26th Apr 2022, 4:07 pm | #61 |
Tetrode
Join Date: Dec 2020
Location: Dungannon, County Tyrone, Northern Ireland, UK.
Posts: 76
|
Re: 1936 Marconiphone 365 Radiogram
C60,C65 and C84 are encased inside a metal box filled with a black substance. I have replaced C65, C84 and R43 (R43 measured 84,000 so replaced with 48,000) but HT voltage remains around 30V with 2/7 link connected. Resistance at point 2 to chassis is 17,000 ohms with link removed. R51 reads 150 ohms but I do not have a 100 ohm resistor at the moment.
|
26th Apr 2022, 4:39 pm | #62 |
Moderator
Join Date: Jan 2003
Location: Ipswich, Suffolk, IP4, UK.
Posts: 21,192
|
Re: 1936 Marconiphone 365 Radiogram
As I said, something West of Point 2 is pulling down the HT. The fact that you measure 17,000 Ohms from Point 2 to chassis suggests that the problem is with a leaky capacitor. The fault may only present once the valves have warmed up and started to conduct.
Try removing V1, V2, V3 and V4 one at a time. How did you replace C84? Was the old cap left completely disconnected, or did you connect the new cap in parallel with the old one? EDIT. Given that R41 and R42 total 11,420 Ohms, I'd expect the resistance from Point 2 to chassis to be less that 18.000 Ohms.
__________________
Graham. Forum Moderator Reach for your meter before you reach for your soldering iron. |
26th Apr 2022, 5:26 pm | #63 |
Tetrode
Join Date: Dec 2020
Location: Dungannon, County Tyrone, Northern Ireland, UK.
Posts: 76
|
Re: 1936 Marconiphone 365 Radiogram
Thanks Station X. Both old caps C84 and C60 have been completely disconnected.
I will remove one valve at a time as suggested and get back with results. Can you clarify please what you mean in edit re resistance 18,000 Thanks again |
26th Apr 2022, 5:44 pm | #64 |
Moderator
Join Date: Jan 2003
Location: Ipswich, Suffolk, IP4, UK.
Posts: 21,192
|
Re: 1936 Marconiphone 365 Radiogram
R41 consists of three 23,000 Ohm resistors in parallel, so the total resistance is 23,000/3 Ohms or approximately 7,670 Ohms.
R42 consists of two 7,500 Ohm resistors in parallel, so the total resistance is 7500/2 Ohms or 3,750 Ohms. R41 and R42 are in series, so we can add their values together giving 11,420 Ohms. R41 and R42 are connected from Point 2 to chassis, so the resistance from Point 2 to chassis must be 11,420 Ohms or less.
__________________
Graham. Forum Moderator Reach for your meter before you reach for your soldering iron. |
26th Apr 2022, 7:13 pm | #65 |
Tetrode
Join Date: Dec 2020
Location: Dungannon, County Tyrone, Northern Ireland, UK.
Posts: 76
|
Re: 1936 Marconiphone 365 Radiogram
Ok, thanks for the breakdown re the resistance. That makes sense to me.
Removing V1 and leaving all other valves in place the HT voltage rises to 38V Removing V2 and leaving all other valves in place the HT voltage rises to 60V then drops to 40V Removing V3 and leaving all other valves in place the HT voltage rises to 40V Removing V4 and leaving all other valves in place the HT voltage rises to 40V Removing V5 and leaving all other valves in place the HT voltage rises to 44V with V1, V2 & V3 removed HT is 87V. With V1 - V4 removed HT is 111V. All readings taken with limiter I will check the total of the resistors |
26th Apr 2022, 8:43 pm | #66 |
Tetrode
Join Date: Dec 2020
Location: Dungannon, County Tyrone, Northern Ireland, UK.
Posts: 76
|
Re: 1936 Marconiphone 365 Radiogram
R41 total 11,500 ohms & R42 total 4700 ohms. Combined 16,200 ohms - almost 5000 ohms over.
|
26th Apr 2022, 8:52 pm | #67 |
Moderator
Join Date: Jan 2003
Location: Ipswich, Suffolk, IP4, UK.
Posts: 21,192
|
Re: 1936 Marconiphone 365 Radiogram
Don't worry about it, it's not what's causing the problem.
__________________
Graham. Forum Moderator Reach for your meter before you reach for your soldering iron. |
27th Apr 2022, 8:35 am | #68 |
Moderator
Join Date: Jan 2003
Location: Ipswich, Suffolk, IP4, UK.
Posts: 21,192
|
Re: 1936 Marconiphone 365 Radiogram
Is the fault present regardless of the settings of the wave change, Radio/Gram and tone control switches?
You could try disconnecting one end of R6 which would tell you whether the fault is East or west of that point.
__________________
Graham. Forum Moderator Reach for your meter before you reach for your soldering iron. |
27th Apr 2022, 10:23 am | #69 |
Tetrode
Join Date: Dec 2020
Location: Dungannon, County Tyrone, Northern Ireland, UK.
Posts: 76
|
Re: 1936 Marconiphone 365 Radiogram
Yes, the fault is present regardless of the settings of the wave change, Radio/Gram and tone control switches.
My next question was going to be if there was another point I could disconnect safely "west" of point 2 to narrow down the problem area. I will disconnect R6 Is it safe to say that all is well with the LS? I have good HT voltage on all three wires coming back to the radio when point 2 is disconnected |
27th Apr 2022, 10:47 am | #70 |
Moderator
Join Date: Jan 2003
Location: Ipswich, Suffolk, IP4, UK.
Posts: 21,192
|
Re: 1936 Marconiphone 365 Radiogram
The fact that you have HT on the anode of V5 suggests that the primary of T1 the output transformer is OK. You should be able to hear some sort of hum or hiss from the speaker. If not check that S18 is set to the internal speaker position and clean its contacts.
__________________
Graham. Forum Moderator Reach for your meter before you reach for your soldering iron. |
27th Apr 2022, 12:09 pm | #71 |
Tetrode
Join Date: Dec 2020
Location: Dungannon, County Tyrone, Northern Ireland, UK.
Posts: 76
|
Re: 1936 Marconiphone 365 Radiogram
I disconnected R6 but there is no change in HT voltage. It rises to 40V then slowly drops to between 20 and 30V.
I have had no sound from the speaker at all and I have cleaned and toggled the internal/external switch many times but no sound. |
27th Apr 2022, 12:58 pm | #72 |
Moderator
Join Date: Jan 2003
Location: Ipswich, Suffolk, IP4, UK.
Posts: 21,192
|
Re: 1936 Marconiphone 365 Radiogram
We're getting side tracked here.
With the 2-7 link removed what's the voltage across R51, V5's cathode resistor (100R)? A voltage here indicates that V5 is conducting, which is the correct state of affairs.
__________________
Graham. Forum Moderator Reach for your meter before you reach for your soldering iron. |
27th Apr 2022, 2:18 pm | #73 |
Tetrode
Join Date: Dec 2020
Location: Dungannon, County Tyrone, Northern Ireland, UK.
Posts: 76
|
Re: 1936 Marconiphone 365 Radiogram
Agree!
NO voltage across R51 with link 2-7 disconnected. R51 reads 127 ohms |
27th Apr 2022, 3:13 pm | #74 |
Moderator
Join Date: Jan 2003
Location: Ipswich, Suffolk, IP4, UK.
Posts: 21,192
|
Re: 1936 Marconiphone 365 Radiogram
It's not conducting then. Is it lit up? What DC voltages do you see on the other electrodes?
http://www.r-type.org/exhib/aad0061.htm So long as it lights up there's no need to check the heater pins.
__________________
Graham. Forum Moderator Reach for your meter before you reach for your soldering iron. |
27th Apr 2022, 4:55 pm | #75 |
Tetrode
Join Date: Dec 2020
Location: Dungannon, County Tyrone, Northern Ireland, UK.
Posts: 76
|
Re: 1936 Marconiphone 365 Radiogram
Yes it is lit up along with V4 but I can not see any glow from the other valves. V2 is definitely heating but V1 & V3 ( both VMP4G) are noticeably cooler than the rest. However I think they are both heating.
I have just tried a NOS U14 which I had ordered before Easter and hey ho I have 160V HT at the rectifier with 2/7 link connected. I only tested briefly as I want to be sure that it is compatible before leaving the set running for any length of time. There was no voltage across R51 (link 2/7 connected) |
27th Apr 2022, 5:30 pm | #76 |
Moderator
Join Date: Jan 2003
Location: Ipswich, Suffolk, IP4, UK.
Posts: 21,192
|
Re: 1936 Marconiphone 365 Radiogram
Let's have some voltage readings for V5 please.
__________________
Graham. Forum Moderator Reach for your meter before you reach for your soldering iron. |
27th Apr 2022, 7:03 pm | #77 |
Tetrode
Join Date: Dec 2020
Location: Dungannon, County Tyrone, Northern Ireland, UK.
Posts: 76
|
Re: 1936 Marconiphone 365 Radiogram
Reading pins as per your link -
pin 1 = 0 pin 2 G1 = 0 pin 3 = G2 118V pins 4&5 Heater can't get a reading but glowing pin 6 Cathode G3 = 0 pin 7 Anode = 113V With NOS U14 Rectifier fitted |
27th Apr 2022, 9:17 pm | #78 |
Moderator
Join Date: Jan 2003
Location: Ipswich, Suffolk, IP4, UK.
Posts: 21,192
|
Re: 1936 Marconiphone 365 Radiogram
Either V5 has zero emission, which would be unusual, or there's a dodgy connection between R51 and chassis, or between R51 and the cathode pin.
Also anode and screen voltages are abysmal. Do they rise with the 2-7 link removed?
__________________
Graham. Forum Moderator Reach for your meter before you reach for your soldering iron. |
28th Apr 2022, 9:02 am | #79 |
Tetrode
Join Date: Dec 2020
Location: Dungannon, County Tyrone, Northern Ireland, UK.
Posts: 76
|
Re: 1936 Marconiphone 365 Radiogram
Connections between R51, chassis and cathode seem good.
With link 2/7 removed I have 345V HT (with limiter) at the anode of V5 but nothing at the other pins. With link in place the limiter is brighter than I would be comfortable with. |
28th Apr 2022, 9:10 am | #80 |
Moderator
Join Date: Jan 2003
Location: Ipswich, Suffolk, IP4, UK.
Posts: 21,192
|
Re: 1936 Marconiphone 365 Radiogram
The lack of voltage on grid 2 (screen grid) is explained by the fact that it's fed from a point West of the 2-7 link.
__________________
Graham. Forum Moderator Reach for your meter before you reach for your soldering iron. |