47/47_Motorola_65T21_106.djvu" TARGET=_blank>http://techpreservation.dyndns.org/schematics/47/47_Motorola_65T21_106.djvu</A> <P>Unfortunately, it doesn't have a voltage chart.<P>You'll need to find and download the DjVu reader to see it. It's available here: <A HREF="http://techpreservation.dyndns.org/schematics/" TARGET=_blank>http://techpreservation.dyndns.org/schematics/</A> <P>Another thing you can do is to see if your radio's local oscillator is working. Find a working radio, put it very close to your Motorola, then tune the Motorola and tune the other radio to 455KHz higher. You should hear a whistle and/or heterodyning as you approach that point. <P>Start there.<BR>Good luck<BR>Regards,<P>------------------<BR>Paul


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 Post subject: Motorola 65T21 Question
PostPosted: Feb Mon 16, 2004 6:48 am 
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Paul Dietenberger wrote:
<font>quote:</font><HR>Originally posted by rodn:<BR><B>Before you go to a lot of trouble if you have flouresant lights in your shop turn them off and see if the static lessons in your radio.<P></B><HR>
<P><BR>Heh. Yeah, check the simple things first.<BR>Ditto if you have a dimmer switch nearby. Those things throw off a lot of RF interference too.<BR><P>------------------<BR>Paul


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 Post subject: Motorola 65T21 Question
PostPosted: Feb Mon 16, 2004 7:27 am 
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Check the IF transformers. I have two vintage Motorola sets, and both of them fouled out an IF transformer. A set I had a number of years ago did the same thing. In each case, it was the IF transformer between the converter stage and the IF stage. Good luck and all the best <IMG SRC="http://antiqueradios.com/forums/smile.gif">, Larry <P>------------------<BR>


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 Post subject: Motorola 65T21 Question
PostPosted: Feb Mon 16, 2004 7:33 am 
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Since you did not mention it, I have to ask a very basic question. Are you certain that all the tubes are good, and in the correct sockets? You mention that the 6SG7 tube is "dim". Do you have access to a tube tester? <P>------------------<BR>Dennis


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 Post subject: Motorola 65T21 Question
PostPosted: Feb Mon 16, 2004 3:49 pm 
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<BR>Thanks for everybodies response... I really appreciate the help. I think I should try to define my problem a little better....<P>The static I am talking about sounds more like a 'dirty pot' being turned (without me touching any of the pots). It does not increase with the volume control. In fact, the level of noise/static remains the same throughout the volume range. The static I am talking about is the only noise I hear in the speaker.<P>Let me see if I can answer some of the replies I received:<P>I have flourscent lights but these have electronic ballasts with T8 lamps. I am familiar with the type of buzz you can get from flourescent lamps and this isn't it. Also, I do not have any wall dimmers that would cause interference.<P>As for the tubes, I do not have a tube checker so I am not sure if they are OK. The 6SG7 is a bit dim and I can only see the glow at the bottom on the tube and not the top like the others. It does not get as warm as the other tubes either. I might be able to locate a tube tester though.<P>As for schematics, I have copies of both Riders and Sams. I believe the Sams has the voltage values for each pin of the tubes. Should I be looking there?<P>Also, how can I check if the IF transformer is bad?<P>Thanks again the help...<P>------------------<BR>Tom M.


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 Post subject: Motorola 65T21 Question
PostPosted: Feb Mon 16, 2004 5:43 pm 
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If your Sams has a voltage chart you will find that helpful. With the radio running and warmed up, measure voltages at any point where a voltage is given in the chart and note any with substantial deviation from the chart. That should give a clue.<P>Measuring voltage on pin 2 of the 6SQ7 should produce a hum or buzz in the speaker that varies with the volume control. If it does, your audio stages and power supply are working fine, and you can limit troubleshooting to the mixer/osc, IF and detector.<P>------------------<BR>Paul


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 Post subject: Motorola 65T21 Question
PostPosted: Feb Mon 16, 2004 7:07 pm 
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Hi Tom, It may be helpful to try to isolate which section of the radio is causing the problem. Static that does not change with the volume control is usually an indication of a component breaking down under voltage, or a corroded or otherwise intermittent connection. Try pulling out the 6SG7 IF tube and see if the static remains. If it is still present, the problem is after that tube, and if it disappears, the problem is in or before the IF stage. <P>As Paul suggested, the next place to go is to check the presence of B+ voltages on the appropriate points in the circuit.<BR>You can check the voltages on the plate (P) pin 8, and screen grid (G2) pin 6, of the mixer and IF tubes to see if they are present, and also the plate pin 3 of the 6J5.<P>The easiest way to check the IF transformers for problems (jokingly referred to by many as the dreaded silver mica disease) is to pull the tube following the IF transformer you want to test. Then with the set turned on, measure the secondary side of the IFT for any positive voltage referenced from the chassis. If you are checking the first IFT, pull the 6SG7 IF tube, and measure at pin 4 of its socket. If you are checking the second IFT, pull the 6SQ7, and measure the voltage on the diode plate pins of its socket, 4 and 5. <BR>Usually a bad IFT which is breaking down will cause loud static similar to what you describe, but it will be affected by turning the volume control, so your problem may be in the circuitry after the volume control.<P>------------------<BR>Dennis


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 Post subject: Motorola 65T21 Question
PostPosted: Feb Mon 16, 2004 9:56 pm 
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Heres a dumber question, how is the antenna? No antenna, no signal. If there is static means there is a clear path to the output section. The front end is missing somewhere in the line. Touch grids starting from the output working to the front. Where the 'bzzz' stops is where the trouble usually follows.


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 Post subject: Motorola 65T21 Question
PostPosted: Feb Mon 16, 2004 10:07 pm 
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That's a point - this radio uses a loop antenna with a plug and socket (good idea, unless you lose it), which must be connected and have good continuity on all pins otherwise you get no sound. <P>Touching grids would work if you have small fingers or a multimeter probe - this radio has all single-ended tubes (no grid caps). One false step and *zap* <IMG SRC="http://antiqueradios.com/forums/eek.gif"><P>------------------<BR>Paul


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 Post subject: Motorola 65T21 Question
PostPosted: Feb Tue 17, 2004 6:48 am 
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Ok, here are the voltage values I measured at the tubes. The actual reading will follow the reading given by Sams. (in the HS67 chassis)<P>5Y3GT<BR>1. 0V 0V<BR>2. 294VDC 296VDC<BR>3. 0V 0V<BR>4. 275VAC 306VAC<BR>5. 0V 0V<BR>6. 275VAC 306VAC<BR>7. 0V 0V<BR>8. 294VDC 294.1VDC<P>6K6GT<BR>1. 0V 0V<BR>2. 0V 0V<BR>3. 208VDC 225VDC<BR>4. 217VDC 233VDC<BR>5. -15VDC -18.8VDC<BR>6. -17.5VDC -19.2VDC<BR>7. 6.1VAC 6.7VAC<BR>8. 0V 0V<P>6SQ7<BR>1. 0V 0V<BR>2. -11V -3.2V<BR>3. -1.15VDCV -2.06VDC<BR>4. -1.15VDC -2.9VDC<BR>5. -1.15VDC -2.6VDC<BR>6. 188VDC<BR>7. 6.1VAC 6.3VAC<BR>8. 0V 0V<P>6SG7<BR>1. 0V 0V<BR>2. 0V 0V<BR>3. 0V 0V<BR>4. -.25VDC -2.36VDC<BR>5. 0V 0V<BR>6. 107VDC 137VDC<BR>7. 6.1VAC 6.7VAC<BR>8. 217VDC 232.3VDC<P>6J5<BR>1. 0V 0V<BR>2. 0V 0V<BR>3. 195VDC 211.4VDC<BR>4. 0V 0V<BR>5. -.2VDC -3.1VDC<BR>6. 0V 0V<BR>7. 6.1VAC 6.7VAC<BR>8. 0V 0V<P>6SG7<BR>1. 0V 0V<BR>2. 0V 0V<BR>3. 2.7VDC 4.45VDC<BR>4. 0V -1.3VDC<BR>5. 2.7VDC 4.46VDC<BR>6. 107VDC 136.8VDC<BR>7. 6.1VAC 6.7VAC<BR>8. 197VDC 211.2VDC<P><BR>I also tried the following suggestions:<P>>Measuring voltage on pin 2 of the 6SQ7 should produce a hum or buzz...<P><BR>I didn't get any buzz at all when I tested pin 2.<P>>Try pulling out the 6SG7 IF tube and see if the static remains.<P>I also tried this with no luck.<P>Any help would be appreciated!<P>Thanks!<P> <BR><P>------------------<BR>Tom M.


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 Post subject: Motorola 65T21 Question
PostPosted: Feb Tue 17, 2004 7:20 am 
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Those voltages aren't bad, but you should definitely gotten a loud buzz when probing the 6SQ7 pin 2. Maybe your 6SQ7 is bad. Got a spare?<P>------------------<BR>Paul


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 Post subject: Motorola 65T21 Question
PostPosted: Feb Tue 17, 2004 7:51 am 
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Hi Tom<P> Noticed you only show 188 volts for pin #6 of 6SQ7. Is this your reading or Sam's? This pin will be supplied through a resistor and must be positive. If not, resistor may be open?<P> Pin #2 of your 6SQ7 has -11 volts as the first reading. The stage will be in cut off if voltage is this negative. Second reading is about right.<P> Is there a cap between pin #6 of your 6SQ7 and pin #5 of 6K6?<P> Since you don't hear hum either your 6SQ7 or 6K6 circuit is a problem. Voltages look good on the 6K6.<P><P>------------------<BR>Norm


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 Post subject: Motorola 65T21 Question
PostPosted: Feb Tue 17, 2004 4:10 pm 
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Hey guys, sorry for the confusion, but I gave you the wrong info for tube 6SQ7. It was late last night when I checked the replies so I went out this morning before work and checked it again. <P>Here is the right info:<P>6SQ7 (actually 6SQ7GT)<BR>1. 0V 0V<BR>2. -11V -3.2V<BR>3. -1.15VDCV -2.06VDC<BR>4. -1.15VDC -2.9VDC<BR>5. -1.15VDC -2.6VDC<BR>6. 188VDC 7.1VDC<BR>7. 6.1VAC 0V<BR>8. 0V 0V<P>So this looks like the obvious problem. Do you recommend I replace this tube or all of them. I checked yesterday and I think I can get all the tubes for under $40.<P>Thanks again for your help... I appreciate it!<BR><P>------------------<BR>Tom M.


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