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Old 20th Jul 2020, 12:18 pm   #21
Ambientnoise
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Default Re: DAC90A Front End Woes.

David, well done for persevered! L1/L2 have been mentioned and you have said they are factory sealed but they seem about the only thing left. Have you checked them for continuity ? Maybe the Q has reduced for some reason ? I think this has come up on other sets, not DAc90A but I guess the only test would be substitution though.
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Old 20th Jul 2020, 12:53 pm   #22
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Default Re: DAC90A Front End Woes.

Hi David

Just catching up with your thread. Must admit I am baffled with the problem but have a scrap chassis here which I'd be happy to pop round if you need any spares.
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Old 20th Jul 2020, 5:07 pm   #23
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Default Re: DAC90A Front End Woes.

Hi David, You're going to hate me now but I appear to have persuaded my troublesome DAC90A to oscillate all by it's self ! not 100% certain how but there was a lot of valve pin cleaning involved. Then I discovered a faulty 2nd I.F. transformer, probably the fixed cap strapped across the winding as it wouldn't peak so out came the scrap chassis again and the transformer nicked from that. Then it was very sensitive on medium wave to any vibration near the wavechange switch which turned out to be a mis aligned mounting bracket which caused the knob screw to touch the aerial coil when in the MW position. All this is on a chassis that worked beautifully when fed with an external Local Osc ! I'm sure they do it on purpose so they can enjoy their retirement for a little longer. I know this doesn't really help you with your front end woes but at least one of us is having positive results.
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Old 20th Jul 2020, 7:25 pm   #24
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Default Re: DAC90A Front End Woes.

Quote:
Originally Posted by Ambientnoise View Post
David, well done for persevered! L1/L2 have been mentioned and you have said they are factory sealed but they seem about the only thing left. Have you checked them for continuity ? Maybe the Q has reduced for some reason ? I think this has come up on other sets, not DAC90A but I guess the only test would be substitution though.
Thanks for reading and your response. The service data only gives the continuity - not the inductance of the oscillator coil, and continuity wise, both windings of coil are fine. It, and the trimmers, show no sign of disturbance from new and the trimmers still have the blobs if white paint on them applied at the factory, and as I mentioned earlier, have now been tested and exonerated as the culprits.

I haven't quite re-wired and refitted the trimmer bank today due to other priorities, and tomorrow we'll be out walking on the Wolds - it will do me no ham to take a break till Weds! Two forum members have very kindly offered to supply me with parts from scrap chassis if need be, including Howard 'Half a Mullard' who lives local to me, so as the options narrow, I can envisage a replacement valve-holder for V1 being a possibility for this 'Franken-Bush'. Should be fun!

I do have the working one alongside the non-worker for comparison, but though I'm not really much of a fan of DAC90As, I decided to buy the 48-page 'e-book' by Yannis Karalis, which has very clear excellent layout drawing and lots of other info to make life a bit easier.

https://www.yanniskaralis.com/bush-d...sy-restoration

Quite a guy!
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Old 20th Jul 2020, 7:26 pm   #25
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Default Re: DAC90A Front End Woes.

Quote:
Originally Posted by Half a Mullard View Post
Hi David

Just catching up with your thread. Must admit I am baffled with the problem but have a scrap chassis here which I'd be happy to pop round if you need any spares.
Thanks for reading the thread Howard and for your very kind offer, which at the right things are going, I may well wish to take up!!
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Old 20th Jul 2020, 7:28 pm   #26
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Default Re: DAC90A Front End Woes.

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Originally Posted by AD360 Rob View Post
Hi David, You're going to hate me now but I appear to have persuaded my troublesome DAC90A to oscillate all by it's self ! not 100% certain how but there was a lot of valve pin cleaning involved. I know this doesn't really help you with your front end woes but at least one of us is having positive results.
Cheers
Rob
High fives all round Rob, from a 'kindred spirit'!
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Old 20th Jul 2020, 7:48 pm   #27
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Default Re: DAC90A Front End Woes.

It's not a core/coil gone pourus is it? I've just remembered that epic thread, from a few months ago, about an AR88 🙂

Cheers

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Old 20th Jul 2020, 8:55 pm   #28
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It's not a core/coil gone pourus is it? I've just remembered that epic thread, from a few months ago, about an AR88 🙂

Cheers

Aub
It doesn't look any different physically from the working set Aub, so hopefully it's OK.
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Old 20th Jul 2020, 10:09 pm   #29
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Default Re: DAC90A Front End Woes.

Quote:
Originally Posted by David G4EBT View Post
Quote:
Originally Posted by Half a Mullard View Post
Hi David

Just catching up with your thread. Must admit I am baffled with the problem but have a scrap chassis here which I'd be happy to pop round if you need any spares.
Thanks for reading the thread Howard and for your very kind offer, which at the right things are going, I may well wish to take up!!
Hi David

You would be most welcome. From memory it is missing the OP TX, dropper and volume control but should include everything else which looks like the bits you are likely to need anyway. You know where to find me and I'm sure a COVID friendly drop off could be arranged!!
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Old 20th Jul 2020, 11:04 pm   #30
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Default Re: DAC90A Front End Woes.

Sometimes I found it useful to prove the front end was OK by feeding a sig gen into the radio and looking at the signal at the 1st valve Grid with a scope. You can verify the signal is not only getting through but that the aerial circuits are also tuning and all done with no power on the radio. If you have already proved the I.F. amp then it just leaves the oscillator to sort. But I know too well how disheartening it can get when a radio refuses to work. I had one myself and replaced virtually all components around the local oscillator, cannot remember the make/model. My mate took a look for me and returned it next day working. I was stunned and begged him to tell me where the fault was. He said, with a grin, that he saw I had replaced the usual suspects except for a tubular ceramic and that was the culprit. I always accepted that ceramic capacitors were pretty reliable, I was caught out by this one
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Old 22nd Jul 2020, 9:00 am   #31
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Default Re: DAC90A Front End Woes.

David,

Looking at one of your photos, are they rust streaks around the mountings of L1/L2?

I only ask as a set recently gave me the run-around, and metering from chassis (ground) to some of the metal mounting brackets shown very low mV on the DVM. Not sure my AVO would've seen these low levels, since it may have loaded the circuit. A good cleanup of all earthing tags, through-bolts & bracket mounting points etc, and she sing's like a canary!

Measuring on low ohms showed a good contact, yet on the mV range, there was definitely somethin awry.

Will keep an eye on this post

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Old 22nd Jul 2020, 8:13 pm   #32
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Default Re: DAC90A Front End Woes.

Thanks for reading Mark and for your comments.

I think it might just be a reflection in the photograph. The mountings of L1/L2 aren't grounding points - they just secure the coil in place. (The coil has four tags). I haven't had chance to do any work on the chassis today, but have collected the 'donor' chassis kindly loaned by Howard - 'Half a Mullard' from which I can use any parts that I may need, such as a replacement valve-holder or any other front end components. While I've got the oscillator coil and associated wiring removed, I may as well replace the V1 valve-holder with one from Howard's chassis, all of which look in excellent condition. Howard suggested the easiest one to remove would be the rectifier as it has fewest connections. A nice bit of lateral thinking!

A bit of DAC90A trivia: Having handled many DAC90As over the years, one thing I'd never noticed till now is the the valve-holders for V1,2,4,5 each have one rivet and one 6BA nut and bolt to secure them to the chassis - the nut and bolt holding ground tags. (V3 just has two rivets). With Mark's comments in mind, I'll check any ground tags to ensure they are making good contact.

Howard's chassis still has the green 'Datum Sticker' on the rear of the metal tuning scale as an aid to alignment, which is missing from mine. I made some waterslide transfers of the sticker a while ago, so I'll now know exactly where to fit it. Although I can only check the resistance of the oscillator windings to compare with the datasheet, which are fine, now I have two oscillator coils, out of interest, I'll check the inductance of the windings, which will no doubt differ a little due to having been aligned on different sets.

As soon as I've fitted the replacement V1 valve-holder and rewired it and the oscillator coil, I'll do a little more testing and maybe hook up my scope.

A little voice in my head keeps saying 'It's not an AR88 - it's a DAC90A - how hard can it be?'
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Old 22nd Jul 2020, 8:45 pm   #33
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Default Re: DAC90A Front End Woes.

Checking those ground tags is a good idea. I had a tuning indicator once (an EM84) that refused to work correctly where the bars were always nearly closed and the edges of the bars were 'fuzzy'. Obviously I'd tried a new one with the same result and it was only by chance that I was prodding about with the DVM when I found a few hundred millivolts on the earth tag that was riveted to chassis via the valveholder. On a whim, I gave the rivet a whack with a screwdriver and a light hammer....not very professional perhaps but the result was a working tuning indicator. I later drilled out the rivet and replaced it with a 6BA screw, nut and star washer.

I had a similar problem on my Leak Troughline Stereo tuner when I first got it. It had always (apparently) suffered from intermittent gain and sensitivity and after about two hours, the gain did drop. Again more by luck than logical fault-finding I was prodding about underneath looking for dry joints etc when I prodded an earth tag on the ECF80 IF amp valveholder and it all burst into life again....another rivet, another high resistance connection. Same treatment as above (without the whack with a screwdriver)...! A 6BA screw, nut and star washer and a perfect tuner that has now worked for 5 years without a hitch.
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Old 23rd Jul 2020, 8:51 am   #34
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Default Re: DAC90A Front End Woes.

Slightly off subject , but just to show the earthing via rivets can be a problem on amplifiers to, I restore Juke Box amp's and have found this problem causing int' hum, having a steel chassis a permanent fix is to clean the bracket and adjoining chassis with a small wire brush in the drill ,then solder the bracket to the chassis using a bit of plumbers flux , I do all the earthing points , I have a new phrase! ,Future proofing Mick.
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Old 13th Aug 2020, 5:34 pm   #35
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Default Re: DAC90A Front End Woes.

It's been far too hot to be in the workshop this last week or two, but here's an update of what I've been able to do in spasmodic bursts of activity.

Howard, ( 'Half a Mullard') kindly loaned me a 'donor' chassis to remove any parts I may need.

It's often been said that valve-holders on DAC90As can be problematical, but I think that's generally related to the forked prongs of the Amphenol McMurdo bases. The bases in my non-working set are ceramic, and visually and electrically looked fine. Nevertheless for no other reason than to eliminate that as a possible source of the fault, I removed a valve-holder from Howard's donor set and replaced the V1 holder in my set. Quite a fiddly task, but successfully completed, albeit the front end fault persisted.

I've rebuilt/rewired the front end, (all except the wiring associated with the tuning cap/frame aerial/band-switch) and I think the triode section of the frequency changer is oscillating as I'll explain below. I think it can really only be the hexode (Aerial) section of the front end. If need be, I'll remove the turning cap, frame aerial and rewire it all, but of course, finding time to spend a day in the workshop is next to impossible

I removed and tested the trimmer bank to see if any of the trimmers had shorts, but they all tested fine and similar to those on Howard's donor chassis. I removed the oscillator coil from both my non-worker and the donor chassis to compare them. Both tested fine and their inductances measured the same - L1 75uH, L2 49uH. I removed the first IF can from the donor set, connected the primary to the signal generator and the secondary to my 'scope. Swept the signal generator across 465 kHz to see how the IF peaked at 465 kHz and fell off either side.

To check that there wasn't a fault in the IF cans of my non-working set, I checked both IFT1 & 2 in the same manner and they performed exactly the same as the donor one. Using my signal generator, with the little audio signal strength meter (designed by Paul Stenning) I recently made across the speaker, I peaked up the IFs so they're more or less spot on. (Could just have peaked them by ear, listening to the speaker output, but as I'd built the little signal strength jobbbie, I thought I'd 'road test' it. As an aside, the link to that in the homebrew section is here:

https://www.vintage-radio.net/forum/...d.php?t=163530

I've never had to align the IFTs on a DAC90A before so have never looked closely at the adjustment arrangement until now.

At first sight, there's what looks like a small hard-to-get-at locknut on a brass adjustment screw, (8BA?) with a flat on the end of the screw. I discovered that it isn't a 'locknut' but a 'captive' nut, and that the core adjusting screw will move freely with a trimming tool. Such a tool would need to have a 1.2mm wide slot but I didn't have one in my collection of trimming tools so I made one. I dare say that many DAC90A restorers will either leave the IFs well alone or use a small pair of pliers - clearly not ideal. I've attached a sketch of the IFTs and the trimming tool I made from a piece of 5mm diam brass rod and a 7cm long piece of plastic rod. I cut the slot with a hacksaw and widened it to 1.2mm with a needle file.

On my working DAC90A I tuned to a station on MW which consisted of two argumentative guys who could bore the boobs off a buffalo, nattering on about sport. I checked the oscillator coil frequency on my frequency counter with an 'RF sniffer'. It was 1518 kHz. With my non-working set alongside the worker, I tuned across the MW band on the non worker till it produced a heterodyne on the working set. I checked the oscillator frequency of the non worker at zero beat, and it was 1053kHz - 456kHz below the oscillator in the working set. I looked that frequency up and it turned out to be that the station I was hearing was 'Talk Sport'. (Saints preserve us - more rabbit than Sainsburys!).

I quickly tuned off that station and tuned in another MW station. Checked the oscillator frequency of that station and it was 1626 kHz. Tuned the non worker to zero beat with that station and checked the oscillator frequency of the non worker. It was 1161 kHz - again, 465 kHz below the oscillator of the working set. 1161 kHz turned out to be 'Greatest Hits Radio' - much more to my liking.

I take it from the results of these tests show that the oscillator section of the non-worker is actually working as it should, and that the fault lies in the aerial tuned stage. To align the first IF involves injection a 465kHz signal into the control grid (pin 6) of the hexode section of the UCH42, and given that the IF stages work fine, I presume that both the hexode and triode sections of the valve are working, and that what is lacking is the absence of an RF signal into the control grid from the aerial tuned circuit.

In pic 3 below I've highlighted in orange on the frequency changer part of the circuit where - as I see it - the fault must surely lie.

Hopefully, I'm on the right lines with the above tests and remedial work and in my view that all but the aerial stage is working as it should.

All the aerial stage connections including the frame aerial coils and wave-change switch check out fine for continuity. The only thing I feel I can do now, is the replaced the wiring connections in the aerial stage, and maybe remove and test the tuning cap because somewhere in that area seems to be the only place the fault can lie.

Unless anyone has any other ideas?

In my earlier post I said: 'Its not an AR88 - it's just a DAC90A - how hard can it be?'

The answer to that question seems to be: 'well 'ard'.

I'm not much into this lockdown malarkey and I like to be out and about, but this has kept me off the streets for a little while.

'Care in the Community' I guess.

Pic 1 Sketch of IFT trimming tool.
Pic 2: Sketch of oscillator coil connections.
Pic 3: Front end circuit highlighted in orange where I'm assuming the fault lies.

If you've read down to here - thanks - it's more than I could expect!
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Old 13th Aug 2020, 6:20 pm   #36
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Default Re: DAC90A Front End Woes.

I did enjoy that read David I am with you regarding Talk Sport that's all it is , well you have got it down to a few components ,you have got it cornered don't let it get away. Mick.
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Old 13th Aug 2020, 8:37 pm   #37
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Default Re: DAC90A Front End Woes.

Hi David
I admire your perseverance!! I’m also glad that the old chassis has proved some use but this really is a puzzle. It does sound as if the fault must lie in the RF side as you have checked everything else. I do have an NOS UCH42 which you would be most welcome to try but you did mention that the valve performed OK in your working set. I do have several DAC90A’s so if you wanted to double check anything please do let me know. I must admit I’ve been less inclined to be indoors with the weather of late! As Mick says don’t let it get away! Please keep us posted. Good luck!
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Old 16th Aug 2020, 8:23 pm   #38
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Default Re: DAC90A Front End Woes.

Thanks for your kind offer of assistance with the valve Howard, and other DAC90As if needs be. I've had a chance to nip out to the workshop for a few minutes this afternoon between downpours (of much needed rain!) and using my scope have confirmed what I believed to be the case. Namely, that the oscillator section frequency changer is actually working correctly.

The attached scope trace of the non-working radio is the same waveform and amplitude as on the working DAC90A alongside it. I didn't get chance to check waveform of the output of the hexode section of the worker into the 1st IF, as a benchmark for the non-worker, but as I said earlier, I'm pretty sure the fault lies in the aerial tuned circuit highlighted in orange on the part circuit in my earlier post.

Given that the DAC90A is a live chassis radio, I should stress that while carrying out these tests, the radio was powered via an isolation transformer as the scope is of course earthed and must not be directly connected to live chassis equipment. I also had a 0.1uF 630V capacitor in the test lead of the scope.

After all these years, I didn't realise that a DAC90A could lead such a merry dance!
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Old 16th Aug 2020, 9:02 pm   #39
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Default Re: DAC90A Front End Woes.

Hi David, Just a suggestion but have you swapped the actual aerial coils ? it's a long shot but worth a try as it's a quick job. Further to my saga, The set hadn't actually given up causing me troubles. I jubilantly went to demonstrate the now working chassis to a good friend who has been involved in many a session on that chassis, powered it on and waited. . . and waited. . . nothing, not a peep or even a crackle from the test speaker ! I was mortified as you can imagine. To cut a long story shot, the new fault turned out to be the UL41 output valve had lost all emission, or more likely had suffered an internal O/C. A replacement returned normal results.
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Old 16th Aug 2020, 9:57 pm   #40
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Thanks for reading Rob and for your suggestion.

Yes, though the aerial coils tests fine, changing it would as you say be a simple task and I do have Howard’s donor chassis to provide a substitute to see if it helps. I’ll keep bimbling on and it will greatly help by having the superb full colour drawings from the download from Yannis Karalis, about whom I can’t speak too highly.
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