UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Radio (domestic)

Notices

Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only.

Closed Thread
 
Thread Tools
Old 20th Oct 2020, 2:59 pm   #41
Herald1360
Dekatron
 
Herald1360's Avatar
 
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,526
Default Re: Motorola 5A7A dead.

Quote:
Originally Posted by Station X View Post
Well spotted Trevor! It looks like the OP was numbering the pins anti-clockwise.

Err.... Trevor?
__________________
....__________
....|____||__|__\_____
.=.| _---\__|__|_---_|.
.........O..Chris....O
Herald1360 is offline  
Old 20th Oct 2020, 3:00 pm   #42
daveo23
Banned
 
Join Date: Oct 2020
Location: Tampa, Florida, USA.
Posts: 33
Default Re: Motorola 5A7A dead.

tube in
daveo23 is offline  
Old 20th Oct 2020, 3:05 pm   #43
ms660
Dekatron
 
ms660's Avatar
 
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
Default Re: Motorola 5A7A dead.

The filament supply voltage looks too low.

Lawrence.
ms660 is offline  
Old 20th Oct 2020, 3:18 pm   #44
daveo23
Banned
 
Join Date: Oct 2020
Location: Tampa, Florida, USA.
Posts: 33
Default Re: Motorola 5A7A dead.

Here is the 1R5, looks bad, - = some mv

1 Filament = -
2 Anode =66vdc
3 Screen grids =44vdc
4 Grid of triode = -
5 Filament = -
6 Grid 3= .6vdc
7=Filament =.9vdc
daveo23 is offline  
Old 20th Oct 2020, 3:52 pm   #45
Station X
Moderator
 
Station X's Avatar
 
Join Date: Jan 2003
Location: Ipswich, Suffolk, IP4, UK.
Posts: 21,192
Default Re: Motorola 5A7A dead.

Wot no filament voltage? The voltage on the screen grid suggests the tube is passing current though.
__________________
Graham. Forum Moderator

Reach for your meter before you reach for your soldering iron.
Station X is online now  
Old 20th Oct 2020, 4:31 pm   #46
daveo23
Banned
 
Join Date: Oct 2020
Location: Tampa, Florida, USA.
Posts: 33
Default Re: Motorola 5A7A dead.

I wiggled the tube around and was able to get .9vdc but thats still too low, right
daveo23 is offline  
Old 20th Oct 2020, 5:04 pm   #47
Station X
Moderator
 
Station X's Avatar
 
Join Date: Jan 2003
Location: Ipswich, Suffolk, IP4, UK.
Posts: 21,192
Default Re: Motorola 5A7A dead.

Check the voltage between pins 1 and 7.
__________________
Graham. Forum Moderator

Reach for your meter before you reach for your soldering iron.
Station X is online now  
Old 20th Oct 2020, 6:00 pm   #48
daveo23
Banned
 
Join Date: Oct 2020
Location: Tampa, Florida, USA.
Posts: 33
Default Re: Motorola 5A7A dead.

I checked all the tube filaments, 1&7, same at .9vdc so I looked at the voltage at the end of r-6(double 1000ohm res) it was 6vdc.
daveo23 is offline  
Old 20th Oct 2020, 9:34 pm   #49
Station X
Moderator
 
Station X's Avatar
 
Join Date: Jan 2003
Location: Ipswich, Suffolk, IP4, UK.
Posts: 21,192
Default Re: Motorola 5A7A dead.

Before tackling the heater problem I'd check the voltages on the other two valves to make sure plate and screen grid voltages are present. Let's not forget that the original problem was the the B+ voltage was being pulled down.

If the 1S5 (audio amplifier) and 3S4 (output tube) are working correctly you should hear a buzz or click from the speaker if you touch the centre tag (wiper) of the volume control with the metal blade of a screwdriver.

Don't forget that the service manual linked to in post #3 gives resistance values from every tube pin to chassis. It would be useful to check these.
Attached Thumbnails
Click image for larger version

Name:	Readings.jpg
Views:	73
Size:	86.6 KB
ID:	218455  
__________________
Graham. Forum Moderator

Reach for your meter before you reach for your soldering iron.
Station X is online now  
Old 20th Oct 2020, 10:56 pm   #50
daveo23
Banned
 
Join Date: Oct 2020
Location: Tampa, Florida, USA.
Posts: 33
Default Re: Motorola 5A7A dead.

That screw driver trick worked..bzzzz
Now the crazy question about the 1s5, how can you have 2 different voltages for pin 1 and 2 when they are connected?

Here are the 1u4 and 1s5 voltages:

1U4
1 Filament=4
2 Anode =67
3 Screen grid =67
4 = .6
5 Internal connection=4.1
6 Grid 1=1.3
7 Filament =3.2

1S5:
1 Filament=.9
2 NC =.9
3 Anode of diode=.7
4 Screen grid =20
5 Anode=20
6 Grid 1=.4
7 Filament=1.8
daveo23 is offline  
Old 20th Oct 2020, 11:41 pm   #51
Station X
Moderator
 
Station X's Avatar
 
Join Date: Jan 2003
Location: Ipswich, Suffolk, IP4, UK.
Posts: 21,192
Default Re: Motorola 5A7A dead.

Pin 2 isn't connected to anything inside the tube. It's connected to pin 1 externally to provide a convenient termination point. US terminology is Tie Lug or TL.
__________________
Graham. Forum Moderator

Reach for your meter before you reach for your soldering iron.
Station X is online now  
Old 21st Oct 2020, 1:14 am   #52
daveo23
Banned
 
Join Date: Oct 2020
Location: Tampa, Florida, USA.
Posts: 33
Default Re: Motorola 5A7A dead.

Look at the sams photofacts for the 1s5 and you can see what iam talking about
Attached Thumbnails
Click image for larger version

Name:	20201020_200742.jpg
Views:	73
Size:	53.5 KB
ID:	218467  
daveo23 is offline  
Old 21st Oct 2020, 9:06 am   #53
Station X
Moderator
 
Station X's Avatar
 
Join Date: Jan 2003
Location: Ipswich, Suffolk, IP4, UK.
Posts: 21,192
Default Re: Motorola 5A7A dead.

There's probably been a production change at some and the routing of wires has been changed. There are many different ways of connecting a number of points together.

The bottom line is that the set worked before with pins 1 and 2 linked together, so it should work now barring any other faults.

The main problem at the moment is that filament voltage is low at 0.9V when it should be 1.4V. There are several resistors in the filament circuit which could be causing the problem. I would suggest taking resistance readings on pins 1 and 7 of all valves at the very least and comparing them with the service sheet. Note that there are two sets of readings, one for the set switched to mains supply and one with the set switched to battery supply.
Attached Thumbnails
Click image for larger version

Name:	Readings.jpg
Views:	61
Size:	86.6 KB
ID:	218469  
__________________
Graham. Forum Moderator

Reach for your meter before you reach for your soldering iron.
Station X is online now  
Old 21st Oct 2020, 1:20 pm   #54
Station X
Moderator
 
Station X's Avatar
 
Join Date: Jan 2003
Location: Ipswich, Suffolk, IP4, UK.
Posts: 21,192
Default Re: Motorola 5A7A dead.

Given that you only have 6V on pin 7 of the 3S4 when you should have 7.9V, a likely scenario is that R17 1000R + !000R or R4 150R have gone high in value.
__________________
Graham. Forum Moderator

Reach for your meter before you reach for your soldering iron.
Station X is online now  
Old 21st Oct 2020, 1:42 pm   #55
daveo23
Banned
 
Join Date: Oct 2020
Location: Tampa, Florida, USA.
Posts: 33
Default Re: Motorola 5A7A dead.

Before taking the filament resistance, pull the tube right? The directions say not to measure the filaments with an ohm meter.
daveo23 is offline  
Old 21st Oct 2020, 1:52 pm   #56
Station X
Moderator
 
Station X's Avatar
 
Join Date: Jan 2003
Location: Ipswich, Suffolk, IP4, UK.
Posts: 21,192
Default Re: Motorola 5A7A dead.

Yes pull the tubes.

I'd suggest moving straight on to checking R17 and R4 though.
__________________
Graham. Forum Moderator

Reach for your meter before you reach for your soldering iron.
Station X is online now  
Old 21st Oct 2020, 2:36 pm   #57
daveo23
Banned
 
Join Date: Oct 2020
Location: Tampa, Florida, USA.
Posts: 33
Default Re: Motorola 5A7A dead.

r-4=184ohms
r-17=1meg
r-8=980 each side

resistance on 1r5 looks good except for pin 6=0

1r5 resistance
1=0
2=5k
3=29k
4=106k
5=0
6=0
7=9
daveo23 is offline  
Old 21st Oct 2020, 2:49 pm   #58
Station X
Moderator
 
Station X's Avatar
 
Join Date: Jan 2003
Location: Ipswich, Suffolk, IP4, UK.
Posts: 21,192
Default Re: Motorola 5A7A dead.

Quote:
Originally Posted by daveo23 View Post
resistance on 1r5 looks good except for pin 6=0
0 what? If it's ohms the pin is short-circuit to ground.

There are a whole load of resistors in series from 1R5 pin 6 to ground ie R1, R10, R12, R13 and R14. Their total value is about 9 Mohms.
__________________
Graham. Forum Moderator

Reach for your meter before you reach for your soldering iron.
Station X is online now  
Old 21st Oct 2020, 3:05 pm   #59
Station X
Moderator
 
Station X's Avatar
 
Join Date: Jan 2003
Location: Ipswich, Suffolk, IP4, UK.
Posts: 21,192
Default Re: Motorola 5A7A dead.

The valve filaments should draw 50mA which will give a volt drop of 98V across R8 (1960R). There'll also be a volt drop in R4 (184R) of 9.2V. In practice the volt drop will be a bit more as R4 also carries the B+ current. Even so with 120V mains there should still be enough voltage for the heaters.

Check the volt drop across R8 and the voltage wrt ground at the junction of R4 and R8.
__________________
Graham. Forum Moderator

Reach for your meter before you reach for your soldering iron.
Station X is online now  
Old 21st Oct 2020, 3:36 pm   #60
ms660
Dekatron
 
ms660's Avatar
 
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
Default Re: Motorola 5A7A dead.

The current flowing through R4 would be typically at least twice the value of the total load current.

Lawrence.
ms660 is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 10:05 am.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.