Dynatron amp output transistors - incorrect voltages - UK Vintage Radio Repair and Restoration Discussion Forum

Emibell · 29th Mar 2020, 11:08 am

On an SRX26 I have come by the output transistor voltages should be about 30 and 15 volts for each pair. ( TR7 and TR8 on the schematic.)
They were correct on one good channel and 15, 15 on the other with complete loss of this channel.
I attempted to resolder TR4 on the bad channel as one of the legs was broken and they are now approx 15, 15 on the good and 22, 22 on the bad.
Clearly I may have disturbed something and my soldering hasn’t worked (very difficult to get decent access without dismantling the whole amp!)
My testing skills are limited so I would like to ask do these equal voltages are likely point to any specific component failure, before I finally give up.

P.S. testing was done with the copper heat sink removed from TR5,TR6
so the possibility of ‘tin whiskers’ interfering here was minimised.

John_BS · 29th Mar 2020, 11:37 am

First of all, please put the heatsinks back on TR5/6 or they might overheat during testing!

I assume the voltages are measured on the cases of the output transistors relative to the 0v rail? In which case, TR8 and it's equiv on the other channel should measure the same as the HT: check the voltages on both sides of F2 and F3, which should all be the same. If either Tr8 or it's mate are different, you have a lack of contact via the screw to the pcb.

There are a lot of electroytics in the amp , and if one or more is particularly leaky you might have problems with the DC voltages internallly. Once you know the power-supply (F2/3) is about right, check the node R18 to R19 (+ve of C19) which should measure about half the HT voltage.

John

Emibell · 29th Mar 2020, 2:15 pm

Thanks John - yes, voltages measured on cases of o/p transistors..

29th Mar 2020, 11:08 am	#1
Emibell Tetrode Join Date: Jan 2016 Location: Shepperton, Surrey, UK. Posts: 91	Dynatron amp output transistors - incorrect voltages On an SRX26 I have come by the output transistor voltages should be about 30 and 15 volts for each pair. ( TR7 and TR8 on the schematic.) They were correct on one good channel and 15, 15 on the other with complete loss of this channel. I attempted to resolder TR4 on the bad channel as one of the legs was broken and they are now approx 15, 15 on the good and 22, 22 on the bad. Clearly I may have disturbed something and my soldering hasn’t worked (very difficult to get decent access without dismantling the whole amp!) My testing skills are limited so I would like to ask do these equal voltages are likely point to any specific component failure, before I finally give up. P.S. testing was done with the copper heat sink removed from TR5,TR6 so the possibility of ‘tin whiskers’ interfering here was minimised. Attached Thumbnails

29th Mar 2020, 11:37 am	#2
John_BS Octode Join Date: Dec 2008 Location: Wincanton, Somerset, UK. Posts: 1,780	Re: Dynatron amp output transistors - incorrect voltages First of all, please put the heatsinks back on TR5/6 or they might overheat during testing! I assume the voltages are measured on the cases of the output transistors relative to the 0v rail? In which case, TR8 and it's equiv on the other channel should measure the same as the HT: check the voltages on both sides of F2 and F3, which should all be the same. If either Tr8 or it's mate are different, you have a lack of contact via the screw to the pcb. There are a lot of electroytics in the amp , and if one or more is particularly leaky you might have problems with the DC voltages internallly. Once you know the power-supply (F2/3) is about right, check the node R18 to R19 (+ve of C19) which should measure about half the HT voltage. John

29th Mar 2020, 2:15 pm	#3
Emibell Tetrode Join Date: Jan 2016 Location: Shepperton, Surrey, UK. Posts: 91	Re: Dynatron amp output transistors - incorrect voltages Thanks John - yes, voltages measured on cases of o/p transistors..