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 Post subject: General Electric C-404 low volume question
PostPosted: Jan Thu 12, 2017 2:10 am 
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Location: Didsbury Alberta Canada
Hello all

I'm working on a Canadian General Electric C-402 which has two issues. The first being low volume. It has about 1/4 the volume it should. It also has the symptoms of silver mica disease which I will tackle as well. I have completed the following, but could the silver mica disease be the cause of general low volume? It does crackle and cut out after a bit but the volume never does adjust to a normal volume. I have had other units with SMD that crackle and cut out but have great volume.

-recapped
-all tubes checked and good
-out of spec resistors replaced
-loop antenna has no shorts or open
-speaker and transformer values in spec

I'll fix the IF cans anyway but I'm interested in views. Any other suggestions welcome.

Rod


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 Post subject: Re: General Electric C-404 low volume question
PostPosted: Jan Thu 12, 2017 2:37 am 
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Location: San Jose, CA USA
Yes, one of the possible situations that can arise with silver mica disease is that one or more of the IF tuned circuits no longer resonates close to the IF frequency. When that happens, those tuned circuits greatly attenuate the signal, giving rise to the type of symptom you describe. Have you tried peaking the IFs? If you are able to get a clear peak on all IF adjustments, then this problem is not occuring at a severe level. If any of the adjustments does not give a nice peak, or the max signal occurs at the end of the adjustment range, then that tuned circuit is not able to resonate properly at the IF frequency, and one of the causes of that can be severe silver mica disease (which only applies to slug-tuned, not trimmer-tuned IF transformers).

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 Post subject: Re: General Electric C-404 low volume question
PostPosted: Jan Thu 12, 2017 3:15 am 
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Thanks for the reply Tom. I can get a peak on both IF transformers. They are the 119 type I hear fail frequently. I'll fix them and see if it takes care of the issue. Thanks again!


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 Post subject: Re: General Electric C-404 low volume question
PostPosted: Jan Thu 12, 2017 3:17 am 
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Can you get a peak on all four adjustments (both the top and bottom slugs of both transformers)? If you're getting a nice peak on all four, then if you have silver mica disease, it is probably mainly causing static, but sensitivity should still be decent. There might be an additional problem affecting the signal strength.

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 Post subject: Re: General Electric C-404 low volume question
PostPosted: Jan Thu 12, 2017 6:19 am 
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Location: Didsbury Alberta Canada
Well, I rebuilt the first IF can with great improvement in the crackle-pops but volume still low. I replaced all the resistors that were even a bit out of tolerance and managed to get a bit more volume but not strong as it should be. I could peak with all 4 slugs so the search goes on. An interesting situation I came across was that pin 2 and 7 seem to be reversed in voltages in the 50L6. Pin 2 states it should have 35 volts ac but it has 85 volts. Pin 7 states 85 and it has 40? Pin 6 of the 12SQ7 should have 27 volts but it never gets that high. When it popped or crackled it would go to 0 and then slowly climb up as reception increased. I'll get this yet!


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 Post subject: Re: General Electric C-404 low volume question
PostPosted: Jan Thu 12, 2017 7:07 am 
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Since pins 2 and 7 are for the filament, it doesn't matter that they are reversed. It's even possible that the guy who made the schematic didn't even bother to confirm exactly how the two pins were wired.

I can't find a schematic for this radio, although it sounds like a typical AA5 from what you've described, and the info at Radiomuseum. This is a Canadian model (http://www.radiomuseum.org/r/canadian_g_c404.html).

The low voltage on the plate of the 12SQ7 is a little odd, but probably isn't an issue. If you turn up the volume all the way, and touch a piece of wire a couple feet long to the center terminal of the volume control (and not touching anything else), do you get a pretty loud buzz in the speaker? If so, your audio amp section is OK, and the problem is likely in the RF or IF stages. If you only get a weak buzz, focus on the audio stages to find the problem.

If you tune in a weak station near the top of the dial, do you get a clear peak within the adjustment range of the antenna trimmer on the tuning capacitor (that's the trimmer on the larger of the two gangs)? If so, the loop antenna and front-end alignment are reasonable. If not, focus on front end alignment and make sure the loop is wired properly and has continuity.

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 Post subject: Re: General Electric C-404 low volume question
PostPosted: Jan Thu 12, 2017 3:12 pm 
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Wrenchtwister wrote:
Pin 6 of the 12SQ7 should have 27 volts but it never gets that high. When it popped or crackled it would go to 0 and then slowly climb up as reception increased. I'll get this yet!

I'd consider that a major clue... Since pin 6 is plate, something is either effecting tube current draw or there is a intermittent condition in the plate supply... If grid voltage is steady, I be suspect of resistors/caps in plate circuit... Does this set have a couplate?


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 Post subject: Re: General Electric C-404 low volume question
PostPosted: Jan Thu 12, 2017 3:54 pm 
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I uploaded the CGE C 402 schematic for anyone that would want to reference it.

https://app.box.com/s/2bkeyhju6v3zu561s8bubmbyfvzohxng

Regards

Ken


Last edited by Tarpawns on Jan Thu 12, 2017 5:32 pm, edited 1 time in total.

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 Post subject: Re: General Electric C-404 low volume question
PostPosted: Jan Thu 12, 2017 4:46 pm 
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Location: Didsbury Alberta Canada
Oh no! I made a typo in the model number. It's 402 not 404. My apologies for your time spent on the 404 :( I'll upload it shortly. The heater voltages make sense and likely an easy one for you wizards:) I do get a peak with stns on high end of dial. Not sure what is meant by couplate? Thanks to all for assistance on this.

Rod


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 Post subject: Re: General Electric C-404 low volume question
PostPosted: Jan Thu 12, 2017 5:34 pm 
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Rod


Edited my link above to CGE C402 schematic 8)


Ken


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 Post subject: Re: General Electric C-404 low volume question
PostPosted: Jan Thu 12, 2017 6:37 pm 
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Joined: Sep Sat 10, 2016 10:30 pm
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Location: Didsbury Alberta Canada
Tarpawns wrote:
Rod


Edited my link above to CGE C402 schematic 8)


Ken



Thanks Ken! I have just spent last 30 min trying to upload it.


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 Post subject: Re: General Electric C-404 low volume question
PostPosted: Jan Thu 12, 2017 6:45 pm 
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Joined: Sep Sat 10, 2016 10:30 pm
Posts: 124
Location: Didsbury Alberta Canada
I do not get a loud buzz when touching center terminal of volume pot. Makes no difference in sound. I don't get it though as I have replaced all resistors and caps in the audio section. Output tube tests strong. I fed a signal into the audio transformer and speaker and they are good. Has me stumped.


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 Post subject: Re: General Electric C-404 low volume question
PostPosted: Jan Thu 12, 2017 8:03 pm 
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Location: San Jose, CA USA
A few things to try:

As 35Z5 pointed out above, your plate voltage on the 12SQ7 is low. Check that 470 K resistor on the plate of the 12SQ7. If that's not it, try a systematic routine, starting from the output. Try subbing the speaker, and then the output transformer. Then we'll work our way back through the circuit.

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 Post subject: Re: General Electric C-404 low volume question
PostPosted: Jan Thu 12, 2017 8:46 pm 
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Location: Cleona, PA
Re: couplate...the internet is your friend.

https://www.google.com/?gws_rd=ssl#q=couplate

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Reece


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 Post subject: Re: General Electric C-404 low volume question
PostPosted: Jan Thu 12, 2017 9:57 pm 
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Location: Didsbury Alberta Canada
wrnewton wrote:
Re: couplate...the internet is your friend.

https://www.google.com/?gws_rd=ssl#q=couplate



Hello Reece.

I did a google search and it kept showing results for copulate (of which before marriage I was quite familiar with). Thanks for the link. No, this machine does not have one though.

Rod


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