Hi Chris,
Well I've replaced the A498s on the power amp board. I checed their hFEs; one pair 156, 143 and the other 170 164.
I haven't turned the amp on, because I was wondering whether adjustments like DC offset and quiescent current need to be checked first, given the much higher gain of the A970s.
Happy New Year
Roy
Well I've replaced the A498s on the power amp board. I checed their hFEs; one pair 156, 143 and the other 170 164.
I haven't turned the amp on, because I was wondering whether adjustments like DC offset and quiescent current need to be checked first, given the much higher gain of the A970s.
Happy New Year
Roy
you could use a pair of NTE290a transistors, or back-reference the NTE290a transistors to another 2SA part number.
one thing you might find useful is the following
2SA pnp high freq (ft generally above 5Mhz)
2SB pnp low freq
2SC npn high freq
2SD npn low freq
2SJ pch fet
2SK nch fet
generally the 2S is assumed on parts, so you will see the markings usually as "C1318", "A970", "K1592", etc....
one thing you might find useful is the following
2SA pnp high freq (ft generally above 5Mhz)
2SB pnp low freq
2SC npn high freq
2SD npn low freq
2SJ pch fet
2SK nch fet
generally the 2S is assumed on parts, so you will see the markings usually as "C1318", "A970", "K1592", etc....
Hi Roy,
I normally use a variac to power up these things and watch the bias current as I bring it up. If the bias starts going high I adjust it down, if it won't go down then there is something wrong (but no smoke yet). So the power is reduced a bit and I can troubleshoot at that point in time.
-Chris
I normally use a variac to power up these things and watch the bias current as I bring it up. If the bias starts going high I adjust it down, if it won't go down then there is something wrong (but no smoke yet). So the power is reduced a bit and I can troubleshoot at that point in time.
-Chris
Hi unclejed613,
NTE (or ECG) stuff is a joke, an unknown. Do not use that stuff. If you don't know where to get original parts, you should not touch a soldering iron!!!!!! They are the worst parts I have ever seen. If found in a repair, they are on the estimate for replacement with a real part.
-Chris
Anyone who knows me understands what I am about to say next. Don't take it personally.
NTE (or ECG) stuff is a joke, an unknown. Do not use that stuff. If you don't know where to get original parts, you should not touch a soldering iron!!!!!! They are the worst parts I have ever seen. If found in a repair, they are on the estimate for replacement with a real part.
Now here is where the trouble really starts. Do not ever cross in and out of these "replacement manuals" That is worse than stupid. If you can't find real specs to use, turn the job over to someone who knows what they are doing. I really mean what I say here.
Yes.
and yes again.
-Chris
Hi Chris,
I don't have a Variac, is there an alternative to its use? (I've read that light bulbs can be used to limit current but are not ideal). Otherwise, I'll check out the Variacs locally. We are on 240VAC here. If possible could you please give me some specs. I would imagine that I don't need a high current model.
Also the power amp board has 2 trim pots in each channel. In the schematic they are R334 and R312. I'm guessing but does R334 adjust the bias current? And R312 ?
Roy
I don't have a Variac, is there an alternative to its use? (I've read that light bulbs can be used to limit current but are not ideal). Otherwise, I'll check out the Variacs locally. We are on 240VAC here. If possible could you please give me some specs. I would imagine that I don't need a high current model.
Also the power amp board has 2 trim pots in each channel. In the schematic they are R334 and R312. I'm guessing but does R334 adjust the bias current? And R312 ?
Roy
Hi Roy,
R312 will adjust the DC offset and R334 will adjust your bias current. Normally you could power up if there was nothing wrong with the controls set to their old positions. The current may be high, but not destructively so.
Is there a variable bipolar DC power supply you can use? Even + and - 20 V will get you out of trouble here.
One note though. Those bias controls are arranged poorly. If the wiper goes open, you will lose bias control and end up with a blown amp. It's better to set the adjustable resistor in series with the resistor already there in the base-emitter leg. A fixed resistor would go from base to collector. This way, if the wiper were to go open, your bias transistor woulod turn full on and bias current would drop to zero. No damage, just bad sound.
-Chris
R312 will adjust the DC offset and R334 will adjust your bias current. Normally you could power up if there was nothing wrong with the controls set to their old positions. The current may be high, but not destructively so.
Is there a variable bipolar DC power supply you can use? Even + and - 20 V will get you out of trouble here.
One note though. Those bias controls are arranged poorly. If the wiper goes open, you will lose bias control and end up with a blown amp. It's better to set the adjustable resistor in series with the resistor already there in the base-emitter leg. A fixed resistor would go from base to collector. This way, if the wiper were to go open, your bias transistor woulod turn full on and bias current would drop to zero. No damage, just bad sound.
-Chris
Hi Chris,
As always I have a couple of questions!!
I know what you are talking about in general, but not in detail (if that makes any sense!!).
So here goes, R360 is connected directly to the base of Q310; the wiper of R334 is connected to the base of Q310 also with one other leg connected to R332 and the other not connected at all.
Would the values of any of these need to be varied?
Sorry for all these naive questions, but I'm not sure what is referred to here. I can buy quite a large range of electronic parts locally. What would be needed to get a variable DC power supply?
I really don't mind buying extra parts and equipment as needed for my limited aims i.e, fixing up this amp and later upgrading of electrolytics and other minor work on this and the other Monarch. I also have a Rotel RA-970BX which I will look at later.
Roy
As always I have a couple of questions!!
I know what you are talking about in general, but not in detail (if that makes any sense!!).
So here goes, R360 is connected directly to the base of Q310; the wiper of R334 is connected to the base of Q310 also with one other leg connected to R332 and the other not connected at all.
Would the values of any of these need to be varied?
Sorry for all these naive questions, but I'm not sure what is referred to here. I can buy quite a large range of electronic parts locally. What would be needed to get a variable DC power supply?
I really don't mind buying extra parts and equipment as needed for my limited aims i.e, fixing up this amp and later upgrading of electrolytics and other minor work on this and the other Monarch. I also have a Rotel RA-970BX which I will look at later.
Roy
Hi Roy,
R360 connects to R334 as it does now. The wiper of R334 connects to the other end of R334, this connection also connects to the base of Q310 and R332.
-Chris
No.
R360 connects to R334 as it does now. The wiper of R334 connects to the other end of R334, this connection also connects to the base of Q310 and R332.
There will be a small difference, but you should still be able to adjust the bias to the correct value. If you have problems with the range, just post.
Sometimes it's better to buy test equipment new. You will be sure it works then. An old HP power supply like a 6236A would be very useful to have for all you projects (as an example).
-Chris
Hi Chris,
I've just moved house so I've only just been able to get back the amp and the replacement of the transistors. Well, sad to relate, I've encountered a problem here.
After replacing the transistors and checking that the leads were inserted correctly ( I took pics and drew diagrams of the layout), I connected the amp up to some old speakers I had around the place (nondescript with no brand name) and using a CD player as the source, and turned it on. I got no sound in the R channel and some faint music in the L channel. I then turned up the amp to about 9.
Big reaction!!!! the tweeter in the R speaker gave a very loud squawk and died as did the woofer. I'm not worried about the speakers, but I at a loss to know what to do next.
I really do want to get this amp going again, so I want to go very cautiously (obviously a good idea. given my state of electronic ignorance).
Any help etc. Thanks
Roy
I've just moved house so I've only just been able to get back the amp and the replacement of the transistors. Well, sad to relate, I've encountered a problem here.
After replacing the transistors and checking that the leads were inserted correctly ( I took pics and drew diagrams of the layout), I connected the amp up to some old speakers I had around the place (nondescript with no brand name) and using a CD player as the source, and turned it on. I got no sound in the R channel and some faint music in the L channel. I then turned up the amp to about 9.
Big reaction!!!! the tweeter in the R speaker gave a very loud squawk and died as did the woofer. I'm not worried about the speakers, but I at a loss to know what to do next.
I really do want to get this amp going again, so I want to go very cautiously (obviously a good idea. given my state of electronic ignorance).
Any help etc. Thanks
Roy
Hi Roy,
Well, call that a lesson you won't forget.
Starting up your amp is done with no load connected. I use a 'scope and DVM to get an idea of what's coming out. Also the bias is checked as I slowly increase the supply voltages.
It sounds like your amp broke into HF oscillation. If the DC offset went nuts, it may be harder to find.
Slow and steady. One thing at a time, one channel at a time.
-Chris
Well, call that a lesson you won't forget.
Starting up your amp is done with no load connected. I use a 'scope and DVM to get an idea of what's coming out. Also the bias is checked as I slowly increase the supply voltages.
It sounds like your amp broke into HF oscillation. If the DC offset went nuts, it may be harder to find.
Slow and steady. One thing at a time, one channel at a time.
-Chris
Hi Chris,
I'm sure about that.
So, to really proceed I need some equipment to supplement my meagre collection which comprises a Mastech MAS830L DVM, a third hand holding tool with magnifier and a soldering iron!
First off, the variable bipolar DC power supply. I've looked around on the internet but haven't seen anything that fits, except:
http://www.electro-tech-online.com/electronic-projects/24291-variable-bipolar-power-supply.html
But I'd like to either buy one or build a kit.
Secondly, an oscilloscope - I'm reluctant to try my luck on ebay, with a 2nd hand one - maybe DOA apart from the hassle (and cost of shipping). So maybe a PC based, USB model might do the trick, like:
http://cgi.ebay.com/60M-2CH-PCBased...yZ104247QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
Would the DC volts range be a limitation?
While all this is being sorted, I'll pull the power board from the amp and carefully check out all the new work.
And is there any thing else I can check, like the DC available to the power amp board?
regards
Roy
I'm sure about that.
So, to really proceed I need some equipment to supplement my meagre collection which comprises a Mastech MAS830L DVM, a third hand holding tool with magnifier and a soldering iron!
First off, the variable bipolar DC power supply. I've looked around on the internet but haven't seen anything that fits, except:
http://www.electro-tech-online.com/electronic-projects/24291-variable-bipolar-power-supply.html
But I'd like to either buy one or build a kit.
Secondly, an oscilloscope - I'm reluctant to try my luck on ebay, with a 2nd hand one - maybe DOA apart from the hassle (and cost of shipping). So maybe a PC based, USB model might do the trick, like:
http://cgi.ebay.com/60M-2CH-PCBased...yZ104247QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
Would the DC volts range be a limitation?
While all this is being sorted, I'll pull the power board from the amp and carefully check out all the new work.
And is there any thing else I can check, like the DC available to the power amp board?
regards
Roy
Hi Chris,
I've been checking out 20 mHz oscilloscopes on the net and locally.
Would you have any suggestions as to make, model and functionality? I haven't been able to locate any online reviews to guide me here, if they exist.
I would prefer to buy good second hand but that may be a little difficult given my location.
I'm a bit concerned about damage during shipping. Some ebay sellers have pretty rudimentary ideas about what is good protective packing. So I'll keep trying locally as well.
Thanks for all your help
Roy
I've been checking out 20 mHz oscilloscopes on the net and locally.
Would you have any suggestions as to make, model and functionality? I haven't been able to locate any online reviews to guide me here, if they exist.
I would prefer to buy good second hand but that may be a little difficult given my location.
I'm a bit concerned about damage during shipping. Some ebay sellers have pretty rudimentary ideas about what is good protective packing. So I'll keep trying locally as well.
Thanks for all your help
Roy
Hi Chris,
I got a quote from a local supplier (in Bangkok) for what seems to be a high end item - Instek GOS-620 (20MHz Analog Oscilloscope) - $414.00
This is too pricey for me. I would like to locate a cheaper new model or a good quality 2nd hand one.
I've been looking on ebay, here's some of the 2nd hand offerings -
BK precision 20MHz oscilloscope model 2120 $42.50 ships only to US
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&rd=1&item=270085119247&ssPageName=STRK:MEWA:IT&ih=017
NEW 20MHZ ANALOG OSCILLOSCOPE, MADELL 5020 $195 no bid ships to US only
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&rd=1&item=280074927595&ssPageName=STRK:MEWA:IT&ih=018
B&K PRECISION 2120 20MHZ OSCILLOSCOPE VGC $9.99 no bids ships to US, Canada only
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&rd=1&item=110084616508&ssPageName=STRK:MEWA:IT&ih=001
Oscilloscope...Goldstar OS-7020A 20Mhz w/Warranty sold $117.00 worldwide shipping
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&rd=1&item=320077709298&ssPageName=STRK:MEWA:IT&ih=011
The problem for me for most of these is the seller will only ship to the US.
Also, shipping by air will be quite expensive.
I'll keep looking around locally and maybe I might be able to get a good deal on ebay.
Just BTW tho', any advice on ones I've listed (or others) would be welcome.
I'm using the other Monarch amp I own at the moment and I am really pleased with the sound. So I would like to go over the one I attempted!! to fix and do a more competent job this time.
Roy
I got a quote from a local supplier (in Bangkok) for what seems to be a high end item - Instek GOS-620 (20MHz Analog Oscilloscope) - $414.00
This is too pricey for me. I would like to locate a cheaper new model or a good quality 2nd hand one.
I've been looking on ebay, here's some of the 2nd hand offerings -
BK precision 20MHz oscilloscope model 2120 $42.50 ships only to US
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&rd=1&item=270085119247&ssPageName=STRK:MEWA:IT&ih=017
NEW 20MHZ ANALOG OSCILLOSCOPE, MADELL 5020 $195 no bid ships to US only
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&rd=1&item=280074927595&ssPageName=STRK:MEWA:IT&ih=018
B&K PRECISION 2120 20MHZ OSCILLOSCOPE VGC $9.99 no bids ships to US, Canada only
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&rd=1&item=110084616508&ssPageName=STRK:MEWA:IT&ih=001
Oscilloscope...Goldstar OS-7020A 20Mhz w/Warranty sold $117.00 worldwide shipping
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&rd=1&item=320077709298&ssPageName=STRK:MEWA:IT&ih=011
The problem for me for most of these is the seller will only ship to the US.
Also, shipping by air will be quite expensive.
I'll keep looking around locally and maybe I might be able to get a good deal on ebay.
Just BTW tho', any advice on ones I've listed (or others) would be welcome.
I'm using the other Monarch amp I own at the moment and I am really pleased with the sound. So I would like to go over the one I attempted!! to fix and do a more competent job this time.
Roy
Hi Roy,
Any of those should be fine. There are a couple reasons I suggested you to buy a new scope. 1.) the shipping issues and 2.) you will own this thing for over 20 years. This my seem like a long time to you, but it goes by very quickly. Trust me on that.
If you are looking used, avoid Philips Combi - scopes. They run their parts far too hot. Same for a PM3070. I have one and the deflection amp PCB keeps dying from excessive temperatures. Tektronix 'scopes like the 22xx series are good, but may need work to be useful.
'Scopes from schools will be in tough condition, you may want to look at units from repair shops (maybe) or industry. I personally think a new oscilloscope is the safest and most useful way for you to go.
So, keep your eyes open and good luck.
-Chris
Any of those should be fine. There are a couple reasons I suggested you to buy a new scope. 1.) the shipping issues and 2.) you will own this thing for over 20 years. This my seem like a long time to you, but it goes by very quickly. Trust me on that.
If you are looking used, avoid Philips Combi - scopes. They run their parts far too hot. Same for a PM3070. I have one and the deflection amp PCB keeps dying from excessive temperatures. Tektronix 'scopes like the 22xx series are good, but may need work to be useful.
'Scopes from schools will be in tough condition, you may want to look at units from repair shops (maybe) or industry. I personally think a new oscilloscope is the safest and most useful way for you to go.
So, keep your eyes open and good luck.
-Chris
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