Hi all,

Back again with an early sixties Panasonic RF-811, a small pocket AM-FM radio with slide rule tuning. I got this on eBay, along with an Airline ‘Transistor Six’ that we’ll tear into later. I asked several questions about condition and the nice seller offered a significant discount to shut me up. I took the bait and got a couple very nicely packed projects a few days later.

Lately, I’ve been looking for these features for vintage transistor sets…
>> Include FM (since AM is increasingly becoming a wasteland)
>> Horizontal slide rule tuning (cause I like the appearance of more precise tuning)
>> Black and Chrome (looks cool)
>> Channel Master & Panasonic (robust designs and builds)

The Panasonic has a hinged metal door for battery access. Haven’t seen that before, very nice feature. The plastic case has no cracks; the chromed metal front is in very nice shape. I wonder what metal is used for casting the front panel, its very heavy. I want to say cast iron but that doesn’t sound right.

The RF-811 has a few issues…
>> Antenna broken off – Ill post a WTB in classified
>> AM band not working – corrected with recap
>> FM band not working – TBD, recap didn’t help
>> Band switch broke in a strange way, when I slid the switch, the end of the plastic actuator snapped off, it’ll be an easy fix.

I replaced the 5 electrolytic caps and the radio began playing well on AM band. FM is completely dead, however. I cleaned band switch, no help. I’ll need to start checking with meter and scope, but a board layout would be a big help. The components are packed so tightly its very difficult to identify where components connect on foil side.

I do have a schematic from RM.org but that’s not really enough to troubleshoot. I notice that the voltage measurements are stated as negative on the Sam’s schematic. It appears that the measurements are taken relative to the +9V terminal rather than common, which is tied to –9V terminal. I get the feeling that PNP vs. NPN circuits caused some confusion as to how to best measure voltages. I get confused enough working with PNP without this. When I learned electronics, NPN was used in 98% of the circuitry, so I have become programmed to think in NPN terms.

More to follow, Tim

The schematics at radiomuseum.org are from a 1965 British service manual.
same: https://www.vintageshifi.com/repertoire ... =rf811.pdf

I will scan the eight-pages from Sams TSM-61 tomorrow and post them here.

Greg.

Thank you Greg!

As always, I appreciate your help.

Interesting that the schematic was an English publication. I wonder if our Brit brothers looked at voltage measurements a little differently... i.e. measuring from B+ rather than B- common.

Thanks, Tim

Incoming...

Thanks Greg,

Very nice quality scan!

Have a great day, Tim

Hi to all,
Hi egg (Greg): Wow, Thanks! for the service manual.

I have several of these Panasonic RF-811 radios.
Beautiful (heavy) construction. The epitome of quality Japanese AM/FM sets from the 70s.
Another favorite is the Sony TFM-825 shirt pocket AM/FM.

https://www.doctsf.com/sony-tfm-825-l/f27114

Best Regards
jhalphen
Paris/France

Hi jhalphen,

That Sony looks like a very attractive set, it has a similar look to the Panasonic.

Something about black & chrome

Thanks for posting it.

Tim

Hi all,

The RF-811 is now operating again. It was surprising that 3 semiconductors were bad…

If you hate to see old Ge semiconductors subbed with modern Si devices, you may want to skip this post.

In this case, all three subs seemed to work well without significant issues with biasing. I believe the real challenges with Ge > Si substitutions tend to be in circuits which use voltage divider or diode bias for a fixed bias voltage. With that said, I am looking at adding a 68K resistor from AGC line to common, it seems to give a better sweet spot for IF gain. I used a 250K pot to find a value which seems optimal, some of this probably is compensating for the different semiconductors.

Although the selected Si transistors seemed to have pretty high hFE compared to old Ge devices, things seem pretty stable without signs of oscillation. I did try 3 different Si transistors for the IF transistor (X4), hFE's ranging from 160 – 360, results were all pretty similar and I stuck with the intermediate 190 value.

X1 FM RF Amp - 2SA241 (replaced with an A733 PNP Si – hFE 360)
X4 FMIF #2 & AMIF #1 - 2SA70 (replaced with 2N3906 PNP Si – hFE 190)
X9 AM Overload FM AGC - OA70 (replaced with 1N5819 Schottky Diode – Vf .25 V)

I’d guess that the FM antenna on this set got a good static whack earlier in its life. All three components are pretty close to the antenna in an electrical sense. Hitting X1, through X9, then on to X4

X4 had an interesting failure, in addition to testing out as two diodes instead of a transistor, the 4th wire which is a shield had an 80 ohm short to the collector. Since the shield wire was connected to B+, this created an interesting mystery which was difficult to understand without pulling the transistor.

Interesting that the bad IF transistor (X4) was handling AM & FM, and AM was working in a minimal way without it??

I enjoy the troubleshooting, it’s a good challenge, always fun to see the receiver start working again.

Next, it’ll get an AM alignment and I’ll be replacing the dial cord since it got in the way of the soldering iron and snapped. After that, final cleanup and assembly.

Thanks, Tim

Hey Tim,

Glad you have it working and another very good presentation !

Looks like you've also experienced a "tin-whisker" with X4 ! Think I've seen that very type Matsushita transistor fail in at least one of my early Panasonics. Glad the silicon parts worked ! Will keep that in mind...

I also agree that you can't beat a sharp-looking black & chrome device...

If I don't remove a dial-cord, I usually wrap a thickness or two of printer type paper around it to act as an insulator when soldering nearby. Really bad when you burn one apart !

You're gaining transistor-radio project momentum---good to see !

Do you still have that Hallicrafters S-38 you bought from me many years ago ???

John

Hi to all,

@Tkilboy : Thanks! for the update on the Panasonic RF-811.

The 4th lead (shielding) showing low Ohms to either E/B/C lead is usually a sure sign of tin whisker internal short. These relatively "large can" transistors are filled with a compound which seems to favor whisker growth. In Europe, this particularly affected the series AF114 through AF118.
More modern replacements AF124 to AF129 have a smaller case and do not show whisker growth.

The UK vintage radio forum has a good article on the topic including a link to NASA's extensive research about this matter:

https://www.vintage-radio.net/forum/sho ... hp?t=76440

Best Regards
jhalphen
Paris/France

Yes John, the condom over the dial cord would have been a big help!

Many radios have come and gone but the S-38 has stayed around, the design is really a work of art, looks great next to the National SW-54.

I would have never thought of tin whiskers, I just assumed it was some static punch through.

Jhalphen, thanks for the link. Lots of good background there, particularly on Ge-Si conversions.

Thanks, Tim

All completed now, with the exception of a replacement antenna.

I found a Chinese seller on eBay that has a very similar antenna, same length, same internal screw thread for mounting, and just a bit skinnier at 5.5 mm vs original 7.5 mm.

So it’s all reassembled, sans antenna,

I was surprised to find a crack in one corner of rear case, one of those that you never notice until you happen to be putting a bit of force on and you see light through the corner. Easy repair with acetone based model cement, polished up and now about 95% undetectable.

Set performs well… fair to good on AM band, very good on FM. I imagine that FM audio quality must have been quite a treat in those early days.

I’ve attached a couple pictures of the finished radio.

Thanks to those who helped me get this done.

Tim

Looks great, Tim---and glad you kept the S-38 ! Still remember when a high-school friend gave it to me when we were kids (1975-76)---think he said an older neighbor had given it to him previously.

For the "skinnier" FM antenna, a layer or two of black heat-shrink sleeving may help physical fit...

Also noticed how much more professional your and others' photos look when a backdrop is used.

John

Looks great Tim! I agree with John, your backdrop really shows off your radio.

That was an interesting read on your success with substituting silicon transistors for the failed germanium ones. I also clip in pots to find the sweet spot for the bias.

I looked up the 1N5819 Schottky Diode you used to replace the OA70. I have a bunch of BAT46 Schottky diodes I use for similar replacements that also has a low Vf. I'd be curious to know if there is any difference in performance when a 1N5819 vs a BAT46 is used for a detector diode. Looking closely at the Vf charts, the 1N5819 has roughly half the Vf for the same current. If I extend the Vf chart, at 2ma the 1N5819 Vf is ~0.15V, and the BAT is ~0.3V

Nash

Nash---That is interesting about a 1N5819 comparison to the BAT-46. I purchased several BAT-46 Schottky diodes years ago after reading that they were a good replacement for detectors. In a couple cases, they actually performed better than originals, so would definitely like to know if the characteristics of the 1N5819 might make it superior.

John

Thanks for the kind words, I just use a piece of poster board alligator clipped to a shelf above my workbench. It makes a nice natural curve for corner free background. Sets up in about 30 seconds.

I just grabbed a 1N5819 from my stash in hopes of getting a low forward drop. I had bought a bag of these a while back when I was building a sensitive rectifier for a passive audio VU meter circuit.

Thanks, Tim

The antenna came in and is now installed. It has dimensions that are pretty close to the original. I was able to use the old antenna’s shell with the new antenna by using epoxy to attach them. This allowed for diameters to match! Both the old antenna and new use a 2mm screw, I was able to rummage to find a longer one in stock to make everything work.

Overall, it worked out pretty well for a modern antenna retrofit.

I’m attaching a few photos…

Thanks to all who helped!

Tim

Great job---antenna looks like it belongs !

John

Thanks John,

I did not expect to find a modern antenna that would work out so well.

Did a little DX evaluation of the AM section last night, not too shabby. I considered trying some alternate detector diodes as you guys discussed in another thread, but didn't feel like pulling the pcb again. Maybe a future project... Lesson learned, I guess if you're going to go with non-original diode, it makes sense to try a few

Take care, Tim