[Amps] DEAD Alpha 91B
thomascarl at gmail.com
Mon May 18 08:33:19 PDT 2009
Mike,I have also had experience with this resistor and also found a design
falw in this circuit.
You also better check if the diode D1 is OK.
It looks to me that the relay K1 (AC ON) is designed for 120VAC operation,
with 220VAC, the current in the circuit is way to high. R2 1K/10W and R3
600/10W generate un-necessary heat and I do not understand how K1 survives
almost double the required current - but it does.
K1 keeps operated by the circuit:
220VAC (one side) - operated contact on K1 - R3 600/10W - K1 coil - diode D1
- J1/7 - cover switch - TB1/7 - R2 1K/10W - 220VAC (other side).
Total restinance in this circuit is then: 600 + 1000 + approx 350 in K1 coil
= 1950 ohm and with 220V gives a current of 113mA.
Relay K1 is 24V/approx 350ohm, so it should easly operate on approx. 69mA.
So I put a 680/10W that I had in the junk-box in series with R2, so
teoritically that will reduce the current to 84mA, and in reality, I measure
some 70mA. And it works OK.
The Alpha 91b is a great amplifier, I like it a lot. Only other modification
is that I have added a fan that fits exactly to the air-intake and pushes
more ait into the box.
Well, if we shall cover all of it: There is a drawing mistake: See schematic
Figure 10, HV Filter and Screen Supply (where the HV capacitors are). On
that drawing the vire from R1B shall move from the break contact to the make
contact. This is the "Hard Fault" relay that operates when the HV (+2850V)
has to hign current. When this relay operates, it short-circuits (with R1B,
15ohm) the K1 coil (AC-ON) and the mains voltage is disconnected from the
73, Thomas, PY2ZXU, SM0CXU
On Mon, May 18, 2009 at 1:32 AM, Michael Tope <W4EF at dellroy.com> wrote:
> Kimo Chun wrote:
> >Did you get your problem fixed? I am not a subscriber to the list but just
> happened across the thread...
> >Kimo KH7U
> >I am about to pull my Alpha 91B out of the operating desk to do a post
> >mortem. The amp has AC power and the 4 fuses on the rear apron appear
> >good (buzzed them out with my Fluke meter), but nothing happens when I
> >push the on/off rocker switch (not even a single flash from any of the
> >LEDs). Any idea where I should look first for a problem.
> >Mike W4EF........
> I am embarassed to admit, but I just got after it this evening, Kimo.
> I've had a bunch of other things going on, so I just kept putting it off
> - that is - until my backup amp died last night (looks like something
> arced over on the high voltage rectifier board). I figured that was the
> signal that I needed to start troubleshooting the primary amp :-) :-).
> The problem looks like a 1K ohm x 10 watt resistor R2 on the "MAINS
> BOARD" PCB. It reads on open circuit on my DVM. I think this would be
> consistent with the observed symptoms. In the troubleshooting section of
> the manual under "91B will not turn on; nothing happens when ON switch
> is pushed", the manual says "Step-start resistor R6 or R7 open". I don't
> see an R7 on the mains board, but this may be a typo. With R2 open, I
> don't see how K1 could energize.
> The front-panel rocker switch that some suggested might be the culprit
> checks out okay with the DVM.
> 73, Mike W4EF
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