Hi Everyone,
New to the forum as I need some help..... :)
Manufactured in the 1970's I have had this little Vox amp a long time. Saying that for a number of years it has sat in the corner in the room and the last time it was switched on it worked. Of course it doesn't now. :( I can a hear very faint hum from it. And I mean really faint. I have to put my ear against the speaker cloth to hear it. Other than that there is no sound at all. I have done some basic troubleshooting using a DVM.
Checked input jacks - ok
Checked speaker - ok
Checked voltage output from transformer - ok
Checked PCB fuse ok.
Checked output from bridge rectifier - ok I think. Getting to the limit of my knowledge.
So, I think the problem is on the PCB. I have inspected it and I can't see anything obvious i.e. Component damage, dry joints etc.
I could take it to some repairer but I don't think it would be cost effective.
Anyway, here is the question -
For the components on the PCB which ones are the most liable to fail to end up with no output all?
I'm just thinking of replacing some of the components as they will be cheap and I'm not too bad with a soldering iron.
It's old school electronics so it shouldn't be too difficult to solder in new components.
I did manage to find a schematic of the PCB but I haven't done much troubleshooting of PCB's before.
Anyway, If anyone has any ideas how I can bring this little old amp back to life I would appreciate it.
Thanks,
Steve
Before replacing Anything, check that the speaker lead has a good connection to chassis ground.
The Ext Spkr Jack has a Normally Closed contact that provides the speaker it's ground connection.
Insert a cord in the Ext Jack & jumper the tip to ring.
If the sound is then good, the jack needs work.
Thanks for getting back to me with you suggestion which I hadn't tried. I did what you said but still no sound. I also tested the continuity from the PCB to the speaker and that was good. For good measure I fed the external output into another amp and still nothing.
The Boost Output is another Normally Closed contact.
Use a stereo jack & jumper tip to sleeve.
Or send the tip & ring to another amp.
The Recording/ Headphone is an open contact jack so there are not any issues with the contacts preventing signal from passing through.
That jack could be sent to another amp or headphones to see if it is a preamp problem or not.
I would expect about + - 18Vdc coming out of the bridge rectifier.
Hi. I am following this thread with interest as I too have a mains/battery Escort amp. I dont recognise the schematic posted previously though. The Vox Escort circuit I know uses a single transistor preamp and an old school TBA810 power amp chip. If the TBA810 is found to be duff it can be replaced (still available mail order I think) but is tricky to desolder/solder as the pins are staggered.
This is the schematic....
Quote from: solderer25 on May 25, 2018, 03:04:30 PM
Hi. I am following this thread with interest as I too have a mains/battery Escort amp. I dont recognise the schematic posted previously though. The Vox Escort circuit I know uses a single transistor preamp and an old school TBA810 power amp chip. If the TBA810 is found to be duff it can be replaced (still available mail order I think) but is tricky to desolder/solder as the pins are staggered.
The chip is as cheap as chips on flea bay.
Thanks for the info, Stevo.
Sorry if I got the amp model incorrect.
Hey Jazz, no problem at all. I should have posted the schematic on the opening post.
Steve.
You could use a volt meter to find out what is wrong.
First, check the output ic to see if it has the correct voltages.
TBA810 power pin: 1 is +Vdc & the tab should be grounded.
Pin #4 is the bootstrap pin. It should have +Vdc on it.
Set your meter to read Volts ac & look at pin #8: that is the input pin.
You should see some action there when the input is fed a signal.
Pin #12 is the output pin.
Same thing: Volts ac at the output with a signal.
The output is capacitor coupled to the speaker.
So if you have signal at Pin #12 but no signal at the speaker, then that cap (or the speaker ground connection) is suspect.
Hi Jazz, I understand all that so I will check it out in the next few days. I did have a quick look but it's hard to see the orientation of the op amp in relation to the pins. I might make a drawing to help with that and also rig up some sort of constant source input signal to aid troubleshooting.
Thanks again,
Steve
"it's hard to see the orientation of the op amp in relation to the pins"
Every IC ever made has a marking at one end. (in this case, a divot)
That end has pin #1 on the left.
Yes, I know that. :) I just can't see it because of the heat sink (?) or grounding arrangement on top of it.
Hi. The copper coloured thing is just a heatsink and is clipped on to the IC. It should just pull off. Probably some white heat conducting paste between it and the IC as well. Do not be tempted to scrape this off as it helps heat conduction if IC is found to be ok.(IC = chip!)
If I am not mistaken, the heatspreader has tabs that solder to the ground tabs.
"If I am not mistaken, the heatspreader has tabs that solder to the ground tabs." - Correct. Hopefully I'll be able to have a good look soon as I'm a bit busy at the moment.
Thanks for all your input.
Steve
"If I am not mistaken, the heatspreader has tabs that solder to the ground tabs."
Better not try and pull it off then - my mistake sorry. xP
No problem. Hopefully I can have a go at this come the weekend.
I think I have manged to work out which pins are which. There is no voltage on pin 1 or 4. i.e.no supply voltage to the chip. From the bridge rectifier (18Vdc) it goes to a 1000uF 25v capacitor. I get 18v on one leg and 0V on the other leg. That's how far I have got....
Just to double check that you have the pins correctly identified:
Amp Off. Meter on Ohms scale.
Pin 1 & 4 should be 100 ohms apart.
If Pin 4 is at the same potential as any of the tabs, that would possibly make it pin 10 instead of 4.
Checks out as 100ohms between pin 1 and 4. :dbtu:
So, either the power supply is not supplying or the switch is bad.
Of coarse you could have an open connection from the switch to the ic.
Quote from: Stevo1987 on May 28, 2018, 05:29:04 PM
Yes, I know that. :) I just can't see it because of the heat sink (?) or grounding arrangement on top of it.
Ok, do this:
1) check that you have proper voltage at the power supply, we expect something around +18V DC
2) you have 2 *possible* Pin 1 , on opposite diagonals.
*One* of them will have same voltage as from supply ... that is Pin 1.
Mark it somehow, a felt marker dot, a drop of nail enamel, and you´re done.
Then count other pins relative to this one, following the datasheet.
3) measure DC voltage at ouput pin 12, we expect 1/2 the supply voltage, so some +9V DC
4) measure bootstrap volotage on pin 4 , we expect 3/4 of supply voltage or around +12/13V.
5) now to preamp: we expect some 6 or 7 V on Q1 collector.
IF DC voltages are reasonable, then active components are working, lack of audio most probably comes from connection problems, dirty/open pots and jacks, etc.
Do NOT replace parts at random, no matter whether they are "cheap" or even free, you are damaging the PCB, the *only* component you can not buy over the counter or even at the (long defunct) Factory.
***THE PCB IS THE AMPLIFIER, PARTS ARE JUST ACCESSORIES***
Showing 18V at power supply.
1 17.8
4 17.8
8 0
12 0.22
collector - 9.9V I think this is correct very hard to see. The two other legs had about 0.2v. The transistor is a PN930.
Pins 1/4 showing 17.8V is a change though. Before I couldn't get anything I did have a look at the batt\mains switch as it was loose.
Quote from: Stevo1987 on June 04, 2018, 04:51:11 PM
Showing 18V at power supply..... check
1 17.8 ..... check
4 17.8 ..... would have expected between 9V and, say, 12 or 14V
8 0 ...... check
12 0.22 ..... too low.
Quote
collector - 9.9V I think this is correct very hard to see. The two other legs had about 0.2v. The transistor is a PN930.
Check.
QuotePins 1/4 showing 17.8V is a change though. Before I couldn't get anything I did have a look at the batt\mains switch as it was loose.
Good.
Pity chip amp looks dead.
IF you can get another for a reasonable price, order it.
I have ordered two. :) I'll let you know how I get on. Thanks for everyone's help. Could be a week or so before I know but I will be back.
Replaced the chip - still the same. >:(
Just in case we are missing something , let´s recheck voltage on all pins.
It doesn´t hurt and we might get a surprise.
So check and post voltage on *all* pins, even those which "officially" are not connected.
Main problem on the old one was 0.22V on Pin 12, which is speaker out, and should be around +9V, but something I found somewhat weird was you found full 18V on Pin 4 ... it´s a bootstrapped output, is fed by a 100 ohm resistor and some current passes through it, so thanks to the series 100 ohm resistor it should be at least 1V below 18V ... yet you reported 18V with no losses.
Of course, since chipamp was damaged, that was possible.
But with a new presumed good amp it should show what I expect.
In fact in my original post I suggested "3/4 of 18V".
So remeasure and repost, but feel confident that absolute worst case we can Mod this amp to use an easy to find TDA2030 or LM1875.
But let´s try once more to make the original work. :tu:
Almost forgot: use a loupe or strong looking glasses (+3 or +4) and look around all chipamp solder pads and tracks leading to them ... we might have a broken/cracked track.
Will do. I'm not for giving up. Thanks for getting back to me.