no output from russian portable amplifier. - Page 1
Forum section: electronics / repair

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: December 21, 2019, 01:59:21 am
content: 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.

author: shakalnokturn
author_rank: Super Contributor
timestamp: December 21, 2019, 09:50:21 am
content: 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...

author: magic
author_rank: Super Contributor
timestamp: December 21, 2019, 11:21:48 am
content: 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.editMaybe check if C35 isn't shorted though, that could be a bad day for the speaker when you get the amp running.

author: magic
author_rank: Super Contributor
timestamp: December 21, 2019, 05:13:06 pm
content: 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.

author: Yansi
author_rank: Super Contributor
timestamp: December 21, 2019, 07:39:17 pm
content: Quote from: ELS122 on December 21, 2019, 04:44:52 pmbut the dashed rectangle are AC VOLTAGES, not DC.

author: Yansi
author_rank: Super Contributor
timestamp: December 21, 2019, 08:04:52 pm
content: What voltages (DC) are at points 20 (base of VT8), 21 (output) and 23 (base of VT9)?

author: Yansi
author_rank: Super Contributor
timestamp: December 21, 2019, 08:26:45 pm
content: 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).

author: Yansi
author_rank: Super Contributor
timestamp: December 21, 2019, 08:34:56 pm
content: 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)

author: Yansi
author_rank: Super Contributor
timestamp: December 21, 2019, 08:46:54 pm
content: 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.

author: magic
author_rank: Super Contributor
timestamp: December 21, 2019, 10:26:13 pm
content: I'm pretty sure the base of VT9 should be 8.4V (9-0.6V) rather than 3.4Similarly, 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?

author: shakalnokturn
author_rank: Super Contributor
timestamp: December 22, 2019, 11:30:16 am
content: 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.