sx-42 low sensitivity revisited

[WB4AQL]

I gotta admit guys, this thing is tough to work on. I made a new discovery today. Removing the first 1st RF tube (6AG5) has no effect on this receiver. Yes, I`ve substituted numerous good tubes. Removing the 2nd RF tube (6AG5) shuts down what little sensitivity I do have. That ain`t right, is it?

I`ll need to remove the chassis panel on that side to check voltages, but I`m still wondering if this is an AGC problem somehow. I`m working from 2 schematics, neither of which exactly match this radio.

Any ideas? Thanks!

Mike  WB4AQL

[KD6VXI]

Sounds like you're only getting feedthrough power out of the 2nd stage.

With it, you have to interelectrode capacitance to couple the signal from input to output.  With it out of stage, it's a stopblock to the input signal.  I bet a few pf from input to plate would provide about the same symptopms.

NOT familiar with the receiver, but is the AGC tied into that stage?


--Shane
KD6VXI

[w3jn]

Someone could have bypassed the first RF due to a bad bandswitch or something.  Also as I recall the first RF stage may not be active on the AM BCB.

[WB4AQL]

I will make sure and check other bands today. That may explain it.

Thanks, Mike

[The Slab Bacon]

Also as I recall the first RF stage may not be active on the AM BCB.

A lot of the older RXs that had multiple RF amp / preselector stages, didn't use all of them on all of the bands. It's been  while since I was in a 42, but IIRC, they did the same thing.

An easy way to tell is to count the number of antenna inpoot coilz in the first stage compartment. Some of them also didn't use the first RF amp on the first SW band as well.

the last 42 that I worked on was a couple years back. It also had a low sensitivity problem.
Seems like I remember there was a small dogbone resistor in a wierd place that supplied s positive biass to the sensitivity pot and it went way, way out of tollerence. That resistor was physically located in an oddball location, where you wouldn't be looking for it.

But it sounds like you need to do an overall voltage and resistance check, at least to the RF and IF sections.

[ke7trp]

I just completed an SX42.  ALL of the paper caps where bad.  If you have not replaced them all, You will never have a correctly working radio.  Every one of them was leaky, Many where open. 

1. Take an ohm meter and go across each paper cap.  Rule of thumb.  The shorted ones will show 1 to 10 ohms. You know those are band. If they show 2meg or higher, they are good. If they are in the 100k to 500K they are leaky. No reading is open. Understand that the circiut around the cap could effect readings but this is a very fast way to find shorted caps! Will take 5 minutes to check them this way.

2. The early version had band switch troubles. Make sure this is not the case with your rig.  You can go through the band switch with a meter to be sure the contacts are good.

3. Alot of these have a BAD Sensitivity pot.  DO NOT trust it. Take the pot out and test it. Or use a fixed resistor on the line. This pot varies voltage and the radio will be dead if its bad. Mine had a non stock pot that was normal rotation!!!  This means that when its set to 10 or full up, The reciever is DEAD with no RX.. You have to turn the control to 0 to get RX.  This pot is a 10k REVERSE pot with 1 inch shaft.  Not availabel anywhere. I found a supplier for 2 inch shaft that you can cut down.

Make SURE THE CORRECT jumpers are there on the back ACC SOCKET!! otherwise, the radio is dead.

Hope this helps!

Here is mine in all its Glory! 

http://youtu.be/pZ11QXVRWYM

Clark

[The Slab Bacon]

1. Take an ohm meter and go across each paper cap.  Rule of thumb.  The shorted ones will show 1 to 10 ohms. You know those are band. If they show 2meg or higher, they are good. If they are in the 100k to 500K they are leaky. No reading is open. Understand that the circiut around the cap could effect readings but this is a very fast way to find shorted caps! Will take 5 minutes to check them this way.

Clark,
         Who taught you to check caps like that   

First of all it is impossible to determine what is up with them while they are in the circuit other than seeing a dead short. You should snip 1 leg of the cap and then check it.
An ohmmeter across a wax cap should show a complete open after the cap charges to the voltage of the ohmmeter. Any leakage is UNACCEPTABLE short, sweet, and simple. A meg or 2 of leakage across the agc line can make a difference in it's operation. A lot of caps with slight leakage across the B+ or regulated low B+ will pull that line down. And any leakage at all will cause the cap to heat up. Dont kid yourself, a leaky line bypass cap will go off just like an M-80, I've had it happen to me.

Dead open on an ohmmeter does not necessarily indicate a problem, especially if the cap value is small. An open reading on an LCR meter however, does indicate a problem. An ohmmeter is not really the best animal for checking caps, it just works to find the leaky / shorted ones.

Your "standards" for checking caps may well explain why you have many mysterious problems with your radios. The best thing to do is simply to replace all of the paper caps and be done with it. Not replacing all of them is just simply asking for problems later.

[ke7trp]

 Its a fast way of checking for shorted caps or caps with a huge leak.This works perfectly and takes about 5 minutes.  I found 3 shorted caps in the HRO 50 this way. 1 ohm dead short. Clipped the lead, Tested with Cap checker. BAD.   Same Drill in SX42. Shorted Bypass caps. Showed less then 10 ohms on meter, Clipped lead, Checked with cap checker. BAD.   ALL 33 caps in his SX42 are bad or leaky if its anything like mine.

C

[KA3ZLR]

Hi,

Here's what I use for the little ones:






73
Jack
KA3ZLR

[WB4AQL]

Thanks guys. Good suggestions all. Long weekend coming up, so hopefully I can make some progress.

This receiver has had all the paper and electrolytic caps replaced as well as about 80% of the resistors. Strange thing is that it was said to be working when I got it. I know the person well and he is reliable and a good tech.

I just can`t figure out what happened when I got it home and powered up. I cleaned the bandswitch and this seemed to help. It`s almost like it needs aligning, but when he went through it that was done.

The RF pot checks OK. I think it is going to require signal tracing and a `scope to isolate the problem. I may just pass it on to someone willing and able. I`ve got other projects more fulfilling and I`ve spent a fair amount of time on this beautiful old `42.

Anyway, I sure appreciate y`all taking the time to comment.

73, Mike  WB4AQL

[The Slab Bacon]

Mike,
        DO NOT SPRAY ANYTHING OILY ONTO THE BANDSWITCH or you'll be sorry! ! ! ! They have full B+ present on some of the wafers. They will literally go up in flames! !  I had it happen to me around 20 years ago!

[WB4AQL]

OK. Thanks Slab. Always verry careful!

Mike

[KM1H]

The first RF is disabled on the 2 lower bands, just like the SX-28.

Start on the BC band and get it working as it should before bouncing around.

And yes operating with paper caps is an easy way to destroy the bandswitch, power and audio transformers. DeWrexIt or any other spray cleaner is another way.

Carl