Adding AVC Mod to Philco 70 - Printable Version
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RE: Adding AVC Mod to Philco 70 - Ed Locker - 05-27-2019
WHEW !
Hopefully that's it for revisions!
RE: Adding AVC Mod to Philco 70 - Ron Ramirez - 05-27-2019
...indeed...
Looking through my stash of resistors, I was dismayed to find that - of course - I do not have most of the resistors (1W and above) needed for this project.
So another online order is forthcoming, this time from Mouser.
Ugh...
Oh well, I won't be able to do anything with this until next weekend anyway.
RE: Adding AVC Mod to Philco 70 - Ron Ramirez - 05-27-2019
Ed or Rob
Where did you find 850 ohm 5W resistors? It appears Mouser does not carry that value...Digi-Key doesn't either...It looks like I will have to order a 2K and a 1.5K, adding them in parallel to achieve close to 850 ohms (857.14, close enough).
They also do not stock 5.6K 5W resistors...but Digi-Key does...
RE: Adding AVC Mod to Philco 70 - rfeenstra - 05-27-2019
I couldn't find one either. Used 820 ohms.
RE: Adding AVC Mod to Philco 70 - Ron Ramirez - 05-27-2019
Thanks Rob. I'll try ordering again later.
RE: Adding AVC Mod to Philco 70 - Ed Locker - 05-27-2019
I ordered the 850 ohm 5 watt from Mark Oppat.
The other resistors I found at the place I worked.
Ed
RE: Adding AVC Mod to Philco 70 - Ron Ramirez - 05-27-2019
Order made with Mouser. Like Rob, I ordered 820 ohm instead of 850 ohm.
And I did find the 5.6K 5 watt resistors, and ordered a few. It's funny, I could not find any upon my initial search but when I first specified 5 watts THEN the resistance value, 5.6K came up as being in stock.
RE: Adding AVC Mod to Philco 70 - KCMike - 05-27-2019
Mouser search engine took me awhile to figure out, still have trouble finding what I need at times but overall I like it.
On the 70 mod. I just want to thank you Ed and Ron for taking time to make this mod. available and details of how and why it works.
RE: Adding AVC Mod to Philco 70 - Steve Davis - 05-28-2019
I have been watching this thread with great interest. I have been thinking about doing this mod for some time. It seems like there is always something more pressing. Ron, I know you did one of these in the past and was disappointed in the performance. That made me start thinking about going with a converter tube rather than separate oscillator and mixer tubes. This would free up a socket to add an extra stage. I had thought something like a 91 with 2.5 volt tubes And no P-P output.If this mod works well I will copy what you have done.
Steve
RE: Adding AVC Mod to Philco 70 - Ron Ramirez - 05-28-2019
Steve
The circuit I tried in the past was simpler than Ed's mod...and performed poorly. Ed's mod seems to be a much better solution. Rob Feenstra has already implemented Ed's mod on his 570 and he is pleased with it, so I'm looking forward to trying it for myself.
RE: Adding AVC Mod to Philco 70 - Ed Locker - 05-28-2019
Yes, it's a real PITA and a lot of work, but you'll be pleased in the end.
RE: Adding AVC Mod to Philco 70 - rfeenstra - 05-28-2019
It took a little time but wasn't really that hard/bad! I managed the coil mods without removing them. I left no longer needed parts in the chassis in case someone wants to return it to original in the future. I will probably not modify my 70 cathedral as it is more original than the 570 and don't really care if I listen to it or not. Just so it works!
RE: Adding AVC Mod to Philco 70 - Steve Davis - 05-28-2019
I have two or three early 70 chassis with no cabinets so if I modify one it will go in a reproduction cabinet so originality is not very important to me.
Steve
RE: Adding AVC Mod to Philco 70 - Ron Ramirez - 06-01-2019
With needed parts now in hand from Sal Brisindi and Mouser, the work on the 70 now resumes.
First of all, I need to point out an error in the under-chassis drawings in Service Bulletins 57 and 57-A.
In these drawings, the 13K, 1 watt resistor (16) is shown as connecting from the plate of the 27 oscillator tube to the end of the 5K resistor (14) which also connects to the cathode of the 27 oscillator tube. This is incorrect. The correct hookup is shown in the revised drawing, made by me, from Bulletin 57 as shown below.
Please note that the higher numbered parts in Bulletin 57 do not match up with the information in Bulletin 57-A (which our modified schematic is based on). However, resistors (14) and (16) do match up.
I replaced bakelite block (12) with a newer "universal" type block, as I needed a ground lug to attach the grounded end of the added 270 ohm resistor from the front end cathodes. I kept bakelite block (5) stock. Both have been restuffed with new capacitors.
I've rewired the cathode circuit of the front end tubes to match Ed's revision.
Here's this evening's progress.
RE: Adding AVC Mod to Philco 70 - Ron Ramirez - 06-01-2019
Question for you, Ed L. -
Your revised schematic has one section of capacitor (5) and one section of capacitor (12) in parallel. This effectively makes a 0.2 uF bypass capacitor between the new cathode circuits and ground.
Just curious - is 0.2 uF necessary, or would 0.1 uF be sufficient?