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 Post subject: Amp Repair
PostPosted: 14 Jan 2016, 11:38 
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Joined: 14 Jan 2016, 11:15
Posts: 7
Hi
I have an INTERM PA-1000B amplifier that is not putting out audio. When I listen very close I hear sound. I bought this from a closing out store. It appears to be brand new.I downloaded a service manual for it also but cant seem to find what could be the problem. When testing the volts on the power transisters one is a B688A and the other is C5197 the voltage on the colecter is -29.16 on the B688A and 29.04 on the C5197. Also checked the voltage from the emitter pin to the base and get 592mv. At that point shouldn't the transister be conducting? How do those transistors fail? Also there are 3 test pins what is the voltage supposed to be there? Any help would be appreciated.

Thank You, mosa


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 Post subject: Re: Amp Repair
PostPosted: 16 Jan 2016, 18:10 
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Joined: 14 Jan 2016, 11:15
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Update. After a lot of testing the amplifier works from the RCA line inputs to the speakers. So it seems there is something wrong with the mike inputs. When applying a signal to the ling inputs it works. Anybody have an idea where I should look?


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 Post subject: Re: Amp Repair
PostPosted: 16 Jan 2016, 21:05 
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Joined: 04 Jun 2008, 20:59
Posts: 3649
Location: Arizona, USA
Hi, it seems you have a schematic for it....can you scan it and post it? That would be a great help.

Good listening
Bruce

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 Post subject: Re: Amp Repair
PostPosted: 16 Jan 2016, 23:15 
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Joined: 14 Jan 2016, 11:15
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Thank you for the reply. I attached the schem for the amp. Hope it helps.


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 Post subject: Re: Amp Repair
PostPosted: 16 Jan 2016, 23:34 
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Attached is page 2 of the manual. The other page is no good. The manual is over 500kb to attach.


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 Post subject: Re: Amp Repair
PostPosted: 17 Jan 2016, 10:02 
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Location: Arizona, USA
Hi, A real multi purpose device. By any chance are you using an incompatible mic? It appears to handle magnetic ones on one input and condensor ones on another. I would test the input circuit by feeding a signal into the jacks. A bare piece of wire held in the hand ought to produce a great deal of hum if the circuit is operational. If that doesn't work then you will probably have to work back from the connection at the line inputs where the other sections feed into it stage by stage with a signal generator to find out where is doesn't go through. Looking at the control circuitry, my suspicion is that that it is not enabling the front end stuff. Otherwise something would work in that section.

EDIT....a real dumb question here....it appears that in order to use the one set of functions you will need to put a jumper between the line in and line out jacks. I have Marantz gear that is like this. The two parts can operate independently but must have the jumper to work as a single unit.

Good listening
Bruce

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 Post subject: Re: Amp Repair
PostPosted: 17 Jan 2016, 17:36 
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Joined: 14 Jan 2016, 11:15
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Hi Bruce
Thank you for the reply. When looking at the circuit isn't that what the 2 caps do c601 and c602 couple the input stage to the power stage? I agree that the problem is in the first stage. The mike is a dynamic mike it should work. The mike works but is very very light. What do you think would burn up if somebody would apply a signal to high to the mike inputs? That's what I think what happened because it looks brand new.
mosa


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 Post subject: Re: Amp Repair
PostPosted: 17 Jan 2016, 19:24 
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Joined: 06 Apr 2009, 10:08
Posts: 1421
Location: US Pacific Northwest
mosa wrote:
When looking at the circuit isn't that what the 2 caps do c601 and c602 couple the input stage to the power stage?
Actually, no. C601 and c602 are capacitors which bypass high frequency noise to ground. For the unit to work you need a bridge between the two RCA jacks at the point labeled "RCA JACK 106" on the schematic. Otherwise, nothing gets to the power amp.

Back in the 1970s this was done a lot. It was usually to allow a frequency equalizer to be inserted into the signal path. Often the bridge between the connectors was nothing more than a little piece of ≈1/8" bar stock bent into a "U" shape that inserted into the center of the two RCA jacks.

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 Post subject: Re: Amp Repair
PostPosted: 18 Jan 2016, 00:24 
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Joined: 14 Jan 2016, 11:15
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Well I'll have to check this tomorrow.
Mosa


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 Post subject: Re: Amp Repair
PostPosted: 18 Jan 2016, 10:28 
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Joined: 14 Jan 2016, 11:15
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Well I feel so silly. Yes I installed a jumper and the amp works fine. I sure do thank you all for the reply.

mosa


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