GroupDIY Bridge Compressor.

[DaveP]

The 26W schematic showed modifications from the original GR tube which was a 6N7.

The bridge resistors were originally 22k for the 6N7 tube (in the unaltered parts list)  These were changed to 6.8k with the 5814 tube.

Have these resistors R119/120 been changed to suit the 5965 tube?

DaveP

 

[emrr]

Beat me to it...R-119/120 are 18K.

Interesting the release timing resistor string is different between the two. 

 

[abbey road d enfer]

I just hopped the bus today on this project, read from the beginning.
It seems to me like a major issue has not been solved, i.e; the concern expressed by Jonte Knif regarding CV feedthrough in the first page of this thread.
According to the schemo in post #54, applying a fast-changing CV to the grid of the attenuator tube will produce an equally fast drop in the plate voltage, that would be transmitted to the grid of the 3rd triode, resulting in a huge thump at the differential output.
I haven't be capable of asserting that this schemo is only half of a symmetrical path, but even in that case I doubt CV feedthrough would be completely eliminated.

 

[DaveP]

Abbey you are six years late!

I think that Jonte was probably right, but that design was discarded in favour of a 26C, it was finally sold this year and a donation made to the forum.

DaveP

 

[abbey road d enfer]

Abbey you are six years late!

Yes, I know, but it's been unearthed yesterday.

 

[emrr]

After looking at all 26W mentions in threads, it was the best place to drop the later tube info. 

Looking back at reply #26, I can add the actual 5965 unit before me has an original 0.5mfd in C109 position, like the drawings, though the parts lists say 0.25mfd. 

Release times are listed as 0.1 / 0.5 / 1 / 2.5 / 5 seconds, but the two shortest are not wired in stock, thought they could be connected on the switch. 

The 6N7 drawing shows this R string for release times:  390K / 1M5 / 2M2 / 5M6 / 10M
The 6N7 and 5814A parts lists both give the above values. 
The 5814A drawing shows this R string for release times:  390K / 1M5 / 3M3 / 5M6
This 5965 version in front of me has 390K / 1M5 / 2M2 that are original, and someone has changed the last two positions to very long release times.

Trying to be clearer about R-119/120 values:
6N7 and 5814A parts lists both say 22K.
6N7 drawing shows 18K, and that's what's in this 5965 version.
5814A drawing shows 6K8.

Also standing out, R158/159 aren't on the parts list but are on the drawing, loading the secondary halves of the interstage transformer.  Drawing says 100K, unit in front of me has original 150K.  Full secondary winding is 40K spec. 

Date codes on the caps with this 5965 version are 3/56, so final version near the end. 


FWIW.....definitely some crossed signals in the literature department at Collins on this one. 

 

[DaveP]

As you say, the literature dept. were not too good at keeping the manual up to date with revisions!

You can get useful info from odd part numbers appearing like R158/R159.  Maybe there was a change of interstage which required those additions, or after complaints from customers or even customer mods that fed back to the company, we will never know.  We can surmise that these additions improved the design, maybe the frequency response perhaps?

They obviously found it necessary to change the bridge tubes several times and the capacitor value for the time constant, whether these changes were to cure problems or make improvements I can't be sure

Thanks for reporting

DaveP

 

[DaveP]

Date codes on the caps with this 5965 version are 3/56, so final version near the end.

The manual I have is dated 19Dec1957 and this had the 5814 mod.

While we are on the subject, I made a list of the changes and the V103 6N7 originally had 47k plate resistors, not sure why they would change down to 22K,  The voltages in the manual refer to these older parts I think.

As we are on the drawing board,  it would be possible to reconfigure the bridge resistors so that they null at the cut-off on the tube load-line instead of at 0V.  The tube could be quiescent at say 1V and then null when its rp reached say 20-40k , (depending on chosen tube) It would not then be necessary to have a separate -ve supply.

DaveP

 

[emrr]

The manual I have is dated 19Dec1957 and this had the 5814 mod.

While we are on the subject, I made a list of the changes and the V103 6N7 originally had 47k plate resistors, not sure why they would change down to 22K,  The voltages in the manual refer to these older parts I think.

I see that now, on the addendum.  The 26U replaced it in July 1958. 

Found another manual problem:  figure 2-6 for the metering circuits is wrong, one of pairs of wires to the test switch is drawn reversed from where it should be. 

 

[emrr]

What the heck, this is the best existing place to put this:

Note the meter circuit attachment below is corrected.

complete reference schematic:
https://groupdiy.com/index.php?topic=60313.msg903696#msg903696

At a loss troubleshooting the mA meter circuit on a 26W.

The unit is confirmed as compressing 15-20dB based on finding threshold based on A/B switching and then turning input up 15-20dB with virtually no increase in peak level or distortion, and diminishing dynamic range.....and you can hear it, and any time constant position changes. 

The only reading it shows is the B+ position.
I have seen the Gain Reduction position intermittently show something very briefly, then disappear. 
No other cathode current measurements have functioned.

The meter works fine out of circuit.  I can pass current through it from a battery and a decade resistor and go smoothly from 0 to full scale. 

I get correct mA readings from the test points using an external meter. 

Ground reference shows continuity from all relevant points to all relevant points, using both R-149/C-111 as principal ground point and both ground lugs (checked individually) next to audio input and output. 

All switch positions show 0Ω continuity where they should, both the rotary switch and the Normal/Test switch. 

I can't find a short anywhere, nor am I identifying one that makes sense for the conditions described. 

If the existing meter is disconnected from the switch circuit and then directly connected with jumpers to the relevant test points, the result is the same, no reading anywhere other than B+.   

At this point I've lifted dang near every path one at a time and re-checked with test leads with no change. 
The only thing I've not done is replace the 2P6T rotary switch, because it measures fine and because I haven't found another yet with a bushing thick enough to provide mounting threads through the thick 3/16" panel. 

What big obvious am I missing? 

 

[scott2000]

I can't help but figured I'd put this section of the manual in in case it has something..... I'm sure you already have read it a million times but can't hurt .....not sure it's even relevant to this version...??


edit////just saw your nice drawing.........at least the text makes sense to me now....lol

Good luck!

 

[abbey road d enfer]

At a loss troubleshooting the mA meter circuit on a 26W.

What big obvious am I missing?

Indeed it's puzzling.
I would suggest you disconnect the meter and check the readings with a standard multimeter on its low-volt setting.

 

[DaveP]

From the symptoms it looks like an earthing problem (I know you've checked that)

There is a resistance there, that is too high for the low voltages to overcome, only the B+ can do it.

How are the various chassis parts earthed?  After 60 years there is bound to be some oxidation and nicotine!

Bonne Chance!

DaveP

 

[emrr]

I would suggest you disconnect the meter and check the readings with a standard multimeter on its low-volt setting.

I think I've done this; I get correct voltage and current readings in all positions with an external auto-ranging multimeter. Both directly across the resistors to their immediate ground points and also using several different 'master' ground points in different locations. 

From the symptoms it looks like an earthing problem (I know you've checked that)

There is a resistance there, that is too high for the low voltages to overcome, only the B+ can do it.

How are the various chassis parts earthed?  After 60 years there is bound to be some oxidation and nicotine!

I agree, and it's not shown itself yet.    The immediate resistor ground points are on terminal board, and all tied together with wire....and all those read 0Ω to any of the master ground points.  I will revisit their solder joints again, I know I've touched some of them but possibly not all.  I did find a number of joints that NEVER had solder, working fine only because of the expert tightly wrapped leads.

I'm very suspicious of the rotary switch, and may sub in another to see, once I find a suitable candidate.  It's possible when I have the pressure of a meter probe on the terminals that I'm forcing a better connection than normally exists. 

I was suspecting a breakthrough issue with the meter itself, and I suppose that could still be it.  I daisy chained 9V batteries together to create a 36V source, and with a multimeter in path measured roughly 1mA at full scale reading with roughly 26K6 resistance in series.  The particular decade resistor topped out at 100K with the meter showing roughly 20% deflection.  What I did NOT try was a shunt resistance. 

 

[abbey road d enfer]

I think I've done this; I get correct voltage and current readings in all positions with an external auto-ranging multimeter.

That would suggest an abnormal contact resistance in the rotary switch. Autoranging meters usually have several Megohm impedance.

 

[emrr]

I've finally gotten back to some tests, and the meter itself seems the problem.  Proper results with another subbed in, and with an auto-ranging multimeter.  This after I replaced the rotary switch!

The manual describes it as a 0-1mA DC movement.

I've done a little work on the inside of the meter with no luck so far; tightening all the various screw contacts, tiny amounts of deoxit applied, also retouched the solder joints in it.  There are two resistances inside it. 

What's most confusing is B+ (the only position that indicates) comes up slowly at power on, there's no jump in meter response to suggest an intermittent being overcome by voltage.  I mention that because the potential at the 500K B+ series metering resistor is about 305VDC before contact is made with the meter terminal, but it drops to 0.9VDC with the meter attached and reading.  The other test positions are all lower voltage. 

Maybe I'll sort the problem with this meter, maybe I'll find another to replace it with.  I'd like to understand why this meter is failing in this way. 

 

[abbey road d enfer]

What's most confusing is B+ (the only position that indicates) comes up slowly at power on, there's no jump in meter response to suggest an intermittent being overcome by voltage.  I mention that because the potential at the 500K B+ series metering resistor is about 305VDC before contact is made with the meter terminal, but it drops to 0.9VDC with the meter attached and reading.  The other test positions are all lower voltage. 

OK, I think I have the answer; the B+ test indicates that the meter nedds 0.9V to draw 0.6mA (305V into 500k). All the other positions do not deliver this voltage; typically the voltage in teh other positions should be about 20-40mV (assuming the idle current in teh output tubes is about 10-20mA, that's  20-40mV into 2.04 ohms).
So my guess is that there is an resistive element in series inside the meter that has drifted significantly.

 

[emrr]

That makes sense, thanks Abbey.  I found another identical movement in my stash, so I'll see if I can make any comparisons between the resistances after making sure it's a good replacement part. 
------EDIT----------

.......and the new meter did it.  What a ghost chase.  Nothing like a partial failure to have you chasing your tail. 

 

[emrr]

crude iPhone video of the meter in action.

https://youtu.be/TroLuhRTbZ0

 

[DaveP]

You done great!  ;D

DaveP