Parabee resistance check

MarimbaMan · 29126

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Offline Doc B.

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Reply #45 on: April 07, 2011, 08:03:51 AM
No, 1T is the ground tap, and that's OK. The rectifier's are OK, the high voltage secondary is putting out about the right voltage, it's just these resistance readings that are weird and there doesn't seem to be any good reason for them to be that way other than the meter being fooled by something in the circuit. Without actually having eyes on the amp I can't really come up with anything else to test that would be very relevant.  At this point if it was in my lab I'd probably fire it up and see what kind of voltages I get, particularly if all resistance measurements are now the same on the good amp and this one (and honestly I'm a little lost on that point since the thread has grown...)

Dan "Doc B." Schmalle
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Offline MarimbaMan

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Reply #46 on: April 08, 2011, 07:15:39 AM
To recap:

The original issue was that resistance checks on both amps were spot on except for T1 and T2 on both.  This led me to believe I made the same bonehead mistake on both amps. 

I installed the tubes, put a shorting plug on the rca jack, and did a voltage check on the right amp:

Terminal                 Spec                    Actual
1                         365VAC                 33VAC
2                         365VAC                 5.5VAC
15                        465VDC                 5mVDC
A1                        75VDC                  0VDC
A2                        450VDC                 11mVDC
A4                        75VDC                   0VDC
B6                         187VDC                 1.4VDC
B8                         3VDC                     11mVDC


Doc, I will be happy to provide pictures if that would help.  Thank you.



Offline Doc B.

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Reply #47 on: April 08, 2011, 12:03:38 PM
OK, a few pages ago you had readings on the unloaded high voltage secondary (1S, 1F) that looked pretty reasonable. Now with the amp circuit reattached to the power transformer the voltages look like there is a short somewhere that is pulling things way down. Try pulling the tubes and checking the voltage at terminal 15 again. The voltage may rise higher than 465V without the tubes loading it.

Dan "Doc B." Schmalle
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Offline MarimbaMan

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Reply #48 on: April 08, 2011, 08:11:58 PM
I removed the tubes, plugged the amp back in, turned on the power, and got:
T15  9mVDC


Offline MarimbaMan

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Reply #49 on: April 10, 2011, 07:47:59 PM
I decided to do a voltage check comparison with the left amp, with the shorting plug, but without tubes.

I switched off the amp as soon as I turned it on.  There is a buzz and a bright arcing at the connection of 1T and the hair-fine wire going into the transformer


Offline Grainger49

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Reply #50 on: April 11, 2011, 03:19:02 AM
That sounds like the transformer wire isn't continuous to the transformer lug.  The high voltage would arc across the break point.

I'm just stabbing around in the dark as I know nothing about a Parabee.  I suppose it is an older 300B design.


Offline Doc B.

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Reply #51 on: April 11, 2011, 05:19:05 AM
Quote from: MarimbaMan on April 10, 2011, 07:47:59 PM
I decided to do a voltage check comparison with the left amp, with the shorting plug, but without tubes.

I switched off the amp as soon as I turned it on.  There is a buzz and a bright arcing at the connection of 1T and the hair-fine wire going into the transformer

Look at that connection closely, preferably with a magnifier to see if the wire is disconnected or otherwise damaged.

Dan "Doc B." Schmalle
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Offline MarimbaMan

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Reply #52 on: April 11, 2011, 10:42:35 AM
I used a 10X jeweler's loupe and a Maglite, and it's still pretty hard to make out what's going on.  The left amp seems to have two hair wires coming out of the transformer, and the right amp seems to only have one.  Could the arcing be between the two wires?  There also seems to be more varnish around the two wires on the left amp.  I am very hesitant to mess with them too much, I don't want to break them or melt them.


Offline MarimbaMan

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Reply #53 on: April 11, 2011, 11:02:15 AM
Ok, I found the arcing problem, the hair wires are disconnected from the 1T tap


Offline Doc B.

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Reply #54 on: April 11, 2011, 12:16:16 PM
Ah hah, that would certainly account for the infinite resistance readings, assuming it was like that before the spark (which I suspect is likely as discussed back a few pages). With some care one can usually tack solder them back on. If you are successful be sure to check the T1/T2 resistance again to see if it reads correctly, before giving it power.

Once again I am sorry, I'm having a little trouble keeping track of the events. Has the other amp been given the voltage test yet?
« Last Edit: April 11, 2011, 12:22:12 PM by Doc B. »

Dan "Doc B." Schmalle
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Offline MarimbaMan

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Reply #55 on: April 11, 2011, 12:28:04 PM
To clarify, the low voltage readings above were for the right amp.  I wanted to do a voltage reading on the left amp for comparison, and that's when I found the arcing problem on the left amp.  I'll attempt a repair on this now


Offline Doc B.

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Reply #56 on: April 11, 2011, 12:31:17 PM
Sorry, you did indeed say the left amp. Definitely focus on that amp first.

Dan "Doc B." Schmalle
President For Life
Bottlehead Corp.

Offline MarimbaMan

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Reply #57 on: April 11, 2011, 02:27:38 PM
Good news!

Left amp:  I tack soldered the hair wire, and did resistance readings from 1T to T4, 0 ohms.
resistance on T1 = 157 ohms
resistance on T2 =  167 ohms

all resistance on the left amp is now within spec.

Bad news!

I've blown two 2A fuses when powering on to do the voltage check


Offline Doc B.

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Reply #58 on: April 11, 2011, 03:57:30 PM
Are the fuses fast blow or slow blow?

Dan "Doc B." Schmalle
President For Life
Bottlehead Corp.

Offline MarimbaMan

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Reply #59 on: April 11, 2011, 04:03:31 PM
They are fast blow


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