Welcome Guest! Be sure you know and follow the Phorum Rules before posting. Thank you and Enjoy! (January 12) x

Thread Closed
Thread Rating:
  • 0 Vote(s) - 0 Average
  • 1
  • 2
  • 3
  • 4
  • 5

Modern troubles again
#14

I spent a little time with this radio yesterday, trying to track down the initial problem which seemed to be a misalignment in the 455 kHz IF. It is quite reproduceable, and at first, seemed to be apparent only with the radio in AM mode, and using the wide audio filter. It sounds slightly more "tuned in" and indicates slightly higher signal strength with the dial set ~ 1-1.5 kHz below the true carrier frequency. With eyes closed, if one rocks the dial and listens for best tone, the result is the same.

The general scheme of the IF is as follows. Just like a tube radio, there are several filter/amp stages:
1st IF is mixed down to 455 kHz>Transformer>20 kHz crystal Filter>Noise blanker>Transformer>6kHz or 3 kHz filter>2 Stages of amplification>Detector. The output from the last IF amp is also buffered and detected seperately to generate the AGC line.

I went through the alignment stage another time yesterday, and may have made an improvement, but I still think something is not quite right. OCD is the enemy of good enough and I like the detective work. Investigation shows that the effect occurs with either the wide (6 kHz) or narrow (3kHz) filter, not just the 6 kHz as first suspected. I tried injecting a 455 kHz signal at various points along the IF chain, thinking this might expose the culprit. One suspect could be the 20 kHz crystal filter - I have seen one other case where a part of this vintage and manufacture had gone bad. However, injecting downstream of this filter, and tuning the 455 kHz signal +/- 2 kHz also seems to show the offset.

I am now back to a close study of the circuit, to see if there is anything else amiss! This is one situation where an IF sweep tool would be very useful.

I don't hold with furniture that talks.


Messages In This Thread
Modern troubles again - by EdHolland - 07-09-2020, 04:11 PM
RE: Modern troubles again - by Simon Wa3uay - 07-11-2020, 10:34 PM
RE: Modern troubles again - by EdHolland - 07-11-2020, 11:06 PM
RE: Modern troubles again - by EdHolland - 07-12-2020, 04:32 PM
RE: Modern troubles again - by EdHolland - 07-13-2020, 01:38 PM
RE: Modern troubles again - by EdHolland - 07-22-2020, 03:35 PM
RE: Modern troubles again - by EdHolland - 07-29-2020, 12:21 AM
RE: Modern troubles again - by EdHolland - 08-15-2020, 10:30 PM
RE: Modern troubles again - by EdHolland - 08-23-2020, 07:39 PM
RE: Modern troubles again - by EdHolland - 08-25-2020, 12:14 PM
RE: Modern troubles again - by EdHolland - 09-06-2020, 01:37 AM
RE: Modern troubles again - by EdHolland - 09-14-2020, 01:23 PM
RE: Modern troubles again - by EdHolland - 09-14-2020, 03:31 PM
RE: Modern troubles again - by EdHolland - 09-21-2020, 02:51 PM
RE: Modern troubles again - by EdHolland - 09-28-2020, 07:19 PM
RE: Modern troubles again - by EdHolland - 10-27-2020, 02:51 PM
RE: Modern troubles again - by EdHolland - 11-01-2020, 09:05 PM
RE: Modern troubles again - by EdHolland - 11-02-2020, 07:17 PM
RE: Modern troubles again - by EdHolland - 11-02-2020, 07:25 PM
RE: Modern troubles again - by Paul Philco322 - 11-02-2020, 08:25 PM
RE: Modern troubles again - by EdHolland - 11-19-2020, 07:46 PM
RE: Modern troubles again - by EdHolland - 11-23-2020, 01:38 PM



Users browsing this thread: 1 Guest(s)
[-]
Recent Posts
Need to purchase some accessories for restoration of my Old Philco Radio
Hi Paulo, Welcome to the Philco Phorum. I can see why you want to find good reproduction parts. That is a bea...MrFixr55 — 11:10 PM
HiFi (Chifi) tube amp build - but my own design.
Tim, I have some bakelite sockets in my work shop that I can dig up. I believe quite a few are NOS. I will take photo...RodB — 10:03 PM
Philco 610B oscillator wiring
> The cathode as noted is wired directly to the ground side of the heater filament for that tube, not to the #17 and ...Radioroslyn — 08:12 PM
HiFi (Chifi) tube amp build - but my own design.
Tim; I noticed your post a little late, but I would take one of the junk sockets and try to break a chunk out of it wi...Arran — 08:07 PM
Need to purchase some accessories for restoration of my Old Philco Radio
Here is a list of resources found in our online library that you might find useful. Mike's Gobs of Knobs email addres...klondike98 — 01:46 PM
First Radio restoration
Hi Tubeman, Welcome to the Philco Phorum.  Phamily Phriendly Pfun with Phine Pholks Phull of Philco Phacts. (See a p...MrFixr55 — 12:33 PM
First Radio restoration
You could post in the WANTED ADs section here on the Phorum and see if anyone has an RF generator that they want to sell...klondike98 — 11:55 AM
Zenith H725
Good ideas, thank you Arrange and Rich. I have the adhesive aluminum foil already and can try that immediately. More ...EdHolland — 10:18 AM
Graphics for majestic 1050 dial glass.
Murf; I found this thread on the ARF, the first photo has a pretty good view of the dial glass. Regards ArranArran — 01:12 AM
Zenith H725
hello Ed, how about that speacial tape used for ducting it's like foil or how about thin piece of aluminum roof flash...radiorich — 12:19 AM

[-]
Who's Online
There are currently no members online.

>