[TV MAN]

Hello all! I finally have the 70 chassis on the "bench". I thought it might to be interesting to parallel Mike on a restoration, having 2 radios being rebuilt at the same time and comparing notes. Heres my story. This will be my first attempt at a full rebuild v.s. a repair, and this is the oldest I have ever attempted. It has been a lot of years since I have worked on anything tube, but I have dusted off the old testers and Riders books, gotten my glasses , notebook and am ready to tackle this.

The radio looks largely undesturbed and clean. I only found one cap that has been replaced. There's a LOT of room under this thing! Much better than those cramped electronics I used to repair! The previous owner told me that the radio powered on, had a low hum and had static. I havent powered it on yet. I am going to check some things first, like the coils, resistors, and will replace the electroytic caps in the power supply. If things check out, I am going to power it on with a variac and "see what I have".

I am sure some will tell me to replace all the caps and test the tubes, etc. But, I want to see whats there first, and that may help prevent me from "causing" a problem while rebuilding. If it worked to some degree before I replaced a part, then suddenly stops, or has a new problem when I test it, I can stop right there and track down the problem, rather than try to figure it out after a full rebuild ... oh-no, what did I do???

Heres a couple of shots of the start of this adventure. I am sure to many this will be a familure sight.
   
   
I suspect this is an early model, but I am not sure. The number stamped on the rear of the chassis is: 433317, and the sticker on the tube shield calls it a model 70, 70A, 270, 270A, 370, 370A. Any input and advice is welcome, as is any advice as to what to look for that are common problems and failures.

Thanks all!

[morzh]

Tim

As we spoke of it,

start with removing the tubes, checking all coils for opens.
Remove the electrolytics,
Remove the tuning cap and old grommets,
and then you will have an easy to clean chassis.

Check the big multi-tap resistor at the bottom (in your photo - on the right) for being intact.
I think we have different versions - it seems to me you have the resistor that has the chassis connect to its very first pin and I have the version where it is the second pin.

Otherwise they look very much the same, even those two overlapping caps at the top's middle are the same (I was wondering if this is how it was originally).

I do not necessarily do that every time, but when the tubes are out, restuff only the backelite that is the closest to the electrolytics (AC filter caps) AND add a 1W 100K or even 0.5W 1M resistor across the filter, solder a good chord, and bring it up on Variac measuring the transformer output voltages at the same time (start from the rectifier voltages).
(before you do it check the chord and the cap for shorts - though the Variac should help with short without creatin any much smoke).

When bringing it up if you see the voltage rising unimpeded (it does not struggle to get to the value listed in the manual, say at 20V it should be the (manual value/110V)*20V - then bring the Variac all the way to 110V, see if it is where it should be, then check the other voltages (I think there are just two - filaments for RF and filament for the output tube), and should they all be OK, proceed with recapping.

Pay attention to the power switch - it could be oxidized after that long and it is not easily disassembled, but in my experience it self-heals once you apply real voltage to the chord and turn it of an on several times (not too fast).

If not - start thinking if you want to get the replacement transformer or maybe a new chassis.

Then proceed with recapping.

[TV MAN]

After some initial checks (not powered), here are some findings: The RF coil(#6) seems to have an open primary winding, like Morzh's. So far, all the other (IF) coils check ok. The large wire wound resistor checks out ok, as do the power trans HV & primary. I havent pulled the tubes yet to check the fil. windings. The choke also checks out. So far, aside from tubes being replaced, the only repairs I have seen have been a cap replaced on one block, and looks like the volume control has been serviced or replaced.

Mike, my resistor has the 2nd tap going to ground like yours, so we have the same (early, I believe) version. I have never wound a coil before. It sounds simple, but any tips would help greatly. Do you know what gauge wire to use (and where to purchase), and the number of turns - in case I mis-count? I havent removed the coil yet, but it looks like the lower and smaller winding.

I think I read somewhere that the lower value (Pf) mica caps are ok and wont need replaced. Also, those large metal rectangular cans under the chassis, a .5 and .25 (also seem labeled wrong on the websites sch.) are just common caps? Anything I should know about restuffing these? Before I order the caps, are there any reccomendations? I read not to go too big on the power supply elect's, they are both 6mf, so about the closest I have found is an 8mf, double, or a 10mf single. The paper caps I am planning to order those little yellow tubulars from AES. I certainly understand replacing those .015 line caps before I power up - seen those on other radios pop! Just curious, what purpose will the 100k or 1meg resistor on the filter serve?

[hsusmann]

I have what looks like the same radio right now on my bench. It is the early 70 model (24A tubes, no AVC). I am having a problem with my tuning knob in that it slips at various points on the dial. The knob will turn but it slips where it rotates against the big metal disc attached to the tuning capacitor shaft.

I was wondering if yours does this also.

I have not figured out a way yet to keep mine from slipping.

Do others with this radio have this problem?

Mine came to me all re-capped already. I just had to replace the tuning capacitor rubber suspension grommets and deal with this slipping issue.

Herb S.
Ithaca NY

[gregb]

Tim:
You should consider a 6.8uf poly cap for your power supply caps. They are close in value, they should fit in your existing cans, they will far outlast an electrolytic, and they arn't polarity sensitive.

Gregb

[morzh]

Herb

Yes my 90 in fact has the same exact dial problem (in one spot) and the 70 (which has the same exact system to turn the dial) also seems to suffer from it.
I think first thing to do is to really try to remove any grime and grease from that groove and clean the metal edge of teh dial


Tim

1. Rewinding the coil is simple - it is a one-layer coil. Remove the old winding carefully counting the turns (I think it was 75 turns). Use the wire of a similar gauge, preferably the same as the larger might not simply fit - mine did, I happened to have some.
The wire is secured by wrapping around a small tab in the plastic film underlying the coil; when removing the first and the last turns notice how it is secured.

Also notice the winding direction and what lead attaches to what wire (top or bottom).
I simply drew the picture.

I started rewinding from the top (the furthest from the bottom rim of the coil, the one where the metal bracket is attached to) - I wrapped one turn around that smal tab to keep wire from sliding and wound 75 turn, then simply threaded it through the eyelet at the bottom for the repective lead where it will solder and then wrapped it around the lead a few times.

Then I used fabric glue (you could use whatever - candlewax or what have you, as long as it does not damage the enamel) to fi the winding.

Then Iouted the top wire on top the winding to its respective lead through the eyelet and wrapped it. I kept it taught and glued it.

Then I soldered the wires.

This is it.

2. DOn't change mica caps. If you need to, it will be obvious during startup and troubleshooting/alignment.

3. I used 2 12uF electrolytics of high ripple current in series to make up 6uF.

4. Rectangular caps are regular caps in cans. When you open them and discard the tar (they open easy, you do not have to heat it or pry it) you will see that they are wrapped in some special cardboard insulator and a bare wire connects them to the case.
I discard the lead wires, solder the bare wire to one end of the cap (or caps if more than one), solder the lead wires, thread them via the front piece of cardboard with the holes for the wires, simply wrap the cardboard around the caps and without doing much more feed them back to the can and then bend the tabs back to hopld the front cardboard.
Then I take my meter and measure the capacitance between the can and the wires making sure everything is copacetic, and then solder it back.

[TV MAN]

Thanks for the info. They have a spool of 32 gauge wire from AES that might do the job. I am guessing finer is better. Without seeing it it is hard to compare. May get the 28 gauge also and try to match.

Herb, my dial doesnt slip any, but it looks like it has some kind of glue or something put on to help the slipping and the shaft bracket looks like it has been bent down towards the dial. I thought originally this was shipping damage. I did discover that when I tune up, the dial hits the chassis. I'll have to address this. I need to pull the tuning cap anyway to clean and replace rubber mounts.

[morzh]

Tim

In 1 minute I will place the photos of the rewound coil in my 70 topic, so take a look.

[klondike98]

Tim, here's some info from Chuck's site that might be useful: http://www.philcorepairbench.com/tips/svctip36.htm

[TV MAN]

Just the info I needed - Thanks Bob for the link, and Thanks Mike for the pic. Now I know what to to on this step of the rebuild.

[klondike98]

Bob Andersen has done a youtube video (one of many) that includes rewinding a coil. About 2min 20sec into this he starts talking about it. http://www.youtube.com/watch?v=RRYr8pY_ViY

[morzh]

Tim, found a discrepancy (a non-functional one) in teh sch; look in my thread, you might have the same. Though you might also not.

[morzh]

One more thing:

Your electrolytics look like the one I had in my 90, probably an original, with this thick aluminum rod and backelite insulator.

In case you have trouble extracting that rod and/or broken insulator, in my 90 topic I had rebuild described for that cap, could be useful for you - I used a Home depot / Lowes part for the insulator which works quite nicely in the setup.

[TV MAN]

The adventure has begun. The parts have arrived and I have started the recap part of the rebuild. So far, I have rebuilt the line filter (47) and (34) of the bakelites. morzh's trick of using a heat gun to warm and push the guts out with a small allen wrench works good. I got the new caps in and those bakelites back in. I checked the resistors around the #34 bakelite. The #32 & 35 resistors show they should be 250k. They both check about 400k (.39 & .41 meg on my meter), and the #35 to the speaker plug checks about 55k. Neither look like they have been hot, but looks like the value is off quite a bit. Seems I read somewhere these old resistors creep up in value. What do you all think? Replace these? If so, I dont know if this type of resistor is still made, or if I will have to use modern replacements.

Another note, I ordered the yellow tubular caps from AES, which so far, fit into the bakelite blocks, but the larger ones for those metal cans under the chassis are too large around to fit, so I need some flat poly caps. I have some, but they are only rated at 250v, which I feel is marginal. This might save someone else from making the same mistake I did.

[morzh]

Tim

1. The resistor of yore will very likely creep up in value up to 100 or 150%. Ageing.
2. I have ordered tubulars before which fit fine, or those brown-red dipped type which I used this time. See the datasheet every time if in doubt.