Help with the 919 and anyone got a spare npn ?

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parsnip56

New Member
Anyone wanna chime in with my little firestarter, or is anyone on here within a hundred miles of manchester got the equipment and ability to help me sort this bastard out !!
I need 1 npn output transistor however i really need to get this right because i hate destroying parts that are hard to find its terribly wasteful, is it worth me replacing every transistor on the driver boards faulty channel ? should i swap the polystrenes for micas ? im gonna drag the preamp out n look at that, amp has only been used through the aux input fed from a dac wired to the pc !
I measured voltages from the power supply before when i rebuilt it n they were fine ! I've performed diode checks on all transistors/diodes and compared them to the working channel also measured the resistance across the pins bce and theyre comparable ! all resistors were measured and were within tolerance values.

My current failure list R31 R75 R91 R87 c65 Tr29 tr31 n the 4 o.p trannys
 
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Exactly what capacitors did you use to replace the black flags?

And you say you replaced the emitter resistors, like for like? - what did you use there?

Sorry I don't have an 'npn'.
 
Replaced the ones in preamp with Silver micas, amp driver was already done with polystyrenes they're listed in the service manual as polystyrene too, ill replace them if necessary ! The emitter resistors were like for like, 2 were ok but I swapped them all !
 
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parsnip56 said:
Replaced the ones in preamp with Silver micas, amp driver was already done with polystyrenes they're listed in the service manual as polystyrene too, ill replace them if necessary ! The emitter resistors were like for like, 2 were ok but I swapped them all !
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I think you should replace the driver board capacitors with either Silver Mica OR Ceramic NP0 or C0G types.
 
Mornin everyone, i did have a set of st's in there :-( i have 2 nma's at present n 1 nmc for the duff channel, however i'm reluctant to fit these until i can be reasonably sure it's fixed. So i'm after one of the nmc op transistors.(or equiv) I'm going to replace all of the polystyrene caps on the driver board n pull the flat amp out again n have a nosy round that. I haven't changed the pots so these can be done too, I dont just want to go throwing more money at this tho, i'm not a millionaire i'm a single parent ! Seems the mj21193/4 are available so i'll happily bang some of them in for now until i'm sure its stable n the fast ones can be found.
 
Sounds like a plan !

I was going to suggest a 'sacrificial' set of MJ21193/94's but you thought of it first :)

Changing the pots/trimmers is a must.

Keep us posted :thmbsp:
 
with regards to checking this with a scope is it a matter applying a square wave to the input n making sure the amp is outputing a clean squarewave at the output ? and the same with a sine wave swept across the frequency range ? Am i right in thinking it should accurately reproduce whats fed in ? then work backwards checking at the driver transistors and the output from the flat amp ?
 
A reasonable approach with this amp, given the difficulties you are having and the rarity of the output devices, is to get the amp channel working well WITHOUT the TO3 devices in place, not to deliver power, just to monitor the circuit up to the outputs. Unlike some later designs, the emitters of the drivers run against the output to the speakers so you'll at least be able to confirm DC stability with no risk to those precious outputs. A bit unorthodox perhaps but an approach to consider given how difficult it is to source the NMA/NMC devices. Of course this approach presumes proper and reliable outputs will be fitted once the driver circuit is debugged.
 
that's what i did when i rebuilt it the first time i asked echowars wether it would run with out the op trans in, he said it would so i did ! gonna have to look at getting a scope sorted. Can you hear the sound difference with the mj's in one of these ? would be cool if the amp could be modded to take t03p trannies, well it cud obviously but would be crude ! The thing that gets me with this is there was no warning signs, no audible distortion, no excessive heat when on, n it ran for days for 10 n 12 hrs at a time. Its obviously an intermittent problem and a real bastard to find !
 
Just been having a look at those polystyrene capacitors on the driver board there's a few that don't match the service manual, i don't know wether they've been changed before i got the amp or if they were originally fitted. I'd normally go like for like but at the mo n given the probs i'm havin im not so sure !
C23 24 25 26 1pf fitted 3pf in service manual c55 56 6pf fitted 3pf in manual ! c53 54 8pf fitted 3pf in manual did they just run out of 3pf caps ? i also have c65 6pf which is melted but its not in the manual if it was prob b 3pf ;)
Any ideas like for like or go with manual what have u lot got fitted ?
 
EchoWars said:
Leave the polystyrene caps alone.
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I think parsnip is talking about the 'Black Flags' Glenn, which (on his driver board) have already been replaced :scratch2: - what we don't know yet is what have they been replaced with, (polystyrene, mica, ceramic?) but it sounds like someone has been 'monkeying' with the values :yikes:

But Glenn is right (of course) leave the original fit, cylindrical silver polystyrenes alone, some confusion may have arisen because I read somewhere that the Black Flags (shaped like black flags) are actually polystyrene !
 
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Hyperion said:
I think parsnip is talking about the 'Black Flags' Glenn, which (on his driver board) have already been replaced :scratch2: - what we don't know yet is what have they been replaced with, (polystyrene, mica, ceramic?) but it sounds like someone has been 'monkeying' with the values :yikes:

But Glenn is right (of course) leave the original fit, cylindrical silver polystyrenes alone, some confusion may have arisen because I read somewhere that the Black Flags (shaped like black flags) are actually polystyrene !
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They are polysterene john.
 
Hi guys yeah my black flags are the clear cylindrical polystyrene type, amp is 1980 model so don't know wether it left the factory as it is or wether someone else had swapped them out. I was in 2 minds wether to just change them to micas but noticed majority of the 3pf ones were something else ! I changed the preamp ones 4 x 3pf and 2 x 33pf ! I have a technics 8080 n a proton aa1150 that I plan to sell if I ever get this working the 919 sounds better than both otherwise it would b on eBay already ! I'm enjoying the learning experience of this but money is tight n the parts are hard to find !
 
parsnip56 said:
Hi guys yeah my black flags are the clear cylindrical polystyrene type, amp is 1980 model so don't know wether it left the factory as it is or wether someone else had swapped them out. I was in 2 minds wether to just change them to micas but noticed majority of the 3pf ones were something else ! I changed the preamp ones 4 x 3pf and 2 x 33pf ! I have a technics 8080 n a proton aa1150 that I plan to sell if I ever get this working the 919 sounds better than both otherwise it would b on eBay already ! I'm enjoying the learning experience of this but money is tight n the parts are hard to find !
Click to expand...

Well, unless they are radial construction they will likely have significant lead length. When you are dealing with as low as 3pf capacitance, too much lead length could nullify some or all of the capacitance effect. Add to that the relatively 'delicate' nature of conventional polystyrenes when exposed to too much soldering heat, and we may have a reason why your '919 seems to be unstable?

You should be able to see from the markings on the schematic which were Black Flags originally, those are the ones that need changing for radial construction capacitors. I suppose it doesn't matter if they are polystyrene, mica, or ceramic, the second two are much more commonly radial types - and a LOT more robust than the polystyrenes - food for thought.
 
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