42-1008 code 121 - Printable Version
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42-1008 code 121 - rperry - 01-12-2019
Hello ,
I'm in need of some guidance, here is the short of it, I bought a radio 42-1008 code 121, recapped it, Power transformer was bad. Found a chassis on ebay to rob the PT. original PT has 12 wires the donor as 9 assuming its a 122. From what i could tell everything was getting power just in a different path. Once i got it installed and powered it up on a variac, tubes are glowing and all the lights are on but no sound or even static at all from radio or Phono, Can someone point me in the right direction that is familiar with this set? Any help is appreciated Thank you!
I still need to check all the tubes my knight 600b doesn't list the tubes for this one. So i am working on testing those currently
RE: 42-1008 code 121 - Radioroslyn - 01-12-2019
Hi and welcome,
Would check to see if you have HV on pins 2 and 3 of the 41 tubes. Should see abt 225v there.
If not check pin 7 or 8 of the 5Y4 should see hv there.
GL
RE: 42-1008 code 121 - rperry - 01-12-2019
50 volts to those pins
RE: 42-1008 code 121 - Radioroslyn - 01-12-2019
Hmmm Pretty low. Remove the 5Y4 and measure the ac voltage across pins 3 to 5. Should see abt 500vac.
RE: 42-1008 code 121 - rperry - 01-12-2019
641 volts
RE: 42-1008 code 121 - Radioroslyn - 01-12-2019
Ok that's more like it. Two things to check, the polarity of #76 and 30a and b. On the diagram the top connect is the + and bottom is -. The other is #77 check the total resistance should be abt 200 ohms. A backwards installed 76 or 30 will acted like a short thus pulling the hv down. If this is the case the offending cap(s) are junk chuck and replace w/new If 76 is open there's no negative return for the hv.
RE: 42-1008 code 121 - rperry - 01-12-2019
Im scratching my head on #76 I dont see a 12uf cap anywhere.
RE: 42-1008 code 121 - Radioroslyn - 01-12-2019
Just to the right of the rectifier tube on the schematic.
RE: 42-1008 code 121 - rperry - 01-12-2019
10-4, I see it on the schematic but not in the chassis, polarity is good on electrolytic's, but I dont see a 12uf, I have a dual 16-8uf which is #30, then I have a 16uf which is #76, hmm both chassis have a 16uf where its calling for a 12uf, the resistor reads 204 ohms.
RE: 42-1008 code 121 - Radioroslyn - 01-12-2019
Ok so if the caps are good then it would seem that the 5Y4 is very weak. This is what to do. Replace the 5Y4 and connect a voltmeter from the chassis (-) and to pin 8 of the 5Y4 socket (+). Monitor the dc hv when you turn it on. If all is well the voltage start @ 0 and quickly build up to 350vdc or so. Then it will start to drop down to abt 250v or so. The important part is when you 1st turn it on if the hv doesn't rise quickly up and just stays @ 50or so volts turn it off after 15sec. If you don't the plates in the 5Y4 will get RED and VERY ANGRY which is not good.
RE: 42-1008 code 121 - rperry - 01-13-2019
ok, when started it quickly went to 450 volts DC and stayed there.
RE: 42-1008 code 121 - rperry - 01-13-2019
The field coil is not getting voltage it seems
RE: 42-1008 code 121 - Radioroslyn - 01-13-2019
Check resistance it may be open.
RE: 42-1008 code 121 - rperry - 01-13-2019
Im a bit embarrassed!!!
I had the phono and the speaker plugs reversed.... they shouldn't be interchangeable in my mind. So my new issue is NO reception just speaker hum.
RE: 42-1008 code 121 - Radioroslyn - 01-13-2019
Do you have a signal generator?