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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only. |
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12th Aug 2019, 2:06 pm | #1 |
Heptode
Join Date: Jan 2008
Location: Bristol, UK.
Posts: 981
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McMichael Dimic screened 3
Hi chaps sorry to bother you but I have a problem and am stumped.
This has been working fine for a few years but turning it on now all there is is a very loud harsh rough tone. The reaction control acts as a 'volume' control and turns up the volume. This buzz/tone is not like the usual reaction 0scilation. This happens with the tuning/aerial controlls just advanced off zero. Connecting an aerial does not make any difference, there are not any trace of a signal/station at all. I can't find a fault with the power supply ( McMichael mains unit.). Valves 2 and 3 are Mazda AC/P's. V2. Pin 1. 197V. Pin 2. 12.86V. Pin 3/4 4.0V. Heater. Pin 5. 0.0V. V 3. Pin 1. 64.8V. Pin 2. 2.6V. Pin 3/4. 4V. Heater. Pin 5. 0.0V. Probably missing something obvious. Thank you for any help etc. that you may be able to give. Cheers. Geoff. |
12th Aug 2019, 4:27 pm | #2 |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,384
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Re: McMichael Dimic screened 3
Hi Geoff, According to the data that I have got, the first valve should be an HF screened grid valve PM12, followed by an HL210 triode and a PM22 screened grid output valve (although the circuit from the Radio Museum (RM) shows a triode output valve. Presumably yours worked OK in the past with the existing valves? Firstly I don't like the sound of +12.86v on pin 2 (grid1) of V2. Either you have an extremely high voltage grid bias battery connected or more likely there is leakage from the grid coupling capacitors connected to the anode of the previous stage. Looking at the circuit there also appears to be a separate grid bias battery providing dc bias to grid 1 of the HF screened grid valve via the aerial coils. Sometimes small button cells were used which fail after so many years. Does yours have anything of this sort and, if so, is it still producing any volts? Other than that I would check the 0.25uF grid 2 decoupling capacitor on the HF screened grid valve. Also does the intervalve AF coupling transformer have continuity on both primary and secondary? Cheers, Jerry
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12th Aug 2019, 4:48 pm | #3 |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,384
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Re: McMichael Dimic screened 3
Valve anomaly maybe sorted. It would appear that there were 2 versions of the screened Dimic 3 - a mains and a battery model. The data/valves I have listed are for the battery model without the base unit (presumably containing the power supply). As per RM website however the HF screened grid even on the mains version is a battery type operated from 4v mains derived supply. Furthermore, the specified grid bias for an AC/P is -12v so if you are measuring -12v not +12v then that is probably OK.
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13th Aug 2019, 11:29 am | #4 |
Heptode
Join Date: Jan 2008
Location: Bristol, UK.
Posts: 981
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Re: McMichael Dimic screened 3
Hi cathoderay57. Jerry thank you for your reply, there is a 1.5 volt AA sized battery in a separate holder screwed to the alloy screen deviding the 2 halves of the radio.
My bad, yes it should have been a minus, -12.86 and -2.6. The 0.25 cap was checked but I will go there again. Any idea of the pri and sec resistances on the transformer, (1 to 3 ish ?) there is a 1 meg R across it. I will check it again just to make sure. I have tried several valves in V1 position but I don't suspect that as it was working well the last time it was used. Thanks again Jerry I will be back in a few days. Cheers. Geoff. I was wondering if it was interferance but the other radio's are OK. |
13th Aug 2019, 12:29 pm | #5 |
Heptode
Join Date: Jan 2008
Location: Bristol, UK.
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Re: McMichael Dimic screened 3
Hi Jerry, quick one must go out, transformer.
Secondary ( marked grid bias and bias.) 10.02 K ohms. Primary .962 K . Recon there is something not right there. Cheers. Geoff. |
13th Aug 2019, 12:52 pm | #6 |
Dekatron
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
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Re: McMichael Dimic screened 3
The transformer is probably OK, the secondary has many more turns, is normally wound over the primary (taking more wire) and may use a finer gauge. I have some transformers with primaries of 50K+.
V3 AC/P detector V2 AC/P output valve V1 ? RF amplifier The voltages you have given all look OK to me. The 64.8 on V3 anode implies V1 SG should be about right too. First step would be to clean the valve pins and waveband switch contacts. Also measure the voltages on V1 and post here. The design uses a wonderfully mad method of changing the RF gain by lowering the V1 filament voltage. This would be another area where things could go wrong so measure V1 filament volts at high and low volume settings. Last edited by PJL; 13th Aug 2019 at 1:00 pm. |
13th Aug 2019, 9:04 pm | #7 |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,384
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Re: McMichael Dimic screened 3
I guess the other thing to try would be to isolate which stage of the receiver is generating the noise, maybe by removing rf amp and det valves one at a time.
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16th Aug 2019, 2:55 pm | #8 |
Heptode
Join Date: Jan 2008
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Re: McMichael Dimic screened 3
Hi all, I have checked and cleaned all the valve pins, sorted through some valves for V1 position and it is a Cossor 220 SG which is the best I have got. ( found a PM12 but duff.) Found a tiny spot where the reaction control vanes shorted. Sorted.
V1. top cap, anode. 150 volts.(DC) Heater volts. 1.6 volts to 2.2 volts.(AC). Grid 1.6 volts - AA battery. It will not pick up any stations. Just one place where there is this rough/hard 'interferance'. Whether connected to an aerial or my finger there is no difference. Bit stuck at the mo. Pulling V1 and complete silence. Something more to check but I will let you know about that later. Cheers. Geoff. |
16th Aug 2019, 6:07 pm | #9 |
Nonode
Join Date: Oct 2017
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Re: McMichael Dimic screened 3
Hi Geoff, if you have got or can borrow an RF signal generator try injecting a modulated signal somewhere in the 600-700 kHz range onto the grid of V2 via a coupling capacitor and see if any sound comes out. If you can't do that then try injecting an audio signal onto the grid of V3 again via a coupling capacitor. The object is to try to ascertain if the AF stage is working then work backwards to the HF stage.
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17th Aug 2019, 12:09 pm | #10 |
Heptode
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Location: Bristol, UK.
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Re: McMichael Dimic screened 3
Hi all, Jerry well found the signal genny and set it to 650kHz to grid V2 and complete silence, nowt, not a squeek anywhere, swung the aerial and tuning and reaction controls from stop to stop!
Will have a break over the weekend, car service, lets see if I can break that!! (Ha Ha.) Cheers. Geoff. |
17th Aug 2019, 2:39 pm | #11 |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
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Re: McMichael Dimic screened 3
Hi Geoff, we have a seriously ill relative at the moment so I'll be away for a couple of days. However, if you would like to bring it round I'd be happy to have a look at it later in the week if that suits. As Gerry Wells used to say tea on the hour every hour. You have a PM with the address and phone number. Cheers, Jerry
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17th Aug 2019, 6:52 pm | #12 |
Heptode
Join Date: Jan 2008
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Re: McMichael Dimic screened 3
Hello Jerry, thank you so much for your kindness, your PM replied to.
Cheers. Geoff. |
17th Aug 2019, 8:15 pm | #13 |
Dekatron
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Location: Seaford, East Sussex, UK.
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Re: McMichael Dimic screened 3
There seems to be some confusion over which is V2. Your voltage measurements made V2 the output valve but Jerry wanted the signal injected into the detector.
For this to work, the time constant formed by the coupling capacitor and the detector grid resistor must be shorter than the lowest frequency you want to detect. Best bet is to lift the other end of the 300pF cap that goes to the detector grid and connect the signal generator to this. Better still would be to use an audio signal and inject it into the detector grid via a larger capacitor, 0.01uF would do it. |
18th Aug 2019, 10:20 am | #14 |
Heptode
Join Date: Jan 2008
Location: Bristol, UK.
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Re: McMichael Dimic screened 3
Hello everyone. Hello PJLhow are you doing. OK I am using a circuit that I found unfortunitly it has very little information, no voltages etc. No valve types.
I am working from left to right, aerial on the left and power supply on right. V1 is a Cossor 220 SG and it sits through a hole in a screen that devides the radio in half. V2 is a Mazda AC/P, anode connected to the primery of the intervalve trans. V3 is a Mazda AC/P, trans secondary, loudspeaker.etc. Sorry if I have confused matters somewhat but not having any info. regards valve numbers, I am using my 'own' take on valve V1,V2, V3 if you see what I mean. Later on I will have another look see. Thank you. Geoff. |
18th Aug 2019, 12:10 pm | #15 |
Dekatron
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Re: McMichael Dimic screened 3
In that case can you confirm you have labelled the valves in your voltage measurements incorrectly above.
As you have some sound that is not present when V1 is removed, I would say the fault is most likely in the RF section. If you measure the voltage on V2 grid with a DMM, we can check to see if V1 is oscillating madly indicated by a significant negative voltage. |
20th Aug 2019, 1:26 pm | #16 |
Heptode
Join Date: Jan 2008
Location: Bristol, UK.
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Re: McMichael Dimic screened 3
Thank you to all of you. I must apologise, advancing age and a memory that is increasingly flakey means confusion is rife. I suspect that V2 and V3 valves were transposed.
I will be able to confirm something in a week, some 7 years ago the sub station next door burnt out, melting an underground cable and the tarmac. I have just recieved notice that there will be 2 power cuts, 1 to connect a genny while they 'replace equipment' and 1 when reconnecting the station. I walked about with a transistor radio and the amount of very heavy interferance from the sub station was unbelievable. It looks as if I owe you an apology, I should have checked better before asking for help. Great forum, many thanks. Cheers. Geoff. |
23rd Aug 2019, 1:10 pm | #17 |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
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Re: McMichael Dimic screened 3
Hi Geoff, I wondered if it is a simple case of shorting variable capacitor vanes. If you are getting the buzzing sound only at one tuning point is that when the vanes are fully open, that would indicate at least the set is capable of picking up local interference, then it's a matter of straightening the bent vane(s)? Cheers, Jerry
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23rd Aug 2019, 1:40 pm | #18 |
Heptode
Join Date: Jan 2008
Location: Bristol, UK.
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Re: McMichael Dimic screened 3
Hello Jerry further investigation shows that I can tune in the 'interferance' with both the aerial and station variable capacitors.
I have a number of other McMichaels all the TRF's show the same symtoms but the Dimic is by far the worst. The battery portable of about the same age, is by far the best, but it has its own frame aerial. I have come down to the conclusion that the sub station is radiating interferance,(see above) the mains supply varies from 247V to 253V. Despite one radio having a main trans and choke about the size of a brick and has a rebuilt power supply it shows a 5.3 volt ripple. When they have replaced what ever is going to be done to the sub station then we will see what happens. Only a week to go!!. Cheers. Geoff. Many thanks to all. Jerry I hope that the M-I - L continues to quickly improve. All the best. |