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Old 15th Oct 2019, 9:38 pm   #1
Dougallh
Triode
 
Join Date: Apr 2012
Location: Manchester, UK.
Posts: 29
Default Goodmans Model 150 Amp Fault

I have a power amp fault on my 1976 Goodmans 150 receiver. This uses monoblock 110 + 110 watts rms power amps, one of which failed. One of the MJ802 (100v 30A) output transistors blew, C-E short. This put +43V dc on the output.

On replacement, I cannot get any quiescent current to flow; the last 4 transistors: BFT37/MJ802, BFT34/MJ4502 are OFF. The previous drivers BC449, BC450 are ON - just, the voltage across the bias voltage transistor feeding the drivers is 1.1V (should be at least 1.2V) - No amount of adjustment will increase the bias current.

The voltage across the BFT34/37 bases - emitters is just 0.2V (fed from collectors of VT808,810)

The top & bottom constant current source voltages all measure correctly.

I have replaced all transistors - NO JOY
I have removed obvious suspects C809, 824 etc - NO JOY
I have disabled protection ccts, VT815,816, diodes WD01,02,03,04 - NO JOY
I have disabled VT801, overcurrent protection to bias current driver VT802 - NO JOY
All resistors are correct & connected (inc the 0.33 ohm emitter resistors)
I cannot see any shorts or burnt out tracks
The +43V & -43V supplies are correct, no noise evident using a scope

I am missing something ?

I have uploaded a schematic

Thanks
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Last edited by Dougallh; 15th Oct 2019 at 9:59 pm.
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Old 16th Oct 2019, 9:44 am   #2
tony brady
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Default Re: Goodmans Model 150 Amp Fault

Have you checked the voltages across R808 & 813? These should both be about 0.65V. If these are wrong the bias will also be wrong. Also check the voltage across R805 - this should be about 0.4V. If the resistor is high in value or the current limiter transistor VT801 is faulty the current though the next stage will be too low so you will not be able to get the bias current high enough.

I presume you have checked the signal (off load) with a 1KHz signal and scope? Does the waveform look basically OK except for some crossover distortion kink being visible? What is the DC offset voltage at the speaker output? For this sort of amp it should be less than 100mV and ideally less than 50mV.
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Old 16th Oct 2019, 5:13 pm   #3
Dougallh
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Join Date: Apr 2012
Location: Manchester, UK.
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Default Re: Goodmans Model 150 Amp Fault

Thanks for your reply; the voltages around the resistors you mention are correct. A 1kHz signal looks OK @ both the C & E of the bias transistor VT809.

There is just 0.5mV DC offset @ the output with ref to +/- 43V.

A fairly pure sinewave appears @ the output but is low level & I believe this has been passed directly by the base/emitters of VT808 & VT810. ; a truncated waveform can be seen @ the collectors of VT808 & VT810 but is not being passed through as the last 4 transistors are still off.

R830 which connects the collectors of VT811 &VT812 seems unusual...

I believe that there are probably 3 levels of DC feedback...(inc R809 + R820 & R819...)

I have replaced both output triplets twice now; no change...

Last edited by Dougallh; 16th Oct 2019 at 5:25 pm. Reason: addition
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Old 16th Oct 2019, 11:31 pm   #4
PJL
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Location: Seaford, East Sussex, UK.
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Default Re: Goodmans Model 150 Amp Fault

Focus on the DC conditions (no signal). You should be able to compare the voltages with the good amplifier. Then markup the transistor voltages on a copy of the circuit noting any differences and post back here, Laborious, but the best way to diagnose the fault.
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Old 17th Oct 2019, 11:01 am   #5
tony brady
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Default Re: Goodmans Model 150 Amp Fault

have you measured the voltage across VT809 while adjusting the bias pot? does it change at all? if not check VT809 and the pot

I would also advise changing both bias pots anyway as if the wiper goes o/c the bias goes to max ( daft design, the pot should be in the lower amp for fail safe)

if the voltage does change I suppose there's an outside chance C812 is duff. The arrangement with R830 is not that unusual, it helps to turn off the o/p trs faster.

what did you replace the MJ802 with? I have to ask is it possible it's a fake?
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Old 17th Oct 2019, 6:46 pm   #6
Dougallh
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Default Re: Goodmans Model 150 Amp Fault

I have managed to find the problem after much trouble:

The most available bias current, from adusting the pot R814, would only turn up the B-E voltage on VT811,812 to 0.3V - not enough to turn them on. I began to think that the replacement BFT34,37 or BC445,450 could be fake...

When the MJ802 blew it took out 10 transistors ! (replacement marked MJ802 Malaysia, originals are Motorola) I replaced the bias voltage transistor VT809 (BC183L) at least twice, I had procured new ones from ebay. Before embarking on re-ordering the triplet transistors from a different source I tried replacing the BC183L with a BC184L - fault fixed !

The new BC183s had a below spec hfe ie < 100 (should be 125 to 900 !) I have been able to set the correct Q current (17mV across the emitter resitors) using 1/3 of the available adjustment & it is stable over a period of 2 hrs; the DC offset is 3.5mV.

I have never had problems with these transistors apart from noise before...

This is a relief as the BFT34/37s are rare & I know of no equivalents, 100V, 5A, TO39.

I agree with you Tony re the pot placing. I have repaired 3 Bush SRP51 record players (one of the nicest portable record players made in my view, superior deck, big speaker, big sound) which have a 50 ohm pot in the top of the divider, these go high over time. 2 suffered loud hum & 1 had blown AD161/162s due to excessive Q current. They all needed 18 ohms so I replaced the presets with a fixed resistor. I may do the same in this case if there is significant drift over time.

Thanks guys for your interest & thoughtful suggestions; this was a new problem for me, working transistor but low current gain (not normally an issue in amplifier feedback ccts).

Andy
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