'84 Marshall JCM 800 2204 PT and OT

[Dixierocker]

How to Test a Transformer: 12 Steps (with Pictures) - wikiHow

Transformers are an electrical component that transmit electrical energy between at least two circuits. Transformers regulate the voltage in circuits, but in some cases these can go bad and cause a circuit to not work. You'll need to...

www.wikihow.com

 

[Grazy]

P = V I
So I = P / V

yes...quite sure I had the right answer , but can't write the formula! ha. Thanks

 

[Pete Farrington]

for a 40 watt bulb, I'd expect 0.21 amps; 0.5 amps for 100W...correct?

So your mains voltage works out to 40 / 0.21 or 100 / 0.5, about 200V in either case, which doesn’t seem likely?

 

[Grazy]

So your mains voltage works out to 40 / 0.21 or 100 / 0.5, about 200V in either case, which doesn’t seem likely?

1) You are right, again, about the amps...I just need to stop doing simple math on the fly - summary below - if this is wrong, I need to give up.

2) Re: PT testing - I think it's shorted - here's what I tested. Let me know if you all agree.

a) As Purgasound counseled: I measured the heater voltage on pin 2 on both V4, V5 - (pin 2 is the heater, correct?) Got 0 v. Same as when I tested across Pin 2, 7; when I flipped on the standby, as mentioned, the main fuse immediately blew. I checked the vottage across the standby when it was off and no reading.
b) Then I hooked up the dim bulb unit and followed the troubleshooting above and also outlined here: https://robrobinette.com/Tube_Guitar_Amp_Troubleshooting.htm

- I traced the voltages and continuity as best I could, so went to the step where it says to disconnect the secondary wires and if bulb is bright, PT is shorted. Bulb was bright.
- I checked the power cord as it says here, and it was fine. Net...need to replace PT?

Thank you.

Volts=	120
	Amps	Bulb Watts
I =	0.33	40
	0.42	50
	0.50	60
	0.83	100
	1.25	150

 

[Grazy]

1) You are right, again, about the amps...I just need to stop doing simple math on the fly - summary below - if this is wrong, I need to give up.

2) Re: PT testing - I think it's shorted - here's what I tested. Let me know if you all agree.

a) As Purgasound counseled: I measured the heater voltage on pin 2 on both V4, V5 - (pin 2 is the heater, correct?) Got 0 v. Same as when I tested across Pin 2, 7; when I flipped on the standby, as mentioned, the main fuse immediately blew. I checked the vottage across the standby when it was off and no reading.
b) Then I hooked up the dim bulb unit and followed the troubleshooting above and also outlined here: https://robrobinette.com/Tube_Guitar_Amp_Troubleshooting.htm

- I traced the voltages and continuity as best I could, so went to the step where it says to disconnect the secondary wires and if bulb is bright, PT is shorted. Bulb was bright.
- I checked the power cord as it says here, and it was fine. Net...need to replace PT?

Thank you.

Volts=	120
	Amps	Bulb Watts
I =	0.33	40
	0.42	50
	0.50	60
	0.83	100
	1.25	150

Appreciate any thoughts on this. Thanks

 

[Purgasound]

Let me make sure I am verifying your conditions...

If the amp is drawing current with -
1) 6.3VAC Heater secondaries disconnected
2) Power Switch in the ON position
3) Standby Switch in the OFF position

If the answer is "yes" to everything above then that is a confirmation the power transformer has a shorted winding and should be replaced.

There's one more thing you can do which probably won't save it but it's not going to hurt to try. At this point you need to pull the PT anyway so go ahead and get it out of there but don't hastily cut any wires. Unsolder them if you can to preserve the length. Take the bell end covers and check the connections of the wires under the cover. Something could have come loose depending on how the wire ends terminate. I can't recall how the insides specifically look on a Dagnall or Drake PT. I have had a few come through the shop that people cut out of amps but they were still good and I needed to open them up to attach new leads under the bell end covers. Sometimes it was possible, sometimes it was easier to just splice new wires on the ends of the old cut leads. The lead connection to the winding may be covered so in that case there's nothing to come loose. Like I said, probably not the case but I would look just to be sure.

Now the fun part, before you power on the amplifier with a new PT you should go through and figure out what caused the old one to fail. It may be an isolated incident but if it's something like a shorted tube socket you are going to need to identify that first or risk the new PT as well. Make sure there is no continuity between the heater pins and any other pins on the sockets (with all the tubes removed). It also doesn't hurt to take a heat gun to heat up the sockets and measure continuity between pins again. Partial shorts or arcing can sometimes be really hard to pin down. There are conditions where it's possible arcing doesn't occur until a certain voltage threshold or temperature is reached. If there is continuity between the heater pins and any other pins on the socket then that socket should be replaced. If there is any carbon buildup on the top of the socket it is evidence that a tube shorted and arced across the pins.

I don't want to send you down a rabbit hole chasing a bunch of possible scenarios but you should at least make sure there's not another underlying issue before powering on the new unit. When we need to identify if fault conditions are present we'll always use the bulb limiter on startup on the new equipment as well. Did you say you had a Variac?

 

[Grazy]

With the dim bulb tester/40 w bulb connected , I confirm the 3 conditions you laid out above.

I also checked for shorts/continuity on V4, V5. as you stated above. Had both the on and standby on, with dim bulb tester, all tubes removed.
V5 and V4 showed similar results as follows:
- pin 7,2 showed continuity - which I believe is to be expected; but pin 7 to 8 and 1 (jumpered) also showed continuity. All others measured OL, except pin 5 - the DMM flashed between 0 ohm and OL (?)

Before the mains fuse blew, V4 was red plating. I exchanged V4, V5 tubes and V4 continued to redplate...and eventually the mains blew (originally owner had a 15a fuse in it, and I never checked it - but it ran great for the last 10-15 years)

I do not have a variac. I saw some variacs on ebay/amazon and they were not that expensive depending on max voltage - do I need one?

Based on this, I guess I need to replace PT - I was thinking Heyboer - or Mojotone. (a few bucks cheaper)...

Appreciate any further thoughts - thank you very much.

 

[Grazy]

With the dim bulb tester/40 w bulb connected , I confirm the 3 conditions you laid out above.

I also checked for shorts/continuity on V4, V5. as you stated above. Had both the on and standby on, with dim bulb tester, all tubes removed.
V5 and V4 showed similar results as follows:
- pin 7,2 showed continuity - which I believe is to be expected; but pin 7 to 8 and 1 (jumpered) also showed continuity. All others measured OL, except pin 5 - the DMM flashed between 0 ohm and OL (?)

Before the mains fuse blew, V4 was red plating. I exchanged V4, V5 tubes and V4 continued to redplate...and eventually the mains blew (originally owner had a 15a fuse in it, and I never checked it - but it ran great for the last 10-15 years)

I do not have a variac. I saw some variacs on ebay/amazon and they were not that expensive depending on max voltage - do I need one?

Based on this, I guess I need to replace PT - I was thinking Heyboer - or Mojotone. (a few bucks cheaper)...

Appreciate any further thoughts - thank you very much.

Well. Took out my PT. So sad - it actually looks quite good. I measured resistance across various leads:
Primary: Blu-black: 1.7 ohms; blu- or: 6.7; blu- yellow: 7.7omhs;
Secondary side: or to black or brown: 0.2 ohms; Black to red: 40.8 or 38.6 ohms. Orange to red or black is OL
Any wire on primary side to secondary side is OL

Note: When I put 120v ac to the blue and black primary wires, my dim bulb tester with 150w bulb glowed brightly; when I connected the blue and orange (220v tap) to the 120v source - same thing. When I connected the blue and yellow, my DMM read 36V. not sure what this means other than there is a short somewhere inspite of decent ohms. Someone with lots more experience may be able to explain/understand this. I’m just hacking around at this point. Thanks

 

[Grazy]

Any further thoughts? Thx

 

[Steve Smith]

Contact Martin, he is the guru....

Trained at Milton Keynes under the great man and restored my 76 JMP to perfection..

About Us – Stoneham Amplification

www.stonehamamps.co.uk

 

[Grazy]

Contact Martin, he is the guru....

Trained at Milton Keynes under the great man and restored my 76 JMP to perfection..

About Us – Stoneham Amplification

www.stonehamamps.co.uk

Thank you...not sure about sending this across the pond. But good to see his amps and products - sound very good!