Well, a classic sign of HF oscillation is:-
- if you touch any of the transistor cases (even the TO-92 ones) , you see DC shifts on the output (as measured with a DVM)
- the amp runs HOT
- If you probe around and measure the DC voltages, they look 'kind of right'
Have you measured the quiescent current BTW?
Have you lost a ground connection somewhere?
- if you touch any of the transistor cases (even the TO-92 ones) , you see DC shifts on the output (as measured with a DVM)
- the amp runs HOT
- If you probe around and measure the DC voltages, they look 'kind of right'
Have you measured the quiescent current BTW?
Have you lost a ground connection somewhere?
...thanks Bonsai i actually stuff like that is what i check before anything else ...but double checking will not harm anyone ...will do that later on
hei bonsai ...some time ago i sent you a PM for some reason either you missed it or didnt answer .... would you please e mail me there is something i would like to talk in private with you
thanks
hei bonsai ...some time ago i sent you a PM for some reason either you missed it or didnt answer .... would you please e mail me there is something i would like to talk in private with you
thanks
Hi sakis,
I imagine you're a busy guy in the shop. I'm going to bet you have put this aside for the time just before you posted and are trying to get back into it with all these suggestions in mind. It worked that way for me at any rate.
One question for you. Aren't the emitter resistors 150 ohm right now? If they are, you would use one pair with a 75 ohm emitter resistor. You are passing double the current in the one and must half the emitter resistor to have the same voltage drop. If they were 100 ohm, your new value should be 50 ohm.
I'm wishing you success here. No one likes to have a problem repair, but you will learn something from it.
-Chris
I imagine you're a busy guy in the shop. I'm going to bet you have put this aside for the time just before you posted and are trying to get back into it with all these suggestions in mind. It worked that way for me at any rate.
One question for you. Aren't the emitter resistors 150 ohm right now? If they are, you would use one pair with a 75 ohm emitter resistor. You are passing double the current in the one and must half the emitter resistor to have the same voltage drop. If they were 100 ohm, your new value should be 50 ohm.
I'm wishing you success here. No one likes to have a problem repair, but you will learn something from it.
-Chris
that was correct all the way through Chris..... hardman is aside as we speak ....it will be done in a quiet night with good music ,no telephones ringing , a few smokes and some cofeee
your remark was also correct arround the resistors .... my post was done late at midnight yesterday so dont take everything as a fact from a guy that repaired another 4 amplifiers the same day ....ha ha ha
( by the way check your pm list ...there is a payment waiting for you... for your sevices !!! )
your remark was also correct arround the resistors .... my post was done late at midnight yesterday so dont take everything as a fact from a guy that repaired another 4 amplifiers the same day ....ha ha ha
( by the way check your pm list ...there is a payment waiting for you... for your sevices !!! )
I would personally be very tempted to replace Q421, 423, 425, 427 with a single pair of TO-126 type devices, and fit small heatsinks to them. 2SA1381/2SC3503, or 2SA1209/2SC2911 for example. The speed is only a little slower. These are also easier to source I guess - I've never heard of Isahaya!
Hi jaycee,
It's scary when many people start thinking alike! It appears we are in good company then. 🙂
I don't think the speed will really cost us here, and circuit stability should go up as well. The 160 V ratings also address the other problem of marginal voltage ratings on the original parts. Those 300V 2SA1381/2SC3503 types have a 100 mA collector rating against a 140 mA rating for the 2SA1209/2SC2911 couples. I would lean more towards the 2SA1209/2SC2911 in that case.
I almost recommended 2SA1360/2SC3423, but they are only good for 50 mA, the related pair (2SA1358/2SC3431), are only good for 120 V and 140 mA. I like yours better (much!).
-Chris
It's scary when many people start thinking alike! It appears we are in good company then. 🙂
I don't think the speed will really cost us here, and circuit stability should go up as well. The 160 V ratings also address the other problem of marginal voltage ratings on the original parts. Those 300V 2SA1381/2SC3503 types have a 100 mA collector rating against a 140 mA rating for the 2SA1209/2SC2911 couples. I would lean more towards the 2SA1209/2SC2911 in that case.
I almost recommended 2SA1360/2SC3423, but they are only good for 50 mA, the related pair (2SA1358/2SC3431), are only good for 120 V and 140 mA. I like yours better (much!).
-Chris
Main reason I'd lean towards the 1381/3503 is that Fairchild do a good second source of them that is easy to get... but the newer sanyo pair is good too. I belive the 1209/2911 is specifically made with predriver use in mind.
I've got two pair of the 1360/3423 lifted from an old Teac amp.. going to put those to use in a 50W amp as the VAS parts.
I've got two pair of the 1360/3423 lifted from an old Teac amp.. going to put those to use in a 50W amp as the VAS parts.
Hi jaycee,
You should find those to be pretty good in that application. They are good quality parts, which is why I bought some for stock. The fact you can easily mount a heat sink on these is bonus, or you can screw them down on the main heat sink.
-Chris
You should find those to be pretty good in that application. They are good quality parts, which is why I bought some for stock. The fact you can easily mount a heat sink on these is bonus, or you can screw them down on the main heat sink.
-Chris
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