[Amps] Palstar HF2500 amp problem

[Carl]

Those amps have a history of cooking the plate blocking capacitors. They 
heat up, detune the amp which reflects back to the input since there is low 
reverse isolation in a GG amp.

Id start there before worrying about soft tubes. Those can be checked one at 
a time with 30W drive on 20M. Stuff a paper towel in the empty hole

Carl
KM1H



----- Original Message ----- 
From: "Jim Smith" <jimsmith at shaw.ca>
To: <amps at contesting.com>
Sent: Tuesday, January 15, 2013 8:10 PM
Subject: [Amps] Palstar HF2500 amp problem


> I'm trying to get a Palstar HF 2500 amp working that I bought off e-
> bay.  I need help with this.
>
>
> Problem is lack of output power.
>
> I ran some tests as follows:
> * Drove the amp with an IC 706MkIIG (My MkV is broken).
> * Measured amp input SWR with ancient Heath SWR bridge.
>     Still works, though.  Shows SWR of less than 1.1 to 1 on my dummy
>     load.
> * Measured amp Po and load SWR with new Daiwa 3 kW power meter.
> * Used my antennas as loads (Dummy load can't handle 1500W)
>     On 10-20m antenna is R5 vertical
>     On 40m antenna is a too short 40m dipole.
>     No functioning 80 or 160m antenna.
> For each band with an antenna
>   Set VFO to 1 kHz above bottom of band, tuned for max Po as follows:
>     Reduced 706 output to min
>     Went into xmit mode
>     Increased 706 output to get some output power
>     Adjusted Tune & Load for max Po
>     Increased 706 output while adjusting tuning for max Po
>       In many cases couldn't get more than 40W or so out of the 706.
>
>   Measured Ig, Ip, Pin (according to 706 meter), SWRin, Pout and
>   Preflected
>   Ditto for VFO at mid-band.
>   Ditto for VFO at 1 kHz below top of band
>
> The only frequency I got rated Po (1500W) was 7299.  SWR 1.5:1
>   Suggests tubes are OK
>
> Got 1050-1100 W at a couple of others
>
> Lots of 700-900W readings
>
> Quite a few 300-500W readings, some into 1.0:1 SWR
>
> No correlation that I can see between max Po available and load SWR.
>
> However, on every band the input SWR is about 2:1.  It was worse on
> some bands before I adjusted the input tuning on each band (mid-band -
> as much Po as I could get) for lowest SWR.
>
> Since then I put it on a friend's 1500W Termaline dummy load and drove
> it with his FT 1000D (with ant tuner).
>
> Only tried it on a couple of bands but couldn't get more than about
> 1000W out.
>   That was with a Pin of 50-60W max.
>
> Po sagged about 30% over about a 1 minute period of key down.  Input
> SWR went up and Pin went down over this same period.
>   Didn't record or remember how much.
>   Didn't retune 1000D tuner to see what happened - should have
>
> Ig was typically 30-40 mA.
>
> Factory tune chart shows:
>   Ig of 8-30 mA for, depending on band, Po of 1125 to 1950 W.
>
>   Pin of 20-60 W
>
>   SWRin ranges from 1.0 to 1.5.  Average is probably about 1.25
>
>
> Some anomalies we noticed.
>   Red Transmit LED comes on sometimes but not others and brilliance
>   varies under key-down cndx.
>     This scares me because it is driven directly by an output of the
>     PIC.  If the PIC goes, nothing else matters.
>
>   Idle Plate current
>     Manual says that it should be approx 120 mA
>     Actual wanders between 20 mA and approx 200 mA.
>       Need to recheck the 200 mA.  Definitely at least 120 mA
>       Haven't noticed any correlation with anything else.
>
>   Tune capacitor settings
>     Actuals very different from factory settings on many frequencies
>     and quite close on some others.
>     At 29.699 actual Tune setting was 0.  Factory didn't test that
>     high but they got 18 at 28.500
>
>   Load capacitor settings
>     Actuals were somewhat different from factory settings.
>     At 29.699 actual Load setting was also 0.  Factory didn't test
>     that high but they got 7 at 28.500
>
> I'm hoping that someone will be able to suggest reasons for the
> behaviour I'm seeing and/or things I might try to get this functioning
> properly.
>
> 73, Jim Smith VE7FO
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