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Sx-28 problems




 
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Author Topic: Sx-28 problems  (Read 9541 times)
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ve8xj
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« on: September 29, 2010, 09:15:54 PM »

Hello

  
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KM1H
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« Reply #1 on: September 29, 2010, 09:58:43 PM »

Ive done a couple of dozen over the years and there is no such thing as a Halli of that era that doesnt need some resistors replaced.

The problem itself sounds like a tube going out with below usable emission, start swapping one at a time with known good ones if you dont have a method to signal inject and trace. Im assuming this is on all bands? If only Band 6 swap 6SA7's with the mixer.....it could still be the culprit on all bands.

Carl
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WQ9E
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« Reply #2 on: September 30, 2010, 12:34:36 AM »

Agree with Carl on this one, every Halli of this era I have worked on has seriously out of spec resistors-especially the 1 watt and up sizes in my experience.

You could well have a tube internally shorting as it warms up given the consistent failure mode.  What does the voltage do when the receiver dies?  Have you tried different bands?  Any crackling as it goes?  What does your S meter read as this occurs?
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Rodger WQ9E
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« Reply #3 on: September 30, 2010, 12:45:55 AM »

Just to add a little, you can quickly test the resistors without unsoldering one side.  You are looking for ones more than 20% above marked value.  This works on 90% of them.  Sometimes if you get a reading below the marked value, it means something is in parallel lowering the reading and then you will need to undo one leg to accurately measure (or just replace anyway since you're halfway done at that point.)  It would be hard to imagine that there won't be a handful of "gone high" or even open resistors in an SX-28.

A total failure like this is a textbook case for dynamic signal tracing.  That way you find where the signal is stopping without just randomly throwing parts and time at the issue. The fault could be any of a dozen things and the signal tracer will find it in short order.

These days I would expect B+ to be too high rather than too low.  A soft rectifier tube is one possibility, although I would think the tester would have picked that up. Another one, more alarming, is that the B+ rail has too much load, in other words there is too much current being drawn for some reason.

Mica capacitors are no longer beyond suspicion either.  It's such a pain to mess with those that most of us try to leave them until they fail, but they have to be suspect these days as well.
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Geoff Fors
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w3jn
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« Reply #4 on: September 30, 2010, 07:00:24 AM »

Sounds like a screen resistor heating up and drifting WAY above tolerance, combined with a weak toob perhaps.

Anyway, welcome to the forum!  I can't think of a problem we haven't collectively licked here, so you're in the right place.
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The Slab Bacon
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« Reply #5 on: September 30, 2010, 08:12:11 AM »

Welcome aboard Smiley

I concur with everyone else so far. I have also done a handful of these receivers. Sounds like you have a screen or plate dropping resistor that is going high as it heats up. Check your plate and screen voltages AT THE TUBE SOCKETS when this happens and see if you are losing voltage. You may have a resistor that got cooked by a leaky cap that you allready replaced.

It is also a good idea to replace all of the resistors in the front end while you have the coil catacombs out to replace the caps. It is a cheap insurance policy that you wont have to pull them again.

the best and easiest way to recap the front end is to remove the coil catacombs, rather than trying to play surgeon with tweezers and hemostats. It isnt really that hard to do. Leave the oscilator section in place, once you remove the other coil racks, you can easily get to anything in the oscilator section.

                                                              The Slab Bacon
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ve8xj
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« Reply #6 on: September 30, 2010, 10:47:32 AM »

Thanks
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WQ9E
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« Reply #7 on: September 30, 2010, 11:36:02 AM »

Tim,

Before you go further, put a pair of diodes across the meter terminals (one each direction in terms of polarity).  This will help prevent meter burnout from circuit failure and the SX-28 meters are a bit hard to find.  In fact I would disconnect the meter while you are doing some basic troubleshooting to avoid damage.  Leave the diodes in place once the circuit is repaired.  They won't affect normal operation but may save your meter when a component fails.

Does the AGC system seem to be working?
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Rodger WQ9E
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« Reply #8 on: September 30, 2010, 05:11:21 PM »

If it truly is a NOS 6B8 Id suspect tube pin or socket corrosion which is fairly common. I use a brass wire brush on tubes and 2-4 reseats in the socket and almost never use DeWrex-It on anything to do with RF or voltage.

Carl
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Rob K2CU
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« Reply #9 on: October 01, 2010, 09:44:21 AM »

Note that in tube testing a 6B8, the cap is a grid 1 connection and not a plate cap. Tube tester notes should have noted this. Many tube testers merely test emission and not mu or Gm.

I am thinking that it might be a worthwhile project to figure how to combine my transistor curve tracer with a standard tube tester to get characteristic curves. Use tube tester to power filament and provide connections for cathode, gird, plate etc. I know my curve tracer provides "Collector" sweep voltages up to and over 200V so it might be usable with low power tubes.... a curve tracer for tubes as it were.
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KB2WIG
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« Reply #10 on: October 01, 2010, 12:44:13 PM »

R,

Look fer a schizamatic for a Tektronix 570, or  buy one if you can find one...
I remember seeing one at school in the early 70's.

klc
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KM1H
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« Reply #11 on: October 01, 2010, 02:48:20 PM »

Alan Douglas went thru that in his Test Equipment book.
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ve8xj
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« Reply #12 on: October 01, 2010, 06:21:31 PM »



Roger

The diodes are a good idea and I will get on that.  

No AGC only AVC and ANL ,neither of which seem to be functioning as well as they should.Any thoughts?
Waiting on a signal generator which i hope arrives soon to help in some fine tuning.

Cheerz


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WQ9E
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« Reply #13 on: October 01, 2010, 06:45:39 PM »



The diodes are a good idea and I will get on that. 

No AGC only AVC and ANL ,neither of which seem to be functioning as well as they should.Any thoughts?
Waiting on a signal generator which i hope arrives soon to help in some fine tuning.

Cheerz


Tim,

AGC  (automatic gain control) and AVC (automatic volume control) are two acronyms that mean the same thing.  AVC is probably the more traditional vintage term.

Unless the AGC is working properly the S meter won't work correctly either.  You are back to your problematic 6B8 circuit since this provides the AVC and S meter function.  If you have already replaced all of the capacitors in the receiver then zero in on the resistors in the 6B8 circuit.  For the S meter R-28 is particularly suspect and this is one of the resistors where a fault can easily lead to a dead S meter since it is the shunt resistor for the meter movement.
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Rodger WQ9E
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« Reply #14 on: October 02, 2010, 07:23:36 AM »

If you look at Phil's old Radio site you'll see he had a similar problem with the AVC in his SX-28, which I helped thim thru.

First on the order of business is to make sure that the wirewound S-meter pot is good - continuity end-to-end, good contact with the wiper, and no shorts/leakage to ground.  90% chance it's bad - they almost always are.  90% chance you can repair it, though, unless the nichrome wire has burned thru in the middle... that stuff is impossible to solder.

Per the AVC trouble, one reason the SX-28 sounds so fantastic is that it has amplified AVC - ie, there's a separate amplified IF stage just for the AVC channel.  The AVC then has its own independent detector so the audio detector isn't loaded down with excessive capacitance.  This is what causes distortion on high-modulation index signals in diode detectors.

If someone aligned it without understanding the circuit, they most likely buggered it up.  The AVC detector transformer needs to be aligned for a *dip* in S-meter reading (or max AVC voltage), *not* a peak.  It's not a very sharp dip, but it's an important alignment step if the thing is ever going to work properly.  Phil's SX-28 had a bad mica cap inside the AVC transformer which prevented adjustment to resonance.
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WQ9E
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« Reply #15 on: October 02, 2010, 07:53:16 AM »

John,

Great catches on noting the amplified AGC and sending him to Phil's site.  I had forgotten about Phil's site and he does have good writeups on a number of restoration projects.  I came across his site when I was looking for information on my Scott 800B hamfest find (chassis and speakers only, no case). 

Another interesting restoration site, although much of it is in Japanese, is noobowsystems labs:  http://www.noobowsystems.org/restorations/index.html   Look for the English comments links on several of the projects.  I found several schematics there I couldn't locate elsewhere and his writing is interesting.  I would like to find one of the interesting looking National (by Matsushita) CRV-1 radios to play with:  http://www.noobowsystems.org/restorations/crv-1/crv-1.html
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Rodger WQ9E
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