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Old 2nd Aug 2010, 10:34 am   #1
yestertech
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Default RR/Baird model 218AC - NO FM

A quick repair, or so I thought.
This one has begun to do my head in ( not that it takes much nowadays ! )

Previously repaired and many of the usual suspects already changed.
AM fine
FM fine as far as 10.7 MHz goes. All the IFTs and ratio detector align OK with 10.7 applied to the ECH81 grid.
Very very faint signals from the FM front end, with no background hiss.
When operating the FM/AM switching, signal momentarily increases ( still not that loud ) before falling back again.
All voltages seem normal. Valves tested by sub.
When a large signal at around 88MHz is applied to the aerial input, the signal can be tuned in, all the tuner L and C alignment points respond, and the output can be heard, but once removed there's virtually no gain/noise/signals.
It seems likely to me that there is little or no gain bein added by the EF80 RF stage, but I can't fathom why.

I have reproduced the relevant part of the circuit below. The FM signal exits at the top via C13 , whence via a series inductor ( L10 ) which is correctly peaked at 10.7MHz and onto the ECH81.
Changing parts is tricky as the tuner head is built onto an inverted U shaped chassis, and below the valves sits the tuning gang. I've annotated what I've measured so far. Most of the resistors are 20% and whilst they're slightly off I don't think it's enough to cause the fault. None of the caps appear leaky, but then I can't get at that many....
HELP !
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Old 2nd Aug 2010, 11:14 am   #2
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Default Re: RR/Baird model 218AC - NO FM

I would be suspicious of the oscillator. It may not be oscillating. FM signals can still get through, weakly, if two are offset by something close to the 10.7 IF (One station takes the place of the oscillator). The voltage on the grid of the oscillator doesn't look right and should be more negative (-4 vdc), but it is tricky to measure directly.

First verify if it is oscillating. If it isn't check the components around it. I had similar problems with a Telefunken, which ended up being a faulty mylar cap.
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Old 2nd Aug 2010, 11:39 am   #3
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Default Re: RR/Baird model 218AC - NO FM

Try putting the signal into the grid of V1. (L1 may be faulty.) Then go forward to the grid of triode B of the ECC85.

I know it's difficult to change parts on these but you may well have to. Regardless of the fault. But check things like wavechange switches first. Easier to sort out...

This may well not have worked from new, so try resoldering and tapping things.

Cheers,

Steve P.
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Old 2nd Aug 2010, 1:06 pm   #4
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Default Re: RR/Baird model 218AC - NO FM

Quote:
Originally Posted by setsappeal View Post
When operating the FM/AM switching, signal momentarily increases ( still not that loud ) before falling back again.
I'm at work at the moment so don't have access to any circuits ( assuming it's on Paul's DVD). There doesn't seem to be much in the way of switching in that part of the circuit but I wonder if there is a clue in the AM/FM switching as in the quote above. I'm thinking AGC, limiting....briefly changing when the wavechange switch is operated. Just idle thoughts during lunchbreak.



SB
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Old 2nd Aug 2010, 1:51 pm   #5
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Default Re: RR/Baird model 218AC - NO FM

I'm glad I sold it and didn't get round to putting it on the bench

Best of luck in getting her going again!
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Old 2nd Aug 2010, 3:50 pm   #6
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Default Re: RR/Baird model 218AC - NO FM

Thanks All - this one becomes ever more irritating ! To ensure I couldn't have missed something further down the IF, I connected a spare VHF front end which I kept from a scrapped Pye set, to the FM input in the direction of the ECH81 ( this is powered from my Bench PSU. )
That brought forth some signals which ,although not deafening ( only 1 IF in this set ! ) were more or less normal given my internal workshop aerial.
So , back to the errant tuner head and time to rip out the tunung gang and inspect what was beneath. Checking most of the caps gave more or less on spec. results until I got to C2. This measured all of 7.2 pf, so was changed. C! and C3 were of the same type so these were duly measured. C3 was as bad at just a few pf, and C1 had gone high and was reading 1.7 nF.
Having changed all these, checked most of the other caps and resistors and carefully resoldered all the parts which had to be moved out of the way , I finally reattached the tuning gang and switched on, full of hope !
NOTHING - no hiss, just the same as before.
Again, I can inject 88MHz to the aerial input and tune it, but there's no station output.

I removed the connection to the EF80 grid from the input transformer and it seems very unresponsive. Seemingly the same symptoms, ie lack of any gain from the RF stage

Beats Me. It's now seriously in danger of becoming an AM only set. I'm out of time and inspiration, alas.


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Old 2nd Aug 2010, 3:52 pm   #7
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Default Re: RR/Baird model 218AC - NO FM

Quote:
Originally Posted by setsappeal View Post
Previously repaired and many of the usual suspects already changed.
By yourself or persons unknown? If by persons unknown, then be prepared for wrong components, wiring....anything really.



SB
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Old 2nd Aug 2010, 4:21 pm   #8
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Default Re: RR/Baird model 218AC - NO FM

Try another ECC85. And the sockets.

Cheers,

Steve P.
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Old 2nd Aug 2010, 4:44 pm   #9
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Default Re: RR/Baird model 218AC - NO FM

Quote:
Originally Posted by Sideband View Post
Quote:
Originally Posted by setsappeal View Post
Previously repaired and many of the usual suspects already changed.
By yourself or persons unknown? If by persons unknown, then be prepared for wrong components, wiring....anything really.



SB
The set was previously restored by another collector and a BVWS member. I bought it with a lot of other sets which belonged to a chap who collected them and got them fixed by said BVWS member.
I've got several of the other sets still and all work on them seems good.

Josh.
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Old 2nd Aug 2010, 6:25 pm   #10
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Default Re: RR/Baird model 218AC - NO FM

Have you determined that the oscillator is oscillating?
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Old 6th Aug 2010, 3:14 pm   #11
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Default Re: RR/Baird model 218AC - NO FM

Well, after much digging, I found that the grid pin of the EF80 seemed to be making poor contact, and that a heater wire which ran underneath it had flaky sleeving and could have been touching. Having cleaned and re-soldered these and other pins and reassembled the tuner unit, I then realigned the whole lot. This brought quiet FM signals, which were improved on tweaking L10 which for some reason peaked in a different place when fed with 10.7 MHz directly from the generator. This then produced fairly pleasant FM results, but only just loud enough at full volume on the quieter stations. Stations towards the lower end of the band seemed to be accompanied by a LF signal , like a rumble in the background. To my surprise this was present with the volume down. Having checked all the other valves anyway by substitution, a new EABC80 was fitted, and this seems to have cleared that problem

Progress at last.

The overall quality and volume now yields a very similar result to that obtained with my spare tuner head hooked on, so I now have to turn my attention to the apparent lack of IF gain.
There remains one odd symptom - The 2 pin power plug is slightly loose, and if disturbed, ie the power briefly interrupted, there seems to be a momentary jump in gain. I only notice this on FM ! Shorting out the AFC Cap makes no difference
I also discovered that if the 1k resistor in series with the 2nd IFT ( EF85 stage ) is bypassed, the gain shoots up , accompanied by much distortion ( poss a red herring ? )

I am certain that, given there is an additional EF80 stage on the tuner head, this set is capable of better performance ?
It really needs to be at or near maximum volume for many stations
Does anyone have one of these who could comment on the performance ?

Andy
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Old 6th Aug 2010, 3:32 pm   #12
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Default Re: RR/Baird model 218AC - NO FM

Try another EF85 first of all. There is quite a difference between 10.7 mHz and 460 kHz.

Then check the biassing of the IF stages on FM as well as AM.

Cheers,

Steve P.
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Old 6th Aug 2010, 6:10 pm   #13
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Default Re: RR/Baird model 218AC - NO FM

Thanks Steve, I've subbed all the valves just in case, but no difference.
A.
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Old 6th Aug 2010, 7:37 pm   #14
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Default Re: RR/Baird model 218AC - NO FM

Have you checked the resistor and the HT rails. I notice from the diagram that the 250v rail goes to the IF and the VHF can. Is it low.

Check R43.

Cheers,

Steve P.

(Baird sheet.)
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Old 10th Aug 2010, 10:47 pm   #15
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Default Re: RR/Baird model 218AC - NO FM

Well after a short break, I've decided to mothball this one for now.
No obvious faults
Good quality FM, but seemingly low IF gain on FM resulting in very average volume levels.

The difference between Am and FM at the same volume setting is about 20db !!

Some you win.....


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