Hello,

@LesJones , in post 23 the ts stated this:

7815 output 1.7
7915 output -2.2

It looks like s short is coming from the preamp/mixer board.

Bertus

 

I finally got the schematic for the mixer part…see if it’s readable:

The circuit schematic is quite readable, just turned a quarter turn off of where it should be.

 

Les and all:
Tonight I revisited some tests…
Power supply board only:
7815 pin 3 out was 15vdc
7915 pin 3 out was -14.8vdc
THEN…I plugged in the 5condr cable from the preamp board.
Turned on power and voila! Power LED lit!
7815 measure only 3.2 vdc and 7915 measured -3.2 vdc.
UPDATE: a few minutes later I put unit back together and applied power. I heard a slight “tick” in speakers, and turned back off. Turned on again and same thing. Turned off. Was going to then try and put some audio into it but this time no power-ON. Pulled back off cabinet again and now I smell something burnt.
Back to outputs of 7815 and 7915 and 0 vdc, with and without preamp board plugged in…
I then noticed the 7294 I had screwed back onto heat sink plate, frame holder, and thermal paste, wasn’t completely tightened up. (Earlier I had applied a slight bit of thermal paste to what was there to ensure a thin, even coat)
Time to sleep a bit and will check back later…

 

Do not attempt any audio test until you fix the power supply problem. Premature testing is asking for trouble.

 

Now that you have given us the facts clearly I agree with bertus that the fault is likely to be decoupling.
Do the following to confirm that R12 and R13 have not changed their value.
With the mixer board connected measure the voltages with respect to ground at each end of R12 and R13 (4 readings.)
Switch the power off and leave it for bout 10 minutes for the capacitors discharge. Then measure the resistance of R12 and R13. You will need to lave the DMM connected to the resistors until the reading stops changing. (The drift is cause by the capacitors connected to the ends or the resistors.)
Les.

 

Hello,

@LesJones , i also mentioned the condition of R12 and R13 in post #66.
When the output of the 7815 and 7915 where low, those resistors got more power to dissipate than rated.
See the measurements made in post #33.

Bertus

 

Update: got sidetracked with other projects…
About that TDA 7294 audio amplifier from earlier…I also noticed that I hadn’t seen that I’d omitted the tiny rubber “sleeve” that prevents the bolt and nut from contacting the sink/mounting plate. I put it back in but haven’t gotten any further yet. I’ll look at those R12 and R13 resistors shortly and report back.
(on a side note—I have received the new power amp board from Kustom…I can’t seem to just put the new piece in and let go the search of what went wrong…I can’t help myself )

 

Persistence is the helper towards success on many occasions.

 

Now that you have given us the facts clearly I agree with bertus that the fault is likely to be decoupling.
Do the following to confirm that R12 and R13 have not changed their value.
With the mixer board connected measure the voltages with respect to ground at each end of R12 and R13 (4 readings.)
Switch the power off and leave it for bout 10 minutes for the capacitors discharge. Then measure the resistance of R12 and R13. You will need to lave the DMM connected to the resistors until the reading stops changing. (The drift is cause by the capacitors connected to the ends or the resistors.)
Les.

Ok here goes…
R13 35 vdc one way, 4 vdc opposite end
R12 same as R13…

resistance:
Meter on 2k ohm setting (no read on 200)

R12 .324 or .318 when swapped

R13 .318 or .324 when swapped.

Also, from the last night episode of slight smell (Post 87) checked fuse and it had blown. Replaced with spare.
Then first thing tonight I used my Dim Bulb tester to supply the power and cautiously turned it on. Blue LED came on and no smell or heat anywhere. And the mixer board was plugged in as well. Then I took the above measurements…

And…power on and took dc measurements at the two speaker outs…about 34vdc on each…
Holding tight for the moment.

 

Can I assume that the voltage readings without a sign are positive ?
The voltage readings on the ends of R13 should be negative. (As when tested in an erlier post.)
R12 and R13 do not seem to have changed value significantly. I had thought that the burning smell had com from them but now you are measuring 35 volts (Either polarity.) on the speakers I think the smell was the voice coils on the loudspeakers burning out. Shorting the tab on the TDA7294 to thr heat sink shorted out the negative supply which was probably the reason the fuse failed.
Les.

 

Can I assume that the voltage readings without a sign are positive ?
The voltage readings on the ends of R13 should be negative. (As when tested in an erlier post.)
R12 and R13 do not seem to have changed value significantly. I had thought that the burning smell had com from them but now you are measuring 35 volts (Either polarity.) on the speakers I think the smell was the voice coils on the loudspeakers burning out. Shorting the tab on the TDA7294 to thr heat sink shorted out the negative supply which was probably the reason the fuse failed.
Les.

Hmmm…I would have thought if I’d seen a minus I would have noted, but thinking about it I’m not sure…so I’ll look again.
Hmmmm…yikes…again thinking back, I believe I did have the speakers hooked up for that earlier test of “oh wow—it powered on with the Blue LED staying lit; maybe it was a better connection somewhere, so I’ll hook it up and see if it makes sound!” But, I had the power/mixer part laying on top of the backside of the speaker cabinet, kinda blocking the speakers, and thinking the smell was coming from the power board. Geez—I may have just ruined it beyond any sensible repair if I killed the speakers…there’s a simple but valuable lesson for me—a substantial DC reading on the speaker outputs…I had no idea until reading about it late last night. I’m gonna go back down and do a little more investigating of the speakers…

 

Voice coil burnout is the frequent hazard with direct DC connected speakers. And often the most expensive error possible to make.

 

Voice coil burnout is the frequent hazard with direct DC connected speakers. And often the most expensive error possible to make.

You got that right! If these speakers were shot it would render the whole thing an expensive, stupid mistake…I’ve already spent 50 bucks on a factory replacement power board…I think the whole unit might have been around 170 bucks when new…
I think I might have gotten lucky …I hooked up the two little woofers and one tweeter in the cabinet to a little power amp, and ran audio from a boom box headphone jack to it…sounds great! Clear, no distortion. I believe this little amp is 20 watts so not as much as the Kustom 50 watts. But I think it’s ok?

 

The remaining ckeck is to clip a DC voltmeter across the speaker . It should show very close to zero volts DC. If not, there is a problem somewhere.

 

The remaining ckeck is to clip a DC voltmeter across the speaker . It should show very close to zero volts DC. If not, there is a problem somewhere.

Ok I think I did that last night and came up with the 34vdc readings on each of the two plugs…thinking maybe I destroyed that TDA7894?

 

THIS problem is why I usually choose a circuit that uses a transformer, or a SERIES capacitor to feed the speakers. The tiny loss of bass is much less painfull than burning out a set of speakers.