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Project Dentron MLA-2500

Discussion in '"Boat Anchor" & Classic Equipment' started by N2RDQ, May 10, 2018.

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  1. N2RDQ

    N2RDQ Ham Member QRZ Page

    project dentron 2500 - i am just getitng myself familair with this amp, and reading a lot of writeups on it. I'm really looking for input from those with experience and knowledge about the amp. I'm calling it a project, not becasue i am set on making changes, but becasue the amp its new to me, and like any bone stock vintage eqipment, it should be gone though no matter what, plus, I need it to be usable with my gear.

    little background: I'm getting back into amateur radio, and have an IC-7300 which id love to use on AM but it is only 25W on AM. i'm working on a better antenna setup but also i have been looking for a good deal on an amp, and found a deal i couldn't pass up (some might say I should have passed up lol) ... a dentron mla-2500 (not the b model) Aug, 77 date code on the fan, 5th week, 78 date code on the tubes. so 40 years old.

    Yes i am aware the 8875's are no longer in production. From what I have checked so far, the amp seems to be bone stock, but does have a CM-2 with it. It was claimed to be in very good working condition, no one could speak for it first hand on it when i got it, so its a bit of an unknown.

    All that being said. I want to start going though the amp. First makign sure it functions, then doing any nessisary changes to insure its longjevity and its compatability with my ic-7300.

    I am looking for input on all this this.

    I've yet to put power to it. it is wired for 240, and i will run a 240 circuit for it eventually, but will temporarily rewire it for 120.

    i have an inital plan of attack to verify the tubes are ok.
    1. open it, clean it out (minor dirt accumulation, looks like from the dirt roads at the hamfest), and check for any mods, or obvious broken/cracked or burnt components.
    2. try reforming the caps buy rewiring it for 120, pulling the tubes, then using a variac to ramp up the voltage.
    2a. if the caps fail - replace them.
    3. assuming all goes well, and the voltage comes up, the next check would be to put the tubes back in fire it up into a dummy load driving it with the ic-7300 and slowly ramping up the power to see if its behaving normally.

    This is strictly to see if the tubes are ok or not. (little concerend that my dummy load is a heathkit 1kw cantenna. can the HN-31 cantenna can be pushed to higher wattages in short bursts perhaps? i know tis clones are rated at 1.5kw)

    assuming the tubes are ok. then the question is what to do to it for use with an ic-7300 and to help insure tis longevity and protect the tubes.

    if the tubes are bad then that is a whole other can of worms.. which gets into likely using alternate tubes and if its cost justified continuing....

    there are a few things i am already considering solely based on some reading i've done.

    1. adding softstart - seems like a smart thing to do - 30 bucks likely use the harbach or a diy solution
    2. replacing the 40 year old PS filter caps - just seems prudent even if there good.
    i see 3 possible options
    repalce with screw termanal caps (kind of expensive) ~ 120 bucks+
    repalce with caps using adapter boards off ebay (pretty ecnomical) ~75 bucks
    repalce the ps board and delay board with the harbach PM-2500 kit ~ 100 bucks - seems like a good bang for the buck as it also replaces the delay timer board.
    3. soft key - or maybe jsut a simple relay isolation, i am unsure what the current draw is on the key circuit, but between the 1 relay in the cm-2 an the 2 in the amp, i am a bit concerend it might be a bit much, plus there is no flyback protection on any of the relays... though I can add that.. i'll know more when i can measure it (maybe somone knows off the top of ther head what the current is) ... id want to be sure its not more then the ic-7300 can handle safely.
    4. maybe add changes made on the 2500b - most interesting of which is the grid metering. i also heard there was a grid metring retrofit kit offered for the 2500, id love to know what that was, i could find no documentation on it.
    5. grid current limiting? seen some writeups on this to prevent accedental over current on the tubes
    6. alc circuit change, - ok well first, i have no idea what the alc output voltage range is on the mla-2500, but i do know the icom-7300 wants to see -4v max so id like to protect it from voltages that might damage the ic-7300.

    soming i've read that combines 4,5,6 i found interesting...

    anyway, please fire away with coments, ideas and suggestions, recommendation's on ways to not destroy anything :) or links to other peoples mla-2500 projects that seem relevant.

  2. KA9JLM

    KA9JLM Ham Member QRZ Page

    Sounds like you are on the right track.

    ALC should not be necessary if you watch/control how you drive it.

    25 watts on AM should be plenty.

    Have Fun.
    VU2YK likes this.
  3. N2RDQ

    N2RDQ Ham Member QRZ Page

    HORRORS ... Well, I didn't expect everything to be perfect in Mudville, but I was hopeful.

    Somone has been in the box and did repairs. probably not a shock considering the age. looks like there was a hv ps issue, 1/2 the caps (3) were replaced with 125uf/500v the other half are the original sprague 150uf/450v ... i'm assuming the person who did this did not take into account Kirchoff's voltage law :rolleyes:. If my calculations are correct, those original spagues would be at or over to their working voltage, depending on the exact voltage the transformer is putting out.

    I'll see what happens when I go to reform the caps, but, I have my doubts that I can test without fixing that.

    Also looks like the 12v DC section also was fried at one point with evidence of a new diode and cap, and scorched pcb. they wired the cm-2 into the 12v inside and the wire seems to have shorted to ground where it leaves the chassis, they did tape up the wire where it shorted at least... o_O I hope the 12v winding is good.

    Going through the wiring, a little, it seems to have the grid metering like the 2500b, so i am assuming this late run has component updates seen in the 2500B. All the HV rectifier diodes are different than listed on the BOM for the 2500, Higher current, 3A at 600 PIV.

    If anyone has the component list for the 2500B please post. - I've yet to locate a 2500B manual as all the ones online that say 2500b online are actually 2500 models when you get to the schematic and component lists. I've found the 2500b schematic though, just no associated parts list.

    well, I have it rewired for 120v, reforming tomorrow....Smoke test #1 coming up....:D
  4. N2RDQ

    N2RDQ Ham Member QRZ Page

    survived the reform. 12v is ok too.
    tube power test next.
  5. W2VW

    W2VW Ham Member QRZ Page

    I had a pretty much all original MLA2500 and brought it up very slowly using a variac. Kept checking the filter capacitors for signs of heat. No heat.

    One tube was kaput so I replaced it with one that looked completely different. Worked great.

    I used it for a season on 10 meter AM at 175 watts carrier out sometimes as much as 8-900 watts P.E.P. using a low level AM exciter.

    Kept it away from the microphone and had an auxiliary fan on top of the cabinet.

    Sold it to a new ham. It worked for another 6 months and one of the filter capacitors failed. Started blowing fuses.

    Replaced the capacitors.

    Moral of the story is capacitors in a voltage doubler circuit especially are best replaced with age. 40 years is a long time.

    On/off switch is a known failure part.

    Thermistor in series with mains can limit ON current and help extend life.
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    It's funny the 8875 was a total dud for Eimac. As far as I can remember, nobody used them in anything except Dentron.

    However, the 8874 lives on, and if you modify the cooling system they can be used. 3CX800A7s use the same socket and I don't know if they'd fit physically, but maybe. 3CPX800A7s are often found on the surplus market pretty cheaply.

    Denny had a lot of weird ideas. But...he's still on the air, and I work him on 40CW. He's a good CW op.
    VU2YK and NJ9P like this.
  7. N2RDQ

    N2RDQ Ham Member QRZ Page

    @W2VW: Thank you for giving me your experience with it. I did trip a 15A breaker a few times just powering it up wired for 120, so I can see where the power switch would have issues with that initial current. Some kind of soft-start is needed just a pracitcal matter at 120v. Thermister inline is an interesting idea, I'll look into that. And..I'm with you, the caps have to be replaced.

    Tube Test Results:
    Tube test ends up at ~ 1100W deadkey with about 60W in after that any additional power in doesnt net any additional output.
    This sounds right though. I know the transformer is rated at like 900ma at 800vac, so back of the matchbook math makes 450ma @ 2400VDC or about 1080 watts. this also matches what i've seen some others test at so, seems in line. if anyone can comment on their experiences that be helpful to compare.

    so all looks good so far with the tubes.. that was my biggest fear!

    Next Steps:
    I am defiantly going for replacing the PS caps. I've decided on the Harback PM-2400 replacement power supply module.
    considering the issues this had in its history, it seems like a good way to go. repalces the recifiers for the hv and 12v plus gets rid of the dealy tube along with new HV caps .. that will give me some peace of mind.

    The grid shunt resistor does need to be changed to a .462 ohm. I did read where others had commented on this and done this.. and now having run it, i can see the grid current reading is useless with the .0462 resistor it comes with. changing the resister will make full-scale 100ma and not 1A so you can actually measure the grid current.

    I did trip the 15A breaker a few times powering it up, the inrush is high, so that's an additional reason for a soft start along with the longevity aspects. I am defiantly adding some kind of soft-start. but undecided on how to implement it. I'm open to ideas. Id like it to not need any changes of someone changes from 120 to 240.

    The thermistor idea series seems like it might be viable and could be implemented inside the voltage jumper box if i wanted to. Also, the Hardback module for Softstart seems ok, but the resistors need to change if I change the supply voltage. for 30 bucks, and like 2 bucks for the extra resistors for the alternate voltage might be at fast a way to add it.

    Power output reading on the panel was way-way off, I'll look into how to correct that. I think it might be as simple as a pot adjustment.

    now the wait for parts :) ...

    comments or recommendations anyone?
  8. N2RDQ

    N2RDQ Ham Member QRZ Page

    oh wow! that's pretty cool that he is still on the air.

    I tell ya, I know some people complain about dentrons, but, I like how this amp is put together. very compact.
    could they have done additional things or different things, sure, but you can probably say that about anything I suppose. .. hey this one lasted 4o years.. and will likely last at least another 40.

    I've seen write-ups where people have used 8874/3cx400a7/3cpx400a7's and 3cx800a7/3cpx800a7's and they all will fit. ... If the tubes were bad, i was going to save my hobby pennies and eventually get some 3C(p)X800A7 medical system pulls.

    I figured they would last forever, and probably not a bad change anyway in the long run.
    the real funny thing is the 2 good 8875's would sell for more than what a new amp would cost me to buy.. go figure. it is a tempting thought to sell the 8875's and buy 2 3CPX800A7's :p

    if dentron had continued, and the 8875 was discontinued, it would not have been a major rework for them to switch to 8874's tubes.
  9. KM1H

    KM1H Ham Member QRZ Page

    Actually Denny badgered Eimac to make them and eventually didnt finish paying.......:eek: They tried to shop them elsewhere but no go. It was a good idea IMO. The horizontally cooled 8121 tetrode was used in several TV transmitters
    There are two 8875 versions, one with a standard type of anode cooler and the other made from a heavy aluminum casting.

    I have tw0 2500's one a mint one owner and the other a survivor of several years of Pacific island DXpeditions. I rebuilt the latter and often use as an AM linear. The other sits next to the second station Alpha 76PA as a backup that hasnt been used yet except for occasional use to keep the new filter caps happy. It loafs at 1000W and I do have NOS and excellent used spares.

    KD2ACO likes this.
  10. KA9JLM

    KA9JLM Ham Member QRZ Page

    Why was a grid current reading not included on Dentron amps ?

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