FM Alignment on Tube Radio with Modern Equipment - Page 1
Forum section: electronics / beginners

author: tgr131
author_rank: Contributor
timestamp: July 13, 2022, 01:58:36 pm
content: Hello everyone!I've been trying my hand at restoring tube radios, and have several AM radios under my belt. The current device on the bench is an AM/FM Zenith J733 tube radio, built in May 1952. The unit is recapped, I've repaired a number of the IF cans that had SMD, and the unit is performing ok, but needs an alignment.When I started this hobby, I got burned buying test equipment on FB Marketplace and eBay that "Powers on, but I don't know how to test", only to find that the unit did power on, but had problems. I now have modern equipment Rigol 1054z Scope and Rigol DG1022z Generator.Things I have done:Read numerous old service texts on FM alignment (with instructions for vintage equipment)Watched numerous youtube videos on the subjectWorked my way through a YT video on using generator and scope to create Lissajous patternsTried to recreate what Photofact says I should see by connecting generator directly to the scopeAttached my equipment as described in the Photofact for this model - didn't see anything close to what I expected, so I decided not to tinker until I had a better idea of what I need to do.The photofact says "Use Frequency Modulated Signal with 60~ modulation and 450KC sweep. Use 120~ sawtooth voltage in scope for horizontal deflection."It seems like I have two paths:Try to set up the modern equipment in a way that it operates like vintage stuffDo something similar to M Caldeira in this video:https://youtu.be/FzWohAQ7PrAI've set up a sweep in the generator, but I'm just missing some key piece to give me that "aha" moment to move my project forward. I was hoping you folks could help.I need some tips on setting the equipment up. I can get the sweep going, but what should I set the sweep time to? How do I set up the 120~ horizontal deflection?Thanks so much!David

author: edavid
author_rank: Super Contributor
timestamp: July 13, 2022, 02:43:07 pm
content: I think you are overcomplicating things.Just set up the DG1022z to do a sweep over your IF frequency range, and use the sync output to trigger your scope. There is no need to use X-Y mode.The scope timebase should match the sweep time - adjust it until you can see the whole sweep. Since you have a digital scope you can go slower than 60Hz if that gives better results.

author: mag_therm
author_rank: Frequent Contributor
timestamp: July 13, 2022, 03:00:27 pm
content: Get a circuit diagram if you have not already. I see that might be paywalled, but low cost.And if this is transformer less, use isolation 120:120 V transformer unless you are really familiar with the potentially live chassis these old circuits.For an initial check, before any alignment or touching the chassis, I suggest just a test as follows:connect sig generator cable with clip leads to ant terminals VIA A CAPACITOR in each lead preferably X rated, small value.Other-wise shock hazard or the sig gen may be damaged.FM deviation is 75 kHz , so I suggest initially set deviation to about +/- 30 kHz, 1kHz audio tome.Set carrier 10.7 MHz. Enough of the IF will leak through, so you can check the IF is basically working by audio in speaker.Then you can move carrier +/- 100 kHz steps (I suggest steps are better than sweep here) to see if the IF is approx peaking at 10.7 MHz.If that all works, then go up to say 92 MHz with a level of -30 dBm or so, and see if it will tune to that, giving audio .These non invasive tests might give an indication of what is required in way of alignment.

author: tgr131
author_rank: Contributor
timestamp: July 13, 2022, 05:13:54 pm
content: Quote from: edavid on July 13, 2022, 02:43:07 pmI may need the "Dummies" version of your explanation. I'll try to explain back what I think you're saying. I'm attaching pics of the signal generator so you see what settings I have available.Generator Setup:Sweep Time: (same as scope timebase. Any suggested starting point?)Return: 0.0Center: 10.7 MHzSpan: 450 kHz (recommended by Photofact)Type: LinearTrigger: leadingFreq: 60hz ?Ampl (currently set at 5 Vpp. Recommendations)The generator will output a Square Wave trigger signal out the back. I assume that will go into a channel on the scope that's set to trigger the other channelThe sweep goes into the radio at connection point spelled out by the Sams through a .05MFD cap. Scope attaches to connection point spelled out by SAMS thru 47kOhm resistor (again, from SAMS).Am I even in the ball park so far?D

author: tgr131
author_rank: Contributor
timestamp: July 13, 2022, 05:16:20 pm
content: Quote from: mag_therm on July 13, 2022, 03:00:27 pmI am working with a SAMS Photofact Schematic and an isolation transformer!I'll parse through your guidance. I'm really at "Dummies" level right now on this. Thank you.D

author: edavid
author_rank: Super Contributor
timestamp: July 13, 2022, 06:47:16 pm
content: Quote from: tgr131 on July 13, 2022, 05:13:54 pmPlease repost them right side up.Quote100ms?QuoteFrequency of what?QuoteSeems much too high. Hard to know without the schematic though.QuoteThat all sounds fine.

author: tgr131
author_rank: Contributor
timestamp: July 13, 2022, 07:07:55 pm
content: I'm not sure what happened to the photos - they are oriented correctly on my computer.on the Freq setting -Selecting the SINE function offers these parameters:FreqAmplOffsetPhaseSelecting the Sweep Function adds these parameters:SweepReturnCenterSpanMarkI assumed the Freq would be the 60~ referred to in the SAMS, as the Sweep Frequency would be the 10.7 MHz.Thanks again for your help, and sorry to be so, well, green.D

author: edavid
author_rank: Super Contributor
timestamp: July 13, 2022, 08:01:06 pm
content: The photos are rotated because you took them with your phone held the wrong way, and didn't resave them.Once you choose the sweep center and span, that should override the frequency setting.The 60Hz in the Sams document is the sweep rate. As I said, since you have a digital scope you can slow that down.

author: CaptDon
author_rank: Super Contributor
timestamp: July 14, 2022, 01:01:53 pm
content: You will be able to set the bandpass response of the 10.7Mhz I.F. but you will be stuck to align the 19Khz/38Khz stereo subcarrier phase alignment unless you have the proper equipment. Your stereo separation may suffer.

author: tgr131
author_rank: Contributor
timestamp: July 14, 2022, 01:56:06 pm
content: Quote from: CaptDon on July 14, 2022, 01:01:53 pmGreetings -Thanks for the info -- this is a mono clock radio with a 5 inch speaker.D

author: mag_therm
author_rank: Frequent Contributor
timestamp: July 14, 2022, 02:28:06 pm
content: According to Wiki, the first stereo broadcasts in USA were in 1961 from WEFM Chicago and WGFM Schenectady

author: tgr131
author_rank: Contributor
timestamp: July 14, 2022, 04:45:54 pm
content: I tried again and I am just lost.I know you all need more details to assist. I'm a newb here and don't want to run afoul of any forum rules. Is it permissible to post relevant portions of my photofact schematic so you all can assist?Thanks!D

author: tgr131
author_rank: Contributor
timestamp: July 14, 2022, 05:06:38 pm
content: Here is what I tried last night -I set up the generator to do a sweep per parameters in the photo. Per the photofact, I connected the signal to pin one of the limiter (12AU6) through a .05MFD cap. In this setup, I ran the trigger signal supplied from the back to CH2. Ch1 of scope is connected to connection point B in the radio through a resistor, which is prior to the signal entering pin 2 of the discriminator (19T8). I've attached pics of the setup and the results.

author: mag_therm
author_rank: Frequent Contributor
timestamp: July 14, 2022, 09:53:58 pm
content: Why are you trying to use sweep for this?As I mentioned, I would just set the generator to 10.7 MHz with about +/- 30 kHz deviation and set 'scope in auto trigger initially.After all, the circuits might not even be working.So you should trace the injected IF signal from mixer forward with your scope probe. Set probe 10:1, AC coupled check its voltage rating is more than about 300 V,First get the scope stable directly on the injected signal, then you can probe the IF tubes and discrim, directly on their anodes, one by one, step by step.Add: The if stages and discriminator might have their tuned frequencies drifted off from 10.7 MHz, so you at each stage you would click 10.7 slowly up and down in 100 kHz steps to see where the peak is,.Take care about jammed slugs, easily broken in these old radios.

author: edavid
author_rank: Super Contributor
timestamp: July 14, 2022, 10:08:05 pm
content: Looks like you are getting pretty close. You have the scope shots right side up, hurray! You might want to read about storing screenshots to a USB stick next.Anyway, you want the scope to show one sweep, not 12. So you could set the generator to e.g. 120ms sweep and the scope to 10ms/division.Also, you want to increase the channel 1 gain so the signal fills the screen. Then you are set to do some (cautious) tweaking.And yes, it's perfectly OK to post excerpts from the Photofacts so we can follow along.

author: tgr131
author_rank: Contributor
timestamp: July 15, 2022, 04:35:48 pm
content: Greetings Guys -@mag_therm - The radio has been electronically restored and functions ok, but when tuning I hit two peaks, and to get ok sound I have to be dead on the second peak. 3 of the IF cans have been rebuilt, so an alignment is in order. As I understand it, there are two paths to alignment - the step method you suggest, and a "visual" alignment per the Photofact which I have attached. One of my goals for this restoration was the learn visual alignments, and the instructions tell me to use a sweep. If I can't master that, I very definitely will fall back to what you suggest, and I am appreciative of your help!@edavid - I've attached the alignment instructions from the photofact, and the section of the instructions for the visual alignment.I'll be working on it over the weekend, so I'm sure I'll have some newbie questions for you all.

author: tgr131
author_rank: Contributor
timestamp: July 17, 2022, 01:14:30 pm
content: Greetings -I was able to get the IF alignment done using M. Caldeira's technique:https://youtu.be/FzWohAQ7PrANote this photofact starts with the limiter and works backward to the mixer. Caldeira starts at the mixer and makes all his adjustments.Turns out I had several issues going on.I had tacked in resistors at points B and C per Sams to connect the scope. Misread the schematic and tacked them in the wrong locations.I had used 2.2 M resistors instead of the 47k ones recommended by SamsIn the video, Caldeira lightly couples the sweep signal in by clipping to the tube shield over the mixer tube. Clipping to tube pin through a cap (per Sams) wasn't working for me, but using the tube shield technique worked greatThe radio is much improved. Signals previously missing or weak are coming in strong.Still some work to do on aligning the limiter, and tuning the antenna, but I'm light years ahead of where I was a week ago.David

author: tgr131
author_rank: Contributor
timestamp: July 17, 2022, 01:30:14 pm
content: Do you all have any suggestions on how to set up modern equipment to per step 4 in the Sam's instructions included in my previous post?I've figured out how to do the sweep, but getting the 120~ sawtooth voltage set up stumps me.D

author: edavid
author_rank: Super Contributor
timestamp: July 17, 2022, 02:02:37 pm
content: Quote from: tgr131 on July 17, 2022, 01:30:14 pmWhy do you think you need that? The scope's sweep is fine.

author: tgr131
author_rank: Contributor
timestamp: July 18, 2022, 12:27:53 pm
content: Quote from: edavid on July 17, 2022, 02:02:37 pmHey,I wanted to do it because the instructions specified it, and I wanted to see if I could do it.I ended up overlaying the "S curve" from the sweep on top of the square wave trigger output, knowing that the middle was 10.7. I adjusted the trimmers to get the middle at 10.7, and for maximum deflection. That appears to have done the trick. Radio is very sensitive and sounds great.I didn't mess with the RF alignment - Dial alignment is almost spot on, and my generator doesn't do the 98 Mhz called for in the photofact. It sounds great though.Thanks for your encouragement and help!David

author: Tom45
author_rank: Frequent Contributor
timestamp: July 18, 2022, 02:21:47 pm
content: Quote from: tgr131 on July 13, 2022, 01:58:36 pmI'm late to this discussion and you now know how to use your modern test equipment to do the alignment.The reason for the confusion is that in 1952, scopes were quite different. In particular, triggered sweeps were new, rare, and expensive, while today all scopes have triggered sweeps.Any scope used in radio and TV repair shops in the 50s and 60s would have had a recurrent sweep (aka synchronized sweep). The sweep was a free running oscillator that could be adjusted by hand to match the frequency of the input signal. These recurrent sweeps were fiddly and just didn't work all that well.Hence, the alignment instructions to use X-Y mode and supply a sawtooth sweep signal that was synchronized to the sweep generator. That is a special case that is the functional equivalent of a triggered sweep.Wikipedia has an article about the history of oscilloscope.https://en.wikipedia.org/wiki/History_of_the_oscilloscopeMy first scope was an Eico 460 with recurrent sweep that I built from a kit in the early 60s.