Have an SWR Workingman's 160 solid state bass head with no power amp output, any help troubleshooting this appreciated.
It powers on and the preamp section works perfectly. The front panel lights for preamp clip and master limiter are working.
some voltages B/C/E:
Q10 = -63/-3/-63
Q4 = -3/63/-3
Q9 = -3/-63/-3
Q3 = -3/63/-3
Q5 = -3/-3/-3
...and so on. From the looks of it, the previous owner shorted a speaker lead and things blew up. The rectifier was blown short. I replaced that and it powers on normally. What next?
It powers on and the preamp section works perfectly. The front panel lights for preamp clip and master limiter are working.
- On power on, I get about 30v DC on the speaker out that runs down within a minute or so to to about 3-5v and stays there.
- Can't find a shorted transistor. Can't find a shorted diode. Have checked them all about three times but I could still be missing it.
some voltages B/C/E:
Q10 = -63/-3/-63
Q4 = -3/63/-3
Q9 = -3/-63/-3
Q3 = -3/63/-3
Q5 = -3/-3/-3
...and so on. From the looks of it, the previous owner shorted a speaker lead and things blew up. The rectifier was blown short. I replaced that and it powers on normally. What next?
Hi szegedin,
I suggest measuring a few voltages:
Collector voltage of Q2
b,c,e of Q6 and Q7
b, c of Q8 with reference to Q8 emitter
Voltage across R24 and R30
I suggest measuring a few voltages:
Collector voltage of Q2
b,c,e of Q6 and Q7
b, c of Q8 with reference to Q8 emitter
Voltage across R24 and R30
Transistor testers, and diode scale of DVM, test at 2 v or less. Transistors & diodes have to hold off rail voltage which is +-blot9 volts in this case.
Suggest you test transistor and diodes at 12 or 24 v Iceo test. 24 v power supply series 47k resistor series current scale of DVM. Warning >25 v across your heart will stop it. Use only one hand at a time, and clip leads. I=V/47k, your transistor or diode is toast. + on collector npn, - on collector pnp.
Or put 60 w tunsten light bulb in series with AC line, put ground clip on DVM to speaker return with clip lead, probe diodes & transistor for voltage to see which failed.
Suggest you test transistor and diodes at 12 or 24 v Iceo test. 24 v power supply series 47k resistor series current scale of DVM. Warning >25 v across your heart will stop it. Use only one hand at a time, and clip leads. I=V/47k, your transistor or diode is toast. + on collector npn, - on collector pnp.
Or put 60 w tunsten light bulb in series with AC line, put ground clip on DVM to speaker return with clip lead, probe diodes & transistor for voltage to see which failed.
Good morning - Okay here goes. It's hard to figure out the layout on this one but I think this is right:
Q2 C = 23v
Q6 = 0/0/-62
Q7 = 0/0/-62
Q8 BC ref E = 0/23v
R24 = 0
R30 = 0
Hold all below in abeyance. Q2 collector voltage isn't correct; should be about +59V. Look for reason Q2 isn't conducting.
These results suggest that there is no emitter current being delivered to Q6 and Q7. Probe the path from Q2's collector through R25, D9, R30. An open component is possible or shorted C12, or other defect that I've not thought of.
These results suggest that there is no emitter current being delivered to Q6 and Q7. Probe the path from Q2's collector through R25, D9, R30. An open component is possible or shorted C12, or other defect that I've not thought of.
There may be more than one fault, given that you mentioned that output initially stuck high, then decays toward 3V.
Well R23 looked suspect -- hard to tell whether burned or just flux smeared on it like many components on this board - so I replaced it. This might be related to what you said about Q2 voltage -- but didn't change Q2 C voltage.
Now I see that same 23v at + on the rectifier and Q3 C. Rectifier blew again?
Now I get the same voltage on Q6 & Q7 emitters as the speaker output, which drifts down slowing around 3v.
But the preamp section now stopped working.
Heed last sentence of post # 3 to avoid injecting more damage to components ahead of those already blown. Or those already replaced as the rectifier. You have to limit the current flowing from from the AC line as you trace the troubles to determine what all is bad.
My 60w tungsten bulb is in a grounded steel case with a circuit breaker, power cord and NEMA20 outlet. This is because edison lamp base screws are suitable for solid wire of building wiring, not the stranded wire of electronic components. The wire might pop off internally. The case has a mesh covered hole to allow me to see how bright the bulb is for how long when I turn the AC power strip on. I can change to 100 w bulb for higher wattage amps. The breaker is 5 amps, suited for amplifiers of 100-250 w. For 1200 w up amplifiers I have to use a heating element out of a 1200 watt room heater to limit the AC current.
My 60w tungsten bulb is in a grounded steel case with a circuit breaker, power cord and NEMA20 outlet. This is because edison lamp base screws are suitable for solid wire of building wiring, not the stranded wire of electronic components. The wire might pop off internally. The case has a mesh covered hole to allow me to see how bright the bulb is for how long when I turn the AC power strip on. I can change to 100 w bulb for higher wattage amps. The breaker is 5 amps, suited for amplifiers of 100-250 w. For 1200 w up amplifiers I have to use a heating element out of a 1200 watt room heater to limit the AC current.
BTW, the schematic is drawn incorrectly re BR1. All diodes in the diode bridge should be the reverse of what is shown in the schematic. Actual installation must be correct or the amp would never have worked. All other diodes in the amp look correct.
In post 1, the both 59V rails were apparently working. But not now?
I can't opine about the health of the output devices, but something is definitely wrong in the circuitry controlling Q2, or Q2 itself has failed. The load resistances on Q2 are modest and the Q2 is moderately well protected from faults in surrounding circuits.
I can't be certain about the purpose of the drive circuit to Q2, but I believe it's intended to provide delayed start/quick off response to AC power. If all were working, there would be -59VDC at top of R18, a few volts positive on C7, +0.6VDC on base of Q1, about 0VDC on collector of Q1, about +59VDC on base of Q2, 0.6VDC across R20, and about +59VDC at collector of Q2.
What rectifier was previously blown? What are the readings on the +/- 59V supply rails?
In post 1, the both 59V rails were apparently working. But not now?
I can't opine about the health of the output devices, but something is definitely wrong in the circuitry controlling Q2, or Q2 itself has failed. The load resistances on Q2 are modest and the Q2 is moderately well protected from faults in surrounding circuits.
I can't be certain about the purpose of the drive circuit to Q2, but I believe it's intended to provide delayed start/quick off response to AC power. If all were working, there would be -59VDC at top of R18, a few volts positive on C7, +0.6VDC on base of Q1, about 0VDC on collector of Q1, about +59VDC on base of Q2, 0.6VDC across R20, and about +59VDC at collector of Q2.
Thanks - the rectifier is actually the only component mounted on the underside of the circuit board, which might account for why it's backwards on the schematic, but it's confusing. When I got the amp (another freebie) I found the rectifier shorted and replaced it with another the same as the original. The speaker jacks showed signs of an explosive short - carbon on one.
It seems this amp will probably not be worth the trouble - was not a great amp in the first place - but thanks anyway for the tips. 🙏 Yes, before the rails seemed to have the right voltage but now no, and the 15v rail failed so the preamp section doesn't work. I may salvage this one for parts.