Parabee resistance check

[MarimbaMan]

I finally pulled the Parabees out of the closet and built them after 6 years.  All resistance checks out fine, except

for T1 and T2 on both sides.

Manual spec on these terminals is 155-165 ohms

My readings:



          T1       T2

Left     910k ohms     970k ohms

Right     810k ohms     910k ohms



I double checked all connections, the RRSFs are connected to 1S and 1F on the transformer.  The 10 ohm resistors are

not resting on the paper cover on the transformer.  The UF4007s are oriented correctly, as are the filter caps.  I

double checked the bleeder resistor, it is attached properly.  It appears I made the same mistake on each amp, since

the readings are so close.  any ideas?  thanks in advance

[Paul Joppa]

Fortunately, I found my old copy of the manual! You are measuring the DC resistance of the power transformer high-voltage winding, from each end (through the 10 ohm resistor) to the center tap which is grounded.

I would guess that the connection of the ground buss to transformer terminal 1T (the center tap) is not good.You can check for continuity between the transformer terminal and T4/T14/T16/RCA jack outer conductor. You may find that the connection is intermittent if you wiggle the wires or terminals a bit.

That ground buss magnet wire was very difficult to solder, requiring a lot of heat to break down the insulation, and there is a caution to not overheat 1T. If that joint is bad, you may want to detach the buss wire from 1T in order to tin the buss wire at high temperature, then re-attach it once the insulation has been burned off.

[MarimbaMan]

Paul,

     My kit shipped with naked ground buss wire, not magnet wire.  I checked for continuity from transformer terminal 1T and T4/T14/T16/RCA jack outer conductor.  The connection is good between all terminals, on both left and right amps.  I know my multimeter is good, because all the other resistance checks were right on.  Still scratching my head....

Thanks

[Paul Joppa]

Oh, excellent - your grounds are pretty good now.

Time to trace the circuit. Check resistance from 1S to 1T, and 1T to 1F. Then move downstream, the other end of the 10-ohm resistors and the T1/T2, and down the ground buss.

[MarimbaMan]

1S --> 1T  10 ohms
1T --> 1F  0
1S --> 2T  0
1F --> 2T  10 ohms

both right and left read exactly the same.  so far so good

                    Right                    Left

T1 --> T5  .691 M ohms                .747 M ohms
T2 --> T5  .755 M ohms                .726 M ohms
T1 -->T14  .850 M ohms                .850 M ohms
T2 -->T14  .850 M ohms                .850 M ohms


I checked the diodes again, silver banded side on all four face T4/T5

[Paul Joppa]

Those are startling measurements, so it's time to be very clear and methodical in order to get to the bottom of this. It's probably quite simple, once we figure it out - it usually is.

There is no tap on winding 2 of a PGP8.1 power transformer, therefor no 2T terminal. I can't tell whether you have a different transformer, made a typo when transcribing the terminal identification, or the terminology is confusing - but the latter seems quite likely, so I'll try to be boringly clear:

The power transformer should be a Magnequest PGP8.1, with solder terminals on both sides of the winding. With the chassis upside down, and the power transformer toward the back the rearmost side of the winding from left to right should be labeled 1T, 3S, 3F, 5S, 5F, 2S. Across the front, the labels from left to right should be 1S, 1F, 4S, 4F, 2F. See the manual, page 18. (Incidentally, S and F refer to the start and finish of the winding respectively.)

I have used the terminology T1, T2, T3, T4, T5, etc. to refer to the terminal strip solder lugs - see the manual, page 25. It is easy to confuse T1 with 1T, etc - for which I apologize but we are stuck with this system. Just be careful.

Winding 2 is the primary, connected to the power line and switch. The high voltage secondary is winding 1. Windings 4 and 5 are both 2.5v; they are wired in series to get 5v for the 300B. Winding 3 is the 6.3v winding for the driver filament power.

If all that is consistent with what you have, then you should be able to measure the winding resistance of the high voltage secondary. It should be around 150 ohms from 1S to 1T, 150 ohms 1T to 1F, and 300 ohms 1S to 1F. If your first two measurements use the terminology as above, then they are very wrong. This may be due to the circuitry connected to these taps, so you may have to unsolder and remove all connections to 1S and 1F to make these measurements accurately.

[MarimbaMan]

The trannie is an MQ PGP8.1

You were correct, I did confuse 1T and T1.  My apologies.

I did remove the RRSF from 1S and 1F, and got the following measurements:

1T to 1F = inf
1T to 1S = inf
1F to 1S = inf

no resistance at all between these terminals.

[Doc B.]

The terminals on the PGP 8.1 usually had some varnish on them that could insulate the tab. Make sure you are getting good electrical contact with the DMM probes. Scraping the terminals with an Xacto knife will usually get through the varnish.

[MarimbaMan]

Yes, the varnish is honey-colored, and the terminals appear to be bare metal, but I scraped them anyway. After scraping, I got the same readings, no resistance

[Doc B.]

There are hair fine wires that come out of the coil and wrap around each of those terminals. It's worth looking, maybe with a magnifier to see if they have broken away from the terminals during construction.

[2wo]

Some digital meters get confused when trying to measure transformer windings. I think it is the high inductance...John   

[Paul Joppa]

On the face of it, an infinite resistance would indicate the winding is broken internally. But it seems improbably that both halves of the HV winding would be broken, and even less likely that this would be the case in both power transformers. Try to measure winding 2 - around 10 ohms if I recall correctly. Let's be really sure we are making a good measurement before suspecting a bad transformer.

[MarimbaMan]

Doc -

I have a light/magnifier on an articulated arm, 1T looks like a solid connection of the hair wire, 1S and 1F appear to have the wire connected, although it is much harder to see those due to the angle.  I will probably need to pull the 8.1 to be 100% sure on 1S and 1F.

I am using a Rat Shack digital mm, model 22-805, fresh battery

Paul -

I measured 2F to 2S, and got 5.3 ohms.  I don't know if this is good news or bad news.  I agree with you, though, that it is statistically extremely unlikely that I got two bad transformers, and that they are bad in exactly the same places.

[Doc B.]

Just a thought - are you using the continuity tester setting for the 1S/1T/1F measurements, or autorange ohms setting or a selector setting of around the 20K ohms range? The continuity checker only measures up to about 100 ohms on some testers and calls anything above that infinity.

Anyway I think at this point it might be best to pull the tubes, disconnect the rectifier connections so that the transformer is disconnected from the power supply rectifiers at 1S and 1F and carefully clip meter probes to 1S and 1T. Set the meter to the highest AC Volts setting (which must be at least 500VAC).  Plug in the amp and turn it on and see if you get any voltage.

[Paul Joppa]

Nice meter - I have two of them! It's autoranging on volts and ohms; 750vAC/100vDC maximum. Measure voltage from 1T to either 1S or 1F; between 1S an 1F there will likely be a bit over 750 vAC.