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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only. |
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1st Jan 2017, 4:31 pm | #1 |
Dekatron
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Defiant RGSH 755 EL84 problem
The plate on the back of this radio says it is an RGSH 755 but it is not actually a radiogram
Anyway, I tried it on full mains as the seller said he had plugged it in and got "white noise" and the smoothing caps seemed ok. There is something coming in through FM so I started at the AF end. The AF coupling caps to the EL84s look like some sort of ceramic disk types. The EL84s were getting very hot and there was a positive voltage on the control grids so I replaced the coupling caps with .022uf ones which is the value shown on teh sheet "up top". When I got the old ones out I found that they were actually .005 but I assume this will make no difference. R39 and R40 are not there but there are 1M resistors from the grid tags to chassis. Anyway one of the EL84s is still getting hot and there is a "pulsing" overlaying a signal fed into the PU sockets. The red visible inside the valve pulses in time to this. If I swap the valves the fault stays at the same socket and if I briefly remove the offending valve the pulsing disappears. On checking the HT on the offending valve there is only 40 odd volts and I cannot get a resistance reading between the anode and the HT rail. I am assuming therefore that the output transformer must be open circuit on that side but I cannot understand why the valve glows so hot or pulses. Can anyone enlighten me please? Thanks Paul |
1st Jan 2017, 4:40 pm | #2 |
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Re: Defiant RGSH 755 EL84 problem
Don't have a schematic but screen grid?
Lawrence. |
1st Jan 2017, 4:52 pm | #3 |
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Re: Defiant RGSH 755 EL84 problem
Extract from circuit.
Check the anode and screen voltages with the EL84's removed. You'll soon see if the O/P transformer is o/c. Could the caps from anode to ground be leaking? Is the common cathode resistor reading the correct resistance?
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1st Jan 2017, 5:05 pm | #4 |
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Re: Defiant RGSH 755 EL84 problem
I'll buy the CD one of these days.
Lawrence. |
1st Jan 2017, 5:05 pm | #5 |
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Re: Defiant RGSH 755 EL84 problem
Yes the screen voltage would explain it. The anode of one is at 300 odd volts and the other around 40. I replaced the caps from the anode to ground.
It seemed a promising set as well! Paul |
1st Jan 2017, 5:15 pm | #6 |
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Re: Defiant RGSH 755 EL84 problem
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1st Jan 2017, 5:26 pm | #7 |
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Re: Defiant RGSH 755 EL84 problem
I keep thinking that, but then each set I buy is going to be the last - until i see the next one of course
PS The common cathode resistor is a little high but nothing troubling |
1st Jan 2017, 6:06 pm | #8 |
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Re: Defiant RGSH 755 EL84 problem
The larger value coupling caps fitted by the OP might improve the bass repsonse as those removed were quite a low value.
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Edward. |
1st Jan 2017, 9:07 pm | #9 |
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Re: Defiant RGSH 755 EL84 problem
Yes, but it makes following other's problems much more engaging if you have the circuit available for reference.......
As does the almost complete run of R&TVS books (anyone got any after 1986/87 they'd like to shift?) and the BVWS Trader sheet CDs, not to mention Radiomuseum and Nostalgiaair etc etc.
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1st Jan 2017, 10:09 pm | #10 |
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Re: Defiant RGSH 755 EL84 problem
I don't do repairs at the moment but I might put the CD on my birthday...I want one o'them list in a month or two.
I assume it'll work with windows 10? I know zilch about PC/program compatibility etc. Lawrence. |
1st Jan 2017, 10:13 pm | #11 |
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Re: Defiant RGSH 755 EL84 problem
Works my Windows 10 machines
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1st Jan 2017, 10:26 pm | #12 | |
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Re: Defiant RGSH 755 EL84 problem
Quote:
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2nd Jan 2017, 12:29 pm | #13 | |
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Re: Defiant RGSH 755 EL84 problem
Quote:
I think Paul is working on an update, it will be on my list when it's available. Frank |
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2nd Jan 2017, 10:58 pm | #14 |
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Re: Defiant RGSH 755 EL84 problem
If you can accept a slight reduction in output, consider changing to separate EL84 cathode
resistors of 270 or even 390 ohms, with individual capacitors. The valves will run cooler. |
2nd Jan 2017, 11:14 pm | #15 |
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Re: Defiant RGSH 755 EL84 problem
Yes I was thinking about that. I have to do something about the transformer though. I think I have one from a hi fi ultra linear amp somewhere. I assume it would work if I just left the tap disconnected
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18th Mar 2017, 1:42 pm | #16 |
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Re: Defiant RGSH 755 EL84 problem
I bought a new output transformer advertised as coming from a Fender EL84 amp.
I duly fitted this yesterday but just get a bad "feedback" howling when the set has warmed up. Is this likely to be something to do with the new transformer or should I be looking for something else that has failed or come adrift? The original soldering is not very robust on this set and several joints have come apart. Thanks Paul
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Paul |
18th Mar 2017, 1:58 pm | #17 |
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Re: Defiant RGSH 755 EL84 problem
This suggests that you have the anode connections reversed and therefor are getting positive feedback.
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18th Mar 2017, 2:59 pm | #18 |
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Re: Defiant RGSH 755 EL84 problem
Thank you. I did try reversing the connections to the valves but it made no difference. Sorry to be dense but how would the way they are connected produce feedback?
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Paul |
18th Mar 2017, 4:21 pm | #19 |
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Re: Defiant RGSH 755 EL84 problem
Just trying to get my head round this positive feedback.
I have tried to associate the earthy end of the secondary with the lead from V8 so that the end of the secondary that supplies the negative feedback is associated with V7. I did this by simply observing the leads that were adjacent on the transformer. That didn't help. The howling continues if I disconnect R43 and the feedback loop from the secondary. Paul
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Paul |
19th Mar 2017, 10:47 am | #20 |
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Re: Defiant RGSH 755 EL84 problem
Another snippet if it helps - it doesn't do it with the ECC83 pulled.
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