Author Topic: no output from russian portable amplifier. (Read 2684 times)

ELS122 · « **on:** December 21, 2019, 01:59:21 am »

So I am repairing A VEF 260 Radio (only the amplifier stage) and got nearly 0 output, so I measured some voltages and got almost nothing at the speaker terminal, almost 9v at the base of the KT814A and 0.3v at the 815A. I have about 9.4V from the regulated power supply.
nothing is heating up only the regulator for the PSU but only a bit nothing really hot.
I also already hear some hum but idk if it's 100hz or 50hz.

shakalnokturn · « **Reply #1 on:** December 21, 2019, 09:50:21 am »

First things I'd be checking by order of priority are:
R38 (gone open?), C35, XS5 switch contact.
If the design uses carbon compound resistors, check through the lot...

magic · « **Reply #2 on:** December 21, 2019, 11:21:48 am »

C35 may be good or bad but this is not the problem yet.
Report voltages at all terminals of VT4, VT5 and C22. I think it is prudent to have a picture of what the amp is doing now before monkeying with it.

edit
Maybe check if C35 isn't shorted though, that could be a bad day for the speaker when you get the amp running.

ELS122 · « **Reply #3 on:** December 21, 2019, 04:44:52 pm »

at point 17 I messured 5.5volts, but I found that C23 and R32 were switched, the capacitor went to ground and the positive of the cap went to the resistor.
all resistors were good-ish they were a bit off but that's normal for these old russian resistors.
at point 20 I messured 4.5 volts
at point 22 I got 5.2 volts
at point 23 I got 5.8 volts
at point 18 I got 4.6 volts
at point 19 I got 0.6-0.7 volts
at point 10 I got 0.6 volts
at point 11 I got 4.7 volts (it's the base of the KT3107D (VT2)
what's weird is that it seems to have 2 voltages labeled, 3-4mV and 4.5v at the same point, I got 4.7v at the base of the transistor not 3-4mV!
at point 12 I got 5.8 volts

ELS122 · « **Reply #4 on:** December 21, 2019, 04:47:55 pm »

and at 21 5.2volts

ELS122 · « **Reply #5 on:** December 21, 2019, 04:54:46 pm »

so I disconnected the timbre/volume board and got a massive increase in the output, but still it's pretty silent, so maybe I'll test more things around, replace as many caps as I can.

also it's great to know that there aren't any transistors dead probably

magic · « **Reply #6 on:** December 21, 2019, 05:13:06 pm »

Am I reading correctly that there is 0.8V difference between VT2 and VT3 bases? That certainly shouldn't be so.
Also, you initially said there is only 0.3-0.4V from each rail at output transistor bases, but now it's 0.6?

BTW, VT2 base should be halfway between the rails. It is of course set by those two 100k resistors and the pot between them.

ELS122 · « **Reply #7 on:** December 21, 2019, 05:23:22 pm »

well in the shematic it says that there should be the same voltage on VT2 and VT3 bases but there's not, there is a diffrence. I'm gonna change all the electrolitics in the circuit and recheck it.
and idk why it would have changed but I messured what I messured, could be that I mistaken which point to messure, I'm also gonna recheck that.

ELS122 · « **Reply #8 on:** December 21, 2019, 05:31:14 pm »

well I changed the electrolitic caps and now it's pretty loud, but it's earpiercingly bight. before it wasn't. maybe this is because the caps aren't exactly the right value, 50uF were replaced by 47uF, 200uF was replace whit 220uF, 500 whit 470uF, 1uF was replaced also whit a 1uF.
I'm gonna now messure the voltages also

ELS122 · « **Reply #9 on:** December 21, 2019, 05:38:52 pm »

still 4.85v at VT2 and 5.25 at VT3. I tried conneting the old caps together whit the new caps so to test if it's the lower value on some of them but nothing, I only found that C22 was shorted and caused the no output.

ELS122 · « **Reply #10 on:** December 21, 2019, 05:44:06 pm »

R22 and R32 were good.
R24 seems BAD, it messures somewhere about 4k and rising. ~~I'll replace it.~~
I disconnected R24 and now it messures 8.9k and no weird behaviour.

ELS122 · « **Reply #11 on:** December 21, 2019, 05:57:31 pm »

~~I just found out that the shematic doesn't match whit the circuit, I'll try to find one that does. the base of VT3 goes straght to VT7~~

nevermind I just found out that it doesn't,
I tested R31 tested good, needed to pull it out to test it. none of the transistors have shorts in them, some of them have a stable resistance between 2 points around 4k but I asume it's just cause they are in circuit!

ELS122 · « **Reply #12 on:** December 21, 2019, 06:12:16 pm »

I'm gonna replace C28 which is labeled C25 in the shematic but I checked and it's the same cap, this is a 240nF cap right. my russian electronics book would disagree and state that it's 240pF, so what do I change it to?
also what value are C38, C39? I'm guessing that C39 is 3900pF and C38 is 68nF? this seems really weird

ELS122 · « **Reply #13 on:** December 21, 2019, 06:39:28 pm »

I remesured everything,
Note: the supply voltage is 9.6v rather than 9v. so values should be a bit high

these are the voltages that are off:
point 20 is 4.9v rather than 3.9v
point 17 is 5v rather than 4.5v
point 19 is 0.6v rather than 0.9v
the base of VT9 is 9v rather than 3.4v (seems bad)
point 23 is 4.4v rather than 5v
point 18 is 4.3v which is a bit high from 3.9v

also there is 4.8k resistance between the base and the colector of VT9
I'm gonna change it, I have one spare see if that helps

ELS122 · « **Reply #14 on:** December 21, 2019, 06:58:40 pm »

nope, changed it. still 9v at the base of it perhaps VT7 is bad, also I found out that I have really early transistors there. the KT814A is the 185th ever made I guess by the number on it. one I swapped in is 8150th

ELS122 · « **Reply #15 on:** December 21, 2019, 07:06:53 pm »

so I removed VT7 and got nearly 0V at the base of VT9 so I asume it's bad. but I don't have any replacements. I have a board whit transistors labeled "v, a, g"
but none that are "b"

ELS122 · « **Reply #16 on:** December 21, 2019, 07:35:38 pm »

so I think VT5 is dead, it doesn't drive VT10

Yansi · « **Reply #17 on:** December 21, 2019, 07:39:17 pm »

Quote from: ELS122 on December 21, 2019, 04:44:52 pm

at point 17 I messured 5.5volts, but I found that C23 and R32 were switched, the capacitor went to ground and the positive of the cap went to the resistor.
all resistors were good-ish they were a bit off but that's normal for these old russian resistors.
at point 20 I messured 4.5 volts
at point 22 I got 5.2 volts
at point 23 I got 5.8 volts
at point 18 I got 4.6 volts
at point 19 I got 0.6-0.7 volts
at point 10 I got 0.6 volts
at point 11 I got 4.7 volts (it's the base of the KT3107D (VT2)
what's weird is that it seems to have 2 voltages labeled, 3-4mV and 4.5v at the same point, I got 4.7v at the base of the transistor not 3-4mV!
at point 12 I got 5.8 volts

but the dashed rectangle are AC VOLTAGES, not DC.

ELS122 · « **Reply #18 on:** December 21, 2019, 07:42:47 pm »

Quote from: Yansi on December 21, 2019, 07:39:17 pm

Quote from: ELS122 on December 21, 2019, 04:44:52 pm
at point 17 I messured 5.5volts, but I found that C23 and R32 were switched, the capacitor went to ground and the positive of the cap went to the resistor.
all resistors were good-ish they were a bit off but that's normal for these old russian resistors.
at point 20 I messured 4.5 volts
at point 22 I got 5.2 volts
at point 23 I got 5.8 volts
at point 18 I got 4.6 volts
at point 19 I got 0.6-0.7 volts
at point 10 I got 0.6 volts
at point 11 I got 4.7 volts (it's the base of the KT3107D (VT2)
what's weird is that it seems to have 2 voltages labeled, 3-4mV and 4.5v at the same point, I got 4.7v at the base of the transistor not 3-4mV!
at point 12 I got 5.8 volts

but the dashed rectangle are AC VOLTAGES, not DC.

oh I didn't know that, but anyway why would there be 9V at the base of VT9 I tried replacing it, replcing VT7 and nothing changed

Yansi · « **Reply #19 on:** December 21, 2019, 08:04:52 pm »

What voltages (DC) are at points 20 (base of VT8), 21 (output) and 23 (base of VT9)?

ELS122 · « **Reply #20 on:** December 21, 2019, 08:17:29 pm »

point 20 is 4.9v rather than 3.9v
point 21 is 4.5v

Yansi · « **Reply #21 on:** December 21, 2019, 08:26:45 pm »

If 21 is 4.5V (close to 1/2 of supply voltage), then the amp is working more than likely correctly.

VT9 base indeed shall be lightly below the supply voltage, so if you have 9.4V supply voltage, 9.0V shall be about correct. 3.4V in te schematic is a mistake. No way there could be 3.4V at point 24.

If you measure voltages at the bases of the two input PNP differential pair, those voltages shall also match within a couple milivolts likely and sit at also abotu half the supply voltage (slightly above the output voltage at point 21).

Lift the left side of the input coupling capacitor and injust some audio signal in there. Preferably of course if you have a signal generator, set it to those required 3-4mVrms. If using other source, be gentle with the signal strength, the amplifier has gain of 1+R31/R32 = about 270x, which is a LOT of gain.

//EDIT: Point 20 shall be about 0.6V below point 21 (output), simply because it is one Vbe drop. So 4.9V is not that good - you have probably mistakenly measured the base of VT7 (point 23) which shall be about those 4.9V (point 23 is simply one Vbe above output).

ELS122 · « **Reply #22 on:** December 21, 2019, 08:29:35 pm »

the bases of the 2 input transistors voltages differ more than 1/4 of a volt, the one that can be adjusted has a correct voltage but VT3 has 5v at the base

Yansi · « **Reply #23 on:** December 21, 2019, 08:34:56 pm »

I would not be much bothered by that, although 250mV difference is way too much to be true.

What kind of meter (input impedance) do you use for the DC measurement?

//EDIT: Note the input R18 trimpot is just to set the steady state output voltage to as close as possible to the 1/2 of supply voltage, i.e. 4.5V (which you have set correctly already).

//EDIT2: More thinking about it, check VT3 if it is good. Measure both BE and BC junctions are not shorted (measure in-circuit unpowered with a diode tester). If in doubt, pull the VT3 out of the board and check also the beta. What voltage is at point 12? (their emitters)

ELS122 · « **Reply #24 on:** December 21, 2019, 08:43:05 pm »

point 12 is a bit low, ~4.9v and my supply is 9.6v. also I'm using a pretty cheap uni-t multimeter so that probably is the reason I'm getting these voltages


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Author Topic: no output from russian portable amplifier. (Read 2684 times)

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