[Eliot Ness]

The brown hot glue used to be fairly easy to find, now not so much.  I got my most recent batch from Amazon but eBay might have been cheaper.

Good work Mike!

[morzh]

John,

Yes I bought mine from Amazon, and it seems to be just one vendor.
Might try eBay but I think I looked and at the time with Amazon Prime it came out cheaper and faster on Amazon for me.

PS. You could see that solder bead that solders the case to the chassis. Not sure it is important but possibly reduces some noise which I doubt could be noticed anyway now that I have aluminum caps inside.

[morzh]

Today the second cap came out.
The challenge is, the cap being not the twistlock type, it has to come out not outside but inside, due to the clamps (riveted) and the flange (wider than the clamp). 
Sure enough, lots of wires and parts cross over it inside, some of those are connected to the volume pot.
Inconvenient, should I say.

   


So, had to desolder, cut and photograph.


   

It's out.

   

[morzh]

The cap's entrails came out even easier than expected.
Just slid out leaving no tar or paper behind, so I rebuilt it right away.
Will show the photo tomorrow.

[morzh]

Fnished recap.

   

   

[morzh]

S'more.

   
   

[morzh]

Something I have just realized.
In 7G605 the dual D-size battery holder is in series with the "A" battery. Back in the day it was possible that the "A" battery was more powerful (current-wise) than the "D" size, and so they paralleled two. I cannot say that with all certainty but I do not see other reasons.
However, today in the replica 985 or such batteries, in the "A" section 6 "D" batteries are used, so it makes no sense to serialize them with two paralleled "D" batteries that are exactly the same. One will suffice just fine, and since in the holder they are in parallel, just one could be inserted in either of the two positions and work just fine. As long as it could be reliably mechanically held by the old holder.

[WyTex]

Lots of hard work there Mike. Looks great!

I kept a few of the Zenith labeled caps but didn't re-stuff. Been meaning to offer them up to anyone who might be interested.

[morzh]

I have a couple of my own. I reformed them, but then decided against it.

[morzh]

   

I've been meaning to do that for awhile. I added a zener across the cap that filters the filaments.
The cap is 25V and the voltage is 10.5V when run from batteries. Should any tube go open, the voltage will rise to the previous cap's voltage. Too high.
In goes a small 600W 13V surge suppressor, a high power TVS zener in its nature. It clamps at about 15V. Which is enough to keep the 25V cap safe and the filaments too if inserted hot (never a good idea but....).

That inconspicuous black rectangle close to the chassis edge is it.

[morzh]

After powering it up today I got the local station. The pot was awfully scratchy, and so was the tuning cap.
I Trolmastered the pot with Fader Lube Deoxit, and used Blue Shower to clean the cap.

After some drying I found out the pot is no longer scratchy but there is a lot of hum increasing with pot rotation, which I attribute to some conductivity from power switch to pot through non-dried Deoxit.
I also saw little sparkies occuring between the pointer and the dial while rotating the cap (the dial was very close to the pointer at one side) so I re-seated the dial, and scratchiness got reduced, but a several positions it still occurs. I decided to wait for the pot to dry out before proceeding.

[morzh]

So, to exclude the influence of the power wires in the pot switch to the audio signal, I unsoldered all 4 power wires and connected them together outside the switch, the switch pins now hanging free.

Nope. Still the same hum.
So, have to do more digging. Likely comes from the detector somewhere, as it is increases with the volume and really disappears if the input of the audio section of the Det-Amp is shorted or the scope is connected to it (which shunts 15 MOhms with 1MOhm).
Thought of bad rectification/filtering. Changing tubes (all tubes one by one, starting with the rectifier) yields nothing either. Caps were measured right before being put in the chassis. Scoping shows large ripple after the first cap, C22 (12V or so) but that is expected. Second pole of the filter looks clean.

[morzh]

I have found an abnormality and I think know where it comes from.

I did remember that there was a little arcing between the dial and the hand when tuning.
So, first I put a large (47uF) cap between the chassis and the last electrolytic decoupling the filaments.
The hum went away altogether.
There is a 0.1uF between the chassis and common.

So then I discharged the cap......not lot of spark.
Then I measured the voltage....and saw 120V. Except that the chassis was positive. Good thing I did not hold that cap there for a long time.
I reversed the cap, and this time after discharging there was a large healthy snap.

I measured the current via 10K resistor - just to make sure it is not capacitively coupled ghost voltage, but no - it was 10mA.
So the chassis voltage is almost equal to the rectified voltage.
But there is no measurable resistance between them; the voltage occurs at the same time as on the rectifier tube K pin.

So....I put two and two together, and I think my drop power resistor (the one mounted proud on top the chassis, riveted to it) has the leakage which develops when it is powered up. I desoldered the wire going to the tube and left the wire going to the filter, and replaced it with 82 Ohm 5W resistor. The hum got greatly reduced. (I cannot unsolder it altogether as it houses the second and third resistive sections of 970 Ohms). So I think I will replace the whole thing and see if it gets completely cured.

I thought of just using the electrolytic cap to remedy it as then it would work and most time it would employ the battery anyway....but it is not good to leave it like this.

[Eliot Ness]

........... I think my drop power resistor (the one mounted proud on top the chassis, riveted to it) has the leakage which develops when it is powered up. I desoldered the wire going to the tube and left the wire going to the filter, and replaced it with 82 Ohm 5W resistor. The hum got greatly reduced. (I cannot unsolder it altogether as it houses the second and third resistive sections of 970 Ohms). So I think I will replace the whole thing and see if it gets completely cured.........

Are you referring to the candohm resistor?  I have read in this model they can develop a resistance to the chassis that causes a hum.

[morzh]

Yes, wasn't sure it was a Candohm as it is a vertical thing on top of the chassis, but I guess we are talking of the same thing.
Resistance it is. Not noticeable when cold, in several megaohm but provides almost a short (or a very low resistance) when powered.