Early 400 FM issue

hogwylde09

hogwylde09

Vintage Tube Hi-Fi Rules
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As I prepare install the Radio Daze glass, BTW it is EXACT. Great job RD!, worth the wait. While I have the unit out of it's case, plan on replacing the IIRC low testing 6SH6. Have a replacement now.
I'll go ahead and test all the tubes in the IF/RF section again. Problem being slow warm up time. 10-15 minutes for FM to work normally. Sluggish is an understatement. Then it's good and strong for deep fringe area I'm in.
Just wanting to know what could cause this. I'm thinking bad tube. All of the Telefunken 12AX7's passed the Gm. tests with flying colors.

Also found a NOS Amperex EM84 in my tube stash as my "A" is dimmer than dim. I'll look into the @analogaudioerc mod.

Haven't checked the PI adjustment, the pots haven't been touched. Probably should do that now.

With several more 7868 in my possession, will try to get a little closer there as well. I will say again, the 6.3 ct line bucker drops the temps on the output tubes. Highly recommend it, if you don't have one.

Since the rebuild this thing is jaw dropping with Klipshhorns
 
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I would suggest that if all the tubes are good in the FM tuner section, then some component is being affected either directly or indirectly by the change in thermal conditions that 10-15 minutes of operation produces. In the FM tuner section, this could be anything from a lose connection, to one of the resonant caps within an IF transformer (i.e., mica disease) becoming intermittent. The next time you operate the unit from a cold start, tap or lightly wiggle each IF transformer, and see if the tuner then doesn't spring into normal operation when a particular transformer is targeted. If so, then you've found your cause or area to investigate.

Good luck with it!

Dave
 
might also be worth checking heater voltage in the tuner to make sure its in spec. Tubes that are starting to get marginal might work properly at normal or slightly elevated heater supply but emission will drop like a rock below that point. It also tends to make for very slow warm-up. If your tester can reduce heater voltage while testing, you can use that to see which ones might be ailing. Some testers refer to this as a "life test".


I've had a lot of flaky tuners caused by bad tube contacts too, so clean the pins and sockets if you haven't already. Even just wobbling the tubes in their sockets might snap it back to life if its dirt or a cracked solder joint underneath.
 
Just an FYI. 6AH6 is a direct sub for the 6HS6. A lot less squealing from your wallet too.
 
Yes, thank you @larryderouin, you had already mentioned the 6AH6. But I thought I also understood with a small decrease in performance somehow with that sub. I had time yesterday to test and install the 6HS6 I had purchased. And HH Scott relabled one wich tested well with no shorts & grid leakage as my original had on the Sencore Continental MU-150 Mighty Might. Replaced both 6AU6 as well, about the same with shorts and leakage. 6HA5 tested bad and replaced as well. Was relieved to have several NOS of these. Now the dial pointer is off a few Mhz, lower on the dial. ie: 106.1 is more around 105.7. FM starts well now without the long delay. But I also believe @dcgillespie may be spot on here as well. At some point almost all audio volume was lost. As he suggested when I wiggled Z3, level came back up. For the most part this was intermittent. It's been working pretty good for the rest of the evening. Sensitivity may be suffering a bit also.
I then went on to replace the Radio Daze glass. Looks sharp, same scenario with the dial pointer off. Reads below the actual call numbers.
I suppose I need to have look at the KM-60 alignment procedure.
Still want to do the EM84 swap. Seems to never end for me.
 
Update on tuner. Woke up this morning turned on receiver FM came alive as soon tubes started conducting. Good volume level but beam barely visible at both ends. Tapped Z3 with no improvement. Moved over to Z2 gave it a backwards push. Bingo stereo beam centered. Now from saved pictures this doesn’t look good. I do believe my transformer has the silver mica disease. Also notice the “bubble” on the ceramic cap near it.
I’ve watched a few YouTube videos, X-ray Tony and a few others repair these things. But the math formulas to figure out the Pf capacitance um, well that’s in another league. Above my pay grade.0F86C887-403D-409B-8EAA-516F6B86E3F1.png
 
Is it a bubble on that cap or is it a soldering iron burn? It looks like one or more of those resistors covering the cap may have been resoldered which is why I'm thinking iron burn. Just a thought.
 
Don’t know Thorn. Was nothing that I was around with and iron. Already know from the ugly green crud under that IF transformer, dreaded silver mica disease.
 
The ceramics are dipped in wax at the factory to seal from moisture. Heat will cause it to migrate to the lowest point at the time and develop like a water drop. Most likely scenario. The dark spot on the cap to the left of the PEC (at top circled in GREEN) is a darkening of the casing due to moisture and age. The wax has let loose at that point. The corner of the PEC (circled in RED) breaking down is not unusual. These usually were NOT dipped in wax to seal them and the casing flakes off in places.

Chip on PEC.jpg
 
Paging @larryderouin, so no concern with ceramic caps and PEC. It all boils down to the Z2 IF transformer, and how to proceed with that repair?
I did see a conversation about pF values listed on later unit 400's. 24pF. Still all above my skillset.
Need a parts donor unit now.
Thank you again for having a look at this, Jimmy

Edit once more, Larry maybe you are suggesting that the rocking backwards of the IF can is somehow related to the deteriorating ceramic cap you circled in your last reply?
 
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Not suggesting anything of the sort. Just pointing out that the discoloration is due to moisture, age, and lifting of the wax. Silver Mica Disease, is most likely the cause of your problem, given the symptoms and your rocking, tapping etc. of the can( s).

By the Way, the picture is of Z4, NOT Z2.


I would check with Dave on the size cap to use on that can. 24 sounds in the ballpark, but Dave is the final arbiter on RF/IF circuits here.
 
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Hog -- The problem is not the external ceramic caps -- they are fine. The problem is the internal resonant caps that become intermittent. Getting another transformer is an iffy repair, as it may suffer from the same problem. The only real resolve is to remove the transformer, disable the internal caps (built into the base of the transformer), reinstall the transformer, and then install silver mica caps of the correct value externally under the transformer. Then the transformer must be aligned. The fix is permanent and forever. The only way the transformer can be damaged after the repair is if a tube shorts (virtually impossible) to burn a winding out, or an unfortunate incident happens where a screwdriver accidentally shorts a terminal to ground and burns a winding out that way.

Dave
 
@larryderouin right. I was misled on location Z2. Most off by seeing green corrosion on one. That would be Z4 I realize now. I want to get it back on the bench and carefully chopstick the carbon comp resistors on that tank circuit, trying not to move the transformer posts. Either narrow it down to one of those or mica disease as Dave identified. The problem area is still rocking Z2.
Ironic the FM starts right away now after disposal bad tubes and new installed. 10 minutes after working then looses signal. Dave the Fisher GURU is always right on however. Thanks again for taking time out of your busy schedules to help out the folks here on AK!
 
OK! Found out where I screwed up. Got some light on the transformer for a part number. As a result from cutting a can cap while still mounted on the chassis, the saw frame dented Z2. Using a 90 degree angled pick, I released the tab clamps and attempted to pull upwards the dented case. Afraid to pull with much force I stopped. I gave the cover a little twist. And radio worked with no signal dropout until I left for work. I don’t know if I will be able to successfully remove the dented cover without breaking the plastic coil frame though. B46C005B-2F67-4A1E-8ACB-81FFC6F57DF3.jpeg
 
Release the clamps again(screwdrivers between the clamp and case to hold the tabs away from the case) and wiggle it up. The internals are somewhat smaller than the cover. Pull straight up, wiggleing slightly while pulling.
 
Unfortunately, the slight indentation I'm seeing at the top of the can will have virtually zero impact on the performance of the transformer however -- whether it has mica disease or not. The coil windings only occupy the center most area of the can (wide space between them and the can cover), while the resonant caps are located in the plastic base of the transformer.

Dave
 
I'd be more inclined to suspect broken connections around the base of the transformer if moving it caused it to come back to life. Might be worth a poke in that area with the pokenstik.
 
R & R of the transformer can I saw no problems whatsoever, visually at least. And all is stable now. Maybe the pressure applied on the base caused the mica between the metal tabs and plastic to "settle" in somewhat. Can't say. And "looking" at the carbon comp resistor and ceramic cap at the terminals revealed nothing either. I gave it a few hours to act up, if it was going too, last night.
It's still in need of some alignment though. A local strong signal 107.3 mhZ seems to be have too wide carrier, tends to want to bleed into a weak 106.9 channel. I can usually pick this station up most of the time on my Sansui TU-517. Backup plan as well, have located another Z2. Now to try to find a decent probe for the couple of old VTVM I have around here.
 
you really want a proper sweep generator to align these, not the old fixed frequency and VTVM method. It will give much better performance. On low end gear it doesn't matter so much but on anything good, and especially on stereo stuff, the alignment is way more critical.
 
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