Just Can't Seem to Find a Short

1 meg seems a bit low, but not unheard of. The rails are more of a concern. It looks like they should be +/- 27 VDC, so you've got some investigating to do. I'd be worried about the transformer. If the diodes are OK, and the filter caps are OK, the transformer is suspect. Is anything getting hot? Diodes or caps? Does the transformer get hot, smell or make any noise? You might have to pull and measure the secondary leads, which according to the schematic should be 20 VAC. Anybody got other suggestions?
No signs of anything with the transformer, and it's 20-20 coming out of it like it should be. It's gotta be something strange in the circuit that's causing this, as the one rail and the input AC are both fine. I'll keep measuring around.
 
This just doesn't feel right. There's got to be some problem with ground or with AC on the supply line or something. Have you got a scope?
 
There are three fuses in the power supply for the output stages, two are for the B+ and one for the xfmr secondary center tap. Also in series with that CT fuse is a resistor, R701. Have those parts been checked? The power supply for output stage does appear to be making both + and - voltages though. Also pulling the B+ fuses still leaves the B- still in circuit.

Craig
 
There are three fuses in the power supply for the output stages, two are for the B+ and one for the xfmr secondary center tap. Also in series with that CT fuse is a resistor, R701. Have those parts been checked? The power supply for output stage does appear to be making both + and - voltages though. Also pulling the B+ fuses still leaves the B- still in circuit.

Craig
Resistor and fuse on the center tap both measure ok. Still trying to figure out how to remove the B- from each channel individually.
 
This just doesn't feel right. There's got to be some problem with ground or with AC on the supply line or something. Have you got a scope?
A scope is on my to-buy list, I sort of have access to one at my college, but I have to move the amp there which is a little awkward. I'll check the wiring from plug to power supply. Wondering about the RC filter on the AC line in.
 
I doubt it's primary circuit related. Check anyway, but be careful. Interesting that the center tap is fused. Not sure I've ever seen that.
 
When you pull the B+ fuses the + side of the power supply is now unloaded, higher voltage than the still loaded - side of the supply, lower voltage. Now the question is the - side pulling too much current? I never use a DBT so I don't know what the norm is. I always use a Variac and when turning up the AC voltage slowly many amplifiers will draw more current than normal until enough DC voltage is available for the circuit to balance out. What are the chances of your 75W bulb is too small?

Craig
 
When you pull the B+ fuses the + side of the power supply is now unloaded, higher voltage than the still loaded - side of the supply, lower voltage. Now the question is the - side pulling too much current? I never use a DBT so I don't know what the norm is. I always use a Variac and when turning up the AC voltage slowly many amplifiers will draw more current than normal until enough DC voltage is available for the circuit to balance out. What are the chances of your 75W bulb is too small?

Craig
With it looking like a short without the B+, I can only assuming the B- is pulling too much current. The bulb is for sure not too small based on the size of the amp, I used the same bulb to test a hefty amp rated at 60 watts a channel and it gave me no problems even with a high bias. Thank you for the suggestion though.
 
You never mentioned if this worked before you got into it. Did you try it with all the fuses and a 100 watt bulb for the DBT?
 
You never mentioned if this worked before you got into it. Did you try it with all the fuses and a 100 watt bulb for the DBT?
Bought it non-working. Also now can't find incandescent bulbs of any wattage anywhere, it's all just LED and halogen. If it was working somewhat fine even when failing the DBT, I would maybe think a bigger bulb would help, but nothing gets dialed in and the lights don't come on, etc....
 
ivandezande , I have picked up incandescent bulbs at Lowes. Also try the Dollar General, Family Dollar type stores. Good luck.
 
You ever mess so many things up that you question everything you do? So literally all of the transistors minus four were in backwards (I blame the mirrored SM and the terrible silkscreen) but anyways....

Still shows short BUT power LED now comes on with fuses in.
With fuses out, the preout works perfectly, FM/AM, aux, all the tone controls work and it sounds great.
Bias only gets down to .2 V, while offset can be dialed in perfectly.

Getting there folks. Gonna be a late night.
 
Last edited:
Took out all the outputs....again. No short. Sound is coming through headphones. Offset trimmer works.
TR613 & TR614 are getting very warm and their compliments (612/611). Voltage at the bases is +/-6 volts, can be adjusted by the bias trimmer. This is much higher than the 1.2 volts I should be seeing, I don't really understand that.

All other voltages measure correctly.
 
Installed a pair and back to straight short. The transistors are fine, and the solder joints obviously won't win a beauty contest, they're fine. And the outputs weren't even screwed into the heatsink so there can't be a short there (probably not a great idea, they started getting very hot).
Bias is adjusted to as low as she'll go.

IMG_4689.jpg IMG_4688.jpg
 
Excess voltage is being sourced from somewhere it shouldn't or not being bled off somewhere it should.

Check ALL sources to the overvoltage as well as whether ALL your drops are in fact bleeding off or blocking what and when they should. Make no easy assumptions; check grounds, caps, resistors, diodes, trannies, etc. (out of circuit as needed and under load if possible). If you're unsure what is actually sourcing and what is sinking, separate the circuits to get an idea of their operation in isolation. Be careful, consider the consequences of bypassing and lifting components out of circuit BEFORE you do so. Go slowly and systematically. You've identified a problem, now go find its extent and the reason for it. Eventually you'll come upon the root cause of the anomaly and the obvious solution to remedy it. Good luck.
 
I had a Luxman L210 that was hiding the bad somewhere too, I ended up just shelving it for the time being as it was just consuming too much time with the never ending tail chasing.... I feel your frustrations....
 
Back
Top Bottom