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 Post subject: Dayton Audio PMA-250 plate amp problem
PostPosted: Jul Fri 05, 2019 9:37 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 20207
Location: Warner Robins, GA
Know it's noy vintage, but I need some help with a Dayton Audio PMA-250 full range plate amp.

It has two balanced mic inputs and one balanced line input.

I replaced three electrolytic caps in the line in section to eliminate some phase shift at lower bass frequencies and when I tried the amp I got very little out of it.

Did some testing and found either a diode, transistor or 100K resistor is shorted as they are all in parallel with each other.

The transistor number I can see.
The resistor I know its value.
The diode I have no clue about given it is a clear body and I cannot see any actual markings as to what it is. It is either a zener or another type of diode. It has just D in the board designation and I assume if it was a zener it would have ZD as I have seen that before, but not all use that designation for a zener.

Those parts are after the tone control network and are right before the output that feeds the amp.

Have no clue how those parts got damaged as I did no work near there.

Also I tried to find a schematic and cannot find one readily available online.

Looks like I'll be taking the amp to work Monday so I can remove one diode lead to test it as I don't have a proper desoldering tool at home. If that's fine then it must be the transistor, although I am kind of suspecting the diode anyways given some are a bit more fragile than transistors.


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 Post subject: Re: Dayton Audio PMA-250 plate amp problem
PostPosted: Jul Wed 10, 2019 2:17 am 
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Joined: Jun Wed 08, 2011 2:33 am
Posts: 9142
Location: Ohio 45177
I hate to ask, but have you contacted Parts Express to see if they will release the schematic to you? Well, maybe not. The guy down page asking about the SA1000 amp says they would not give him a schematic for that one.

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 Post subject: Re: Dayton Audio PMA-250 plate amp problem
PostPosted: Jul Wed 10, 2019 3:16 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 20207
Location: Warner Robins, GA
I did post a question asking about the schematic, but no response yet.

I did find out it was a general purpose PNP transistor that went bad. A 2N2907 got it going again.

Replacing all 15 4.7uF 50V caps with 22uF 50V caps took care of the phase shift and the amp actually does sound better in the bass. I just did them all as I had no schematic to know which caps were in the two mic input preamps. So in a way it validates me making a big deal about ensuring amplifiers have little to no phase shift over the bandwidth the amp is required to reproduce.

That said when used in for instance a PA speaker cabinet, the speakers rarely are flat to 40Hz much less 20Hz so the phase shift would not be noticeable and not being completely flat down to 20Hz would actually be desirable as there's no point in having an amp amplify frequencies the speaker cannot reproduce.

I also need to find out what the controls are so I can replace two where the mounting collar was damaged by taking the nuts on and off too many times then tightening them down too hard on top of it all.

I'd just switch to regular panel mount pots, but the preamp board the pots are on is held in place by the pots mounted to the panel. Plus I'd need to know what the pots are anyways to get proper panel mount replacements and then add shielded wire to the pots when the circuit was not designed for the pots to be mounted off the board which could introduce problems.

Now I could disconnect the pots, cut the leads off then reinstall them just to hold the board in place and mount panel mount pots on a metal panel then cut out a space in the wood cabinet for the metal plate. I could then use larger knobs and quality pots.


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