Author Topic: no output from russian portable amplifier.  (Read 789 times)

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Online Yansi

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Re: no output from russian portable amplifier.
« Reply #25 on: December 21, 2019, 08:46:54 pm »
Point 12 is very very unlikely to be lower than point 17. No way hat could happen, as on the other side R31 you have confirmed you have 4.5V.

Could you please measure all the points in the circuit and write the voltage in red text over to the image? That would get the situation way more clear! Thanks!!  :-+

No no, UNI-T is absolutely fine for this job. The concern is just the input DC resistance of the meter. With a modern meter that is quite high. Using an old "analog" type meter would be quite problematic for this job. The absolute precision of the meter does not play much role for troubleshooting this amp.
 

Offline magic

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Re: no output from russian portable amplifier.
« Reply #26 on: December 21, 2019, 10:26:13 pm »
I'm pretty sure the base of VT9 should be 8.4V (9-0.6V) rather than 3.4 ;)
Similarly, VT10 should be 0.6V, not sure why they wrote 0.9.

What's the input impedance of your DMM? 10Meg is OK, but 1Meg could pull VT2 base lower than it normally is. Measure from ground to VT2 base and then from VT2 base to the supply and see if it adds up. Or measure differentially straight between V2/V3 bases, without touching ground.

Is R22 good? No shorts in parallel with it?
 

Offline ELS122

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Re: no output from russian portable amplifier.
« Reply #27 on: December 22, 2019, 12:29:43 am »
so it seems I fixed it, it sounds obsurdly bright but still has plenty of power to play some BASS trough it.

I'm gonna remessure all the voltages now and post them.
also the base of VT9 is 9V cause the supple is 9.6V. 9.7V when I replaced nearly all of the leaking (in both ways) electrolitics.
I now am trying to figure out all the wiring to the volume/tembre control and nearly figured it out.

I also got no output at all when touching the casette player head wires so it would induce some 50hz hum in it. but nothing, but I will probably make a diffrent post about that since this one is already full of comments about the power amp.

EDIT: the uni-t meter was like 25eur was the best meter I could find for when I just started working on electronics, I know that I should get a new meter probably cause this one is pretty inpresise, only thing good about it is that it doesn't care if you pump 1kv in to it when working on tube radios :-DD. and yes I know it's dangerous to do that but I'll make sure not to touch it.
if you want the model for it it's a UT20B
« Last Edit: December 22, 2019, 12:32:50 am by ELS122 »
 

Offline ELS122

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Re: no output from russian portable amplifier.
« Reply #28 on: December 22, 2019, 12:59:28 am »
25 is 9.5v
12 - 5.6v
11 - 4.85v
17 - 5v
19 - 0.4v
20 - 4.8v
10 - 0.6v
18 - 4.5v
21 - 5v
22 - 4.8v is the lowest it can go, but it distorts sound at this voltage, at 5v and up it doesn't distort anymore

VT2:
4.8v to ground
4.8v to positive
VT3:
5v to ground
4.5v to positive

they have 0v between the bases

EDIT: 23 is presisely 5V
« Last Edit: December 22, 2019, 01:32:47 am by ELS122 »
 

Offline ELS122

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Re: no output from russian portable amplifier.
« Reply #29 on: December 22, 2019, 01:06:16 am »
also now that I replaced the caps, nothing at all is heating up.
 

Offline ELS122

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Re: no output from russian portable amplifier.
« Reply #30 on: December 22, 2019, 01:17:17 am »
oh and forgot: the base of VT9 is at 9V
 

Offline ELS122

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Re: no output from russian portable amplifier.
« Reply #31 on: December 22, 2019, 01:24:30 am »
pretty much this part of the circuit is a good bit off.

19 is 0.4v not 0.9v
20 is 4.8v not 3.9v
18 is 4.5v not 3.9v
and 22 distorts if it's anywhere near the 4.4v it needs to be. it stops distorting when it's above 5v
 

Offline ELS122

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Re: no output from russian portable amplifier.
« Reply #32 on: December 22, 2019, 05:25:17 am »
holy crap, adjusting the power supply from 9.5v to 9v actually fixed everything, it's still bright sounding but I'll just use the tone control on 0!
 

Online shakalnokturn

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Re: no output from russian portable amplifier.
« Reply #33 on: December 22, 2019, 11:30:16 am »
What happens if you run it off nice new batteries?
The problematic 9.5V supply voltage would be less than 1.6V/cell, something that could happen with new batteries.
Also check how well it works with supply down to 8V, I'm not sure I'd call it normal if it only operates correctly spot-on 9V supply.
 

Offline ELS122

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Re: no output from russian portable amplifier.
« Reply #34 on: December 22, 2019, 08:54:30 pm »
It worked fine down to 6v, I didn’t check lower. Well it sounds normal when at 9.5v it’s just that the bias is all messed up.
« Last Edit: December 22, 2019, 10:58:40 pm by ELS122 »
 

Offline ELS122

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Re: no output from russian portable amplifier.
« Reply #35 on: December 22, 2019, 11:03:51 pm »
Also new batteries can be even 1.65v a cell, so 6 batteries can be up to almost 10v and all of the caps are rated 10v so it is good that I replaced them whit new ones, also they were leaking in every way.
I guess when it was made there weren’t so powerful batteries?! But I guess if you would have but in maybe overcharged or just batteries whit a really high voltage it could’ve blown up the caps.
 


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