To All,

I have recently picked up a Trio W-50 Stereo AM/FM/SW receiver/amplifier.
It is a nice looking clean unit, but came with 4 bad 7189 output tubes. I
checked all the other tubes in this baby and all checked good, so I picked
up 4 new 7189A's to put in.

When I fired it up without the 4 tubes, I used a variac and brought it up
slowly to 110 AC to make sure I wasn't going to inadvertently let all the
smoke out and there appeared to be no problems. So far so good.

I then put in a 3 amp fuse (replacing the stock 4 amp) and added the
7189A's and a speaker on the left channel and again used the variac.

The speaker was silent and within a few minutes, the fuse blew. I never
saw a serious current load on the variac, but then, I wasn't watching that
closely. I double checked all the tubes at this point, and they all still
checked good (using a Hickok 600A tube tester).

So the question is, does anyone know of a source for a service manual for
this unit?

Regards,

John C.

John,

I do not have a service manual to help you. Even without it, you can
make several checks. It is suspicious that the original 7189's were all bad.
Can you describe how they were bad? Was it leakage, low emission, or
transconductance? The new 7189's should not be sacrificed in the early
troubleshooting stage. If I recall correctly these are basicially a
EL-84/6BQ5 with voltage ratings a little higher.

I am most suspicious of the vintage power supply, and filtering
electrolytics. If the Trio has a tube type rectifier, you can use an old one
with low emission for early tests. A few sparks visible in a 5Y3, or 5U4 are
a big clue, and is often seen way before the fuse pops. My feeling about
electrolytics of vintage gear is that they need to be replaced. Sure many a
Macintosh tube amp restorer gradually form the original electrolytics,
change a few tubes, clean things up, and then sell on Ebay. My feelings are
that if you want your amp to work when you need it, the stock electrolytics
need to go. I just restored a Ham transceiver built in the 1950's, and the
variac forming worked fine. I used a 5U4 solid state replacement while
forming so that the heater lag of the 5U4 didn't get in my way. It worked
fine. I put the 5U4 back in, and two days later the first electrolytic in
the pi (C-L-C) filter shorted out to less than one ohm. The 5U4 sparked
internally, and before I could shut it off the fuse popped.

You did good with the Trio with no 7189's, and a variac. Keep in mind
that the B+ load will be low, so keep the variac low enough to not over
voltage your electrolytics.

You need to determine if the 7189's are biased from a fixed negative
bias supply, or from cathode bias. If the bias is fixed, is there adjustment
pots to set the bias for each tube, or for each channel? If so, consider
setting the bias to maximum (most negative) during your early "blow out the
cobwebs" stage of testing.

I hope this helps.

Regards,
Jim Candela
WD5JKO


"John Cusick" wrote in message
news
To All,

I have recently picked up a Trio W-50 Stereo AM/FM/SW receiver/amplifier.
It is a nice looking clean unit, but came with 4 bad 7189 output tubes. I
checked all the other tubes in this baby and all checked good, so I picked
up 4 new 7189A's to put in.

When I fired it up without the 4 tubes, I used a variac and brought it up
slowly to 110 AC to make sure I wasn't going to inadvertently let all the
smoke out and there appeared to be no problems. So far so good.

I then put in a 3 amp fuse (replacing the stock 4 amp) and added the
7189A's and a speaker on the left channel and again used the variac.

The speaker was silent and within a few minutes, the fuse blew. I never
saw a serious current load on the variac, but then, I wasn't watching that
closely. I double checked all the tubes at this point, and they all still
checked good (using a Hickok 600A tube tester).

So the question is, does anyone know of a source for a service manual for
this unit?

Regards,

John C.

Thanks for the info. As I'm sure you can tell from my posts, I'm really
new at figuring out tube amps, etc, although I have fixed a few years ago,
but now I'm well out of practice :-(

I looked at the schematic of the LR-800 and it is very similar to mine, so
my question is how do I tell if its cathode biased or not. I see no
adjustments for biasing on my schematic, but then I may not be seeing
properly.

I can put a copy of the schematic up on my site if you have the time to
give me a few pointers.

If the output cathodes are grounded (or grounded via small resistors
like 10 Ohms), it's not cathode biased. Look for -15V or so on the
control griods - you can check with the output tubes pulled, but keep
the variac turned down to 110 or below, as the B+ will be higher
unloaded.