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Old 22nd Jul 2017, 11:54 am   #21
Kevin Hoyland
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Default Re: RCA AR88 LF 600 ohm

Hello Colin.
Thank You for all the information regarding the 600 ohms output. If I can not get a 2.5 ohms output on the Audio Transformer tag I will then have go for the matching Transformer.
on ebay I see that some tannoy speakers are rated at 70v have settings for 670 ohms at 7 watts if I get my hands on one I can then start working on the 88 lf and listen at the same time. I will send for the matching transformer 119-Td made by Hammond but it looks like it will have to come from the US
Thank You.
Kevin.
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Old 22nd Jul 2017, 12:06 pm   #22
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Default Re: RCA AR88 LF 600 ohm

The printing on the rear chassis apron looks as though it's original? If so then that would suggest the receiver came fitted with that output transformer fitted as standard, interesting paint/varnish job on T5.

I wonder if the front panel was part of the modification?

Lawrence.
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Old 22nd Jul 2017, 1:17 pm   #23
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Default Re: RCA AR88 LF 600 ohm

http://www.henrys.co.uk/PA/100v_line_transformers.html
I was wondering if this would do the trick.
Don, m5aky
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Old 22nd Jul 2017, 4:14 pm   #24
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Default Re: RCA AR88 LF 600 ohm

For boatanchors a small filament transformer will work perfectly as a 600 to low impedance speaker matching transformer. Voltage ratio is very roughly square root of the desired impedance ratio. Unless you are seriously starved for power a factor of two in the voltage ratio is good enough. You don't need anything fancy unless youneed response to 10 kHz, which won't make it through that sort of radio.

Visit your junk box. People seriously overthink this. And overpay!
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Old 22nd Jul 2017, 5:59 pm   #25
Kevin Hoyland
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Default Re: RCA AR88 LF 600 ohm

Hello.
Just sent for 3 line matching Transformer from the us made by Lowell they are TLM870-70 volt power rating 8 watts- 8 or 4 ohms. They work out £3.50 each postage is just over the cost of the 3 Transformers.

Kevin
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Old 22nd Jul 2017, 8:13 pm   #26
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Default Re: RCA AR88 LF 600 ohm

Aside from the question of impedance matching, does the LF model O/P Tr have the same risk of a cascade failure on the primary as is often reported for others, ie a possible burn out if it's not kept loaded one way or another? That used to be a disaster as replacement originals are scarce but more recently, alternative TR's have been fitted and protective circuit changes suggested to stop it happening in the first place. It was an ongoing topic at one time and put down to a war time production rush creating an avoidable problem but of course, there would be an endless supply of parts in the store then.

Dave W
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Old 23rd Jul 2017, 9:40 am   #27
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Default Re: RCA AR88 LF 600 ohm

Also, does this variant have the potentially destructive "tone correction" capacitor between o/p valve anode and chassis (I moved mine to across the transformer primary - same function but fail safe).
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Old 23rd Jul 2017, 10:46 am   #28
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Default Re: RCA AR88 LF 600 ohm

Taking a close peek at the photos in post 9, particularly the third one, shows two resistors connected between one of the output transformer tags that is also connected to one end of the mentioned 5 ohm resistor. These could well be the audio amp's feedback resistors, R54 (2.7k) and R37 (100 ohm)- they have the same electrical connections and physical location as the ones in my set, the one in the apparent "R37" location is clearly 100 ohms, the possible "R54" ends in red, silver so could well be 2.7k. Further evidence that this set does indeed have a low-impedance loudspeaker secondary that has been left internally loaded but unused for this particular application.

I also note that the noise limiter heater voltage dropping modification has been implemented, with 2x 10 ohm in parallel, and that the original "bogey" grid coupling capacitors have been replaced with decent HV ceramic types, That set does look in good general condition and, with its HM Taxpayer PVC loom, hopefully should give a few more decades good service.
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Old 23rd Jul 2017, 12:39 pm   #29
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Default Re: RCA AR88 LF 600 ohm

Hello.
Thank You to Peter G8BBZ for telling how to work out the M.O.D code.

Kevin.
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Old 23rd Jul 2017, 12:56 pm   #30
Kevin Hoyland
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Default Re: RCA AR88 LF 600 ohm

Hello.
When the Matching Transformer comes from the US I will then put a speaker on to the set. I will be replacing all the Capacitors and Resistors that need to be. When I went to pick up the 88 it had the power on and the sound was good.
From the time i came home it as never been turned on. I have tested all the Valves on my Avo mk 2 valve tester.

Kevin
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Old 23rd Jul 2017, 4:01 pm   #31
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Default Re: RCA AR88 LF 600 ohm

It does impress me that a radio designed in the late '30s with a free-running LO was still considered good enough to be the heart of a Cold War FSK installation two decades on.
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Old 23rd Jul 2017, 9:50 pm   #32
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Default Re: RCA AR88 LF 600 ohm

Quote:
Originally Posted by Kevin Hoyland View Post
I will be replacing all the Capacitors and Resistors that need to be.
The obvious capacitor candidates are the triple "bathtub" types. These can be leaky both electrically and physically. They can be opened and re-stuffed if you wish to retain the original look (caution - may contain PCBs).

The 33K screen feed resistors in the RF section can also go high, reducing the gain. Worth replacing but a PITA to access.
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Old 24th Jul 2017, 8:50 am   #33
Kevin Hoyland
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Default Re: RCA AR88 LF 600 ohm

Hello.

Thanks Keith regarding the Bathtubs capacitors and so on. I will replace the large block Capacitors 4.7 uf as well and hope it will be ok for the next 50 years or so.

Kevin
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Old 24th Jul 2017, 3:54 pm   #34
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Default Re: RCA AR88 LF 600 ohm

We've had many threads regarding these receivers, the search box above will find quite a few, including this one: https://www.vintage-radio.net/forum/...d.php?t=131415
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Old 25th Jul 2017, 7:07 pm   #35
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Default Re: RCA AR88 LF 600 ohm

http://www.vintage-radio.net/forum/a...1&d=1501005940. Hi Kevin . Here is a picture of the transformer cheers Chris .
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Old 1st Sep 2017, 4:09 pm   #36
Kevin Hoyland
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Default Re: RCA AR88 LF 600 ohm

Hello.
Fitted the Audio Transformer to the 8 ohms speaker and changed all the capacitor. Bathtubs and the 4uf ones in the main block. Put power on and sound out of all 6 bands, Band 2 RTE1 v/good but then after 15 to 20 mins the sound starts to distorts badly and the volume gos right down. Turn the set off let it go cool and all is ok for the next 15 mins or so. I have tested all the valves on my Avo mk2 all tested good. I have change some of the resistors that were high. Right at the back of the Rf section are some small Mica Capacitor in pf. I know it is heat related but after 40 to 50 hours working on it I am going walk a way from it for a day or two. On this set it as Diversity if gain were on my other sets it is tone control when the sound gos down you can turn the IF gain up to get some sound back but it is so distorted.

Kevin

Last edited by Kevin Hoyland; 1st Sep 2017 at 4:16 pm.
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