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Old 22nd Apr 2019, 9:41 pm   #21
mike1 ryan1
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Default Re: Bush DAC90A only receives 2 stations

V2 control grid and v1 control grid via a capacitor also earthed to chassis

Earthed via a capacitor too is that correct

10nf capacitor BTW in both
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Old 22nd Apr 2019, 10:45 pm   #22
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Default Re: Bush DAC90A only receives 2 stations

Yes that will be ok, but as I said you can loosely couple it to the radio, that is just holding close to IF stage, in fact you can make a loop aerial and connect that to sig gen and just sit it close to set. It’s worth trying out these different set ups once you have got your set working correctly.
Don’t want to confuse the issue, so good luck with your fault finding.
Cheers
John
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Old 23rd Apr 2019, 8:38 am   #23
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Default Re: Bush DAC90A only receives 2 stations

Hi Mike, I'm thinking you probably already have the service sheet which tells you how to do the alignment but if not you can get one on this website by clicking Vintage Radio Service Data (top right of this screen) or following this link https://www.service-data.com/search....&search=Search Cheers Jerry
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Old 23rd Apr 2019, 8:40 am   #24
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Default Re: Bush DAC90A only receives 2 stations

You mention that adjustment to IFT 1 has very little effect. This could be due to O/C of one or other of the 110pf capacitors connected across the primary and secondary if the I.F. coils. They are numbered C3 and C6 in the Newnes R&TV servicing pre 1953 models. They are probably inside the cans. John.
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Old 23rd Apr 2019, 8:59 am   #25
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Default Re: Bush DAC90A only receives 2 stations

Thanks John Is there a way to check without removing from the can and desoldering. If the signal was too high going in to v1 could that be a problem.
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Old 23rd Apr 2019, 10:47 am   #26
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Default Re: Bush DAC90A only receives 2 stations

You could try tacking a 100 or 110pf cap. across the winding(s) externally. If that allows you to peak the IFT, then the internal cap. is faulty. To save removing and opening the can, replacement capacitors could be permanently soldered in place. Others may disagree with adopting this approach, which I suggest whilst metaphorically wearing my service engineers hat, but if it restores correct operation without dismantling a 60+ years old component, why not do it that way? I have a DAC90A in my 'to do it' pile. Don't yet know what the problem is beyond a broken 2-pole line socket on the mains lead, but I can check if there's space to fit I.F. pader caps. externally when I open my 90A.
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Old 23rd Apr 2019, 11:20 am   #27
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Default Re: Bush DAC90A only receives 2 stations

Quote:
Originally Posted by mike1 ryan1 View Post
If the signal was too high going in to v1 could that be a problem.
Above a certain level (determined by the AGC delay voltage) the AGC will try to maintain a constant signal level input to the detector, this to a certain extent can mask the effect of trying to alter the tuning of the IF transformer(s) for peak response.

Also monitoring a modulated signal by ear when doing the alignment can sometimes be deceptive due to the characteristics of the human ear.

Lawrence.
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Old 23rd Apr 2019, 11:44 am   #28
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Default Re: Bush DAC90A only receives 2 stations

Right another worm can opened up. Had a think and a read and went to have another look. Got everything set up ready and had half hour while kids having breakfast before I had to get them to nursery. Slightly harrased I connected the wrong lead to the set, the one without capacitors in line, doh. Only the negative lead, and only touched the chassis with it....spark between the two and small puff of smoke rising from underneath the upturned chassis. Oh dear! Lesson learned, signal generator checked and ok but now decidedly dead radio. What is most likely to have been damaged by this action? Again just negative lead touched to the chassis.
Thanks,
Mike. ��
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Old 23rd Apr 2019, 12:17 pm   #29
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Default Re: Bush DAC90A only receives 2 stations

First, as a minimum, make sure that the mains supply to the receiver incorporates a working RCD.

Make sure that the receivers mains plug is fitted with a suitable fuse and wired correctly.

Make sure that the receivers chassis isn't live with respect to earth, then troubleshoot in the normal way.

Lawrence.
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Old 23rd Apr 2019, 1:50 pm   #30
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Default Re: Bush DAC90A only receives 2 stations

These things happen Mike, lesson learned. I would strongly suggest you purchase the service sheet and then carry out fault finding, firstly with mains disconnected.
Cheers
John
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Old 23rd Apr 2019, 4:02 pm   #31
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Default Re: Bush DAC90A only receives 2 stations

Hi John I do have the service sheet and have carried out voltage checks which seemed pretty close to where they should be . I have replaced all the wax capacitors on the tag board and checked and replaced resistors that were out of spec. I moved to the alignment because I was suspicious that all the if cores were wound right out. I had ordered a new electrolytic capacitor as this tested high on both sides, . I will wait until I get this and have another look. Bit time poor with kids, work, house, etc but hopefully I'll get there. I'll start at the beginning and check everything again when I get a chance.
Cheers,
Mike
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Old 23rd Apr 2019, 4:17 pm   #32
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Default Re: Bush DAC90A only receives 2 stations

The reason I say pretty close is that it says checks were done with an avo meter set to 1000v and it's a little hard to tell exactly what it is reading while trying not to move hands and short across anything.
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Old 23rd Apr 2019, 7:57 pm   #33
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Default Re: Bush DAC90A only receives 2 stations

What could have got smoked by just touching the negative lead to the chassis? I would have thought it more likely that I'd have damaged the signal generator doing that than the radio!
Mike
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Old 23rd Apr 2019, 9:01 pm   #34
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Default Re: Bush DAC90A only receives 2 stations

Sounds like time for the nose test, with the set unplugged of course. With a small puff of smoke, it could be a coil/inductance which you may be able to smell the demise of. If you're lucky it won't be but have a sniff test to see if you can pin-point anything and then double-check all the coils and output transformer with a meter set to Ohms X1 against the service sheet values. No point in moving forward until they've all checked out OK.

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Old 23rd Apr 2019, 9:27 pm   #35
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Default Re: Bush DAC90A only receives 2 stations

You have probably popped R16 [250 ohm] and the dial bulbs. It is in the neutral return to chassis. Worth a look. J.
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Old 23rd Apr 2019, 9:41 pm   #36
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Default Re: Bush DAC90A only receives 2 stations

I agree with the previous poster's diagnosis. It also means that you had the mains live connected to the chassis via the dial bulbs and their parallel resistor R18 (numbered according to Trader Sheet #1161), so when you touched the earth lead of the signal generator to chassis the radio mains switch must have been on and thus you put virtually full mains across the dial bulbs. You were lucky not to get a severe shock. Make sure that you reverse the plug polarity so that the chassis is at low potential. Should be easy enough to check continuity of the bulbs and R16 with a multimeter, with mains disconnected! Cheers and stay safe, Jerry
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Old 23rd Apr 2019, 9:56 pm   #37
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Default Re: Bush DAC90A only receives 2 stations

P.S. there's also a possibility that one of the mains switch contacts has been burned out so check those for continuity as well.
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Old 23rd Apr 2019, 10:31 pm   #38
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Default Re: Bush DAC90A only receives 2 stations

I agree with previous posts, R15, 250 ohms across dial lamps, according to my Bush service sheet, which is the most likely problem, and probably dial lamps.
Just to add a further word of warning regarding the chassis being being connected to “live”, make sure it is not connected this way, as has already been explained. Take care working on this set.
Cheers
John
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Old 24th Apr 2019, 4:31 am   #39
mike1 ryan1
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Default Re: Bush DAC90A only receives 2 stations

I will check them out when I get a chance again thanks. I also appreciate your concerns for my safety and will reverse the polarity of the plug before I turn it on again. I'm pretty careful keep my hands away from the set, I've been turning it off before touching it to move it, and was attaching small hook tips to the valve pins for connecting the signal generator to before turning it on to keep from touching/shorting anything accidently.
Thanks for the advice,
Mike
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Old 24th Apr 2019, 4:47 am   #40
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Default Re: Bush DAC90A only receives 2 stations

Couldn't wait so at 04:30 on my way to work I went out to the garage to check and sure enough R16 the lamp resistor is blown. This is a large green affair. What watt replacement should I get? I do remember reading one of the mods to get brighter lamps so I shall find that and re-read too.
Thanks again,
Mike
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