Radiogram Power Supply

Thread Starter

redrooster01

Joined Jul 15, 2014
93
I'm having problems with this old power supply giving out the wrong voltages. I was asked to try and fix it by a friend but I have no experience of these radiograms at all and I'm hoping I can get some help with it here ? It seems that I can only upload one file at a time,Ive uploaded a photo so I will post another post Astor Power Supply.jpg Astor Power Supply.jpg with the schematic. I have replaced the 2 x 1000uf electrolytics and 2 x poly caps 1 x 223k and 1 x 274j greencap,otherwise I haven't changed anything. Obviously their is something wrong but I just cant see it? Any help will be greatly appreciated.
 

Thread Starter

redrooster01

Joined Jul 15, 2014
93
Here is the schematic.The radio is getting 9.6v instead of 14.5v? and the driver boards are getting 24.7v instead of 22v? The tone boards are pretty well good.Astor PS.png Astor PS.png Astor PS.png
 

Thread Starter

redrooster01

Joined Jul 15, 2014
93
Hi Dodgy,Sorry for the late reply,I didn't tick the email alert? Duh! Anyway I desoldered the radio power line wire from the RF board 44 at point J Pos, it read 24.7vdc and then I measured from the switch 73 it read 24.7vdc? Does that sound right to you?
 

Thread Starter

redrooster01

Joined Jul 15, 2014
93
That pretty much says the RF board is using too much current.
Ive just finished replacing all the out of spec resistors on the RF board and Ive found one of the three AT319 IF amplifier transistors is no good it reads 850 from base to collector and 850 from base to emitter,otherwise everything else looks OK but its still measures 9volts? I wouldn't have thought that a bad transistor would cause the voltage to drop like this. I tried to upload some photos but I couldn't for some reason?I clicked the upload a file button a few times and it didn't work.
 

Thread Starter

redrooster01

Joined Jul 15, 2014
93
Here is a photo of the RF board and its schematic. Ive replaced everything that was out of spec except for one of the IN295 diodes that measures 340 forward voltage compared to a reading of 440 on the other IN295 using the diode setting on my DMM. I measured resistor #30 3.9k ohms on the power supply out of circuit about a week ago and it was OK. Ive just measured it out of circuit again and it has gone high to 4.39k ohms? Where I live is fairly remote and Ive had to order the 3.9k resistor in,so that will take about two weeks to get here. Actually the radio was working on 9v,its stopped working now because of the transistor I suppose.
can you upload the RF board 44 diagram?
Astor PS#1 003.JPG Astor RF Board.jpg
 

Alec_t

Joined Sep 17, 2013
14,280
Did you check resistor 28 (as per post #3)? If it's gone high resistance it could account for the voltage drop. Check also for dry joints in that area.
 

Thread Starter

redrooster01

Joined Jul 15, 2014
93
Did you check resistor 28 (as per post #3)? If it's gone high resistance it could account for the voltage drop. Check also for dry joints in that area.
Yes I replaced number 28 with a 1 watt 2.7k ohm 1% tolerance resistor and every other resistor that was out of spec on the RF board.Ive also re-flowed all the solder joints. Ive done this to all the boards in this radio.That's why Ive asked for help here because I'm stuck on what the problem is?
 

Thread Starter

redrooster01

Joined Jul 15, 2014
93
Check for leaky power supply bypass capacitors.
Yes Ive replaced them with new ones. Ive spent $85 on new parts,anything even slightly out of spec has been replaced. Could it be the choke? According to the SM schematic the power transformer has 18vAC across it which is right when I measured it it was 18.3vAC,then after the choke instead of being 22vDC it is 24.8vDC? The choke also reads 3.9 ohms I don't know if that is significant or not but it looks high compared to the power transformers 0.6 ohms.
 
Last edited:

MrChips

Joined Oct 2, 2009
30,720
I was referring to the filter capacitors in the RF module.

I assume you are referring to the choke (54) in the power supply.
If you are still uncertain about the current capability of the power supply, disconnect the connection entering the RF board.
Put a load resistor, from 3k to 5k-ohm, of known resistance, from that supply point to ground and measure the voltage across the resistor.
 

Thread Starter

redrooster01

Joined Jul 15, 2014
93
Capacitors 18 & 19 are new, cap 4 is a bit hard to get at to measure so Ill de-solder one of the legs tomorrow and check it.Ill check the power supply as well. Its 12.30 am here and I had better get some sleep.Thanks for the ideas fellas its much appreciated.
 

Thread Starter

redrooster01

Joined Jul 15, 2014
93
I was referring to the filter capacitors in the RF module.

I assume you are referring to the choke (54) in the power supply.
If you are still uncertain about the current capability of the power supply, disconnect the connection entering the RF board.
Put a load resistor, from 3k to 5k-ohm, of known resistance, from that supply point to ground and measure the voltage across the resistor.
I used a 4.7k ohm 10 watt resistor from the RF power supply to chassis ground and it read 41mv across the resistor?
[
 

Thread Starter

redrooster01

Joined Jul 15, 2014
93
On the RF PCB,try unsoldering resistor 31, and then 37, 39, 43, and check the 14v supply voltage at each removal.
I lifted one leg each on resistor 31 & 37, I measured the input voltage at 9v still but when I measured 39 with one leg lifted it reads 18.6v at the power input? I haven't done resistor 43 yet,I thought I had better tell you about this reading first.
 
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