22nd Aug 2018, 7:53 pm | #1 |
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Hmv 1620
Does anyone have a service manual or circuit diagram for this radiogram.
It's similar to the 1622, but there are differences. After a rebuild, I'm getting very low sound, the fault looks like it's in the output amplifier stage. |
22nd Aug 2018, 10:33 pm | #2 |
Octode
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Re: Hmv 1620
I wonder if Old Bell antiques could help. They sold one in 2015 "fully serviced" and will have a paper trail that could get what you are looking for. Call 01-367 -252- 608.
Chris |
24th Aug 2018, 12:22 am | #3 |
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Re: Hmv 1620
Old Bell, couldn't help unfortunately.
Can anyone else assist? |
6th Sep 2018, 4:16 pm | #4 |
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Re: Hmv 1620
I've got the power supply/output stages working.
I've replaced all the old waxies on the radio chassis, I get lots of noise which varies as I tune but no stations are picked up on any of the 3 bands. The valves are X79 and EBF80. There is no service manual anywhere |
6th Sep 2018, 4:25 pm | #5 |
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Re: Hmv 1620
Check voltages/check if the osc is working, should be a few volts -ve on the osc grid (X79 pin 7)
Lawrence. |
6th Sep 2018, 9:24 pm | #6 |
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Re: Hmv 1620
The oscillator is not running. There appears to be a short to ground on the tuning capacitor for that part of the circuit. Its not the tuning capacitor. as I've removed the connection.
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6th Sep 2018, 10:01 pm | #7 |
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Re: Hmv 1620
Trimmer short?
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7th Sep 2018, 9:07 am | #8 |
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Re: Hmv 1620
None of the trimmers are short, There are three, LW MW SW the short is present wherever the waveband switch is.
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7th Sep 2018, 9:09 am | #9 |
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Re: Hmv 1620
Is there HT on any part of the switch wafer that could be tracking across?
I look at 1622 and the switch wafer that switches the HT to the VHF ECC85 also has a contact onto the oscillator grid. Bush had loads of problems doing this. Last edited by Boater Sam; 7th Sep 2018 at 9:17 am. Reason: added |
7th Sep 2018, 9:11 am | #10 |
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Re: Hmv 1620
The oscillator circuit could have a coil to chassis across the tuning capacitor. It all depends on whether it uses identical sections and a padder, or whether the sections are different to give IF offset tracking without a padder.
Is the short maintained between bandswitch positions? David
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7th Sep 2018, 3:28 pm | #11 |
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Re: Hmv 1620
There is no HT on any part of the wafer. It looks like there are coils shunted across the tuning capacitor, I've ordered a new valve, lets see what happens.
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11th Sep 2018, 2:01 pm | #12 |
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Re: Hmv 1620
Fitted a new valve, no change. zero volts on grid 220V on the anode.
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11th Sep 2018, 2:38 pm | #13 |
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Re: Hmv 1620
220 volts on the osc anode suggests that the osc anode is passing very little or no current, with the osc not working I would expect the osc anode voltage to be at least half of that.
Lawrence. |
11th Sep 2018, 3:07 pm | #14 |
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Re: Hmv 1620
Sorry my fault, the anode of the triode is 92V the anode of the hexode is 220V
Other readings: G2/G4 is around 95V, G1 hexode is -06V, G triode -0.06V |
11th Sep 2018, 10:42 pm | #15 |
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Re: Hmv 1620
Taken some more readings and measurements and produced some sort of circuit diagram.
I cannot find anything that uses the X79 valve but I have foun that the HMV 1622 uses the X78 valve which is similar save that it is a B7G valve. I have attached a PDF of part of the 1622, and my own sketch What I can tell you is that the resistance across the tuning capacitor is NOT short circuit but a few ohms depending on which waveband is selected. The HT is 220V. The voltages are in the FAULT condition. |
12th Sep 2018, 12:10 am | #16 |
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Re: Hmv 1620
Are you sure the triode osc section in your diagram is correct? I can’t see any feedback between anode and grid.
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12th Sep 2018, 9:27 am | #17 |
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Re: Hmv 1620
The feedback route is shown in the printed schematics. Both anode and grid circuits are switched to RF transformers for feedback on different frequency bands. The hand drawn sketch doesn't show the anode side switch connection. Unless it's been fiddled with, if the set ever worked, it must be there. Somewhere there's going to be a dodgy component or bad connection that's common to all bands........
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12th Sep 2018, 9:42 am | #18 |
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Re: Hmv 1620
I did wonder had it been fiddled with, hence my question, otherwise yes I agree Chris, it must be there somewhere.
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12th Sep 2018, 11:03 am | #19 |
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Re: Hmv 1620
I think the resistance across the tuning cap, assume both sections, is a red herring. There are coils across in all wavebands. Is there a coil across on gram? Or is there a dead short on gram to silence the radio?
Often the oscillator is killed in gram too, could this be causing the lack of oscillations on radio? |
12th Sep 2018, 11:18 am | #20 |
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Re: Hmv 1620
I wouldn't expect low resistance across the oscillator section of the tuning capacitor unless it's capacitance is lower than that of the ganged RF section, if the capacitance of the gang sections are the same I would expect padders to be fitted in series with the osc tuning coil which would mean no resistance reading across the osc tuning capacitor under normal conditions.
Do the sections of the ganged tuning capacitor appear to be of equal value? Lawrence. |