38-116 Code 121 Powering Up Help

38-116 Code 121 Powering Up Help - Printable Version

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38-116 Code 121 Powering Up Help - AVSorrell - 07-17-2017

Hey folks!
I have just completed a recapping of my 38-116 code 121. I replaced all capacitors including electrolytics and micas. I also replaced all resistors. I took the RF sub chassis apart and replaced all caps and resistors hiding there too as well as the caps in the bakelite blocks. I installed a fuse and a CL--90 and a safety cap on the line cord circuit. I have checked all tubes out on my Hickock tube tester (and purchased a new 5X4 rectifier).

With all the tubes out, I slowed powered the chassis up on my isolated variable AC power supply (a BK precision 1653). Got it up to 110 and dial lamp worked well and was drawing right amperage of 1.4 amps. I checked all the tube filament voltages and they all read 6.3 VAC as the should.

I next measured the B+ voltage on the filter caps - these too read correctly about 390 vdc from positive to the can ground and about 315 vdc from positive to chassis ground. I next put the rectifier tube in and then measured the filter caps again - it started off in the right range but then once the tube warms up it goes down to about 210 vdc for the reading that should be 385 and then about 180 for the reading that should be 315. I put in the rest of the tubes and still had the same low readings on the filter caps - however all tubes glow.

A couple other observations:

--the main dial light works (on at all times)

--the 4 magnetic tuning lights work when magnetic tuning switch it on

--the mixer tube should be a 6L7G (pentagrid mixer) - but it came with a 6K7G (remote cutoff pentode) in its spot and I have not replaced it yet.

--there is no noise from the speaker - not even a hum.

--I had a bad audio interstage transformer which I had replaced with custom made one from heybour.

Any ideas on why the voltage across my filter caps be reading low?

RE: 38-116 Code 121 Powering Up Help - morzh - 07-17-2017

Andy

I am not sure I understood this part:

>>>>I next measured the B+ voltage on the filter caps - these too read correctly about 390 vdc from positive to the can ground and about 315 vdc from positive to chassis ground. I next put the rectifier tube in and then measured the filter caps again -

Without rectifier tube there is no B+. You should've had 0V on the filter caps with no rectifier in.

RE: 38-116 Code 121 Powering Up Help - Radioroslyn - 07-17-2017

Hi Andy,
Just a quick point if the rectifier is in spkr must be plugged in too. If not and you apply power to the chassis you can damage the new electrolytic caps in the power supply. Just saw a yt vid where a guy was measuring dc voltages on a 116X chassis w/o the spkr and couldn't figure why the reading were way high!

GL
Terry

RE: 38-116 Code 121 Powering Up Help - AVSorrell - 07-17-2017

Thanks folks - I have had the speaker plugged in when I have the rectifier in. Also morzh good point - I had the rectifier in when I got the lower readings.

RE: 38-116 Code 121 Powering Up Help - morzh - 07-17-2017

Terry

From what I read I figured he had the speaker in as he said "--there is no noise from the speaker - not even a hum. " Icon_smile

This is why I did not ask.

RE: 38-116 Code 121 Powering Up Help - WallaceRoger - 07-17-2017

Possible hot tube fault?

RE: 38-116 Code 121 Powering Up Help - Radioroslyn - 07-17-2017

(07-17-2017, 08:47 AM)morzh Wrote: Terry

From what I read I figured he had the speaker in as he said "--there is no noise from the speaker - not even a hum. "
This is why I did not ask.

Mike you forget I'm lisdexsic!!![Image: http://philcoradio.com/phorum/images/smilies/icon_eek.gif]

Andy, I would poke around w/ your voltmeter and measure the from chassis to pin 3 of the 6L6's (should see abt 275-300vthere). Check both separately. If not opt is open or not wired correctly . Also check the voltage on pin 5 of the 6L6's ( should see -16 to -20vdc). If not secondary of driver/interstage transformer is open or miss wired. Where the - voltage isn't present and the high voltage is on the 6L6's it causes them to draw excessive current which might be why your hv is reading low. Not having HV on pin 3 is not great, when there is hv on pin 4. 3 and 4 should be fairly close in voltage.
Obtw on #140 the 10mfd cap has the + side connects to the chassis and - goes to the - of the 8mfd. It filters the bias voltage. If you wired it w/the - to the chassis(backwards) it will kill off the -voltage in the set.

RE: 38-116 Code 121 Powering Up Help - AVSorrell - 07-18-2017

Looks like i have mis-wired somewhere as i have replaced all the old caps and resistors.

First 6L6 (farthest left looking from the bottom) I get:
Pin 1 - nc
Pin 2 - Should be: 6.3 vac I get: ~6 vac
Pin 3 - Should be: 290 vdc I get: ~170 vdc
pin 4 - Should be: 300 vdc I get: ~170 vdc
pin 5 - Should be: -20 vdc I get: -340 vdc!
pin 6 - nc
pin 7 - Should be: 0 I get 0 (grounded)
pin 8 - Should be: 0 I get 0 (grounded)

for the secoond 6L6 I get:
Pin 1 - nc
Pin 2 - Should be: 6.3 vac I get: ~5.6 vac
Pin 3 - Should be: 290 vdc I get: ~170 vdc
pin 4 - Should be: 300 vdc I get: ~180 vdc
pin 5 - Should be: -20 vdc I get: -0.48 vdc
pin 6 - nc
pin 7 - Should be: 0 I get ~0 (.0029 vdc) (grounded)
pin 8 - nc

And i checked out the speaker readings too. 345 dc resistance on the field coil (should be 350 dc resistance) and got 3.9 ohms dc resistance on the voice coil - so i think those are ok. However, the primary on the audio output transformer should be 375 dc resistance but I only get 212 - so i expect i may need to replace that one.

RE: 38-116 Code 121 Powering Up Help - morzh - 07-18-2017

Depends where you measure. The primary on the output xfmer is centertapped so make sure you measure from end to end and not to the centertap, as one half is 200 and another is 175. You might be measuring the 200 Ohm half.
I doubt the resistance could go from 375 to 200 as a result of burning.

RE: 38-116 Code 121 Powering Up Help - AVSorrell - 07-18-2017

I double checked and was measuring correctly from end to end and getting 212 dc resistance - and about 105 from end to centertap.

RE: 38-116 Code 121 Powering Up Help - codefox1 - 07-18-2017

Temporarily disconnect ends primary of output transformer and measure again. You can always temporarily sub just about any PP output transformer from the tube era for a test run.

RE: 38-116 Code 121 Powering Up Help - morzh - 07-18-2017

It may be that the transformer was replaced and is good. If both halves are close in value. Another question, if it is a replacement, is it a good replacement.

DC resistance is not the goal. Power limit and turns ratio are.

RE: 38-116 Code 121 Powering Up Help - AVSorrell - 07-18-2017

It appears to be original - I have not replaced it and it is still riveted to the speaker frame.

RE: 38-116 Code 121 Powering Up Help - Radioroslyn - 07-18-2017

So try removing the 6L6's and than measure the voltages on pins 3,4,and 5. I'm thinking that one of them is shorted SG to CG. See if the voltages are more normal w/o these tubes in place.

RE: 38-116 Code 121 Powering Up Help - AVSorrell - 07-19-2017

Thanks for the advice Terry - I removed both the 6L6 tubes and the measurements were much closer to what they should be. Also for 6L6 tubes I have, one is metal and the other a 6L6GT:
First 6L6 (farthest left looking from the bottom) I get:
Pin 1 - nc
Pin 2 - Should be: 6.3 vac I get: ~6 vac
Pin 3 - Should be: 290 vdc I get: ~335 vdc
pin 4 - Should be: 300 vdc I get: ~335 vdc
pin 5 - Should be: -20 vdc I get: .001 vdc
pin 6 - nc
pin 7 - Should be: 0 I get 0 (grounded)
pin 8 - Should be: 0 I get 0 (grounded)

for the secoond 6L6 I get:
Pin 1 - nc
Pin 2 - Should be: 6.3 vac I get: ~6.0 vac
Pin 3 - Should be: 290 vdc I get: ~335 vdc
pin 4 - Should be: 300 vdc I get: ~335 vdc
pin 5 - Should be: -20 vdc I get: .001 vdc
pin 6 - nc
pin 7 - Should be: 0 I get 0 (grounded)
pin 8 - nc

I also now get 350 vdc for the filter cap positive to can ground (should be 385) and 350 for the filter cap positive to chassis ground. I also check that I have the positive of the 10 mfd elecro. cap going to ground and I do. Much better than before! Looks like I need new 6L6G tubes!