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Old 14th Aug 2019, 8:25 pm   #1
RobertF
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Default Bush SUG26

Hi all,
I am working on a Bush SUG26.
I have replaced all the wax caps and the electrolytic.
It is now powering up..

There is no sound on any of the bands.
When I was doing some voltage checks I was getting a hum through the speaker when I touched some off the pins on V6 which is a Mullard EL41. Am I right in assuming the output stage is working?.

I seem to have a problem with the rectifier EZ40 !!!!

The voltage on the cathode is 108v

I removed all the valves apart from the Rectifier and then I was getting 306v

I have checked all the surrounding components which all seem to be within tolerance..
Would one of the other valves be pulling it down ?.


Could anyone point me in the right direction.


Regards

Robert
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Old 14th Aug 2019, 8:32 pm   #2
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Default Re: Bush SUG26

Probably worthwhile to check the work up you have already done and any previous work. Changing many parts it’s easy to connect a component to the wrong tag.
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Old 14th Aug 2019, 8:52 pm   #3
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Default Re: Bush SUG26

Have you tried inserting one valve at a time in addition to the rectifier or inserting valves until the voltage drops? This may enable you to localize the fault to a particular valve or part of the circuit.
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Old 14th Aug 2019, 9:34 pm   #4
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Default Re: Bush SUG26

The manual states that the current through the rectifier is 75mA.

There is a 1.5kOhm resistor between the reservoir and smoothing capacitors.

The HT current to valves V1, V2, V3 and V4 (and V5,V6 screens) passes through it. If you measure the actual resistance of that resistor and the voltage across it you can calculate that actual HT current and compare it to the figure given in the service manual which is approximately 14mA.

The HT current to the two push pull output valve anodes passes through a common cathode resistor.

Measure the resistance of that cathode resistor and the voltage across it. Then calculate the current.
It should be in the region of 61mA.

The 50uF cathode bypass capacitor may be faulty. You could temporarily disconnect it until you have fixed the radio.

The 50uF capacitor connected to V3 cathode could also affect the bias on that valve if it is faulty.
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Old 15th Aug 2019, 7:09 pm   #5
RobertF
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Default Re: Bush SUG26

Thanks for the replies.
I tested the 1.5k resistor which is spot on.
Double checked the caps I replaced are in the right position..
I then took out the valves one at a time then put it back in and took the next one out but it didn't change the cathode voltage very much.
I then took out V5 and V6 out together and then the voltage rose to 260v which is correct according to the manual.
They are both EL41's . One of them is a new one I had on a shelf the other one looks pretty clean and by swapping them over didn't make any difference.
Looking at the schematic is it possible that the output transformer is the problem.

Any advice welcome..

Regards

Robert
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Old 16th Aug 2019, 11:17 am   #6
Silicon
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Default Re: Bush SUG26

The output valves take most of the HT current.

The fact that the HT voltage drops when these valves are back in their sockets suggests that the output transformer is conducting and therefore OK.

The Mains transformer could be faulty. One of the HT windings may be open circuit.

Do you have any high voltage diodes such as UF4007?

You could remove the EZ40 and temporarily wire the diodes to the valve socket.
Ideally they should each be wired in series with a 200 Ohm power resistor because they are more efficient than the EZ40.

Which Electrolytic capacitor did you replace?
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Old 16th Aug 2019, 7:17 pm   #7
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Default Re: Bush SUG26

Hi Silicon,
I have replaced all the electrolytic C26,31,32,33 and 34.
I have checked for resistance on the main transformer. I am getting 130ohm and 140ohm measuring from the centre tap.

I don't have any power diodes.

Regards
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Old 16th Aug 2019, 11:31 pm   #8
Silicon
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Default Re: Bush SUG26

If you have any faulty compact fluorescent lights you could take one apart.

They have several high voltage diodes on the printed circuit board.

They might be enough to find out what the problem is with the radio.
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Old 17th Aug 2019, 4:12 pm   #9
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Default Re: Bush SUG26

I will have a look for some old bulbs, trouble is I usually get rid of them when they go faulty.

Regards

Robert
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Old 17th Aug 2019, 4:22 pm   #10
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Default Re: Bush SUG26

I can't help feeling that the reservoir capacitor is either incorrectly wired or O/C. Worth a double check here. J.
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Old 17th Aug 2019, 7:01 pm   #11
RobertF
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Default Re: Bush SUG26

I have checked the reservoir cap which looks ok. It is a 47mf, the original was 50mf.

When the set is powered on, the cathode of the rectifier steadily rises to 108v then stops. Then I noticed the voltage began climbing slowly, over the course of 20 minutes it rose to 170v then stopped there.

I have checked all components which I have replaced to make sure that they were correct.
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Old 17th Aug 2019, 7:23 pm   #12
Silicon
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Default Re: Bush SUG26

Robert, you appear to be using the Trader sheet schematic.

C33 and C32 are the reservoir and smoothing capacitors. C33 is the most important one.

As John suggests, an open circuit or faulty reservoir capacitor could give the symptoms you are seeing.

Try swopping them over.

(I posted this before seeing your latest post #11. It still might be worth swopping them over)

Last edited by Silicon; 17th Aug 2019 at 7:28 pm. Reason: More info
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Old 17th Aug 2019, 9:25 pm   #13
RobertF
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Default Re: Bush SUG26

I have swapped C33 with a new capacitor I had on the shelf. It didn't make any difference.
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Old 18th Aug 2019, 12:06 am   #14
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Default Re: Bush SUG26

The steady rise of the HT voltage to 108V and then the slow rise over twenty minutes to 170V is something that could be investigated.

It suggests either that something in the power supply is providing more voltage/current, or that something in the radio is initially taking too much current and dragging the voltage down but this lessens with time.

Do any of the valves have a purple glow inside them?

There could be a bad contact in the valve holder of V7. What is the AC voltage at the heater pins of the rectifier valve?

I would still like to know the voltage across, and the current through resistors R30, R29 and R25.
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Old 18th Aug 2019, 6:16 pm   #15
RobertF
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Default Re: Bush SUG26

There isn't any visible purple glow in the valves.

I have done the following measurements:

R29 4.7v 25.6ma
R30 14.8v 7.4ma
R25 57v 4.2ma

The heater voltage is 6v
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Old 18th Aug 2019, 6:25 pm   #16
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Default Re: Bush SUG26

I would try a new rectifier.

Lawrence.
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Old 18th Aug 2019, 6:57 pm   #17
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Default Re: Bush SUG26

This could be a very silly question but have you a lamp limiter in circuit.
I have made this mistake and wondered why the HT was low.

Also have you checked the value or R29 and tested the bypass capacitor C34 that it is not shorted, sometimes new capacitors can be faulty.

Mike
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Old 19th Aug 2019, 8:38 am   #18
RobertF
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Default Re: Bush SUG26

I don't have the lamp limiter connected.
R29 is within 10% tolerence and C34 looks good.

I am going to order in an EZ40 today and try it. It looks like the only thing left to try.

It should be here in a few days. I will let you know the results.
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