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Vox BM1 - battery/mains Escort - no output

Started by Stevo1987, May 25, 2018, 06:51:13 AM

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Stevo1987

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




Jazz P Bass

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.

Stevo1987

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.

Jazz P Bass

#3
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.

solderer25

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.

Stevo1987


Stevo1987

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.

Jazz P Bass

Thanks for the info, Stevo.

Sorry if I got the amp model incorrect.

Stevo1987

Hey Jazz, no problem at all. I should have posted the schematic on the opening post.
Steve.

Jazz P Bass

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.


Stevo1987

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

Jazz P Bass

"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.

Stevo1987

Yes, I know that.  :) I just can't see it because of the heat sink (?) or grounding arrangement on top of it. 

solderer25

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!)

Jazz P Bass

If I am not mistaken, the heatspreader has tabs that solder to the ground tabs.