Surplus store score! Collins 26c and 2x Gilfillan Bros/Bendix filters pics

[emrr]

All the values over 6 mfd are electrolytic.  Check yer email, if you haven't spotted something.  

$10 = $151 in 2009.   Things get cheaper all the time.  

I put an A/R panel on an L bracket inside the door, using the existing screws.   You will probably find the concept of hard bypass to be a little useless, given the gain structure and sonic variance.   At least in regards to evaluation of limiting.  

 

[plumsolly]

I have been looking around for power transformers, and I have found quite a few that have everything except a center tap for the filament voltage. The fellow at http://www.oldradioparts.com/ (which is a really cool site) suggested: "for 6.3 vct use two 22 ohms resistors and tie center point to ground. Actually much better way to get an exact CT, like the old 'hum" adjustments seen on some of the older amps." Will this work for my application? For this one: http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=320507625748&ssPageName=STRK:MEWAX:IT
, is tying pins 3 and 6 together the same as a center tap?
Thanks again for all your help, Ben

 

[emrr]

Yes, for instance, Mouser carries 2W (I think) wirewound 100 ohm pots for $2-$3 which are perfect for artificial filament CT's. 

 

[plumsolly]

Ok, It looks like I've found a power transformer that will work. I've also found a replacement PIO cap; it is 4mfd but, since the 26c-2 used 4mfd I should be ok, no? I am thinking now about attack and release controls. The manual states: "The time required for the gain reduction to become effective upon the application of a large input signal is determined by the charging time of condenser C9. This has been chosen so that the operate time is very short, or of the order of 2 milliseconds." C9 is .25mfd. I am wondering how to calculate attack time with different values for C9. The manual goes on to state that, "The time required for the gain to return to normal after the input signal has been reduced is determined by the discharge time of C9 through resistor R24. With the circuit constants shown, this is approximately 3 seconds." R24 is 10 Megs. It seems like the release time is roughly the rc constant of C9 and R24. It looks like the release time depends on the attack time which wouldn't bother me except that as I slow up the attack, the fastest release I can have gets slower and slower, and sometimes I like to set the attack slowish and the release really fast. I thought a potential solution would be to switch in an appropriate compensation resistor with each step in the attack in order to keep the release constant, but the value of that resistor would change depending on where the release was set, so scratch that idea. I guess the solution would be to make range of the release control large enough so that when I set the attack slow, I can still have a fast release.
Best, Ben

 

[emrr]

There is no resistor specifically dictating attack time; it's set for fastest possible.  You insert a rheostat to decrease attack time beyond the stock fastest value.  Calculation is nothing but academic here; stick various resistances inline until you find the one you like best as slowest value, then divide as you see fit.  I think I used (11) 360K steps, but I don't remember.  I probably staggered the curve as seemed useful by ear.    Release; find a 10M pot and wire it as a rheostat, or subdivide as you see fit.  Again, by ear.  This thing ain't precision rocket science requiring specific calculation; it's much more vague in operation than that.  I don't see any real point in altering C value for the same reasons.  A decade resistance box is your friend here. 

On the PIO cap, note that there's already a spare present; used for preamp B+.  I haven't even looked to see which one you clipped.  If you clipped that one, you don't need it anyway. 

 

[plumsolly]

You insert a rheostat to decrease attack time beyond the stock fastest value.

Oh I see... just in series with C9? That makes sense - I thought I had to switch caps. I will probably just use a rheostat... I don't need the steps.

On the PIO cap, note that there's already a spare present; used for preamp B+.  I haven't even looked to see which one you clipped.  If you clipped that one, you don't need it anyway. 

Oh, damn... That's convenient in my case. 

Thanks again emrr. Best, Ben

 

[emrr]

Don't get ahead of yourself here, thinking about mods before you've heard the thing.  You may hate it.  Get some de-oxit and fader lube to clean and restore the attenuators, the metering switches, the external connections, the tube sockets, etc.  Think long and hard about the proper way to do these things, in the cleanest and least damaging manner.  Replace the (already replaced once) electrolytics on the back.  Check capacitance on the remaining original coupling caps.  When you fire it up (with a 600-1000 ohm load resistor strapped permanently across the output terminals), put it in bypass to listen to the amp stage without the compression stage, to be sure it is working correctly as an amp.  Use a variac on power up slowly to minimize potential problems, while listening.  Let it live at 60VAC input for awhile, then 90VAC, and listen the whole time.  If it gets worse at any point when raising AC input, turn it off immediately and figure out which (probably coupling) cap is leaking DC.  Then listen to the compression switched in.  Figure out the input and output padding and basic gain staging, and how to run it without blowing up the meter.  Make sure all the current metering switches are working; this means even when you aren't pushing them - if they are flakey the tube cathodes won't connect and you won't get sound.    Confirm correct polarity through the unit; compare bypass and non-bypass to see if it's like the others I've seen (different for each setting).  Figure out release options before attack options.  Study the parts layout and figure the absolute least change required to achieve any mods.  Read this 48  more times before proceeding.    ;D

 

[plumsolly]

That all sounds good emrr. Do I have to take the caps out of the circuit to test them? Thanks and best, Ben

 

[emrr]

No need, usually. 

Treat her like you just spent that 1939 inflation adjusted to 2010 $4600 that she cost. 

 

[plumsolly]

I started going through and cleaning the buttons and attenuators. Everything so far looks ok except that the meter source selector is flaky. I think the capacitance meter in my Fluke is badly out of calibration - so I'll need to figure something out to test those caps. Power transformer is on its way from nielsk. On a side note, it looks like this guy: http://www.broadcastdocuments.com/BROADCAST_DOCUMENTS/Collins.html has absconded with my money for the manual. Not a big deal as I now have 2 different electronic versions, but anyone thinking about dealing with him should be wary. Best, Ben

**EDIT** The issue with http://www.broadcastdocuments.com/BROADCAST_DOCUMENTS/Collins.html has been resolved; he refunded my money as well as sending me a pdf of manual and schematic.

 

[emrr]

broadcastdocuments is a member here (can't remember his ID), and I'm sure will surface and supply sooner or later.  If you have two variations, you probably already have his copy anyway.  I can't imagine there are 3 out there.  

Fluke is off when measuring a known new film cap like a 0.1 mfd?   Or when measuring 26C?

The bypass position on mine is flakey; good thing I don't use it bypassed!  It'd be a bitch to replace.  

 

[plumsolly]

broadcastdocuments is a member here (can't remember his ID), and I'm sure will surface and supply sooner or later.  

I hope so. I filed a resolution, or whatever in paypal.
**EDIT** The issue with http://www.broadcastdocuments.com/BROADCAST_DOCUMENTS/Collins.html has been resolved; he refunded my money as well as sending me a pdf of manual and schematic.

Fluke is off when measuring a known new film cap like a 0.1 mfd?   Or when measuring 26C?

Actually, scratch that. I just tested a couple of new caps and they tested right. I had had some trouble in the past, but I think that was do to user error.

I am thinking I may replace the meter source selector switch - it is flakey in the GR position, which is the one I'll be using I guess.  

Bets, Ben

 

[plumsolly]

So here are some capacitance measurements. These were all measured in circuit.
     Measured     Supposed to be
C2 (overload)        .03uf 
C6  1.18uf            .25uf
C7  2.21uf            .25uf
C8  .319uf            .05uf

My meter only reads up to 5uf, so I can't really measure all the 6uf caps. Let me know what you think of these values, whether they should be replaced or not.

Thanks so much again, Ben

 

[emrr]

The point of c2 is to give a high frequency boost, since it's a small value cathode bypass.  V2 current measurement will be seriously off if cap is shorted, if open you will simply lack the high boost.  

C6 value being extra large should be okay.  On power up, I would have the unit in bypass, and check c6/r13 junction for presence of DC.  I'd check it initially, and if it seems okay, again after 20-30 minutes to be sure leakage hasn't increased.    If it were me, I'd tie off the original leads somewhere and put in a new cap.  Probably choose a 0.33/400.  You ABSOLUTELY can't afford leakage possibly taking out the next transformer.  

C7, I'd tie off the old and put in a new, again 0.33/400.  You can't afford leakage possibly taking out the next transformer.  

C8 measures too high, and can potentially let too low a frequency affect the side chain, which can lead to pumping issues.  The other schematic shows it as 0.1 mfd.  For the same reason, you also shouldn't enlarge the value of c5 beyond 22 mfd.  

C1, c3, and c? (c3 companion) can all be made 100 mfd to improve low frequency linearity.  

new c9 with accurate value (0.22 okay; you will probably not ever keep it near 10M r24 value) may also improve limiting consistency.  

The 6mfd cans are 99% likely to be fine.  Just be sure none are shorted or have measurable resistance under say, 1M.   You really shouldn't see a value even that low, but you may, being in circuit.  

 

[plumsolly]

Thanks so much for the detailed explanation, emrr. I really appreciate you taking the time to do that. I guess It looks like I should be replacing all of them, with the exception of 6uf's. I already replaced the 20uf electrolytics, with the same value. Should I change C1, C3, and C4 to 100uf? Would you recommend film caps for the rest? Is it worth tracking down PIO caps? I measured the resistance of the 6uf's and they all measured 100k or so in circuit. I measured the one I had taken out, though, and it was OL.

Thanks again, Ben

 

[emrr]

I would assume OL means out of range, in this case, so no measurable resistance. 

Leave 1/3/4 alone if you've already done them.  Once it's up and running, you could try paralleling more capacitance and see if you like it's effect. 

The others have to be film caps, of course.  I wouldn't worry about PIO here; there's plenty of color already. 

The first restoration I did had all the original square cap cases in place, so I pried them all open and re-stuffed them all with new caps.  Totally original look.  Marked the re-cap date on the bottom of the cans, where it would be seen if removed, but not otherwise. 

 

[plumsolly]

I just took C7 out and measured it at .36uf - quite a bit closer! To be sure I put it back in circuit with alligator clips and got the ~2uf I got before. So I guess that one is good? Should I take out the others and measure them out of circuit? Should I be worried at all about where that parallel capacitance is coming from?

Thanks again, Ben

 

[emrr]

You shouldn't worry, that's pretty normal.  It's seeing parallel capacitance somewhere.  You should only need to lift one leg to get a correct reading. 

 

[plumsolly]

Ok here are the out of circuit readings. Looks much better. 
C2: .08uf
C6: .39uf
C7: .36uf
C8: .06uf

I went ahead and put C6, 7, & 8 back in, but I wanted to see what you thought about C2, as it's further off than the others. I'm getting closer to being able to power it up on the variac. I've still got to clean a few contacts, solder some spades, install a C9, and get an inline fuse, and then I'll be ready for the first try!

Thanks again for all your help, Ben

 

[emrr]

Looks fine, just be sure to monitor the critical points for DC leakage.