24th Jun 2017, 10:12 pm | #181 |
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Re: Bush A.C.91
Not a lot wrong there assuming R17 (cathode resistor) is on value.
What are the readings on V3?
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24th Jun 2017, 10:38 pm | #182 |
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Re: Bush A.C.91
One end of C26, 0.01 uf is also connected to pin 6 on V4 I assume?
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25th Jun 2017, 11:07 am | #183 |
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Re: Bush A.C.91
R17 (150 Ω) is measuring 140.5 Ω (in circuit).
Pin 6 of V4 is indeed connected to one end of C26 (and we have continuity) which of course has been replaced - That Capacitor! Now we move on to V3 (EBC33) - Double Diode Triode - which has a number of Anode connections marked on the Mullard Valve Sheet as a, ad' and ad''
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25th Jun 2017, 11:17 am | #184 |
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Re: Bush A.C.91
V3 is a triode and two diodes in the same envelope, hence the three anodes.
At this stage you only need to check pins 3 (anode of the triode) and 8 (cathode).
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25th Jun 2017, 11:53 am | #186 |
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Re: Bush A.C.91
Check the value of R12 (anode resistor 68k) and measure the voltage wrt chassis on both sides of it.
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25th Jun 2017, 1:53 pm | #187 |
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Re: Bush A.C.91
At last some progress - R12 had been replaced by myself as the original was way over tolerance - can't remember the exact value but I operate a 20 % figure.
Value of new resistor is measuring 67.8 kΩ in circuit. Then measured the voltages wrt chassis on both sides as helpfully suggested by Station X and was puzzled by a reading of Ov on the resistor leads on both sides. When I measured at the base of the connector on the chassis side I suddenly got a reading of 132.1v and a loud hum from the speaker. So I carefully wiggled the connections and got the voltage and hum to remain steady - so a dodgy solder connection! Measuring on the anode side of the resistor is now giving a reading of 61.5v. Tonight I'll remove and resolder R12 and then check the Anode reading on Pin 3 of V3 and the Cathode on Pin 8. Loud hum but still no radio stations on any wave band with aerial plugged in??
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25th Jun 2017, 2:00 pm | #188 |
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Re: Bush A.C.91
Is that a loud hum without intervention or a loud hum when touching grids etc with your meter probe?
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25th Jun 2017, 7:45 pm | #189 |
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Re: Bush A.C.91
Alistair , if you've sorted the supply side and the radio still doesen`t work. Just a thought ,have you tried turning the radio upside down ? to see if one or more of the ift slugs have come detached from the adjusters. hold the chassis upside down and listen for the knock as the ferrite hits the brass spindle. a common problem on the octal valved bush series. regards, Tim
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25th Jun 2017, 8:12 pm | #190 |
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Re: Bush A.C.91
Oh great, now I've got even more problems to resolve!
Turned it upside down and heard two distinctive knocks - presumably the two ift slugs? Meanwhile R12 removed and then re-soldered but still 0v on the V3 Anode and Cathode...!
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25th Jun 2017, 8:24 pm | #191 | |
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Re: Bush A.C.91
Quote:
If there's no voltage on either side of R12 it's become disconnected from the HT supply.
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26th Jun 2017, 11:36 am | #192 |
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Re: Bush A.C.91
Looking back at the measurements for V4 (CL33): -
I'm still not satisfied that I've got the HT supply side sorted?
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26th Jun 2017, 11:48 am | #193 |
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Re: Bush A.C.91
The HT measures high compared to the manual but by not a lot, it's rarely exact, give or take 10 volts or so won't make much difference, the output stage (V4) appears to be basically up and running, need to sort V3's stage next.
If you look, there's voltage errors in the manual anyways, check those in the valve table to those written on the schematic page. Lawrence. Last edited by ms660; 26th Jun 2017 at 11:54 am. |
26th Jun 2017, 11:51 am | #194 |
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Re: Bush A.C.91
If there's no voltage on R12 that's a show stopper and needs to be fixed.
Where's it wired to?
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26th Jun 2017, 12:02 pm | #195 | |
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Re: Bush A.C.91
Quote:
But I need to trace where the other end goes to - from the Circuit Diagram it should be linked to C27 and R18 so should be easy to find?
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26th Jun 2017, 12:19 pm | #196 |
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Re: Bush A.C.91
A circuit diagram does not show actual routings or commoning points. Think of it as being like a London Underground map rather than an OS map.
R18, C27, V4 SG, R12, R9, R7, L1 and R1 should all be connected together, but not necessarily in that order. It might be easiest just to do a continuity test between all those points or, with the set turned on, check for HT on all those points.
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27th Jun 2017, 9:36 am | #197 |
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Re: Bush A.C.91
With continuity and HT testing ongoing I'm hopeful that something will be uncovered in the near future - aargh!
Need to acquire and build a Velleman Signal Tracer/Injector Kit K7000 as David G4EBT has politely reminded me. Meanwhile what about those broken IFT slugs that tim.norris brought to my attention? Moderators - don't know if I should start a new Thread or just Post in this one, but I've attached a picture of one of the IF cans uncovered - where do I even begin?? If I do (ever) get the IFT slugs repaired will I need to have the set re-aligned?
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27th Jun 2017, 9:43 am | #198 |
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Re: Bush A.C.91
Here's one person's fix for the broken slugs:
https://richardsears.wordpress.com/2...m-transmitter/ Yes, the IF transformers will need aligning after being repaired. Lawrence. |
27th Jun 2017, 9:54 am | #199 |
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Re: Bush A.C.91
I've fixed broken slugs by gluing them back together with two part epoxy, namely "Araldite Precision".
Get V3 sorted first though. Making some progress wil give you confidence.
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27th Jun 2017, 10:06 am | #200 |
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Re: Bush A.C.91
Thanks Lawrence and Graham for your almost instantaneous replies!
I will of course persevere with V3 before moving on. I have no idea how to disassemble the faulty IF cans to get at the screw-adjuster and slugs - reading some other Threads you will appreciate that I'm a bit cautious about doing this " ... because you run the risk of damaging a possibly irreplaceable IF transformer by attempting to remove a cracked ferrite core." And for the sake of my ongoing education (!) are these Bush IFT slugs ferrite cores or iron dust cores - just interested.
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