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Old 7th Oct 2007, 10:03 pm   #1
saxmaniac
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Default RA17 Help!

I have a problem with an RA17 which I bought 15 years ago and have only now got round to attempting to cure. It has identical symptoms to an old thread by skywave in 2006 and I thought I was reading about myself as he is using virtually identical test equipment!
I have low 37.5Mhz output from the filter & about a tenth of the required level at the test point 3 at the mixer stage. The stages up to the filter appear to give correct signal levels. Very little signal comes out of filter stage to grid of V10. All trimmers peak nicely & all tuning capacitors were changed, a few gave odd readings on a bridge. V10 stage gives no gain however, ie 100mv in = 100mv out. I have checked DC conditions, all capacitors, valve, anode tuning gives good peak. It is a simple stage so I must be missing something obvious! With artificial 1v signal injection at TP3 Reciever works, so no other faults exist. I do not know if fault exists in V10 stage or filter.Should masses of signal be lost in filter due to loose coupling, to be made up by gain in V10? Any comments appreciated as I am struggling now!
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Old 8th Oct 2007, 12:58 pm   #2
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Default Re: RA17 Help!

Here's the post referred to above:-

https://www.vintage-radio.net/forum/...ead.php?t=9783

On reading through it, it appears that Al fixed his RA17, but never told us the solution, at least not in that thread. Can you enlighten us please Al?
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Old 8th Oct 2007, 7:45 pm   #3
Skywave
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Arrow Re: RA17 Help!

Quote:
Originally Posted by Station X View Post
Can you enlighten us please Al?
Only too happy to oblige!

I looked at that old thread - I can see that I've declared it as "fixed; thread can be closed" - but didn't state what I had done. I was a raw newcomer to the Forum then, and hadn't picked up the Forum's 'culture'!

This is a quick 'just for now' reply to keep this Thread alive - it was quite some time back - and the notes I made at the time are buried in the workshop archive: it'll take some time to dig them out. But for now, from memory . . . .

This was one of those annoying inconclusive repair jobs - you measure this, test that, replace something else - using the manual as a guide (and decent test gear) - but fail to find any one thing that is clearly wrong in a relevant way. What I do recall however is that the input to the 4th. mixer from the harmonic generator is very critical: like any frequency changer, its IF output level is highly dependant on its local osc. drive level. In this case, if the drive level is too low, you get lack of 37.5 Mc. drive to the second mixer, and low O/P from that mixer at 2 - 3 Mc. Poor sensitivity is the resulting symptom, which can be dependant on what '1Mc. chunk' the receiver is tuned to by the action of the first VFO (40.5 to 69.5 Mc)

Not wishing to state the obvious, but to successfully fault-find in the 'front end' of a RA-17 does require a clear understanding of the whole freq. translation process - a.k.a. the Wadley Loop.

---------

For ease of reference, I've copied the relevant bits from my earlier thread below:

However, I seem to be making some headway now. I have been able to get the specified throughput from the HG to the mixer - as referred to above. It was noteworthy that one of the trimmers in the 37.5 Mc filter seems jumpy on adjustment. I've also replaced a couple of valves "on spec." along the way, even though the voltages were about right. (You know & I know that the d.c. op. condx. don't tell you everything that's going on!).

I can only conclude that all that heaving about on the bench (recent repair unrelated to this problem) must have upset something mechanically - e.g. the trimmer or ??
My provisional re-alignment has resulted in different settings in various places (L's and C's).

-------------------------

One point is worth mentioning: make sure that the 1Mc. crystal oscillator is producing full output. If it's low, then the whole set will not work properly. A loose connection in its socket is a real possibility. (See my comment above on 'heaving it about on the bench'). A quick test here is to check the internal calibrate signal when the mode switch is set to 'cal.' It uses the 1Mc. osc. as input to the HG used in the calibrator: weak cal. signal = weak 1 Mc. osc. (probably).

Having said all that, I must now 'come clean' and report that this receiver of mine is not well again. It has now developed a similar fault whereby the AE tune seems progressively less effective as the selected receive freq. is raised above about 17 / 18 Mc. In my 'non-radio life' there are a lot of outstanding commitments at present, so this fix is just going to have to wait for a while.

And there we are. I hope that lot has been of use - even if it went on a bit!

Al / Skywave.
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Old 9th Oct 2007, 6:14 pm   #4
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Default Re: RA17 Help!

Thanks for reply. I assume dodgy trimmer was at least part of your problem, I know all my tuned circuits peak up smoothly.
I really need some idea what level of 37.5 mhz should be present at output of the filter as a massive amount seems to disappear here. also I know that on mine 50mv injected at this point results in only 50mv at the anode of V10 so that stage doesnt appeat to give any gain. Drive into filter is fine, previous stages all work, 1mhz oscillator etc all healthy output
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Old 9th Oct 2007, 10:47 pm   #5
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Arrow Re: RA17 Help!

O.K. - I've now got the maint. man. & cct. diag. in front of me, so let's see what relevant stuff pops up . . . .

You say: "I have low 37.5MHz output from 'the filter' & about a tenth of the required level at the test point 3 at 'the mixer' stage".
Well that's clearly wrong for a start - and, I suspect, will result in a distinct lack of sensitivity. (I assume here that 'the filter' is the 37.5 Mc. band-pass filter and that 'the mixer' is the 2nd. mixer).

First thought: Check that the filter is actually & accurately tuned to 37.5; note that it has a pass-band of 300 kc. (to allow for drift in the 1st. VFO).

Extract from maint. man. P22, para. 5.13 . . .
"The filtered 37.5 Mc. signal is further amplified by V10 . . . ."
So . . . V10 does provide gain . . . more on this later . .

Extract from the maint. man., P41 . . .
"A fault appearing atthe 2nd. mixer stage may be due either to the mixer ccts. or lack of 37.5 Mc. drive (my italics). The latter may be checked by removing the second mixer valve (V9) and connecting a suitable valve voltmeter to the test point TP3. At each setting of the 'megacycles' dial, several volts at 37.5 Mc. should be measured. The absence of this voltage or lack of it (my italics; added comment) will indicate a fault in the harmonic generator and / of amplifier ccts., harmonic filter or possibly the 1st. VFO stage."

On P43 . . .
"Signal input (c.w.) to grid of V10 @ 37.5 Mc., level = 40mV, produces 1v at test point TP3. Test condition: V9 (2nd. mixer), V5 (1st. VFO) and 1 Mc. crystal removed; VTVM shunted with 12 pF" (my italics; added comment). N.B. A X10 'scope probe will have an input cap. comparable to this, if you're not using a VTVM.

If you disconnect the BPF output from the grid of V10 and feed in a 37.5 Mc. signal from the sig. gen., the above implies that you should get the 1v. at TP3 - which, in effect, is the anode of V10. If you don't get this level, check for correct tuning of L50 / C113 (V10 anode load; adjust dust core of L10). Try a replacement screen-grid decoupling cap. (C114); check cathode decoupling cap.; V10 itself may be faulty (CV138, aka EF91) - e.g. simply 'gone low gain'.

Furthermore, the man. also states that " . . . to get this '1v at TP3', you need . . .
25 mV at grid of V8; 2.8 mV at TP1" (harmonic filter feed-point to the harmonic mixer) . . . ."
Armed with this info, you can check for correct levels through the 37.5 Mc. amp. chain.

Note, however, further down on P43, it is stated that . . .
"The level of the 37.5 Mc. drive at TP3 should be between 2 and 10 volts at any 'megacycle' dial setting". I can only assume that the conditions here are now with V9, V5 and the 1 Mc. xtal in situ. (in other words, all valves in place and all settings adjusted correctly; i.e. 'normal operation').

On P47, full details are provided for the re-alignment of the 37.5 Mc. BPF, with typical signal levels to be expected. I'm not going to repeat that lot here, now, [] since I suspect (read: hope) that all the above should enable you to 'home in' on the faulty component(s) and / or settings.

Finally, a word of encouragement.
The RA-17 is a superb receiver when it's 'running to spec.' But it is a complicated beast! Therefore, maintenance tasks like this are NOT 'five minute quickies': patience, persistence and determination are required - but the results are usually worth the effort & time. The only one area that I wouldn't touch is the 40 Mc. BPF - that is without using a good quality spectrum analyser with a built-in tracking generator.

And that's it!
Let us know how you get on.
Good luck!

Al / Skywave.
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Old 10th Oct 2007, 9:26 pm   #6
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Default Re: RA17 Help!

Skywave, thanks for reply, over the next few days I am going to double check everything I have previously done and make detailed notes. The one thing that is glaringly obvious os that 40mV at grid of V10 does not result in anything like 1v at TP3, more like 40mV!. You are right in that the initial symptom is lack of sensitivity. If 37.5MHz is injected at TP3 artificially at the correct level the reciever works fine. I am using Marconi 2022D signal generator and Tektronics 475 scope for voltage measurement. One thing of interest though not the prime cause of this fault is that the internal 37.5MHz signal from M4 is not exact according to the signal generator, but tuning the 37.5 MHz filter to this does not improve results. Anyway, I will let you know what happens when I have double checked everything.
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Old 11th Oct 2007, 10:43 pm   #7
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Arrow Re: RA17 Help!

Hi.
I see that you're using a Marconi 2022D.
First, let me say that I've never heard of the "D" model 2022, 2022C & 2022E, yes. IMHO, 2022's are 'reasonable quality' S.G.s - and quite adequate for the task that you have in hand.

However, the point is this - it is possible, with a 2022, to have a freq. and / or a level offset arranged internally under uP / software control. The full manual tells you all about this. It may well be worth your while checking the freq. of the 2022 with that of another generator - I personally recommend the HP 8640B (my 'default' sig. gen.). Mind you, the 2022 is a much easier S.G to repair!

Further - and a follow-on from my earlier posts - I have now reviewed my 'archive workshop notes' on the fault that I had (v. similar to yours) - and there is nothing new therein to add to what I have stated above. Amongst these notes, there are various measurements of voltage levels in the main signal path, which might be of use to you at a later date, if you need to 'dig in' to those sections.

TTFN.

Al / Skywave.
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