On the NT805, make sure the base voltage is correct. That will affect the emitter voltage. The .2 volt difference should remain constant. If the base voltage is not right, check the bias divider resistors.

Philco's transistor expertise was the invention of the Surface Barrier Transistor. That provided high gain, high frequency and low voltage operation. They proved best for small signal applications and were used by leading manufacturers of high frequency communication equipment.

Another advance was the high speed manufacturing process (FAST line) enabling high yields at low cost. That led to Philco's mainframe computer business. The early contracts were with the US Navy in 1957 and later with the Army for battlefield computers. Ford absorbed the military computer business into Ford Aeroneutronics and soon exited the commercial computer market. The transistor manufacturing business was sold to Sprague.

The transistor technology preceded the computer business, however it was a natural follow on.

Don

Thanks, Don, for the Philco transistor info---it's been years since I've read anything on them and appreciate your reply & thanks also for the factory Philco document scans & tips... The scan quality was excellent---can actually read all detail clearly !

Mescalero, thought about the AVC circuit, but haven't looked into it too deeply yet... Probably be awhile until I get back to radio since there are other plans this weekend... I injected IF signal directly into tuning cap (with small series cap ) before I had correct alignment data and tone was loud---but when I tried "passive" coupling with loop of wire on generator output as recommended, couldn't even receive it... The LO signal on scope didn't seem right, either---amplitude at certain area of tuning seemed way to high, but need to check my work next time as I was really getting frustrated at that point...
Hmmm...

Expect the oscillator level to vary greatly across the band. There is nothing to regulate it and it tunes over a broad ratio, 3:1. You should be able to see the RF at the base and collector of the RF amp, obviously at much lower levels than the converter output. Tune across for a strong station. If you disable the oscillator, you should also see RF at the converter base as well.

Don't overlook the first IF transistor. Make sure it has some emitter voltage and it is ~0.2 Volts less negative than the base. The AVC won't change anything very much in a working radio; tenths of a volt at most.

Don

Thanks Don,

Pretty sure only the RF amp & 2nd IF transistors showed deviation from service data emitter voltages as mentioned earlier & their respective resistors were okay---tried two replacements for RF amp & one for 2nd IF with same voltage readings & performance...
Sure has me baffled...

Probably won't be able to get back to it for awhile, but sure am comtemplating what the failure could be---just hope I don't have an internal IFT cap bad like I've had in a couple other radios... Will continue to dig deeper until fault is found & appreciate all suggestions...

John

Hello All,

Made some progress on the NT-808 this weekend...

Found the source for the intermittent voltage drops---while moving PCB with power applied during troubleshooting, it happened a couple times without turning off & on. Then noticed when I flexed negative lead from battery contact, it happened again---felt wire and thought it seemed a little "lumpy", so pulled on both ends and it came apart full of corroded strands... Even though radio had a lot of battery leakage residue when I got it, no evidence was seen of it being near the electronics or wiring---guess the "fumes" attacked this wire as well as other corroded connections I found...

After checking all IF stages including detector diode, just couldn't find reason for such low output ( audio itself tested fine except volume control---see following paragraph ), so even though it seemed to pass sig. gen output and show gain each stage, decided to remove wax from IFT's and try tweaking---1st & 2nd were nearly dead-on, but 3rd showed large increase when "bottomed out". Retweaked all coils on weak, lower end station by ear & reception seemed close to normal now. Wonder if the fumes from battery leakage attacked the the 3rd IFT ? Resistance measurements on windings okay, but what about the internal cap ?

Now local stations were very loud & volume control had little effect... Cleaned & lubed with Deoxit/Faderlube with no change. Measuring pot showed that element contacts must have been affected by fumes as resistance was much higher than it should have been & wiper measurements at both "stops" were also high... Did see the contacts were
nearly black from corrosion, so control is shot... Now a mission to find good one begins !

Put radio back together until I can find the control & took one more pic... With all the chrome, it just doesn't photograph too well...

Hey John - what's that pot look like? Post a pic when you can.
I'll check the junkbox.

~ Mitch ~

Hey Mitch,

Sure wish I knew pot was going to be bad after cleaning/lube---would have got good pics !

It's a 5K w/switch standard sized chassis mount with split, knurled aluminum shaft about 5/8" long... Shouldn't be too hard to find since only the shaft type/size is critical because of knob design...

Am listening to favorite station WSM 650 tonight---but when signal becomes strong, volume becomes so loud with bad pot that I have to turn radio around to lessen it...
Don't think AVC is working very well, either...

Thanks much for your offer & let me know if you have anything that sounds promising---am going to search on-line first since it seems to be a fairly common part...

Will let you know if I find anything before you spend any time "digging"...

Thanks again...

John

Of course I did some digging... mostly to see if my memory is still functioning!
I have one or two, but they're missing the on/off switch you would normally need.
And you're right, that should be a fairly common part.

BTW, this video of a GE P780 was posted within the past week:
http://www.youtube.com/watch?v=d2RrFBAT2bs

~ Mitch ~

Hey Mitch,

Thought I'd found one at Mouser, but shaft diameter too large @ 7 mm... But I have found some 6 mm split knurled shaft pots, but not right type or with switch... Have to get out calipers & mm scale to check mine accurately...

And I also need to search for a 3rd IFT, have Philco P/N but radio is Toshiba made...

Thanks for link, but I have dial-up here---usually look at YouTube "stuff" while at my sister's with her hi-speed connection...

Did you see post in Clubhouse with that 1955 Chrylser commercial link ? Really neat as it discusses the "new" all transistor car radio using Philco's surface-barrier transistor...

Thanks for looking !

John

Got the new volume pot today ! They had them in stock at Mouser though obsolete...

Bourns 5K, 7 mm threaded bushing, 6 mm split-knurled shaft, with switch...
Mouser P/N: 652-PDA17-SS20-502AK
Price: $1.56

These were only ones of that type I could find and would be great for other transistor projects... Posted above info in case anyone interested...

Hope to get it installed this weekend & "play" more with 3rd IFT...

Damn John... you always seem to have problem with pots, eh?

~ Mitch ~

Well, Mitch, fortunately not that often---just a couple in recent history...

Got new control installed today---had to install a thin "slice" of nylon tubing to center the smaller shaft bushing in chassis & use split lockwasher under nut to hold steady---didn't want to take chance with drilling new locating tab hole...

Shaft length & size the same, so all looks & works well...

Another problem fixed, now to find a new 3rd IF transformer ( 10C1 20891 )...

But at least radio works somewhat now with adjustable volume...

Here are pics of new ctl installed & new/old side-by-side:

What's happening with the 3rd IF xformer?
Is it a standard size as found in normal pocket radios?
Surely you have a junk board floating around, right?

~ Mitch ~

P.S. - Nice job on the replacement. Looks good.

Not sure, but think that the battery leakage fumes may have attacked it like they did volume ctl... After digging out the "wax", found that all adjustments up to 3rd IF were either very close or "right on"---had to "bottom" out 3rd IFT core to get usable signal for detector, but still not right... If you compare schematics, you'll notice that there are differences in these transformers and not all are interchangeable... Really would like to find exact replacement... I did replace the transistor with an unused one in my parts cabinet & performance the same... Other components in circuit appear okay...
Since this radio was made by Toshiba, other sets of theirs may use it...

And thanks...