Possible PSU problem on a micro-tel 1200

[neazoi]

Hi in a Micro-Tel 1200 receiver I have measured the voltages on the PSU (page 99 schematic) and all are close, apart from the 140v which measures only 114v. I tried tuning the R6, and it does vary the output voltage but I cannot go to more than 114v.

Note the receiver operates on 230v instead of 240v, but I do not think this is the problem because the other voltages are more or less ok. There must be something with this 140v line. Can you please let me know what you would check?

--- Updated Apr 6, 2021 ---

I have measured the in-circuit R5 (carbon composite 5%) resistor to be 44k instead of 39k. can the loss of it's value be the cause?

 

[stenzer]

Hi,

I would check the current limiter consisting of the two BJTs Q1 and Q2. Check the voltage drop across R4 to determine the current flowing through Q1. I would suspect Q1 as a possible cause, as it is mounted on a heatsink, thus indicating to be exposed to a high temperature / dissipation a lot of power.

What is the input voltage at the connections E and H, and at the common cathode of DA1. Are both diods still functional?

BR

 

[betwixt]

And yes, R5 could be responsible. An in-circuit test should never show a resistance higher than R5's marked value, it may be lower because of parallel components but not higher. The other possibility is the reservoir capacitor has dried up, it is C1 "part of main assembly" so I guess it is fixed to the chassis somewhere.

Brian.

 

[neazoi]

The other possibility is the reservoir capacitor has dried up, it is C1 "part of main assembly" so I guess it is fixed to the chassis somewhere.

Thanks for all the replies.
I could replace the capacitor temporarily (although if not needed I like to keep the original look of the components) to see if it fixes the problem. I will also replace the resistor there temporarily with the accurate one.
But please let me know, if the electrolytic has dried up, what are the bad things that it could cause to the PSU? For example lower output voltage, higher, more ripple etc etc.?

 

[neazoi]

Hi,

I would check the current limiter consisting of the two BJTs Q1 and Q2. Check the voltage drop across R4 to determine the current flowing through Q1. I would suspect Q1 as a possible cause, as it is mounted on a heatsink, thus indicating to be exposed to a high temperature / dissipation a lot of power.

What is the input voltage at the connections E and H, and at the common cathode of DA1. Are both diods still functional?

BR

Ok here are my measurements:

Voltages out of the psu:
-15.16VDC
19.81VDC
114.1VDC (it should be 140v)
245.1VDC

Voltage across R4.
143.4mV DC

voltage between the base and emitter of Q3
420mV DC (it should be 0.6V?)

voltage at the connections E and H
292.3v AC

input voltage at the collector of Q1 (common cathode of DA1):
130.3V DC (it should be higher than the output 140V, possibly 180V or higher?)
Could it be caused from a faulty C1, somehow leaking DC to the GND?

Note the attached (from the manual) for the 140v

 

[betwixt]

The center tap of the transformer is 'S' so you should measure the voltage between 'S' and E and H to get the AC input but if you measured 292.3V I guess it was across E and H so they would both be about 146V RMS with respect to 'S'.

That means the peak voltage across C1 and therefore at the collector of Q1 should be about 205V. Being a linear regulator the input must be higher than the output so if you are only measuring 130.3V I would say C1 was open circuit. It is unlikely to be leaky as that would make it draw a huge current and probably destroy the transformer or rectifiers, far more probable is it has gone open circuit or very low in value.

Q3 B-E is probably low because the regulator can't operate with insufficient input and the voltage across R4 is fine, it should always be below about 0.6V and would only rise if over-current protection kicked in.

Brian.

 

[neazoi]

I would say C1 was open circuit. It is unlikely to be leaky as that would make it draw a huge current and probably destroy the transformer or rectifiers, far more probable is it has gone open circuit or very low in value.

The problem I have is huge audible ripple/distortion when connect an RX to the 30MHz output even on a dead carrier, without any LO sweep action. This ripple did not occur on feeding a 75MHz signal to IF 75MHz directly, so I suspected the first LO power. Then I measured this lower voltage in the 140v line which also feeds the first LO. Something tells me that I will see a huge ripple in C! as well. I will connect the 1M 10:1 250v max probe on it to see (I hope I won't burn out the probe.
Ok I think I will replace the C1 and see what happens, It is far easier than searching for in-pcb faults, and since the lower voltage is at the input of the regulator...

--- Updated Apr 8, 2021 ---

The problem I have is huge audible ripple/distortion when connect an RX to the 30MHz output even on a dead carrier, without any LO sweep action. This ripple did not occur on feeding a 75MHz signal to IF 75MHz directly, so I suspected the first LO power. Then I measured this lower voltage in the 140v line which also feeds the first LO. Something tells me that I will see a huge ripple in C! as well. I will connect the 1M 10:1 250v max probe on it to see (I hope I won't burn out the probe.
Ok I think I will replace the C1 and see what happens, It is far easier than searching for in-pcb faults, and since the lower voltage is at the input of the regulator...

Ok look at that!
The first picture is from the input of the regulator (C1 point) and the second is from the output of the regulator.
At the bottom of the images the Vmin and Vmax are shown.
It looks like an open C1 to me.
I also notice that the regulator works, since I can see the flat top edges at 148v (I have already peaked the 140v set trimmer to the end)

 

[neazoi]

Yes, C1 was replaced and all the problems (ripple, distortion in the IF) fixed
Thanks for the help!

 

[betwixt]

Well done!
If you look at the scope waveform at the output of the regulator it was producing 140V correctly when it could but when you measured 114V it was the average of a varying voltage.

Brian.