Original Paraglow 2 PowerTransformer issue

[Nutube]

Folks
This just happened
Turned on system to warm up....then thick white smoke coming from transformer underneath. Smell like plastic
Did visual check and nothing appeared blown ( meaning resisters caps etc)
Tubes are fine
Fuse not blown
Except ! PT too hot to touch and smell is definitely from it
I will be doing a resistance check right after this and will post
Question
Is there a PT available for this older amp to match? Or do I have to get a later model pair.
How to test to see if Transformer is shot
Thanks

 

[Doc B]

Sounds like the transformer may be toast. You will want to do resistance tests across each winding to see if any of them have shorted (you can compare against the transformer on the good amp). If shorted,  that is a MagneQuest PGP 8.1 and Mike might be able to make one for you. This is a fairly old amp and you have probably exceeded the service life of the filter caps. So you will want to replace all the electrolytic caps whether or not you have to replace the power trans. And I would recommend replacing the caps in the other monoblock at the same time.

I would recommend this refreshing of filter capacitors in general to anyone who has an older Bottlehead amp like the original S.E.X. amp, Afterglow, Paraglow, Parabee, B-Glow, etc. Caps run close to their max voltage rating have a limited lifetime, and the ones in power supplies do wear out after several thousand hours of use. It is better to replace them before they fail, as they can cause damage to the power transformer, rectifiers, etc.

Wow, how about that - we've stayed in business long enough to have "vintage" amps out there.

 

[Nutube]

Hey Doc
Thanks for the feedback I will do the resistance testing on PT and then post everything at the same time

 

[Nutube]

PGP 8.1, resistence check probed with Black to buss ground red to following terminals
Good Amp / Bad amp 
1S = 152.2 ohm / 74.7 ohm (discrepancy)
1F = 161.2 ohm / 44.6  ( discrepancy )
4S = 3.9K ohm / 3.9K ohm
4F = 3.9K ohm / 3.9K ohm
2F = OL  /OL
2S = OL / OL
5F = 3.9K ohm / 3.9K ohm
5S = 3.9K ohm /3.9K ohm
3F = OL / OL
3S = OL / OL
1T =0.8 ohm / 0.7 ohm

Amp resistance check per manual
Terminal. Manual/Good Amp / Bad Amp
Input 246K / 248.5K
T1. 250K / 171.2 /88.8 ( discrepancy)
T2. 200 ohm /162 /56.9 ( discrepancy)
T3.INF / Fluctuating 3.2M / .8K
T4. 0 ohm / 0.5 ohm / 0.5 ohm
T5.0 ohm /0.5 ohm / 0.3 ohm
T6. INF / Fluct. 0.7 ohm /1K ohm (discrepancy)
T7.INF /OL / OL
T8.INF / OL / OL
T9.0 ohm / 0.5 ohm /0.4 ohm
T10.INF /3.8K / 3.89K
A1. 2.5K ohm or 3K ohm / 3,9K /3.9K
A2.INF / Fluctuating / 1.3K ( discrepancy)
A3.INF / 3.6M /3.6M
A4.2.5K to 3K / 3.9K / 3.9K
B1. INF /3.6M / 3.6M
B2.INF /249K / 249K
B3.332 / 331 / 332 ( this is correct for Tuckers board)
B4.INF / OL / OL
B5. INF/ OL /OL
B6.INF / Fluct./ 16.8K
B7.INF /0,5 ohm / 0.4 ohm
B8. not required
B9. INF / 0.5 ohm / 0.5 0hm
P1.INF / OL /OL
P2. INF /OL / OL

Anyone ?please diagnose......THX

 

[Paul Joppa]

This is preliminary ...

The low resistances measured on winding 1 may be due to a damaged transformer, but they may also be due to a bad power supply capacitor.

To check the transformer by itself, you must measure the resistance of each winding. Winding 1, the high voltage secondary, is the only one which is grounded - 1T is the center tap. (1S is the start of the winding, 1F is the finish.) In addition to measuring resistance from start to finish rather than from terminal to ground, you must disconnect other things attached to the winding to get accurate values. That means unsoldering the wires connected to 1S and to 1F. Measure 1S to 1T separately from 1T to 1F.

Windings 4 and 5 appear to be the 2.5v windings. They are paralleled to drive a 2A3, so you'll have to disconnect at least one of the terminals and remove the 2A3 to test each winding. Both wil be ver low resistance, less than the 0.5 ohms or so residual resistance of your meter - you are just checking them for whether they have opened up (infinite resistance) or not.

That leaves windings 2 and 3. I'm not sure which is the primary and which is the driver heater power, but you just have to remove the driver tube to measure each of them. The primary will be about 5 ohms, the other will be very low.

If the transformer is OK - and it may well be OK, we don't know yet - then you'll have to look for short circuits elsewhere to explain the smoke. In fact, even if the transformer is not OK, it may have been some other short circuit that overloaded it. As Doc said, the power supply filer capacitors are the first place to check.

Since this amp has the Tucker board, that's another thing to check. Because it's direct coupled, a failure of the driver circuit could cause the 2A3 grid to go positive, up to the power supply voltage; that would cause the 2A3 to conduct too much current, overheating the transformer and other components and causing the failure. In that case the cathode bypass capacitor (220uF, 250v?) may also be toasted.

 

[Doc B]

I'm afraid I don't have a Paraglow manual handy, but the 1S and 1F discrepancy indicates that there is a short associated with those terminals. That means that either the rectifiers and/or filter caps are shorted (or something else in the circuit is shorted), or the transformer winding itself is shorted - or both.

The next step is to disconnect the wires connected to 1S and 1F and take those resistance measurements again. If you get the same low readings, the transformer will need to be replaced, and then you will need to replace the filter and cathode bypass electrolytic caps as i suggested in my previous post, because some other failure in the circuit would have caused the transformer failure. They don't die without help. If the numbers go do up to where the good transformer reads the trans is probably OK and you probably have blown rectifier diodes and bad caps to replace  And maybe the 270K bleeder resistors too.

 

[Nutube]

on the suspect PG 8.1
Disconnected resistance measurements
terminal/good amp/BAD amp
1S, / 151.9 / 74.7
1F /161/ 44.3

Bad amp only disconnected;  driver tube removed and 2A3 removed
2s to 2F  = 5.9 ohm
3S to 3F = 0.8 ohm
4S to 4F = 0.5 ohm
5S to 5F =0.7 ohm

It appears that the PG 8.1 at 1S and 1F is off

 

[Paul Joppa]

"...It appears that the PG 8.1 at 1S and 1F is off"

Looks that way all right. It seems odd that both half windings are similarly damaged, but those numbers sure indicate some shorted windings. Better call Mike!

 

[Nutube]

I did leave Mike a phone message and email....since he is recovering and back logged I'll wait .