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 Post subject: Radio broadcast test receiver?
PostPosted: Sep Fri 04, 2020 9:59 am 
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Hi,

I got this old radio device that based on the serial plate was used by the local public service broadcasting company. Late 40's or early 50's.
Device is battery operated and has surprisingly large number of tubes. Based on dial and selector switch it supports FM and maybe different modulation modes. I'm assume it could be kind of broadcast test receiver for field engineers.

Naturally there is no schematic and this was not made for the consumer market. However it appears to be factory made in larger series and uses British components. So it could be just locally cased more common device.

Is there any hope of finding schematics that would match even close what I've? Or at least help to understand what I'm dealing with.

Thanks!


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 Post subject: Re: Radio broadcast test receiver?
PostPosted: Sep Fri 04, 2020 12:52 pm 
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Could this have been used to track down illegal radio users? You had to have a license to listen back then in Britain.



Dennis


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 Post subject: Re: Radio broadcast test receiver?
PostPosted: Sep Fri 04, 2020 1:04 pm 
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It's from Finland, based upon the legible words on the front panel.

Herkkyys = sensitivity

Hehku = glow or incandescent

EI = no, or off

So we know the upper left toggle switch sets the meter sensitivity, and the lower right one is the power switch.

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 Post subject: Re: Radio broadcast test receiver?
PostPosted: Sep Fri 04, 2020 1:59 pm 
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Mr. Detrola wrote:
It's from Finland, based upon the legible words on the front panel.

Correct, it has id plate (low left corner) that marks it as property of Finnish public broadcasting company. However I've reason to believe that the actual chassis is British made and just cabinet has been done locally.


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 Post subject: Re: Radio broadcast test receiver?
PostPosted: Sep Sun 06, 2020 12:04 am 
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Where ever it's from, me likey!

Would love to play with that one. Looks like a three bander. BTW, that would use Finnish batteries, I recognize the battery tags.


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 Post subject: Re: Radio broadcast test receiver?
PostPosted: Sep Sun 06, 2020 1:01 am 
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I think it has 4 bands: LW, AM broadcast, SW, and FM broadcast.

Dave


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 Post subject: Re: Radio broadcast test receiver?
PostPosted: Sep Sun 06, 2020 6:43 am 
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Chances are it uses these kind of batteries...the filament battery would have brass knurled screw terminals, the B battery would have small plug sockets.


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 Post subject: Re: Radio broadcast test receiver?
PostPosted: Sep Sun 06, 2020 10:22 am 
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battery-maker wrote:
Chances are it uses these kind of batteries...the filament battery would have brass knurled screw terminals, the B battery would have small plug sockets.


It came with batteries but they were empty :)

I will try it with lab power to fire up fliaments (+1.5V) and just finished building a power supply to get out ~90V DC. Let's see if there is any life left in this thing.

Here is also picture underside shield, if I'm not mistaken it has 4 IF amplification stages and FM ratio detector? The copper coils going to tube positive fliament pins are for filtering interference I guess?


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 Post subject: Re: Radio broadcast test receiver?
PostPosted: Sep Sun 06, 2020 12:43 pm 
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Has a USA, I presume, Weston uA meter. You could do several things with it. You could do a survey of broadcast signal strength in a particular area, or maybe even check interference with it. What does it do for an antenna?

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 Post subject: Re: Radio broadcast test receiver?
PostPosted: Sep Sun 06, 2020 5:27 pm 
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wazz wrote:
Has a USA, I presume, Weston uA meter. You could do several things with it. You could do a survey of broadcast signal strength in a particular area, or maybe even check interference with it. What does it do for an antenna?

I'm still learning antenna stuff but If I'm interpreting the physical circuit correctly there are 3 different antenna setups.

  • The top left antenna holes "A" and "M" are for the AM antenna. In Finnish parlay the 'A' is antenna and 'M' is for ground ("Maa" = "Ground").
  • "UA" connector top right is for dipole antenna, I've seen similiar circuits and wiring in 50's FM tube radios. ("UA" = "Ultra-aallot" = "Ultra waves")
  • Inside of the cabinet has two sets of copper wire coils around it, shorter and longer. I think there is about 20 and 40 loops in each so total wire length is easily several meters (well beyond 6 feet). Very similiar to AA5 loop antennas.

I don't know what the 'GR' plugs are for (bottom left). One of them goes to chassis and another is shielded wire to bowels of the circuitry. 'KU' is for headphones or extra speaker, connected parallel with audio trasformer primary.

Investigation continues. The elcos and wax caps leak like sieves (so much that capacitor meter refuses to test them). I need to start swapping them, starting from the audio power pentode. Almost all resistors measure 10-20% high but I'll not touch them yet unless I find dead short or open one.

Speaker and audio transformer work, I can get clicks and buzzing when poking transformer primary with different multimeter modes (diode test, resistance, capacitance, ..).


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 Post subject: Re: Radio broadcast test receiver?
PostPosted: Sep Mon 07, 2020 7:38 pm 
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You can usually get away with 20 percent plus resistors in radios. Have seen plenty that are 50 percent high and then is when you can get concerned. Depends on the application to the extent that it affects things noticeably. You certainly would not want a 10 percent error in resistors in a VOM. But a grid resistor that is marked 1 meg and reads 1.2 ought not make any difference. Actual tube biasing resistors may cause a small or moderate reduction in gain, or maybe even distortion in some instances. Depends on where and the design. Maybe in an oscillator, incorrect bias might cause problems with reliable function. Or not.

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 Post subject: Re: Radio broadcast test receiver?
PostPosted: Sep Tue 08, 2020 6:48 pm 
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Have been tracing and disassembling the circuit to reach those leaky caps buried deep under wires and rails.
This is what I've been figured so far, 3 out of 9 tubes. The switch was pretty complex, it is a 3 wafer switch, each wafer 2-pole 5-positions.
One wafer connects ammeter for current measurement. Second enables fliaments for various tubes and adjusts grid bias. Third connects audio input.
I'm bit scared of making mistake or wrong note and if I can ever bring it back to the way it was.

There is something that may be a mistake done by the original builder, the 25uF elco between B++ and B+ is rated 35V but the voltage over this cap can be easily over 35V. Also for some reason *all* the wax caps are rated 1500V. I just used 100V, except 3nF over the audio transformer primary, there I put 1600V one.


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 Post subject: Re: Radio broadcast test receiver?
PostPosted: Sep Tue 08, 2020 7:03 pm 
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Quote:
the 25uF elco between B++ and B+ is rated 35V but the voltage over this cap can be easily over 35V.

Since this is the area the grid bias (across R1 300 ohm) is created, I wouldn't think it would be much higher unless someone (tube) is really conducting hard beyond it's design rating. Feel free to put a higher voltage electrolytic in it's place.
EDIT: Since this is a battery set and the tubes are the 1.5V filament types I would guess the total current of all tubes thru the 300 ohm resistor 10 to 15 ma, so voltage across the 300 ohm biasing resistor would be about 3 volts, about right for tube bias
Nice schematics you are creating.

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 Post subject: Re: Radio broadcast test receiver?
PostPosted: Sep Wed 09, 2020 6:02 am 
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pauls.ironhorse wrote:
Quote:
the 25uF elco between B++ and B+ is rated 35V but the voltage over this cap can be easily over 35V.

Since this is the area the grid bias (across R1 300 ohm) is created, I wouldn't think it would be much higher unless someone (tube) is really conducting hard beyond it's design rating. Feel free to put a higher voltage electrolytic in it's place.
EDIT: Since this is a battery set and the tubes are the 1.5V filament types I would guess the total current of all tubes thru the 300 ohm resistor 10 to 15 ma, so voltage across the 300 ohm biasing resistor would be about 3 volts, about right for tube bias.

I meant the cap on supply side, between B battery voltage and adjustment pot. See at top center of the ”AF and power” schematic. Cap over grid bias resistor you’re referring to is rated 50V and as you said it has only few volts over it. Maybe builder swapped these two by mistake?

You’re right that the total B amps are around 10-15mA. That is the number I got summing up tube plate and screen currents from datasheets.


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 Post subject: Re: Radio broadcast test receiver?
PostPosted: Sep Fri 11, 2020 9:22 am 
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I had to disassemble almost all AM circuitry to reach and swap bunch of leaky caps. The mixer tube circuitry is more or less clear but the band selector is a separate module with large number of coils and trimmer caps. Trimmer caps have been sealed shut with glue. Not going to reverse engineer it, if it doesn't work then it doesn't work. The module includes AM antenna input and fixed loop antenna input.


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 Post subject: Re: Radio broadcast test receiver?
PostPosted: Sep Tue 15, 2020 6:48 am 
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Audio end including the transformer, speaker and tubes work. I built basic 90V supply for testing but found out the radio circuit is very sensitive to any voltage ripple, I will need to rework my supply design and add some regulation. After that's sorted out next is reassembling the AM circuitry.


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 Post subject: Re: Radio broadcast test receiver?
PostPosted: Sep Mon 21, 2020 7:23 pm 
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FM circuit looks like this. Coils and transformers are off course guesswork, the components are sealed.

Circuit has few weird things I don't get.

FM detector has two 100k open resistors (section A5). They are soldered to empty tabs on the other end that connect to nothing.

Dipole antenna secondary coil has two parallel somethings connected to ground (section D2), I think they may be 100pF ceramic caps? Never seen that kind of component before. [Edit: they are ceramic capacitors, apparently known to be very stable]


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File comment: Mystery component
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Last edited by tikonen on Nov Tue 24, 2020 7:11 pm, edited 1 time in total.
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 Post subject: Re: Radio broadcast test receiver?
PostPosted: Sep Fri 25, 2020 8:23 am 
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Resistors that connect to nothing might be test points?
(Used during factory testing)

Nice looking radio- it is compact.

David

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 Post subject: Re: Radio broadcast test receiver?
PostPosted: Nov Tue 24, 2020 7:10 pm 
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Got this radio finally working. AM works in theory but there are no AM radio stations in my part of the world so that's pretty useless. FM is surprisingly sensitive, I can listen stronger stations without antenna.

There are some sensitivity and interference issues i.e. its very picky about where I'm standing in the room. I need to reattach shielding plates, check soldering and get proper dipole antenna.

Here are the final reverse engineered schematics in case anybody is interested. I did not bother to draw open the AM band selector coil+cap assembly. Schematics are made with TinyCad open source tool.


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File comment: FM receiver
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File comment: AM and AF
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File comment: AM bandswitch blackbox
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File comment: new components
yleisradio_completed.jpg
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 Post subject: Re: Radio broadcast test receiver?
PostPosted: Nov Wed 25, 2020 1:15 am 
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tikonen wrote:
but there are no AM radio stations in my part of the world so that's pretty useless.

So where is your world. Please add your general location to your profile to help people like me out.

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