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Atwater Kent 328 Early - Printable Version

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Atwater Kent 328 Early - Eric - 09-24-2013

I have an Atwater Kent 328 (early) Rider 6-18 for the schematic and 7-34 (bottom) for chassis layout. http://www.nostalgiaair.org/PagesByModel/334/M0001334.pdf

I think I have a bad power transformer but want to be sure so here is what I have done.

Recapped, changed some out of spec resistors and rewired the Selective Fidelity Switch back to original and replaced the 5Z4 tube, got the tuner mechanism working, had to find a speaker and output transformer which I did I'm 250ohms light on the field coil and use a resistor to bring it up.

When I powered up it started drawing high current 1amp at 15v(I use a varac with a digital meter set to read amperage) so I backed it down and tried it with all tubes removed, same thing. I pulled the transformer and powered it up and on the plate leads I got 700vac and 350 & 345vac each side to the center tap.5.3vac for the 5Z4 heater and 6.3 for the tube heaters and dial lamps current was minimal (.o6-7). That looked good so I reinstalled and still had high current draw I stop at 1amp but I'm only at 15v so I disconnected the leads one set at a time and when I disconnected the center tap things got better but still high about 1.7amps but at 110v the set says 90watts so I figure around 1 amp or less I don't leave it on for long as I don't want to damage anything not sure what the effect of leaving the center tap open? Checked R26 and it is good and c27 has the +on the white and the - to the center tap
I did get some stations briefly then faded out.

Any Ideas?

Thanks Eric

RE: Atwater Kent 328 Early - Radioroslyn - 09-24-2013

If you leave the CT ungrounded you will have disconnected the B- side of the HV. Set will not work this way. Does seem like your transformer is shorted. The good news is you don't have to find a transformer that has a 2.5 filament winding.
Terry

RE: Atwater Kent 328 Early - Eric - 09-25-2013

Terry
So see if have this right
7tubes
6H6 .3a
6F5 .3a
6K7 .3a
6K7 .3a
6K7 .3a
6A8 .3a
6F6 .7a
2-lamps .3
total 2.8amps @ 6.3v
5Z4 2a @ 5v
now I have 2 unites I'm looking at
1 DX270 Fil1 2a@ 5v, Fil2 3a@ 6.3v and the HVAC 550v CT 104ma
2 DX272 Fil1 3a@ 5v, Fil2 4a@ 6.3v and the HVAC 600v CT 144ma
not sure about the HVAC rating?
There is plenty of room for the larger
Eric

RE: Atwater Kent 328 Early - Mondial - 09-25-2013

Did you say that the transformer did not draw high current when you removed it from the chassis and powered it up? I am a little confused by your description.

If you have the proper secondary voltages when powered up outside the chassis and it does not draw excessive current, then there might be a short from the metal case or core laminations to one of the windings. You might check with an ohmmeter for a short from each winding to the case. If there is a short, it could be something as simple as damaged insulation on one of the leads under the housing or where they exit the case.

Might be worth checking before you order a new transformer.

RE: Atwater Kent 328 Early - Eric - 09-25-2013

You gave me pause for thought and I went back and cut the xformer loose and started checking. the xformer is on my bench with the end caps off and what I found is the lead marked shield is shorted in varying degrees to the center tap 200ohms as well as the HV Plate leads 24ohms. This is probably why on the bench it tested fine as it was isolated, but when installed it grounded it to the chassis. Can I run it with the shield cut out? Or just order a new one?

RE: Atwater Kent 328 Early - marty - 09-25-2013

Hi All;
Eric, I would carefully take off the outer covers and look for internal wire shorts, caused by broken wires or broken insulation.. One possible way to fix it after the covers are removed is to use Liquid Tape arround the various wires and then put the covers back on.. And recheck for any shorts.. Look under Joernone for his fixing a transformer..
http://www.youtube.com/watch?v=UN4e855AeY4&list=PL36346738A28F2222&index=11
http://www.youtube.com/watch?v=0kB5zn3LHYM&list=PL36346738A28F2222
http://www.youtube.com/watch?v=2NkGw-kw7tE&list=PL36346738A28F2222
http://www.youtube.com/watch?v=IQlLKDMlaEk&list=PL36346738A28F2222
http://www.youtube.com/watch?v=a31C5MsH-sI&list=PL36346738A28F2222
THANK YOU Marty

RE: Atwater Kent 328 Early - Eric - 09-25-2013

I have the covers off and the xformer looks very good with no exposed or bare conductors, insulation looks good and is not flaking and almost as important it smells ok. from looking at the print the shield is between the pri and sec. It was probably there to take static to ground or for noise reduction my guess. I know the new ones do not have them, I will if I have time tomorrow is reinstall and see what happens with the shield disconnected and taped off.

RE: Atwater Kent 328 Early - Arran - 09-25-2013

If the high voltage winding is leaking to the electrostatic shield then that would cause your high current draw problem, very likely it's caused by rotted insulation on the power transformer leads. The electrostatic shield is normally grounded to the chassis so having 200 ohms between it and the center tap would make sense if the A.K has bias resistors in series with teh center tap and the chassis. You could probably fix it by slipping some heat shrink tubing over the respective leads, unless they are really crappy then replacement of the leads is probably the best alternative. I had to do this on a transplant power transformer and it's a bit tricky, the old wires need all of the tar and varnish cleaned off in order to solder to them, and the copper sanded thoroughly as well. Most larger power transformers, even new ones, have that electrostatic shield, but they use copper foil for that purpose and ground it to the lamination of the transformer, it's a way to eliminate power line noise.
Regards
Arran

RE: Atwater Kent 328 Early - Eric - 09-25-2013

The problem is not in the leads as they are soldered to tabs on the face of the coil and one of the HV leads only measures 24 ohms and the other is 400 ohms to the shield which is the listed resistance of that winding. That tells me that the short is very close to the one lead the question is can I dig into it and clear it, so if I can't repair it I picked 2 Hammond xformers to replace it
1 DX270 Fil1 2a@ 5v, Fil2 3a@ 6.3v and the HVAC 550v CT 104ma
2 DX272 Fil1 3a@ 5v, Fil2 4a@ 6.3v and the HVAC 600v CT 144ma
I have room for the larger is the HVAC voltage ok?

RE: Atwater Kent 328 Early - Mondial - 09-26-2013

The HV windings on both replacement transformers is a bit low as I believe you mentioned you measured 700 VAC on the original transformer. The bigger one is closer at 600 VAC, but otherwise larger than needed for the actual load.

You also mentioned that your speaker field is a bit lower in resistance than the original, so you might have to take out the added 250 ohms, giving you less voltage drop which would compensate somewhat for the lower HV. You could also increase the capacitance of the first filter capacitor which would increase the DC output voltage a bit.

I would try isolating the shield and reinstalling the original transformer first. I doubt you would notice any appreciable difference in reception with the shield left insulated and floating with respect to ground. At least you would be able to measure the actual operating DC supply voltages which would give you a better idea of what transformer you might need to order.

RE: Atwater Kent 328 Early - Arran - 09-26-2013

The H.V winding is too low on either of those replacements, it should be a 350-0-350, the filament current ratings are fine on the second transformer though. You have to remember that there will be a 10-20 volt drop across the rectifier tube, and then another drop across the field coil of the speaker, so that 300 volts will be something like 200 volts or less by the time it gets to the screen of the power output tube. By those measurements the H.V winding itself is not shorted, is the transformer running hot as well with no load?
Regards
Arran

RE: Atwater Kent 328 Early - Eric - 09-26-2013

Reinstalled minus the shield and current draw is .7amps @ 110v so that looks like it will work. At least I can start trouble shooting the rest. Also I picked out another xformer just in case it is a Hammond 273DX 700vCT 103ma, 5v@2a, 6.3v@3 or 273X 700v CT@126ma, 5v@2a, 6.3v@4a.
going out of town till Sunday and hate not being able to work on this. Am I addicted?
thanks Eric

RE: Atwater Kent 328 Early - Eric - 10-06-2013

Now that the power issues appear to be taken care of I need to start troubleshooting. All caps have been changed except mica ones. First questions
1) I located all resistors and checked for value and changed those over 20% but #7 10k is not there just a jumper on the #4 pins on the 6k7 1st & 2nd IF. Should I install one?
2) Socket markings? What are they?
P-Plate
F-Filament "Heater"
G-grid?
S-screen?
C-cathode
SL-?
CS-?
OP-? 6A8
OG-? 6A8

RE: Atwater Kent 328 Early - Mondial - 10-06-2013

If it is not there I would install the 10K resistor between the screen grids of the 1st and 2nd IF tubes. This acts as a decoupling filter and serves to isolate the two stages, preventing instability.

6A8 OP = Oscillator Plate
6A8 OG = Oscillator Grid
CS = Suppressor Grid (G3)
SL = Metal Tube Shell (grounded shielding)

RE: Atwater Kent 328 Early - Eric - 10-07-2013

Got some data now here goes. The numbers in parentheses are what the print calls for. The ** are way out of spec. I do get a station very weakly and the volume control makes no difference then it fades away after a minute or two. Need all the help I can get.
5Z4 P1,2 - 343,342. F1,2 - 380,380
6f6 pin3 plate 362v (255)
pin4 screen 310v (255)
pin5 grid cap .04v (4)**

6F5 pin4 plate 116 (150)
grid cap -.6 (1) Loud hum when connected to meter

6K7 RF pin3 plate 316 (255)
pin4 screen 60 (80)
grid cap 2.6 (2)

6k7 1st IF
pin3 plate 237 (240)
pin4 screen 80 (80)
grid cap 278 (1)**

6k7 2nd IF
pin3 plate 306 (250)
pin4 screen 91 (70)
grid cap 2.5 (5)

resistors
R6 54v (3)**
R10 8 (5)