17th Jul 2017, 10:32 pm | #221 |
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Re: Bush A.C.91
There are 2 anodes to v1.
anode (hexode) is pin 3 anode (triode) is pin 6 We could do with both measurements as well as the triode grid on pin 5. If your lucky pin 5 will measure -ve indicating the oscillator is working. If so then you could try running the set upside down so the cores are back where they should be! |
18th Jul 2017, 9:09 pm | #222 |
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Re: Bush A.C.91
Did some re-measuring tonight but first went back and checked the heater voltages again. Measuring these heater voltages in AC of course.
V1 Heater : 7.66 V2 Heater : 6.5 V3 Heater : 6.58 Heater Chain (Pin 2 on V5 to Pin 7 on V1 : 69.6 So moving on to the individual Valves:- V3 Anode (Pin 3) 57.0 Cathode (Pin 8) 1.07 V2 Anode (Pin 3) 97.2 Cathode (Pin 8) 1.23 Screens (Pin 4) 71.1 V1 Anode Hexode (Pin 3) 131.2 Anode Triode (Pin 6) 68.2 Cathode (Pin 8) 0.63 Screens (Pin 4) 68.4 Triode Grid (Pin 5) -3.68 (yes, minus 3.68) Turned the set upside down, heard the usual two clinks/clunks from the IF cans but no sound on any waveband.
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20th Jul 2017, 4:33 pm | #223 |
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Re: Bush A.C.91
I'd appreciate some feedback on the above measurements and any suggestions as to where I should look next.
If, as suspected, the IFT slugs are broken would that result in the set being silent? I've also installed two new HT line smoothing capacitors C27 and C29 but still got that hum?? Running out of ideas ...
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20th Jul 2017, 4:49 pm | #224 |
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Re: Bush A.C.91
Given that the set is silent, I'd start from the other end ie rectifier V5 and output valve V4. What are the voltages there?
Do you get a buzz from the speaker if you touch the centre tag of the volume control with a screwdriver blade? My standard approach when tackling a radio is to sort the power supply, then work backwards from the output valve and speaker to the aerial socket. That way you sort the simple things first. Other approaches are available. Apologies if you've done all this, but this is a very long thread to wade through.
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20th Jul 2017, 6:21 pm | #225 |
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Re: Bush A.C.91
I've apologised a couple of times about the length of this Thread.
I did start from the power supply end, checked the transformers and rewired everything around the on-off switch/ volume control and worked my way towards the aerial socket. All suspect capacitors have been replaced along with a number of out of tolerance resistors.Crumbling wiring replaced where required. I'll go back again and re-measure V4 and V5 - checking everything as I go. Part of my ongoing concern is that this set had been "got at" previously by the infamous "Phantom" who did many things I failed to understand - all now hopefully undone?
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20th Jul 2017, 7:21 pm | #226 |
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Re: Bush A.C.91
You have already sorted the audio side and I assume you still get a hum if you touch the volume control wiper connection.
The voltages look good and the oscillator is working. Do you get any noise from the speaker when you switch between wavebands? An earlier post gave this link to an article for a repair with a good picture of what to expect. https://richardsears.wordpress.com/2...m-transmitter/ There is a good chance this is your remaining problem. The set will need an IF alignment after the repair. |
20th Jul 2017, 10:37 pm | #227 |
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Re: Bush A.C.91
Thanks PJL - I do indeed get a hum when I touch the volume control wiper connection.
Went back and redid the measurements on V4 and V5 - luckily I'd kept my earlier notes but this time I found a new problem - no surprise there with this set! All seems well with V4:- Anode 260V Cathode 4.4V G2 129V Heater 35.6V But now a surprise with V5:- Cathode 278V Heater 19.7V Anode - initially 5V, then jumped to 236V and then back to varying between 5.8 and 7.4V Clearly a problem with the Anode on Pin 5 - the Pin socket is very loose so I used my thin nosed pliers to crimp the socket onto the valve pin - now tighter but no change to the voltage. Conclusion is that it may be a faulty Mullard CY 31 but I may need to replace the octal valve base first just to be sure. Can't explain the one time reading on the Anode of 236V which would be close enough. To rule out (or rule in) the valve base socket can I just solder this to Pin 5 (Anode) of V5?? The Anode of V5 is connected to R19 (Surge current limiter) listed in the Service Instructions as 150 Ohms - this is reading 152.2 Ohms in circuit which is fine for me. I feel that we are now further away than ever from a working set!
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20th Jul 2017, 10:43 pm | #228 |
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Re: Bush A.C.91
If a valve holder connection is playing up you'll get different voltages on the valve pin itself and the valve holder socket, whereas they should be the same.
What voltage do you get if you measure the voltage on that end of R19 which goes to the valve holder? It'll be AC. I wouldn't recommend soldering a wire directly to a valve pin. You need to squeeze up the pin socket with the valve removed, so that when the valve is inserted there's tension in the socket.
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21st Jul 2017, 8:24 am | #229 |
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Re: Bush A.C.91
""Cathode 278V
Heater 19.7V Anode - initially 5V, then jumped to 236V and then back to varying between 5.8 and 7.4V"" Cathode voltage? Lawrence. |
21st Jul 2017, 9:40 am | #231 |
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Re: Bush A.C.91
Yes, but with 7.4 volts AC on the anode?
Lawrence. |
21st Jul 2017, 9:45 am | #232 | |
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Re: Bush A.C.91
Quote:
I thought it had already been established that the power supply and output stage/preamp stage were working correctly with Voltages that looked OK.
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21st Jul 2017, 9:47 am | #233 |
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Re: Bush A.C.91
""I thought it had already been established that the power supply and output stage/preamp stage were working correctly with Voltages that looked OK.""
Me too... Lawrence. |
21st Jul 2017, 10:06 am | #234 | |
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Re: Bush A.C.91
From post #207.
Quote:
Probably worth getting to the bottom of the strange anode readings on the rectifier anode though.
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21st Jul 2017, 10:42 am | #235 |
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Re: Bush A.C.91
Thanks guys - over the weekend I'll revisit the Anode readings on V5 and get to the bottom of this.
I'm sure it's the valve socket/valve pin connection that is causing the trouble as I did get a reading of 236V at one point on the rectifier Anode.
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21st Jul 2017, 11:32 am | #236 |
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Re: Bush A.C.91
Remember that when selecting the voltage range on the meter, it's AC on the anode and DC on the cathode. Easy mistake to make!
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21st Jul 2017, 12:37 pm | #237 |
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Re: Bush A.C.91
Perhaps you can pinch a connecting "fork" from the socket in an unused position, pin 6 is usually not fitted to the valve so its a new unused one in the holder.
Swapping these is easier than replacing the whole socket. Sam. |
21st Jul 2017, 1:46 pm | #238 |
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Re: Bush A.C.91
Great idea Sam but how easy is it to swap a socket if required?
From another Post I understood that "In a pre-war radio the valve bases have eight brass sockets mounted in a paxolin sheet with another sheet rivetted on top so the bits cannot come apart when the valve is removed."
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21st Jul 2017, 1:54 pm | #239 |
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Re: Bush A.C.91
If the pin sockets are sandwiched between two paxolin sheets they cannot easily be removed and replaced.
https://www.vintage-radio.net/forum/...42&postcount=8 If however the valve holders are of ceramic or plastic construction it's generally possible to bend back a locking tab and slide the pin sockets out.
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21st Jul 2017, 9:48 pm | #240 |
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Re: Bush A.C.91
Hi Donald, You say in a previous post that the IF Cores are broken as you have the familiar clunks when the chassis inverted. If the cores are damaged performance is inevitably effected and may well be the cause of the loss of signal. I had this in my BA91 and several DAC90s.
Whilst these are not to difficult to repair, with care it can be done by stripping the cans down and labeling all wires to aid reassembly. The dust cores can normally be super glued back together taking care to get the broken parts carefully aligned. I would hope once this is done you will be listening to the world on the SW and the UK on LW & MW. Regards Trevor. |