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Success Stories If you have successfully repaired or restored a piece of equipment, why not write up what you did and post details here. Particularly if it was interesting, unusual or challenging. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 7th May 2022, 10:44 pm   #1
Sideband
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Default Homebrew tuner from Richard GM thread

From this thread https://www.vintage-radio.net/forum/...d.php?t=190030. I decided that the builder of this tuner had taken so much trouble over it that it was worth saving. There is a matching amplifier as well which I've not yet looked at.

Well these items arrived a few weeks ago and I've only just got round to having a look. The FM tuner was particularly interesting since the IF unit was built in a tobacco tin.....very popular way back in the 60's and before. They made excellent housings for anything needing good screening.

Having done a few checks on the tuner (the mains lead had been cut) and not finding anything untoward, I decided to replace the lead and just see what happened. I removed the tuner from the home made case, replaced the mains lead, found some leads, connected the output to my headphone amp, plugged in an FM aerial and switched on. Nothing.....no hiss, crackle, clicks or pops. There was supply present at just over 9 volts. Since everything apart from the tuner section was in the tobacco tin I removed this next...very easy since the lid was the base so the tin just pulls off.

I was very impressed with what I found....a very neat and well constructed IF amp consisting of (as I found out later) three OC170 as IF amps and an OC71 as an AF amplifier. A few quick voltage checks soon revealed next to no voltage on the collector of the last IF amp and low voltage (3.2V) on the other two. The OC170 is the same construction as the notorious AF11x series and suffers the same tin whisker problem. It's actually an early version of the AF117.

Disconnecting the collector of the OC170 brought the collector supply voltage up to a more respectable 7.5V and the other two transistors were very similar. I set about replacing the OC170 using an AF125.... any of the usual AF124 to AF127 will replace it but the led-outs are different of course. Having removed the OC170 I confirmed from the data book which lead was which on the AF125 and used the coloured sleeving from the old transistor on the new AF125. Despite the shorter leads on the AF125, it fitted the space and I was able to locate it more or less where it's larger counterpart had fitted.

Next switch on was much more rewarding.....it worked! Only a very quick test as it was getting late but R2 and R4 came in very well.

A few photos showing the general tuner, the well-constructed IF amp in the tobacco tin, the location of the OC170 transistors and the replacement AF125 fitted.

I'll check it over properly next time and probably tackle the matching amplifier.

I'd like to know when this was built. The date on the 3000uF reservoir cap is 1962 which seems a bit early for a transistorised FM IF. The tuning scale has been hand marked with R2, R3 and R4 which means 1967 or later although of course the scale could have been hand marked at any time after 1967 with the tuner being built earlier.
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Last edited by Sideband; 7th May 2022 at 11:04 pm.
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Old 8th May 2022, 1:40 am   #2
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Default Re: Homebrew tuner from Richard GM thread

By the early 70's ceramic filters had emerged, and the dual gate MOSFET had taken over, at least for domestic sets.... there was still a nice where Philips/Mullard reckoned that bipolar front ends would save power compared to MOSFETs in battery sets. This was in the silicon era.

The difficulty here is in knowing whether the builder had bought the optimum parts of the time, or whether he built it around whatever he had in stock. The R2 R3 R4 markings may be the best guide you have.

Here, North of the border, we didn't get R4 on VHF until quite late on... 1980-ish. So it looks to have been made South of the border. The single RF tuned circuit implied by the dual gang capacitor implies that it was intended to receive services of roughly equal amplitudes from a single transmitter site.

It all looks earlier than the dial markings imply... could they have been added later? Technology-wise it could have been perfectly contemporary, receiving home, light and third. It looks early sixties.

People were looking for things better than those alloy diffused transistors and early JFETs like 2N3819 were seized upon as offering improved performance.

OC44 was announced in 1956 The OC170 came along at the beginning of the sixties. An end date can be put on it by the time Philips/Mullard changed their numbering system and OC- parts became AF-parts.

If the set was made by someone after the radio station renaming-fest, it was done by someone with his head still in the early sixties.

David
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Old 8th May 2022, 7:47 am   #3
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Default Re: Homebrew tuner from Richard GM thread

Laurence Nelson-Jones produced a design using both MOSFETs and ceramic filters, which was published in WW. This was around 1970/71, I think.
I've got one of the kit versions here, it works very well.

I would say that Sideband's tuner pre-dates this by quite a bit, possibly mid 60s ?
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Old 8th May 2022, 4:05 pm   #4
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Default Re: Homebrew tuner from Richard GM thread

Well It's for Wrotham I think which indicates London area. The R2, R3, R4 markings must have been added after 1967. I'm inclined to think mid-60's as well. The problem with home brew is that it could have been made anytime during the 60's or even early 70's. possibly the OC170 was being offered cheap by then since the AF11x was appearing. I'm still impressed by the tobacco tin I.F amp though!

I'm assuming the amplifier part was built at roughly the same time so that might give more clues.
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Old 9th May 2022, 8:20 am   #5
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Default Re: Homebrew tuner from Richard GM thread

Changed a couple of electrolytics in the I.F module...one a decoupler the other was in the FM detector. It sounds better but I have to say is a little 'bright'....I wonder about de-emphasis...or lack of! It appears to have some sort of AFC although it can't be switched out. It tunes in stations with a 'plop'....seems to have quite a wide 'pull-in' range and makes tuning close stations at the top end of FM quite difficult. It doesn't like the 400khz separation of some of the locals. It was obviously designed in the days when 2.2Mhz separation was the norm.
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Old 11th May 2022, 9:05 pm   #6
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Default Re: Homebrew tuner from Richard GM thread

I had a quick look inside the amplifier unit today. Unfortunately the pictures I took are not very good so I'll get some more later. However it uses silicon transistors throughout, the output pair are BD121. It has a separate pre-amp/tone control unit built on .15matrix veroboard and the main amplifier is built on .15 matrix plain perfboard. I was hoping to get some idea of build date from any electrolytics with date codes. I have found one which might be March 63 (63/3). However I think 1963 would be a bit early for silicon transistors. It looks late 60's to me. I've not yet identified the smaller signal transistors...maybe that will give a clue. They are plastic encapsulated in the pre-amp.

More when I've delved inside further.
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