Print

[WX2S]

Hi, all,

I acquired a new-to-me HF-2500 and fired it up for the first time today. The plate voltage should be 2600, per the manual, but it reads about 1850. The amp keys but produces no output for a 20w input signal.

I know the HV power supply is going to need some attention, but should I be able to get any power out at all with the lower plate voltage?

73, -Steve WX2S

[K1VSK]

Hi, all,

I acquired a new-to-me HF-2500 and fired it up for the first time today. The plate voltage should be 2600, per the manual, but it reads about 1850. The amp keys but produces no output for a 20w input signal.

I know the HV power supply is going to need some attention, but should I be able to get any power out at all with the lower plate voltage?

73, -Steve WX2S

Can we assume you have it powered by the correct line voltage for which the transformer is wired?

[WX2S]

Hi, all,

I acquired a new-to-me HF-2500 and fired it up for the first time today. The plate voltage should be 2600, per the manual, but it reads about 1850. The amp keys but produces no output for a 20w input signal.

I know the HV power supply is going to need some attention, but should I be able to get any power out at all with the lower plate voltage?

73, -Steve WX2S

Can we assume you have it powered by the correct line voltage for which the transformer is wired?

Yes, you can assume that.

73, -Steve WX2S

[W1QJ]

Those amps used old carbon comp resistors in the series metering resistors.  Over time they change value a lot and the  panel meter can’t be trusted.  The very first thing that needs doing is to verify what the actual HV really is.  An alternate means of measuring in in order.  Don’t assume anything.  If it turns out that is correct, chances are the amp needs a power supply rebuild.  Since Command Technology went out of business I’ve worked dozens of these amps.  The amp needs to be throughly checked out.

[WX2S]

There has to be something else wrong with the amp, other than the metering resistors, or it'd be producing power. I've got a HV probe on order, so I can check that when I get it.

Also worth noting is that when I key the amp, I'm getting a slight dip in plate voltage and a slight increase in plate current.

I've also got replacement caps on order for the HV supply. Another thing I want to check is the output relay. I'd thought that maybe the Molex connector from the transformer might have failed, but if it had wouldn't the plate voltage be zero?

[K6AER]

This sounds like an amplifier wired for 240 volts being run on 120 VAC.

What is the filament voltage?

When you key the amplifier does it draw some current?

Does RF go through in the bypass mode?

Do the tubes get warm?

[WX2S]

This sounds like an amplifier wired for 240 volts being run on 120 VAC.

What is the filament voltage?

When you key the amplifier does it draw some current?

Does RF go through in the bypass mode?

Do the tubes get warm?

Again: The amp is wired for 240 AC and is being run on 240 AC.

I haven't measured the filament voltage, but I'm sure it's not zero. See next.

When I key the amp with about 7-8 watts, plate voltage goes down slightly, plate current and grid current go up slightly. (The Transmit light also goes on.)

RF does go through in standby.

The tubes do get warm when the amp is run.

[N0XWR]

Just a suggestion from an experience I had with my 2500.  It required some attention and I had dropped the front panel to gain access to the board under the meters so I could put in LED’s for the meter bulbs.  After getting it back together and operating it for a period and using the band switch, I had no output. After some head scratching, I discovered the nut on the shaft to the band switch was not snug and the input wafer just behind the front panel was able to slip out of position so the contacts did not line up correctly.  The wafer and its index will slip a notch if the nut is not tight.  I would agree the low voltage is probably a metering resistor or resistors.  Mine does read 2700.  Good luck.  73  Jerry N0XWR

[W1QJ]

Make sure you know the procedure for getting to the power supply underneath.  There’s a wrong way and a right way.

[VE7RF]

Keyed, but with NO drive, does it draw normal idle current ?   If so, the small drop in B+ is normal.

I have had nothing but trbl in the past, using carbon comp resistors for meter multiplier resistors. They all drifted way up on the high side, like as much as 30%.  Which probably explains your lower than normal B+.   The 2 watt and also 3 watt MOF types are no better...since the 3 watt mof's are only rated for a max of 500 vdc....and will break down internally. 

3 kv rated resistors are available, buried in the Mouser online catalog.

The wafer misalignment was a good suggestion also.  That would kill the drive right there.

[W1QJ]

All possible problems have symptoms.  If the symptoms aren’t there you are probably barking up the wrong tree. If the input switching network is out of sync with the tank and switch you experience an infinite swr on the input as the radio sees it.  It would have to happen on all bands most likely.  Low reading B+ voltage is common due to those metering resistors.  The 3cx&00 tubes have lots of gain and the loading changes drastically with small increases in drive power so be aware to advance the load every time.  If you absolutely have to get to the power supply make sure you do it the right way. 

[N0XWR]

Lou, I would love to know the best way to get at the power supply.  I have heard you drop the front and back panels and remove the entire RF deck, center section and blower in one piece.  Is that correct?  Thanks.  Jerry N0XWR

[W1QJ]

Yes that’s the way.  Just remember how things come apart. Before you go through all the trouble, first pull down the rear panel.  CAREFULLY exam the black terminal strip for HV breakdown.  The secondary HV AC passes into the underneath area through that terminal strip.  The terminal strip is not rated for the 2000 volts connected to it.  I’ve seen one terminal strip eaten up and it caused problems with the HV.  You may only have to stop there.  Of course if it’s ok then proceed.

[VE7RF]

Take pix as you take things apart etc.

[W1QJ]

It’s too bad that a simple access panel wasn’t incorporated in the HF2500 and the others.  Would make it a snap to repair the power supply.  The Palstar rendition has the access panel so that’s a plus.  If they would have just done that it would have been a home run.  While the access panel was a major improvement, the whole control circuitry was changed and this shifted the problems to that area.  The RF deck for the most part is the same.  No changed to speak of on the RF side.  The control circuits are totally different and much more user unfriendly.  Not enough testing was put in before they were released.  Being pretty much all alone with the amps the warranty work was choking and after the first run of amps they were gone.