Swr red head fuse blow both fuses

[hewo]

Player foiled external fuse blowing the internal fuse.
Do I disengage pt loads to 1st evaluate worthiness pt?
Nothing appears abnormal, no visual discrepancies deficiencies, no odors, nothing out of the ordinary.
Usually the power transistors blow from old age?
Or does this ab class runaway when bias strays off course?
Any help appreciated.
Curtis repairman

[Jazz P Bass]

Fuse blowing can be caused by, well, anything.

You could certainly start at the power transformer, but a couple of quick resistance checks of the output transistors & the power supply diodes would be a better start.

Thinking about why it failed before knowing what failed is kind of futile, no?

[52 Bill]

What Jazz said.

Which two fuses were blown, both the primary ac fuses?

I have seen dead power transformers in SWR amps, but much more likely are output transistors and power supply rectifiers.

[Enzo]

I replied to this in another forum.

The power transformer is last on the failure list. First suspects for blowing fuses are always output transistors and rectifiers.

[hewo]

15amp200vRectifierShortJustOneLeg

I replied to this in another forum.

The power transformer is last on the failure list. First suspects for blowing fuses are always output transistors and rectifiers.

I pulled the square rectifier, isolated, measured, shorted one of four internal diode, funny, negative poled, but the ballast and downstrm feeds all ok.

Do I chk output transistors and feeding intermediate driver transistors?
There's quite a bit of umbilicals to remv to lift out massive heatsink integrated to poweramp board.

[Enzo]

I check them where they sit. Nothing external to a shorted transistor can make it seem not shorted. SO I check for shorted transistors while they are still mounted. Only if they check as shorted, THEN I remove them for further tests.

[hewo]

cursory poweramp transistor offend chk

I check them where they sit. Nothing external to a shorted transistor can make it seem not shorted. SO I check for shorted transistors while they are still mounted. Only if they check as shorted, THEN I remove them for further tests.

Thankyou enzo!
Will go thru one by one, TO-220, TO-92, MT-200, TO-5 (TO-39 can)

If ok, proceed to energize by way of gradual variac utility line power, with 8ohm 100watt resistor engaged to "external spkr", as "internal spkr" is stereo male 1/4" plug hardwired but there's no combo cab to receive three conductor (horn dedicated using amp's variable inductor rheostat that's undocumented schematic.

[Enzo]

DO NOT connect a speaker or dummy load until the amp works with no DC offset at the output.

[hewo]

All transistors, diodes, of poweramp chk'd in circuit, none faults affirmed

DO NOT connect a speaker or dummy load until the amp works with no DC offset at the output.

Ok, semiconductors chk okay.
Soldered two 100watter 8ohm non-inductive power resistors parallel, for dummy load.

Variac on utility line readied.

Agree, variac upwards but without dummy load, I.e., open circuit spkr terminal. Verify dc nil, accept tenths or hundreths of 1 dcv.

Inject 12rms volts 1khz sine front end, dials at nominal lunchtime, scope spkr inflection zero crossing satisfactory, UNDER OPEN CIRCUIT SYSTEM STATUS?

After performing open ckt system status, shutdown, connect dummy and repeat system status query of said critereon.

Gameplan sounds ok?
Wisdom tells me to set dials nominal at nine am to affirm inflection zero point crossing. Why? Its because the lower power level "exaggerates" any glitch occurring at inflection zero point crossing.

[Enzo]

Plan approved.

[hewo]

Plan approved.

Great!
System status opened, utility120.0vac, spkr open12mvdcNegative/5mvac
Next system status closed, 4ohm200wattPowerResistorNonInductive
Standby

[hewo]

Ok, SYSTEM status closed (load):

Utility line 120.0vac
Spkr DC 8mv Positive
Spkr AC 5mvac
Knobs assigned lunchtime

Then without front end inject sine, dialed all knobs infinite except mv.
Still stable until dialed mv upwards, then at 4pm heard motherboard ultra high pitched audio parasytic, then variac 5fastblow shutdowns, gotta re-fuse fastblow 5ampere fuse variac.
Standby, this creature can suck utility energy like nobody's business!

[hewo]

Sorry, variac silkscreen 5ampFast
Putting on glasses, the opened fuse reads "250V 312 3A"
So, almost home folks.
Stndby

[hewo]

Confusion exists now as to how such current draw resulted without signal inject demand amplification.
Either variac's 3a fastblow was fragile preexisting condition, meaning it was rated below actual 3a from age and fatigue, or, circuitry experienced parasitic oscillation thereby consuming actual rated fuse 3ampere. That momentary high frequency audio sound emanating from circuitry actually climbed "upwards" fuse blowing variac arrests furtherance. Or, maybe the shrill climbed because of the onset transient of starvation fuse blowing. During current demand rise, the fuse filament experiences thermal escalation, equivalent voltage reduction in pt transient, equivalent pt counter-response falling voltage which in turn disrupts circuitry with counter response to energy suffocation.
Anyhow multiplying 3amps times 120.0vac utility line yields 360watts, kind of unjustified without actually performing amplification of an injected signal at the front end.
What say you to this unjustified 360wattage manifested?

[hewo]

Redhead proofed satisfactorily, installed brass 12ax7wa shield canning (missing historically), as high pitch audio sourced to this culprit when "aural" enhancer knob prescribed too much, spkr ac voltage jumps exponentially under real load, knobs dialed nominal lunchtime, and aural knob increased.
Ac spkr voltage does grow huge, curtailed at about 22vac, dc rails can go further, but dummy resistor 200watt 4ohm gets too hot (drifting impedance?). So curtailed aural and mv to about 12noon and not any further. Panel clipping lights not exceeded (not illuminated).
V=I*R so audio watts apprx = (I^2)*R = { [V/R]^2 } * R = (V^2) / R
So nominal dials audio watts = (22^2)*4 = 120 audio watts
Good enough proofing? Inject frontend was 12mvac rms @ 1khz sine.
Experimented with generator's other waveforms other than sine, wow! Squarewave redhead can duplicate but dunno why spkr ac voltage jumps even bigger, I saw 32vac on fluke79, lots of energy heating dummy load.
Oscilloscope perfection crossover, even when decreased injection strength to 7mvac 1khz sine. This means that open framed bias wheel potentiometer (why not sealed Bjorn potentiometer?) is highly reliable untouched since leaving factory 1986!
Bassist reinstalled redhead into church combo 2x10x1xTweet.
Tweet out of commission, must get equal or better, from whom?
Bassist played beautifully about twenty minutes, spectacular, 1986 design mentality is lucrative, I think its the 250vdc onto 12ax7wa, the real McCoy tension for signal beautification.
Finally instructed church bassist don't ever re-fuse (spares attached inside tray chassis) because redhead is too precious sound signature that we don't want blown apart.