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 Post subject: 1964? Panasonic T-81H AM/FM/SW -- latest arrival (nice) !
PostPosted: Jan Thu 05, 2017 2:44 am 
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Joined: Oct Mon 26, 2009 10:02 pm
Posts: 4381
Location: Indianapolis, IN
Hello All,

This is still another radio that just appeared while browsing Ebay the other evening---just couldn't pass it by for $17.50 "Buy It Now" and seller was only around 10 miles from me ! Shipping was low enough to keep me out of the Indy traffic... It looked so good in initial pic that I "pounced" on the BIN button !

Already have a "plain" T-81 with just AM & FM and it's an excellent little set that I really like with very good performance & sound---it was pretty rough & dirty when I got it and thankfully turned out well.

This one is in a lot better condition and just a bit dirty from use. The battery holder (-) springs were very rusty so simply clipped it out and will install a new 4-"AA" holder when I do the refurb.

Applied power and nothing but a "pop" and a bit of "scratch" from volume control. Noticed that the battery voltage dropped when turned on, so checked current and found it was pulling over 250 mA !!! Hopefully just a shorted electrolytic and not a transistor !

Below are three of the Ebay pics---was concerned about antenna, but it's perfect ! They'd only pulled it up partway...

Stay tuned !

John


Attachments:
arf 81h ebay frt.jpg
arf 81h ebay frt.jpg [ 192.39 KiB | Viewed 885 times ]
arf 81h ebay bk.jpg
arf 81h ebay bk.jpg [ 179.3 KiB | Viewed 885 times ]
arf 81h ebay top.jpg
arf 81h ebay top.jpg [ 118.88 KiB | Viewed 885 times ]


Last edited by xrhonda91 on Jan Sun 08, 2017 4:22 am, edited 1 time in total.
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 Post subject: Re: 1962 Panasonic T-81H AM/FM/SW -- latest arrival (nice) !
PostPosted: Jan Thu 05, 2017 3:45 am 
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Joined: Jan Tue 31, 2012 1:55 am
Posts: 1635
Location: Mountains of Mourne. Ireland.
Very nice John... no duck + cover markings!

Greg.


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 Post subject: Re: 1962 Panasonic T-81H AM/FM/SW -- latest arrival (nice) !
PostPosted: Jan Thu 05, 2017 9:10 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 23451
Location: SoCal, 91387
Slick looking radio there, John! I like the top facing slide rule dial. Yeah, so many of the '60's Japanese made Transistor sets are now dead, and normally requiring just changing the usually 4 Electrolytics to get them going again.

Interesting that it's a 3-bander, kinda unique for that size set.

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 Post subject: Re: 1962 Panasonic T-81H AM/FM/SW -- latest arrival (nice) !
PostPosted: Jan Thu 05, 2017 1:27 pm 
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Joined: Oct Mon 26, 2009 10:02 pm
Posts: 4381
Location: Indianapolis, IN
Thanks Guys !

Greg---didn't even notice the lack of Conelrad markings ! Hmmm... The seller said he found it in a box of his things while unpacking---think I'll contact him and see where he got it ! But it does have the label on back cover...

Richard---more than four in this set and a little more "packed" inside than the AM/FM...

Hope to get to it this weekend !

John


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 Post subject: Re: 1962 Panasonic T-81H AM/FM/SW -- latest arrival (nice) !
PostPosted: Jan Fri 06, 2017 3:09 am 
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Joined: Oct Mon 26, 2009 10:02 pm
Posts: 4381
Location: Indianapolis, IN
Doesn't look good...

Pulled cover and checked power input with VOM---30 ohms forward with speaker noise & 700 ohms reverse... Looks like a shorted audio transistor...

But it is clean inside, though...

John


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 Post subject: Re: 1962 Panasonic T-81H AM/FM/SW -- latest arrival (nice) !
PostPosted: Jan Fri 06, 2017 4:34 am 
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I wouldn't assume anything until the Electrolytics have been changed out.

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 Post subject: Re: 1962 Panasonic T-81H AM/FM/SW -- latest arrival (nice) !
PostPosted: Jan Fri 06, 2017 4:10 pm 
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Joined: Oct Mon 26, 2009 10:02 pm
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Location: Indianapolis, IN
Hey Richard, hope to pull the PCB tonight...

I heard back from the seller and he said that he had it packed away so long he'd forgotten where he got it !

John


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 Post subject: Re: 1962 Panasonic T-81H AM/FM/SW -- latest arrival (nice) !
PostPosted: Jan Fri 06, 2017 7:06 pm 
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Joined: Aug Wed 26, 2009 1:50 am
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Location: Mount Pleasant UT 84647 (from Coventry, UK)
That's a nice looking radio. Hopefully all it needs is caps.

Colin


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 Post subject: Re: 1962 Panasonic T-81H AM/FM/SW -- latest arrival (nice) !
PostPosted: Jan Fri 06, 2017 9:40 pm 
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Joined: May Wed 01, 2013 10:05 pm
Posts: 893
Location: British Columbia, CANADA
Good catch John. Glad to see you found the 3 band version of that.
Hopefully the SW band performs well on that set too.


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 Post subject: Re: 1962 Panasonic T-81H AM/FM/SW -- latest arrival (nice) !
PostPosted: Jan Sat 07, 2017 2:24 am 
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Joined: Oct Mon 26, 2009 10:02 pm
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Location: Indianapolis, IN
Thanks Guys !

Does look like a audio output transistor issue causing fault...

Rocco---was yours the 3-band ?

John


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 Post subject: Re: 1962 Panasonic T-81H AM/FM/SW -- latest arrival (nice) !
PostPosted: Jan Sat 07, 2017 10:15 pm 
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Joined: May Wed 01, 2013 10:05 pm
Posts: 893
Location: British Columbia, CANADA
xrhonda91 wrote:
Rocco---was yours the 3-band ?

John

No, it's just a plain T-81 (AM/FM) same as your first one.

I just noticed, my radio is not working anymore (no audio) on either band, gonna dig into it this weekend.

John, it should be easy locating a shorted audio transistor. hope that's all it is with yours.

Check the headphone jack as well for bent contacts.


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 Post subject: Re: 1962 Panasonic T-81H AM/FM/SW -- latest arrival (nice) !
PostPosted: Jan Sun 08, 2017 4:21 am 
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Joined: Oct Mon 26, 2009 10:02 pm
Posts: 4381
Location: Indianapolis, IN
Thanks Rocco...

Thanks Mitch for the SAMS---didn't realize that the "H" info was available ! Have been using the "plain" T-81 Photofacts... Remember, I bought that radio from you ? Guess I should change the year as it looks like this one is from 1964 since SAMS is 1965 ?

Not a good day...

After the long, tedious task of replacing the twelve electrolytics, radio still bad. Had disabled the audio outputs last night since it appeared they were shorting and the operating current dropped from over 250 mA to around 36 mA with no signal which is still around twice what it should be.

Other than the FM module, other transistor voltages are abnormal---think someone had plugged in a wall-wart of higher voltage or reversed polarity and damaged the transistors in the AM & audio circuitry---the power to FM module is off when AM or SW selected which explains why I could still get a very low level FM signal through with signal tracer. The AM converter doesn't oscillate.

Very discouraged as it'll probably be difficult or impossible to find a cheap parts set to replace these transistors... And it's not an easy set to work with, so don't want to try anything else but the exact, working transistors off of a parts PCB if one can be found...


Attachments:
ARF T-81H INS BEFORE.jpg
ARF T-81H INS BEFORE.jpg [ 244.47 KiB | Viewed 662 times ]
ARF T-81H PCB BOT BEFORE.jpg
ARF T-81H PCB BOT BEFORE.jpg [ 208.99 KiB | Viewed 662 times ]
ARF T-81H PCB BOT RECAPPED.jpg
ARF T-81H PCB BOT RECAPPED.jpg [ 227.83 KiB | Viewed 662 times ]
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 Post subject: Re: 1964? Panasonic T-81H AM/FM/SW -- latest arrival (nice)
PostPosted: Jan Sun 08, 2017 5:31 am 
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Joined: Aug Wed 26, 2009 1:50 am
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Location: Mount Pleasant UT 84647 (from Coventry, UK)
Well that's a shame. Are the transistors specific to this set, or would other radios from the same era possibly provide replacements?

Colin


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 Post subject: Re: 1964? Panasonic T-81H AM/FM/SW -- latest arrival (nice)
PostPosted: Jan Sun 08, 2017 5:47 pm 
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Joined: Oct Mon 26, 2009 10:02 pm
Posts: 4381
Location: Indianapolis, IN
Colin---The converter & IF transistors (2SA70) appear to be special, shielded Matshusita parts. Considering how well the "plain" T-81 performs, think that they must be something special... They are not used in any of the other Panasonic sets that I know of...

Am going to play with it a bit more today...

Mitch---Haven't even looked to see what subs might be for the 2SA70, but it appears to be a unique part...

Thanks !

John


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 Post subject: Re: 1964? Panasonic T-81H AM/FM/SW -- latest arrival (nice)
PostPosted: Jan Sun 08, 2017 6:20 pm 
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Joined: Sep Sat 29, 2012 6:32 pm
Posts: 666
Location: Endicott, NY
I hate when I spend a bunch of time on a radio and it ends up not working anyway due to some unobtainable part (or a fault I just can't seem to locate). Do you use a power supply with current limiting, or do you just use an ammeter when you suspect a problem? (The power supply I use has adjustable current limiting, so I can keep excess current draw from cooking something).


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 Post subject: Re: 1964? Panasonic T-81H AM/FM/SW -- latest arrival (nice)
PostPosted: Jan Sun 08, 2017 7:14 pm 
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Joined: Oct Mon 26, 2009 10:02 pm
Posts: 4381
Location: Indianapolis, IN
Matt, typically just use batteries. When I noticed the voltage drop when switch turned on, then used ammeter to determine current draw...

John


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 Post subject: Re: 1964? Panasonic T-81H AM/FM/SW -- latest arrival (nice)
PostPosted: Jan Sun 08, 2017 7:49 pm 
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Joined: Nov Wed 14, 2007 11:37 pm
Posts: 1197
Get a refund. It's not worth fussing with.


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 Post subject: Re: 1964? Panasonic T-81H AM/FM/SW -- latest arrival (nice)
PostPosted: Jan Sun 08, 2017 11:59 pm 
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Joined: Oct Mon 26, 2009 10:02 pm
Posts: 4381
Location: Indianapolis, IN
Metzman---these radios are real gems when working, so worth a little effort... And they look great, too !

A bit of good news this afternoon !

Found that the "bias regulator" transistor was the "current sucker"---erroneously thought I'd disabled the entire output stage by pulling the 5.6 ohm emitter resistor... After disconnection, current dropped to just a few milliamps.

Decided to go ahead and pull the converter transistor (X3) and install a socket for experimentation. After removal, checked junction resistances and found significant leakage to grounded shield lead---tin whiskers ??? Bent the shield lead over and placed into socket---still no oscillation. Looked around in my transistor drawer and found a clipped-out Delco DS-38 which I think was used as an RF or converter in an old car radio. Plugged it in, applied power, and checked with scope---now oscillating fine !!! Got out the signal tracer and RF probe and connected to the collector pin---several stations were heard fine though there were some oscillations at various points on the dial. Moved probe to detector diode input and WOW---many stations came in fine including my favorite low-power DX station I use as a benchmark !!! Excellent sensitivity & selectivity without any adjustment---but there were still some oscillation issues mid to low dial. The signal sounded pretty good, but seemed to have a bit of distortion probably due to degraded IF transistor. Continued into the audio preamp & driver circuits with the tracer and there was significant amplification each stage, but somewhat more distorted. SW also seemed to work fine with just a short test lead as antenna. At least there's hope !

Pretty excited now to get this done !

John


Attachments:
ARF T-81H PCB TOP RECAPPED X3 SOCKET.jpg
ARF T-81H PCB TOP RECAPPED X3 SOCKET.jpg [ 200.02 KiB | Viewed 596 times ]
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 Post subject: Re: 1964? Panasonic T-81H AM/FM/SW -- latest arrival (nice)
PostPosted: Jan Mon 09, 2017 12:06 am 
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Excellent news.

Colin


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 Post subject: Re: 1964? Panasonic T-81H AM/FM/SW -- latest arrival (nice)
PostPosted: Jan Mon 09, 2017 2:15 am 
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Joined: May Wed 01, 2013 10:05 pm
Posts: 893
Location: British Columbia, CANADA
I knew you wouldn't give up that easily John, you can't stop now.

Today I also found my bias regulator is shorted in both directions (34 ohms) and an audio driver transistor is open.
Using my signal tracer I'm getting good strong audio just before the driver on both AM and FM.

Looks like anything close to a 2SB173 will work, I tried a few subs already, but not sure what to do about the "bias reg"


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