9th Sep 2019, 9:43 am | #41 |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,364
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Re: Bush SUG26
Hi Robert, OK, almost certainly a fault then in either the IF stage (V2, EF41) or the frequency changer V1 (ECH42). As before, cathode voltage measurements will tell us if the valves are conducting. From your previous post we know the anode voltage of the EF41 is 120v - what is the cathode voltage (pin 7)? We know the mixer anode voltage of ECH42 is 98v - what is the oscillator anode (pin 3) and cathode (pin 7)? Also assume you've cleaned the valve pins, valve sockets and wavechange switch contacts? Cheers, Jerry
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10th Sep 2019, 7:59 am | #42 |
Triode
Join Date: May 2019
Location: Cavan, Republic of Ireland.
Posts: 42
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Re: Bush SUG26
Hi Jerry,
V2 EF41 Pin2 = 94v, Pin7 = .8v V1 ECH42 Pin2 = 120v, Pin3 = 75v, Pin7 = 1.1v Regards Robert |
10th Sep 2019, 8:31 am | #43 |
Nonode
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Re: Bush SUG26
Hi Robert, those voltages look reasonable. Unless you have access to a signal generator the next steps are going to be difficult. I'll have a think, but meanwhile somebody else might have an idea or two. I can send you a replacement ECH42 if you PM your snail mail address but I suspect the valve is OK. You might want to check the oscillator coupling capacitor (the one connected in the ECH42 triode anode circuit) usually about 50pF. I 'm not on the service sheet at present so can't give exact details until later this evening. Cheers, Jerry
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10th Sep 2019, 8:37 am | #44 |
Nonode
Join Date: Oct 2017
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Re: Bush SUG26
Ps also worth measuring the grid2 (pin5) voltage on EF41 and grid2&4 (pin5) of ECH42.
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10th Sep 2019, 8:39 am | #45 |
Triode
Join Date: May 2019
Location: Cavan, Republic of Ireland.
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Re: Bush SUG26
Hi Jerry,
I have a Black star Jupiter 2000 signal generator and an 35 year old scope sitting on a shelf. I am pretty sure they are in working order. Regards Robert |
10th Sep 2019, 8:27 pm | #46 |
Nonode
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Re: Bush SUG26
Hi Robert, that's more good news then. I'm not familiar with that Sig Gen but data says it operates over the range 0.2Hz - 2MHz so that frequency range will suffice for generating a sinewave at the IF (465kHz) and MW and LW at least; however, the sig gen does not seem to provide a facility for internal or external AM tone modulation, which will make life more difficult. I think the first thing is to bottom out whether all the DC conditions on the EF41 and ECH42 are within tolerable limits; if the EF41 grid2 and/or the ECH42 grid2&4 voltages are absent or way off spec then you won't receive anything i.e. a likely diagnosis of the fault might have been found. Those voltages should be 60v and 55v respectively. If those voltages are OK, then a sig gen input will be needed. You could inject an unmodulated IF signal at 465kHz via an isolation capacitor to grid1 (pin6) of the EF41 but as there is no tone modulation on the 465kHz sinewave you're not going to hear any output from the speaker even if the IF stage is working. I guess you could monitor the output across L16 with the scope and look for a 465kHz output but there are likely to be issues with loading unless you have an RF probe of some sort for the scope. If anybody else has any better advice on how best to proceed then now would be a great time to post some help! Cheers, Jerry
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11th Sep 2019, 3:25 am | #47 |
Nonode
Join Date: Jun 2007
Location: Coulsdon, London, UK.
Posts: 2,152
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Re: Bush SUG26
The Black Star Jupiter 2000 is a Function Generator.
There is no modulated output. It does have Sine, triangle and square wave outputs. |
12th Sep 2019, 8:42 pm | #48 |
Triode
Join Date: May 2019
Location: Cavan, Republic of Ireland.
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Re: Bush SUG26
Hi Jerry,
I have replaced C10. It didn't make much difference to the voltages on V1 or V2. V1, Pin2 = 120v Pin3 = 78v Pin4 = -1.15v Pin5 = 78v Pin6 = -2.7v Pin7 = 1.1v Pin4 voltages change as I change bands. -1.15v, -9.1v, -4.9v, .6v V2, pin2 = 94v Pin5 = 73.5v Pin6 = .03v Pin7 = .73v I think it is time to invest in a signal generator. What would members advise to look for or stay away from. |
12th Sep 2019, 10:09 pm | #49 |
Nonode
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Re: Bush SUG26
Hi Robert, the voltages look plausible. One thing I overlooked is that it's possible a previous owner has twiddled with the cores in the IF cans. I'm not suggesting you should tweak them without a suitable Sig gen, but does it look as if they have been adjusted recently? Evidence of malpractice is typically that they have all been screwed fully in or almost fully out? As far as Sig gens are concerned there's a reasonable choice available. The problem is, unless you buy from a reliable seller, you will almost certainly end up having to do some resto on the Sig gen itself before you can use it. Even then the calibration might be out unless you also get a frequency counter or meter. If I were to recommend one Sig gen it would be the Advance E2. There are plenty around and they are usually easy to restore and use https://www.radiomuseum.org/r/advance_in_e2e.html. You might want to put a wanted request here on the Forum, as you're more likely to get a good one here than on an Auction website. Sorry I couldn't diagnose the fault for you; from the measurements you've taken I would say the valves are probably OK. Given a known serviceable Sig gen the next thing I would do is inject an AM tone modulated 465kHz signal into V2 grid1 and see if the tone comes through the speaker. If it comes through strong and clear then the IF is OK and the fault lies in the V1 stage so you can inject some similarly modulated RF signals into the aerial socket. Just for the record it would also help if you would list the capacitors you've replaced already. Cheers, Jerry
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13th Sep 2019, 8:53 am | #50 |
Nonode
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Re: Bush SUG26
Another thought is if you have a transistor radio that receives AM then turn it on at the same time as the Bush and place the transistor close to the V1 end of the Bush chassis. Tune both radios to approx. same wavelength (on MW) and then rock the tuning of the Bush one way and then the other. If the Bush oscillator (V1) is running you should pick up a whistle on the transistor as you tune past with the Bush. Cheers, Jerry
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14th Sep 2019, 9:04 pm | #51 |
Triode
Join Date: May 2019
Location: Cavan, Republic of Ireland.
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Re: Bush SUG26
Hi Jerry,
I tuned a transistor radio to a station on 530 MW. On the Bush I was getting a whistle coming in on approx. 300 MW. Am I right in assuming the oscillator is working but the frequency is just way off course. The IF cans don't seem to have been moved. There is a what looks like some locking wax on the under side of screws which is still in place, although there is nothing on the upper screws so I can't tell if they have been adjusted. Regards Robert |
15th Sep 2019, 8:37 am | #52 |
Nonode
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Re: Bush SUG26
Hi Robert, that's interesting. 530m=566kHz. If we assume the transistor IF is 455kHz (in practice it could be anything between 455 and 470kHz) then the transistor oscillator will be running at the dial frequency (566kHz) + IF (455kHz) =1021kHz. That is then the frequency the transistor will be radiating. 1021kHz=294m which is spookily close to 300m and therefore indicates that the Bush is picking it up normally. For that to happen then V1 and V2 on the Bush would have to be working. It would be good to try the reverse procedure and tune the Bush to 530 and see if the transistor picks up a whistle at 300? Can you also advise what you're using for an aerial on the Bush? Cheers, Jerry
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19th Sep 2019, 9:27 am | #53 |
Triode
Join Date: May 2019
Location: Cavan, Republic of Ireland.
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Re: Bush SUG26
Hi Jerry, I have tried the Bush on approx 530 and am getting a whistle on the transistor at around 390.
For an aerial, I have a 40m coaxial cable going around the outside of the house. I also have a 5m coaxial cable in the workshop. I am inserting the braid into the aerial socket. There is no difference to the reception using either cable. I get good reception on other radios using the 40m external cable. Regards Robert |
19th Sep 2019, 11:15 am | #54 |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
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Re: Bush SUG26
It might be worth checking the control grid voltages with a DMM on the mixer, the IF amp and the oscillator.
Lawrence. |
19th Sep 2019, 2:52 pm | #55 |
Nonode
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Re: Bush SUG26
OK, agree Lawrence's suggestion. Meanwhile, to recap (sorry for the pun). We know all is working OK from the control grid (g1) of V3 (1st EBC41 AF pre-amp) because you've played audio from a phone though it. From the front end it appears as if the oscillator is running (at least on MW) as you are picking up from and radiating to a whistle on a nearby transistor portable. Because you are receiving that whistle on or about the expected wavelength that tends to indicate the detector diode in the first EBC41 and the IF amplifier EF41 are also working. The only thing that leaves us with is a fault somewhere in the RF aerial tuning circuit that is common to LW/MW/SW. Here's a few more guesses. RF tuning capacitor C36 has shorting vanes or a broken wire connection. Broken or shorting wavechange switch S1 or S10. C5 open circuit. If none of those come up trumps try connecting the aerial direct to g1 of V1 (pin 6) and see if there is any sound from the speaker as you tune around. Not sure how loud the whistles were that you were hearing with the transistor tests. Should have been fairly loud. If faint then the possibility remains that the detector diode pins/valve socket receptacles on the first EBC41 have a high resistance and need cleaning (pins 5&6). Finally check the detector diode load resistor R12 (330k) hasn't gone o/c. Cheers, Jerry
Last edited by cathoderay57; 19th Sep 2019 at 3:01 pm. |
19th Sep 2019, 3:21 pm | #56 |
Nonode
Join Date: Oct 2017
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Re: Bush SUG26
A couple of additions that I missed. If L14 or L16 are either o/c or the trimmer capacitors have gone short circuit then that could be another couple of possibilities. The whistle tests tend to indicate these faults are unlikely. You'll need to disconnect one wire from each before measuring - both should be 12.5 Ohms.
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