• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

1000A conundrum

gp1680

gp1680

Active Member
So I rescued a 1000a from someone for $50. It powered up but no sound. I checked and replaced a few of the tubes. I took it to my tech and he says both output transformers are shot. :(. Is it worth pursuing replacements? Anyone have two good opts they want to part with?
 
Register to hide this ad
They need to be ohmed out to see if they are bad. A lot of tech's don't know how to figure out tube stuff. I've had one unit with a bad output trans, fisher 500C and it still worked just had distorted audio. I'd be shocked if BOTH output tranny's were open.
 
bktheking said:
They need to be ohmed out to see if they are bad. A lot of tech's don't know how to figure out tube stuff. I've had one unit with a bad output trans, fisher 500C and it still worked just had distorted audio. I'd be shocked if BOTH output tranny's were open.
Click to expand...

This guy builds and repairs tube guitar amps and has done other work on tube amps for me in the past with great success, so I trust him. However, I will double check with him anyhow.
 
Well then he should know what he is doing. I'm just saying it's pretty rare that 2 outputs transformers are dead in a unit.
 
He said there is voltage going into opts, nothing coming out. He also said it appears someone monkeyed around in it before.
 
it is very rare for both OPT to be bad at the same time. Could be some very simple . I don't trust what the guy said.
 
As a 1000A owner of 8 years now, I can believe that both went out at the same time. You get a 50 year old piece of gear with leaky output caps, lousy tubes, and an owner who either doesn't know any better or doesn't care, and it makes for a BAD COMBINATION. I've seen 1000A's with both OPT's done in by blown caps, or shorted tubes. Remember, Sansui ran these things at or past the ragged edge. And with what we would today consider sub-par components. It's a wonder to me that more haven't shown up with bad output's.

Sansui SHOULD HAVE made the 1000A with a larger chassis (both depth, width, and length) or done away with some of the non required circuitry (ie: extras). And re-designed the layout so the transformers weren't being melted by the output tubes. The chassis is just too small for everything it does have. This is one of the very very few receivers I've seen or read about where pretty much everyone runs it without a top and with fans due to the excessive heat.

HEAT KILLS! And the Sansui 1000A will die an early death if steps aren't taken to reduce the heat either thru reduction of heat given off by internal or topside parts or by forced ventilation. You can literally cook eggs on a sansui 1000A's top given an hour or so running without ventilation.
 
Transformers rarely fail - with the exception of earlier Sansui 1000A OPTs. Those fail at a rather alarming rate. I built one complete 1000A out of two damaged early units (one extremely early with the round voltage selectors, another one slightly later with the square voltage selector but only one bias pot). Between the two of them I managed to get a pair of functioning OPTs, both of which were later production. The earlier unit had one original OPT and one later replacement from Sansui, the original had failed. I've rarely heard of an OPT failure from the two and four bias pot production runs, so it seems as if the longevity issues were worked out over the several years of production.

Read through old threads around here, the 1000A is notorious for blowing OPTs. I agree with larryderouin that some of it is due to the overcrowded and overcooked chassis, but I've seen similarly cramped units from other manufacturers jammed into badly-ventilated cabinets without toasting OPTs so often. Something about the design was not quite rugged enough for the application.
 
gp; If you decide to go with the Trancendar's, you can do some improvements/modifications to the topsides to reduce the heatload in the back of the unit.

1.) Cut down the isolation plate between the transformer and the can caps, but only from the inner edge of the Power transformer to the end of the plate. This will improve ventilation around the tubes and transformers in that immediate area.

2.) REPLACE the Power Resistors as shown in this thread....... ......Overall temps will stay about the same but the heat sources will be more forward in a cooler area (directly between the dial cord cover and the 12ax7's up front.) The rear area will now be heat loaded only from the transformers and the output tubes.

3.) If you need to get new output tubes (New Stock) get the Tung-Sols. The EH's will work, but with the larger envelope, the ventilation is reduced some, and the radiant heat transfer to the transformers is increased due to the proximity of the tubes. The Tung-Sols are same size as original 7591's.

4.) a couple of 3" 12v computer fans powered by a 9V wall wart, attached to the back panel of the top will help keep the temps down.

5.) Install 10ohm cathode resistors on the output tubes (1/4W). These are used to check the tube current draw and also act as fuses shutting down the tube and keeping the transformer safe.

6.) Install 100ohm Screen stability resistors. Make sure the pin 4-pin 8 connector between the tubes is cut between the tubes when you do this. This will help keep the output tubes from arcing (which is never good for the tubes.)

Replace ALL SUZUKI PIO caps. No questions. And all electrolytics. And the normal stuff for rebuild rehab.

Bias the tubes to about 32ma with 480v on the plates. This will give you about 14.5 watts on the tubes or about 70 to 75% of max dissipation. This is in the ideal range of 70-80%. it'll still kick ass, and run a lot cooler than if you set it up for 18-19W or about 38-39ma.

If you run into problems that you don't have an answer for, STOP and post the question. Pictures help if needed.
 
I've only restored one 1000A. Really nice sounding receiver. BUT... I concur completely with everybody. I couldn't believe how hot the thing ran. First time we ran it for while I kept monitoring temperature, as we always do, and eventually turned it off. It just kept getting hotter and hotter. Couldn't even lay your hand on any of the transformers. And this was with the cabinet off. I rigged up two muffin fans so we could continue testing.

I recommended to the customer that he not run it with the cabinet on and always with a fan. Never heard from him again. Maybe his house burned down.

That is exactly the opposite of my BA303 and BA202 amps. You can run them all day at full factory bias setting and full volume, and the xfmrs, both pwr and opt, do not even get warm to the touch. I wonder what the primary wire size is in those 1000A xfmrs compared to the BA's.
 
Layout, bias settings and the B+ being 470v to 480v on the plates with 430V on the screens don't help things. That's almost 100V higher than Fisher ran the 800C. Same basic circuit too. Funny thing is, I've never heard of or seen a dead power transformers for these. I just wonder if the output's are actually undersized for the unit,.
 
Some help with heat issues-
Place some METAL FOIL (not cloth or plastic!) duct tape on the sides of the power and OP transformer sides that face the OP tubes and the power resistors.
Put two computer type fans at the back blowing OUT, maybe using a slightly larger one for the power resistor side, running them both on a 9 Volt DC wall transformer (PM me if you want to power them from the 6.3 heater supply).

Mark T. :music:
 
You must log in or register to reply here.
Back
Top Bottom