[criageek]

I meant to do that Modial but forgot.  I'll calculate it the next time I go downstairs to work on it.

Thanks,
Rich

[Mondial]

Rich, I just saw your oscillator voltage readings and they are consistent with a proper operation.

The reason that the grid voltage measures positive when measured as per the test procedure is due to the type of meters used at the time. There were no digital meters available which could read a negative voltage, just the analog type with a needle and scale.
 
In order to read a negative voltage, you reversed the test probes so that a negative voltage reads positive. 
You read a positive voltage on the scale but knew it was really negative because you reversed the test leads. So your positive reading is normal with the test leads connected as instructed in the procedure.

The oscillator grid voltage is actually negative, as you measured with your meter from pin 1 to ground. If you have -4 V
then the osc is working well, provided it is at the correct freq for the dial setting.

[criageek]

Thanks Mondial - I probably should have been able to figure that out.  I started in electronics in the early 70s so I'm quite familiar with analog VOMs and still have a couple around here.

I checked the frequency of the AM oscillator.  I'll do it again a different way to verify my results, but here is what I came up with.  At the very low end of the band it's running at 925kHz and at the very top end it's running at 1600kHz.  I can't do anything based on the dial pointer since I removed it.  One of the first things that happened when I started working on this radio was that I bent the dial pointer, so I took it off immediately  

Rich

[criageek]

I checked the AM oscillator frequency again using a different method.  The first time I counted the number of cycles in 10 divisions on the oscilloscope.  This time I counted the number of divisions for one cycle.  Here are the results of the 2nd test.

Bottom of band:  870kHz
Top of band:  1575kHz

This sounds low to me, but I would think it's close enough that I should get some stations...they'd just be offset on the dial, right?  I'm thinking the 7F8 converter is bad since I don't get anything on any band.  But one thing that makes me think that isn't true is that I was able to inject a 1000kHz signal into the antenna terminal and find it by tuning to it (see post 87).  I've put the dial pointer back on and will try that again and see how close it is to the correct place on the dial, but that's just for info...it really doesn't change the diagnosis.

Any ideas from the experts?  I can get a NOS 7F8 for about $10 so may just shotgun it and see what happens.

Rich

[Mondial]

It seems your osc is working, but the freq range appears to be off. Don't know how accurate your scope freq measurement is, but you might try confirming by listening for the osc signal with a nearby receiver with a digital freq display.

The osc range should be about 995 kHz to 2055 kHz. Its not just a matter of dial calibration because the osc freq ( 455 kHz higher) has to accurately track the antenna tuned circuit at the received station freq. If they do not track, you will have low or no sensitivity. This may be your case since you can receive the strong signal gen at 1000 kHz, but no braoadcast stations.

I would check the connections to the push button and bandswitch. It seems that there may be a problem with the connections to the antenna coil, tuning cap and external antenna loop. If the antenna and osc tuned circuits are not wired correctly, you will have the symptoms you describe.

[criageek]

Thanks Mondial - I appreciate the input!

I bugged out the pushbutton connections when I finished them and all looked good but I'll do it again.  Anytime I redo a test I try to do it a different way so if I made a mistake the first time I reduce the chances of making the same mistake.  I'll also check all of the bandswitch connections.  I was concerned about them since there are so many, but I did take a lot of pictures and think I got it right, but we'll see.


I might also find a way to measure the oscillator frequency with my frequency counter.  At the place I was looking at it on the scope it was a pretty large signal and I'm not sure my counter can handle it, but should be able to attenuate it.

Thanks!

Rich

[Mondial]

Rich, does your scope have a vertical channel output? This is a BNC connector on the back or side which gives a sample of the vertical signal you see on the scope screen.

 Its a very good place to connect the counter, because the signal is of relatively constant and reasonable amplitude and you can actually see the signal you are counting.

You can also connect your counter to pin 5 of the 7F8. This is a relatively low impedance point with a reasonable but not excessive level of signal.

I really don't think the 7F8 is at fault here, because you have a good amount of negative voltage on the grid of the osc. That said, it is probably a good idea to get a spare because the 7F8 is run hard in this radio and they tend to wear out quickly. The tubes condition is not too important on AM, but a weak tube may not oscillate at FM freqs.

I am presently working on a 47 -1227, the cousin of your 47-1230 (without the pushbuttons and SW band). My 7F8 was very weak and did not oscillate at all on FM although AM worked fine.

[criageek]

No, I don't see a vertical channel output.  There is a BNC connector on the back but says it's for a Z axis input.  This is a Tektronix 2215 that I bought new when I was working in a test equipment cal and repair lab and the Tektronix rep gave me a little bit of a break on it.  I had our scope guy calibrate it, but that was in 1983  lol.  Now I wonder how accurate it is.  It looked like the level was reasonable on pin 4 of the 7F8 so I connected the frequency counter there and came up with a range of 934kHz - 2086kHz.  That seems pretty reasonable to me.  I'll check it again, this time connecting both the scope and the counter to pin 4 and see how my calculations compare to the reading.  Is it possible one or the other causes the frequency to skew when connected?  I wouldn't think so.

I checked over all the wiring I messed with when I pulled the tuner sub-chassis and the pushbuttons out and everything checks out right, so now I'm stuck again.  I'll probably go over the wiring again just to be sure.  I think I'll go ahead and get a 7F8 ordered.  Are there any other tubes you'd recommend I get while I'm ordering?  Might as well save on shipping.

Thanks for your help...I appreciate it  ;)

Rich

[Mondial]

Well, from the osc frequency readings to obtained, it appears the oscillator is spot on and can be eliminated as the problem. That, with the fact that you do receive the signal generator well at 1000 kHz points to the antenna circuit as the next place to look. The possible components here are the bandswitch, antenna coil, tuning capacitor, loop antenna and the connections between them.

Have you sprayed  the bandswitch contacts with contact cleaner and "exercised" it? Usually these switches have not been operated in years, and the contacts are coated with an insulating layer of tarnish. I am surprised that you don't get static noises when you rotate the switch though.

[criageek]

I have sprayed and exercised the bandswitch but will do it again.  I feel like I'm a good schematic reader but I'm having trouble understanding the front end on this one.  It looks to me like the only time that the loop antenna and AM antenna coil are connected to the circuit is when the bandswitch is in the PB (pushbutton) position.  I'm looking at the FS1( R ) section.  When it's in the BC (broadcast) position the connection is opened.  What am I missing?

Thanks,
Rich

[Mondial]

Looks as if there is an error on the schematic, as it can't work as shown. The two contacts marked BC and PB should be connected together, so the ant coil is connected to C412 in both positions. The switch section below then selects either the main tuning cap C405 in BC position or the push button trimmers C400 in PB position.

Make sure you have continuity from L401 to C412 in both BC and PB positions of the band switch. In addition check for continuity from C405 to L401 and C412 in BC.

[criageek]

Thanks Mondial - I came to the same conclusion about the schematic after looking at the tabs on the rear of the 1st section of the bandswitch.  Pins 8 and 9 are indeed connected when the switch is in the BC position, which results in the antenna coil being connected to C412.  I didn't want to conclude that the schematic was wrong since I have two different schematics and they both show it that way.  But now that I've done that checking and you've concurred, I'll move on  ;)

Tonight I just did a whole bunch of bugging.  It seems like either something is disconnected somewhere or something is shorted to ground that shouldn't be.  So I spent a lot of time measuring resistance from every connection on the bandswitch to ground.  It can be difficult to determine if the reading is accurate since many are connected to coils which are connected to ground.  But for each connection I verified that the reading I got at least seemed reasonable.  I also did that with many of the tabs on terminal strips.  I've taken the voltage and resistance readings on all of the tube sockets and compared them to the expected values and all seemed reasonable.  Other than the resistance readings on the  filament pins...I assume they are way off because I'm using a DMM as opposed to a 20,000 ohm/volt VOM.

This is starting to worry me.  It's starting to look like one of those things that will make me slap my forehead when I figure it out because it was something dumb I did  lol.

Rich

[jcassity]

keep plugging away rich, i dont think though that the problem is going to be something stupid.
your too detailed , i too tripple check,, verify and verify again, to reduce errors. IM not perfect but i try like H**l to be close enough to it so it reduces problems later. watching this thread, paying close attention to your methods.

hopefully you will find the violator causing problems, and i hope its something that you could'nt have known, atleast then you cant kick yourself.

[criageek]

Thanks jcassity!  Although at my age it's difficult for me to kick myself anyway...I have to get someone to do it for me  lol

Rich

[criageek]

WOO HOO!!  Check it out:

https://drive.google.com/file/d/0B2i1fbB...sp=sharing

I hadn't completed the alignment procedure and after looking at the remaining steps I decided they were probably pretty important in relation to the problem I was having.  Here are the alignment steps from the Sam's Photofact
   
I did step 1.  Steps 2, 3, 4 are for shortwave and I didn't continue on and do steps 5, 6, and 7, which involve adjusting the caps for the AM antenna coil and the AM oscillator (403A, C415B, and C415A in the schematic I posted in post 100)...pretty important.  I made this loop and suspended it inside the radio's loop antenna for these steps.
   
So now I get about 6 stations, but I have to have the volume all the way up to make them listenable.  I'm going to continue trying to improve the alignment but if that doesn't improve the volume I'm going to have questions about how to fix that.  But for now, I'm smiling!

Rich