QRO you say? Dentron MLA-2500 repair




MLA-2500 front
Front face of the Dentron MLA-2500

We just had our yearly dose of hamfest and swapmeet and I couldn’t leave empty-handed. Well, I got my arms full when I had to carry this vintage Dentron MLA-2500 amplifier back to the car. Heavy means good, right? Let’s hope so. I traded it in for a mobile radio, so no expenses so far, but the seller said it was working, but defective.. Uh, what? Well, it can’t be that bad.

 

MLA-2500 original capacitors
MLA-2500 original capacitors

First things first; I didn’t attempt to power it up before properly inspecting it, just in case I made things worse. Smart decision! The old electrolytic capacitors in the B+ line was way, way out. One of them had vented and one was shorted. I immediately started tearing it apart and ordered new caps and bleeder-resistors from eBay

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. Thanks to VE3PVS, Peter, I got a great deal and excellent shipping time for six new Nichicon 150uF/500V caps including six new bleeder resistors

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MLA-2500 HV electrolytics
MLA-2500 HV electrolytics

The new capacitors were of course quite a bit smaller than the original ones, but that shouldn’t matter too much. I just have to figure out a proper way of mounting them. It wouldn’t hurt if it looked original either. I decided to keep the paper insulators that covered the old caps, just had to be liberal with the glue 🙂

 

 

 

 

 

I just tacked the paper forms to the chassis with hot melt glue before I inserted the new caps and glued all around, both at top and bottom. See pictures below.

MLA-2500 paper insulators
MLA-2500 paper insulators
MLA-2500 new caps installed
MLA-2500 new caps installed

 

 

Next up was wiring them in and confirming everything was correct according to the manual.

MLA-2500 new capacitors wired in
MLA-2500 new capacitors wired in
MLA-2500 band selector flashover
MLA-2500 band selector flashover

After connecting everything, I had to check the rest of the amplifier

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. I saw that there had been some flashovers in the band selector switch, especially in the 80m and 40m positions. I had to clean that up a bit, but saw no permanent damage.

MLA-2500 band selector flashover
MLA-2500 band selector flashover

 

 

 

 

 

Nast nasty, that must have made a nice sound 🙂

 

 

MLA-2500 in operation
MLA-2500 in operation

After some gentle cleaning and vacuuming, it was time to power it up. Ideally, I would like to use a variac to slowly increase the voltage, but I don’t have any. Tried to look for a decent 60 or 100W lamp to connect in series, but quickly tossed that idea away (getting eager to try it). I just plugged it in and poked it with a stick. Guess what? No bangs, no circuit breaker blowing! Just a fan starting up and plate voltage meter showing a healthy 2250V+ 🙂

So, will it produce any output? Oh yeah, plenty of it too! I have no problems reaching our legal limit of 1kW on all bands without even being in the danger-zone of the grid current on the expensive 8875’s which lives in this amplifier. Sweet 🙂

Just to be sure my setup was ready for handling the QRO situation, I had to change some coax and secure my windom antenna a bit more. Ok, this is absolutely overkill, but I now use Ecoflex 15 for my HF antenna. This was primarily selected to be my 70cm coax, but as I didn’t want to rip down the mast for that antenna and change out the Ecoflex 10, I keep it this way for now.