[MCC]

I picked up a vintage 1946 Warwick C103A AM radio the other day at a garage sale and I've been evaluating what needs to be done. Most of the resistors have drifted substantially (from 3% to 300%!) and it needs to be recapped (electrolytics with Nichicons, Sprague Black Beauties and Micamold paper in oil with orange drops, micas with new silver mica). I'm also going to fuse the primary and one side of rectifier winding as well as install a CL-90 at the line switch to protect the heaters at the very least. The radio is rated for 53W @ 117VAC- what values of fuses are recommended? Would a 0.63A slow blow on the primary be too aggressive?
 
On to the main reason why we're here. Before I put in a $30 part order I'd like to check if the transformer is OK. I haven't desoldered it but there are no shorts between primary and secondary or primary and ground. I'm concerned by the winding shown with the red dots though:
 
 

 
Is 413Ω a concerning value? Pins 6 and 4 of the 5Y3GT each share a roughly equal split of the resistance to ground as they should.
 
I also plan on grounding the chassis but I've read that this can cause some problems with badly insulated transformers. Should I be concerned about that?
 
Edit: The full schematic for the 103 model can be downloaded here. My C103A has some subtle differences (R2/R6 missing, 100Ω resistor not on schematic on the 6SK7, different capacitance values).

 

[MCC]

Bump. Any opinions?

Although $30 is really insignificant at this point since I just invested in an HP 3312A function generator w/ modulator, Rigol DS1052E scope, and a Precision 612 emission tube tester. The tube tester has a 113 page searchable PDF database of settings available which is absolutely amazing. $65 shipped and accurate against a calibrated military meter isn't bad either on today's market.
 
I also found an isolation transformer for $15 at the ISU surplus sale so that I don't have to keep taking apart my TOPAZ to lift the ground/neutral bond when I work on equipment.

 

[digger945]

I'm not a tube guru, ok, but your dcr reading are all well within "normal", as compared to other, newer, transformers I have checked.

 

[MCC]

Alright, I've ordered the parts. This should hopefully be a cakewalk compared to what I did to my MAC 1500.

 

[WarriorAnt]

Quote:

Alright, I've ordered the parts. This should hopefully be a cakewalk compared to what I did to my MAC 1500.

what did you do to the MAC 1500?  I have to know!

 

[MCC]

Quote:

what did you do to the MAC 1500?  I have to know!

Complete overhaul. New FP style filter caps (CE Manufacturing), modern soft recovery rectifiers, 6N copper signal wiring, gold plated RCA jacks, PRP resistors (made in Iowa!), and various boutique capacitors that I now recognize to be completely overkill. I haven't touched the FM section yet- that comes after I finish with the Warwick. I'll likely opt to replace out-of-spec parts only instead of shotgunning it.
 
It still sounds spectacular. It's the best amp I've tried with the Senn HD-595 but my ATH-AD2000s sound better on my Sheer Audio Dynalo (HA-006+). I did center my component selection around the Senns so that's a factor.
 
With speakers- WOW. It decimates my Harman/Kardon and edges out dad's Fisher KX-100 by a fair margin. He has Magnepan MMGs though. 

 
Thread HERE

 

[WarriorAnt]

Nice!  
 
Got any links for purchasing the replacement components that you'd recommend? 

 

[MCC]

These are some sources that I've used in the past-
 
For the FP style "can" caps and much more: http://www.tubesandmore.com/
Boutique resistors, capacitors, etc: http://www.partsconnexion.com/
"    ": http://www.soniccraft.com/
Miscellaneous (entire parts order for the Warwick radio was placed here): http://www.mouser.com/
 
I've had stellar service from all of the above and highly recommend them.

 

[WarriorAnt]

Quote:

These are some sources that I've used in the past-
 
For the FP style "can" caps and much more: http://www.tubesandmore.com/
Boutique resistors, capacitors, etc: http://www.partsconnexion.com/
"    ": http://www.soniccraft.com/
Miscellaneous (entire parts order for the Warwick radio was placed here): http://www.mouser.com/
 
I've had stellar service from all of the above and highly recommend them.

Thanks, I'm just getting started and the links are helpful.
 
 

 

[WarriorAnt]

Not wanting to derail this thread I was just wondering if you had a few more links for test equipment you might suggest.  Thanks.
 

 

[MCC]

I have very limited experience with test equipment so I can't really give any specific recommendations. I'm just a student at the moment- a more senior individual can provide better advice. Here's a breakdown of what I have:
 
*Extech EX330 multimeter
     -Has extra features like a capacitance meter, frequency counter, temperature sensor, and a diode checker function.
 
*HP 3312A function generator
     -Basic sine, triangle, and square waves with symmetry adjustments and AM/FM modulation as well as sweep capabilities. It only goes up to about 13MHz so it can't be used for broadcast FM radio.
 
*Rigol DS1052E oscilloscope
     -50MHz scope, easily unlocked to the 100MHz model. Brand name parts throughout (even the buzzer is TDK!) but the Analog Devices ADCs are overclocked from 40MHz to 100MHz so that they can get 1GSa/sec. I don't have this yet but it should arrive very soon.
 
*Precision 612 tube tester
     -Tests for shorts and shows how well electrons can flow from the filament to everything else. Transconductance models do a more "real-world" test but cost much more. I also don't have this unit yet but it's in the mail.
 
*Variac model SC-3M
     -Allows me to vary the AC line voltage. I use it to bring up items under test to verify that the voltages are proportionally correct before applying full wall voltage. I also use it to slowly bring up my MAC 1500 to extend the life of the tubes as I had issues with the standard route. Usually you can install a GE CL-90 NTC thermristor in series with the AC line to "soften" the turn-on jolt. It ran so hot in the cramped power section of my MAC 1500 that it melted the solder... very bad. Use only if you have breathing room for it.
 
*375VA isolation transformer
     -Makes operating on live equipment a little safer. See this post on Hack a Day for more info. Operating on live equipment is never 100% safe though so be sure that you're taking proper precautions. I always wear shoes, remove my watch and sit on one of my hands when I'm probing a live circuit.
 
*Hakko 936 soldering station
     -High quality pencil iron with a temperature control. Much more of a pleasure to use than my old Radio Shack model. Well worth the $.
 
*Weller 8200 N 140W soldering gun
     -The Hakko is really only suitable for PCB work. Everything else, like the radio I'm restoring, needs a more powerful tool. My Weller heats up in seconds and can solder to even the largest of metal surfaces if enough flux is used.
 
*Radio shack desoldering iron
     -Great for removing huge blobs of solder, unreliable for removing enough to remove a component.
 
*Techspray #3 solder wick
     -Sucks up solder very well. Works much better than the stuff you can get at Radio Shack. 
 
*Fan
     -Solder fumes aren't the best for your health. I always work near an open window with a fan blowing outside.
 
I hope this a) is helpful and b) doesn't scare you off! I acquired all of the above over a 4 year period. I was able to restore the MAC 1500 with only the multimeter, variac, soldering gun, and desoldering iron. Since I didn't have an isolation transformer I checked voltages by attaching an alligator clip to the point of interest while off and then powering it back on.
 
Edit: a word of advice while we're here. If you ever see a large metal FP style capacitor and you're not 100% sure that it's perfectly operational (current leakage especially) REPLACE IT. Don't power the unit up. They can explode like a hand grenade and send shrapnel flying everywhere. I myself replaced all three such capacitors on my MAC 1500 with brand new (not NOS) units to avoid that possibility.

 

[WarriorAnt]

Thanks again!  Very helpful, and thanks for the metal FP style capacitor tip!

 

[MCC]

I wish I could find it but there was a video on YouTube by a professional that had an FP cap explode while he was working on an amplifier. He wasn't injured but there were paper and metal bits scattered throughout most of the room and embedded in the ceiling. I don't remember specifically but he must have been out of the room to have made it through that OK.
 
The chance of such a catastrophic failure probably isn't all that high since those caps usually have a vent underneath. Yet the possibility exists- don't risk it!

 

[WarriorAnt]

I'm afar id to ask what else can happen!