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 Post subject: Unexpected 2nd Panasonic T-81H project...
PostPosted: Feb Wed 14, 2018 3:13 am 
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Joined: Oct Mon 26, 2009 10:02 pm
Posts: 5467
Location: Indianapolis, IN
Hello All,

Another totally unexpected project---this set just showed up while browsing EBay for $14.99 "Buy It Now" with free shipping and couldn't hit the button fast enough ! Not fun radios to work on, but well worth the effort as they are great performers with very good sound for their small size. This one is missing the battery compartment cover, but can use mine as a sample to make something that will do the job. The battery holder had also been cut off...

It arrived Saturday and when I had the chance, applied power. After a few minutes, FM began to work at low volume---but AM & SW were dead. Tried the adjacent radio "trick" tonight and no noticeable oscillator activity. It was dirtier than it appears in pic, but looked much better just after a preliminary wipe down with damp, then dry cotton cloths. Pulled back cover and found that ferrite antenna rod was broken in half. Have been through that situation a few times in recent history so hopefully it will glue back together as well as the others...

To be continued...

Ebay pic below...

John


Attachments:
ARF T81H EBAY PIC.jpg
ARF T81H EBAY PIC.jpg [ 191.09 KiB | Viewed 964 times ]
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 Post subject: Re: Unexpected 2nd Panasonic T-81H project...
PostPosted: Feb Wed 14, 2018 3:51 am 
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Joined: Mar Sun 30, 2008 1:45 am
Posts: 190
Location: Oakland Township, Michigan
That is one cool looking radio John!

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http://www.smokypond.com/gallery/main.php


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 Post subject: Re: Unexpected 2nd Panasonic T-81H project...
PostPosted: Feb Thu 15, 2018 2:39 am 
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Joined: Oct Mon 26, 2009 10:02 pm
Posts: 5467
Location: Indianapolis, IN
Thanks Matt ! These are solid, heavy, well-made little sets---and are real gems once detail cleaned (and work as good as they look !).

Looked at my folder for the 1st set and discovered that X3, the 1st FM IF amp & AM converter was bad---seems that this radio may share the same fault ! Fortunately, have a newer Panasonic parts set obtained last fall which will hopefully provide a usable part...

John


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 Post subject: Re: Unexpected 2nd Panasonic T-81H project...
PostPosted: Feb Fri 16, 2018 12:06 am 
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Joined: Aug Wed 26, 2009 1:50 am
Posts: 1406
Location: Mount Pleasant UT 84647 (from Coventry, UK)
Nice find!


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 Post subject: Re: Unexpected 2nd Panasonic T-81H project...
PostPosted: Feb Fri 16, 2018 2:00 am 
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Joined: Oct Mon 26, 2009 10:02 pm
Posts: 5467
Location: Indianapolis, IN
Thanks Colin !

Removed cover and straightened ferrite antenna rod support brackets to bring the two halves into alignment---it must have suffered a strong impact as rod sections were sagging and both brackets were twisted slightly. Loosened the right side so that section would move easily, cleaned both broken ends with Isopropyl alcohol and Q-Tips, then applied Superglue Gel to surfaces and held together until it set. Have radio on end vertically for a couple hours to ensure solid bond...

Hope to pull chassis tomorrow evening and begin troubleshooting ! Really like those "hyper" speakers...

Below is a pic just before gluing...

John


Attachments:
ARF T81H ins before.jpg
ARF T81H ins before.jpg [ 202.38 KiB | Viewed 881 times ]
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 Post subject: Re: Unexpected 2nd Panasonic T-81H project...
PostPosted: Feb Fri 16, 2018 3:04 am 
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Joined: May Wed 01, 2013 10:05 pm
Posts: 1793
Location: Rayleigh, BC, CANADA
Wow! another great deal John, looking good.

This one has SW, curious how that turns out.

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 Post subject: Re: Unexpected 2nd Panasonic T-81H project...
PostPosted: Feb Sat 17, 2018 6:32 am 
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Joined: Oct Mon 26, 2009 10:02 pm
Posts: 5467
Location: Indianapolis, IN
Thanks Rocco !

Replaced all twelve electrolytics this evening (not an easy task with this set) and no improvement. Audio still seems okay (normal low-level hiss), but radio wouldn't even pass a 455 kHz signal from signal generator---absolutely nothing made it through to audio stage... Ran out of time tonight, so cleaned up work table for troubleshooting tomorrow.

Stay tuned...

John


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 Post subject: Re: Unexpected 2nd Panasonic T-81H project...
PostPosted: Feb Sat 17, 2018 5:20 pm 
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Joined: Oct Mon 26, 2009 10:02 pm
Posts: 5467
Location: Indianapolis, IN
Got up this morning ready to go on this radio !

X3 converter transistor voltages were correct, so pulled out signal tracer and got good signal at its output ! Amazing that the nearby radio "trick" didn't detect oscillation... But the first IF amp transistor (X4 in SAMS) was a different story---"E" read 1.0 instead of 5V & "B" 2.2 instead of 4.8 ! These transistors have a "shield" lead and have seen shorts or leakage to "can" before probably due to "tin whiskers", so am getting ready to pull it from PCB...

Just a short update as the "story" continues...

John


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 Post subject: Re: Unexpected 2nd Panasonic T-81H project...
PostPosted: Feb Sun 18, 2018 5:24 am 
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Joined: Oct Mon 26, 2009 10:02 pm
Posts: 5467
Location: Indianapolis, IN
All day "adventure" with this radio---too much to write tonight, so will post more detail tomorrow...

X4 was definitely bad---leakage to shield as well as between elements. Installed socket to try and find suitable replacement in transistor drawer---tried a couple "common" Japanese IF transistors with no luck. Went through several others and finally found that an unused RCA 2N1632 that I've had for around 42 years worked perfectly ! Performance very good, but still at low volume... Desoldered and disconnected shield lead of converter---no difference (yet...) Since sensitivity & selectivity excellent, just seemed like an audio issue though sound quality good. Was staring at PCB and the 56K bias resistor in audio preamp stage and it looked warped, so measured voltage and it was high so paralleled with another and volume increased noticeably---but the 56K was fine and the transistor was bad ! Fortunately, had a good 2SB173 (exact replacement) in a Panasonic parts set and it corrected the problem---PLENTY of good, clean audio now ! Was very surprised how loud and clear everything sounded on test speaker...

Got all the detail cleaning/polishing completed also---hope to get all back together tomorrow after resolving a mechanical issue...

More detail & pics tomorrow, so stay tuned !

John


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 Post subject: Re: Unexpected 2nd Panasonic T-81H project...
PostPosted: Feb Sun 18, 2018 10:07 pm 
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Joined: Oct Mon 26, 2009 10:02 pm
Posts: 5467
Location: Indianapolis, IN
Got it completed today !

The tuning knob was rubbing against lower edge of tuning cap pulley---marked contacted area of pulley with silver Sharpie and filed down lower lip as much as possible leaving just enough to keep dial-cord in groove. And when installing knob after cleaning, pulled it as far away as possible before tightening mounting stud to screw.

Cosmetically, need to get a black "lacquer stick" and redo the PANASONIC lettering. Appears that someone had spilled white paint on that corner of radio as I not only found residue in cabinet crevices, but inside and around whip antenna. Otherwise, radio cleaned up very well...

Believe that both defective transistors had "tin-whiskers"---after being handled and dropped on work-table, the incorrect resistances I measured both between the elements and the elements and "cans" had disappeared ! Since I didn't measure anything to converter transistor shield lead after desoldering and performance didn't change, reattached it to ground and it now failed---soldering heat ? So desoldered once again and slid a tiny piece of 24 gauge Teflon insulation over lead and into hole to isolate---all now okay...

Replaced the stiff battery wiring with new Teflon and added new snap connector for a 4 "AA" double-inline holder...

Since the top section of the chassis frame and metal trim at ends of case are used in the FM antenna system, they used a small rubber grommet originally to isolate lower portion where the jack & switch section mounts. The rubber had rotted, so used two small, shouldered Nylon washers which came from a TO-220 power transistor heatsink mounting kit to replace. One between with shoulder up through hole and the other with shoulder towards mounting screw which has sleeve that passed through grommet---worked perfectly... If anyone interested, can post details of this...

After chassis installed into cabinet, applied power and adjusted IFT's (very sharp peaks)---amazingly, they all were still nearly dead-on to factory markings. The ANT trimmer also required very little adjustment. Performance outstanding on both AM & FM---sensitivity/selectivity on AM is up there with some of the best sets with tuned-RF amp stages. Not much activity on SW, but have made no adjustments yet... Amazing how much volume this set is capable of with properly functioning audio system (control still good, too !)---sound quality very good on FM, but has just a bit of that "bottom of barrel" tone on AM when turned up... Since this is noticeably the best performing and sounding of my T-81's, decided to give it the other "H"'s battery compartment cover and might also swap the back cover with label. Plan to eventually make a cover and label for the other... Might also look closer at the audio preamp & driver transistors in the first set as well as deal again with volume control which has become noisy after sitting...

All in all, a little "gem" of a well-built radio that really performs as good as it looks (when working correctly !)...

Project pics below...

John


Attachments:
1ARF T81 OLD CAPS.jpg
1ARF T81 OLD CAPS.jpg [ 224.37 KiB | Viewed 782 times ]
2ARF T81 SPKR FRT HRDWR CLN.jpg
2ARF T81 SPKR FRT HRDWR CLN.jpg [ 200.8 KiB | Viewed 782 times ]
3ARF T81 CAB BK INS CLN.jpg
3ARF T81 CAB BK INS CLN.jpg [ 125.81 KiB | Viewed 782 times ]
4ARF T81 CAB PARTS CLN.jpg
4ARF T81 CAB PARTS CLN.jpg [ 213.92 KiB | Viewed 782 times ]
5ARF T81 CAB W-SPKR INS CLN.jpg
5ARF T81 CAB W-SPKR INS CLN.jpg [ 206.89 KiB | Viewed 782 times ]
6ARF T81 CHS BEFORE.jpg
6ARF T81 CHS BEFORE.jpg [ 202.49 KiB | Viewed 782 times ]
7ARF T81 CHS BOT CMPLT.jpg
7ARF T81 CHS BOT CMPLT.jpg [ 206.43 KiB | Viewed 782 times ]
8ARF T81 INS CMPLT.jpg
8ARF T81 INS CMPLT.jpg [ 214.88 KiB | Viewed 782 times ]
9ARF T81 CMPLT REAR.jpg
9ARF T81 CMPLT REAR.jpg [ 183.26 KiB | Viewed 782 times ]
10ARF T81 CMPLT FRT.jpg
10ARF T81 CMPLT FRT.jpg [ 223.66 KiB | Viewed 782 times ]
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 Post subject: Re: Unexpected 2nd Panasonic T-81H project...
PostPosted: Feb Sun 18, 2018 10:17 pm 
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Joined: Mar Sun 30, 2008 1:45 am
Posts: 190
Location: Oakland Township, Michigan
Very good work John! That's a beautiful radio!

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http://www.smokypond.com/gallery/main.php


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 Post subject: Re: Unexpected 2nd Panasonic T-81H project...
PostPosted: Feb Sun 18, 2018 10:59 pm 
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Joined: Aug Wed 26, 2009 1:50 am
Posts: 1406
Location: Mount Pleasant UT 84647 (from Coventry, UK)
Beautiful. I might need to find me one of these.


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 Post subject: Re: Unexpected 2nd Panasonic T-81H project...
PostPosted: Feb Sun 18, 2018 11:17 pm 
Member

Joined: Oct Mon 26, 2009 10:02 pm
Posts: 5467
Location: Indianapolis, IN
Thanks much, Guys !

Colin---keep your eyes open ! Have seen a couple others on Ebay in the past few months... And they don't take up a lot of space !

John


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 Post subject: Re: Unexpected 2nd Panasonic T-81H project...
PostPosted: Feb Mon 19, 2018 4:00 am 
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Joined: Nov Wed 14, 2007 11:37 pm
Posts: 1475
Nice little radio there. You did a great job with it.


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 Post subject: Re: Unexpected 2nd Panasonic T-81H project...
PostPosted: Feb Tue 20, 2018 4:04 am 
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Joined: Oct Mon 26, 2009 10:02 pm
Posts: 5467
Location: Indianapolis, IN
Thanks Metzman !

John


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 Post subject: Re: Unexpected 2nd Panasonic T-81H project...
PostPosted: Feb Fri 23, 2018 8:42 am 
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Joined: Mar Thu 16, 2017 11:14 am
Posts: 47
Location: Amsterdam
Wonderful moments, when reception comes back, when sound comes back.

Congratulations!


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 Post subject: Re: Unexpected 2nd Panasonic T-81H project...
PostPosted: Feb Sat 24, 2018 5:56 am 
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Joined: Oct Mon 26, 2009 10:02 pm
Posts: 5467
Location: Indianapolis, IN
Thanks Rojosuiza ! You are absolutely right---especially when the radio is dead previously...

John


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