Archive for Repairs

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. 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.

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. 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.

 

FacebookTwitterGoogle+Share

Kenwood TS-2000 delayed transmission fix

Well, I had to do something to sort this out. I can’t cope with a radio that’s not working as intended, it really

TS-2000 with the control board removed.

TS-2000 with the control board removed.

bothers me 🙁 After some digging around, I found a Yahoo-group where this case was discussed (see it here). To sum it up, it suggests that you put control and PTT over to the sub band and the radio will transmit at full power immediately. I tried this, but didn’t work at all – still the same. Seeing that my radio had a pretty old firmware (F095) I decided to try an update. The update procedure is easy enough and was done in no time. After this update, I could do the PTT/control trick – which works (wicked). This workaround creates a new problem though; the log program will save my contacts as worked on the sub receiver, which isn’t very handy. I could of course edit all those logs, but what a pain! At the very least it makes me able to do the important Winlink-tasks over Winmor.

TS-2000 R708 removal

Removed R708 from the control board.

need a proper solution to this, so had a new look at the bulletin from Kenwood. I could easily do the suggested mod, but I miss the flash-IC (IC508). Can’t exactly see why they have to change that one, unless it was for some other preventive measure.. No idea, so let’s try! Opening up the transceiver is trivial, just remember to be gentle with the flat flex cables (there’s plenty of them). I desoldered R708 and put in a 47kOhm resistor between C705 and C701 per the bulletin. I didn’t have any smaller resistor than an 80’s 1/4W device, but it fits ok. I put it all together and fired it up, anxious to see the results, but (bah!) – it’s still the same.

 

 

TS-2000 resistor mod

A new 47kOhm bodge resistor in place.

What now? Should I try to source the flash IC and change it, or would it be a waste of money? I haven’t decided yet.. There’s another “redneck” solution though, making a mike-selector box. Put in a couple of male 8-pin connectors for microphone and interface so I can switch between them. It’s possible, but I don’t like it! It would also require me to feed the interface from the ACC2 or a speaker jack, absolutely not elegant.. A third possibility would be getting a newer control board off ebay, but might have some issues pairing it with the other boards as it could contain different firmwares and of course calibration data. Will consider everything right now. Might also need a new encoder for the multi function wheel as mine is bouncing a bit.

Will come with a new update soon 🙂