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Old 17th Oct 2019, 6:46 pm   #6
Dougallh
Triode
 
Join Date: Apr 2012
Location: Manchester, UK.
Posts: 29
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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