whilst attempting to adjust the bias on this amp, my probe slipped and i shorted the Qe23 side of Re47 to the Qe19 side of Re43. this introduced an immediate hum, and amp now draws excessive current. i immediately shut down the amp. thinking that i may have stressed the smoothing caps Ce29 and Ce30, i replaced them (not cheap-GAK!), but still have the same hum and current draw.
any suggestions you guys might offer would be greatly appreciated.
best,
aidan
(red faced and embarrassed)
http://flipperpie.com/KenwoodKA-3500Service.pdf
any suggestions you guys might offer would be greatly appreciated.
best,
aidan
(red faced and embarrassed)
http://flipperpie.com/KenwoodKA-3500Service.pdf
Not the caps... you will have blown one or more output transistors and possibly the drivers as well. Check all the low value resistors around the output stage for damage too.
(Its a dead easy repair if done methodically but you will need suitable (alternate) replacements and please please please
use a bulb tester to prevent damage)
(Its a dead easy repair if done methodically but you will need suitable (alternate) replacements and please please please
use a bulb tester to prevent damage)
These will give you an idea of what is involved. All similar amps follow exactly the same faultfinding procedure.
http://www.diyaudio.com/forums/solid-state/251459-marantz-pm-66se-ki-bin-repair.html
http://www.diyaudio.com/forums/soli...500-sounds-weird-youtube-link-included-2.html
http://www.diyaudio.com/forums/solid-state/251459-marantz-pm-66se-ki-bin-repair.html
http://www.diyaudio.com/forums/soli...500-sounds-weird-youtube-link-included-2.html
Mooly, thanks very much for your quick response. i have a variac, and can run the amp at a reduced voltage. at 30V the amp hums, and draws .7 amps. before my "repair" (read: blunder), the draw was a comfortable .2 amps.
Question: would blown output or driver transistors cause such excessive current? i am off to work soon, but will check back later today for any tidbits of assistance you can offer. the resistors/transistors mentioned in my post are on the 12th page of the 13 in the pdf service manual, bottom right corner of that page in my link.
again, thanks so much to you any any others for any and all assistance offered. the amp belongs to my son, and i was hoping to bring it to him in toronto when i visit next week. i am so peeved with myself!
aidan
Question: would blown output or driver transistors cause such excessive current? i am off to work soon, but will check back later today for any tidbits of assistance you can offer. the resistors/transistors mentioned in my post are on the 12th page of the 13 in the pdf service manual, bottom right corner of that page in my link.
again, thanks so much to you any any others for any and all assistance offered. the amp belongs to my son, and i was hoping to bring it to him in toronto when i visit next week. i am so peeved with myself!
aidan
A variac is OK (in many ways ideal) but it can still deleiver lots of current even at low voltage output, something the bulb doesn't. Yes its fine... just wind it up slowly.
The symptoms are classic. Measure those transistors in circuit using a diode range on your meter and I'm sure you'll find one or more reading really low across the collector emitter junction.
The symptoms are classic. Measure those transistors in circuit using a diode range on your meter and I'm sure you'll find one or more reading really low across the collector emitter junction.
ok, will do. i'll rig up a dim bulb device tonight and report back then.
if not too much to ask, can i impose upon you to walk me through the checking of these transistors. i am of the vintage where i am very comfortable with hollow state gear, but have little-to-no experience with SS devices.
if you have a moment, could you perhaps take a quick look at the schematic and nudge me as to which components i should purchase today? due to the sentimental value of this amp, cost is of no concern (well okay, of some concern)
again,
aidan
if not too much to ask, can i impose upon you to walk me through the checking of these transistors. i am of the vintage where i am very comfortable with hollow state gear, but have little-to-no experience with SS devices.
if you have a moment, could you perhaps take a quick look at the schematic and nudge me as to which components i should purchase today? due to the sentimental value of this amp, cost is of no concern (well okay, of some concern)
again,
aidan
All these need checking. Tbh I would automatically replace the outputs and drivers (the four on the right). The other transistors may/may not have suffered but they cost buttons.
You won't get the originals, you'll have to fit alternatives. The physical style of the outputs determines what will be suitable, the others its less important.
But you really need to do some basic checks first just to confirm what has happened.
You won't get the originals, you'll have to fit alternatives. The physical style of the outputs determines what will be suitable, the others its less important.
But you really need to do some basic checks first just to confirm what has happened.
Attachments
Shorted transistors in many possible areas such is the Vbe multiplier , driver or output area is the cause of excessive current
In no case i think that an open resistor will cause excessive current draw
I am sorry to say that if you have blown the outputs you are doomed since these are totally unobtainable
In this case choosing replacements will cause mounting issues and the choice of replacements is a very tricky story since this amp is already marginal and poor choice of even good transistors will end up to catastrophic oscillation .
If outputs and drivers are to be replaced both bias scheme and compensation needs to be replaced with other values
You got your self a real trouble ...
Nobody please rushes in to suggest 1943-5200 or MJE 15030 for drivers and outputs been tested a million times and not working properly there ...
Been there a million times
Kind regards
Sakis
In no case i think that an open resistor will cause excessive current draw
I am sorry to say that if you have blown the outputs you are doomed since these are totally unobtainable
In this case choosing replacements will cause mounting issues and the choice of replacements is a very tricky story since this amp is already marginal and poor choice of even good transistors will end up to catastrophic oscillation .
If outputs and drivers are to be replaced both bias scheme and compensation needs to be replaced with other values
You got your self a real trouble ...
Nobody please rushes in to suggest 1943-5200 or MJE 15030 for drivers and outputs been tested a million times and not working properly there ...
Been there a million times
Kind regards
Sakis
I can see the output transistors being a problem because they are physically an old obsolete type looking at the picture in the manual. That means you need to be sure you can physically mount a replacement on the heatsink successfully using a single nut/bolt together with the correct insulating kit for the replacements. Its all easy stuff, just something to be aware of.
Looking at the transistors on this and I think I would go for TIP41C and TIP42C for the outputs and probably MJE340 and MJE350 for the drivers.
Look at the TIP 41C attached. You need to be able to mount that on the heatsink. It would need the appropriate insulating kit too (mica/silicon washer and bush).
Also, don't be tempted to buy transistors off ebay, too many fakes and suspect devices around.
Look at the TIP 41C attached. You need to be able to mount that on the heatsink. It would need the appropriate insulating kit too (mica/silicon washer and bush).
Also, don't be tempted to buy transistors off ebay, too many fakes and suspect devices around.
It will not work
VAs has not enough current to drive MJe 340 or even BD 139 or 140
Bias will be altered like hell and will not work unless modified
Cob is different compensation also needs to be altered
Plus that TIP series is waaaaaaaaay slower than the originals resulting a degrade in the total quality of the amplifier ...
Load resistors for both drivers and outputs will need to be replaced and the all thing will end up to a mess
I can suggest an already tested set of semis if you like that is more "drop in "
VAs has not enough current to drive MJe 340 or even BD 139 or 140
Bias will be altered like hell and will not work unless modified
Cob is different compensation also needs to be altered
Plus that TIP series is waaaaaaaaay slower than the originals resulting a degrade in the total quality of the amplifier ...
Load resistors for both drivers and outputs will need to be replaced and the all thing will end up to a mess
I can suggest an already tested set of semis if you like that is more "drop in "
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1) outputs can be C4467 A1694 one drill per transistor and one insulator and its done the physical size of the trs and the pcb pads will fit with no probs The specific transistors have been outsiders which means that are real performers from sanken and seems that these can easily be found dirt cheap and original
2) drivers should be 2Sb560 2SD438 small and very easy to drive also possible to obtain also given as replacements in some other Kenwood similar models ( by Kenwood)
3) Bias will be fine like that and if not altering Re 27 will bring bias in a reasonable adjustable level
4) in some models i have seen some oscillation with this modification and cured it increasing miller cap on the VAS Ce 11 from 22pf to 33
Been trying with Mje150xx series for outputs as these seem to be almost capable to handle the current but it was impossible to drive them the circuit goes to oscillation
Been trying to place 1943 5200 impossible to drive then went to BD 139-140 for drive then impossible to be driven from VAs + oscillation while in all cases bias went to hell
Circuit is too thin and delicate and needs transistors that are easily driven
Kind regards
Sakis
2) drivers should be 2Sb560 2SD438 small and very easy to drive also possible to obtain also given as replacements in some other Kenwood similar models ( by Kenwood)
3) Bias will be fine like that and if not altering Re 27 will bring bias in a reasonable adjustable level
4) in some models i have seen some oscillation with this modification and cured it increasing miller cap on the VAS Ce 11 from 22pf to 33
Been trying with Mje150xx series for outputs as these seem to be almost capable to handle the current but it was impossible to drive them the circuit goes to oscillation
Been trying to place 1943 5200 impossible to drive then went to BD 139-140 for drive then impossible to be driven from VAs + oscillation while in all cases bias went to hell
Circuit is too thin and delicate and needs transistors that are easily driven
Kind regards
Sakis
Hmmm... an old Towers book gives these as 325mw dissipation in free space, the Fairchild data sheet quotes 625mw. Sounds about right
Very little different to the 2SD438It is a problem sometimes knowing what to recommend for these older designs...
I'd still think modern production TIP41C/42C's for outputs would be OK for this and available anywhere. I'm looking back at its little brother in the link I posted earlier, the KA1500.
well, i spoke to my local electronics shop today. the only transistors still available are the ones for Qe9 and Qe11, being a 2SA733. i was told there are not even substitutes available for the others.
i hate to think i have to scrap the amp due to my mblooper.
any suggestions guys?
thanks so much,
aidan
i hate to think i have to scrap the amp due to my mblooper.
any suggestions guys?
thanks so much,
aidan
whoa, go slow guys!
so, originals were
Qe9 2SA733
Qe11 2SC945 (not available)
Qe13 2SC945 (not available)
Qe15 2SA733
Qe17 2SC1735 (not available)
Qe19 2SA850 (not available)
Qe21 2SD588 (not available)
Qe23 2SB618 (not available)
so, where do i spend my hard(ly) earned monies?
do i buy the available 2SA733's, and sub the others, or do i need a whole new line up?
also, should i do both channels?
you guys are great!
aidan
so, originals were
Qe9 2SA733
Qe11 2SC945 (not available)
Qe13 2SC945 (not available)
Qe15 2SA733
Qe17 2SC1735 (not available)
Qe19 2SA850 (not available)
Qe21 2SD588 (not available)
Qe23 2SB618 (not available)
so, where do i spend my hard(ly) earned monies?
do i buy the available 2SA733's, and sub the others, or do i need a whole new line up?
also, should i do both channels?
you guys are great!
aidan
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