Help fixing a Border Patrol Power Amp

[Patrick Dixon]

I have a prototype Border Patrol PA that I'm trying to fix. First off I should say that although I'm an electronic design engineer, I know very little about valves and I'm very cautious about putting my hands into anything high voltage! I've read all the safety stuff in the sticky, and I'm heeding it.



The valves are:
Input stage:-
EC182CC
ECC82
Power Supply:-
EZ80
EZ81
Rectifier:-
CV378
Output stage:-
2x 300B

I was told the the amp powered up but didn't appear to work - which I assumed to mean no audio output. First off I gave it it a good visual check but couldn't see anything obviously wrong (apart from the general standard of construction!). Resistors and caps don't appear to be short or open circuit, transformers seemed OK, and nothing had obviously dropped off. Powering it up with all but the 300Bs in - everything all seemed to light up correctly.

Adding the 300Bs and with an audio input, there appeared to be audio on the scope trace at the output - before the mains transformer started getting hot and I switched it off fast.



Further investigation discovered that one of the pair of //ed Caps on the HT line appeared never to have been properly soldered, but after correcting this and trying again without the 300Bs, the mains transformer still gets hot - so obviously there is something else wrong.

So I'm looking for suggestions as to how to proceed. I'd rather measure stuff with the power off, and I don't want to damage the mains transformer (which incidentally measures about 4R across the primary at DC). I'm leaning towards thinking the the overheating mains transformer is effect rather than cause ...

Thanks for any help.

[kevinkr]

I noticed that there appears to be a X or Y cap connected across the red wires (High voltage secondary) on the rectifier tube socket. These caps usually can't withstand the likely 600V or more volts across that point so I would remove that.

Next pull the rectifier tube and apply power to the amplifier, measure all of the voltages, and determine whether or not the amplifier still gets warm.

Measure the high voltage secondaries to the chassis ground - they should be within a couple of volts of each other or better. If not this indicates a partial short in the windings and if so the transformer will need to be replaced.

You might expect to see about 300Vrms from each lead to the chassis ground as a guess. YMMV..

A couple of distinct possibilities beyond this are a bad rectifier tube (internal short) or bad psu caps.

Do you have a scope and good dvm?

We'll talk again once you confirmed the power transformer is good.

Best.

[Patrick Dixon]

Thanks Kevin

The cap is actually marked with a 3KV rating (but I removed it anyway).

The high voltage secondaries measure 455V & 310V - so I guess the transformer is bad after all.

Is it likely that a downstream fault has caused it (like half the smooth capacitance on the B+ (?) rail)?

Any suggestions on a replacement transformer - preferably in the UK?

[kevinkr]

Hi Patrick,

Unfortunately that is the classic symptom of a shorted high voltage secondary.. Ouch!

Yeah, I would contact Sowter and have them wind a replacement..
Here is a link: http://www.sowter.co.uk/

Fairly, but not totally unlikely that anything downstream was directly responsible. Usually this is caused either by a defect in the construction of the transformer or by being marginally undersized for the job. This is particularly likely if the amplifier operated normally until it failed. Sometimes another failed component will cause a transformer failure, but in most cases a properly sized fuse should take care of that.

Seems like this transformer might have been a 900VCT type which incidentally is very high for a 300B application. You might want to double check to make sure that the primary voltage was correct. (Not 220V on a 250V mains for example.)

What is the voltage rating on the electrolytics in the power supply?
Knowing this should help to avoid issues with specifying the transformer. Is there any possibility that you could get a schematic of this amplifier - in most cases there will be sufficient information to develop a spec for a replacement transformer.

[tubelab.com]

Before condemning the mains transformer check to see if the center tap is actually grounded (standby switch?). Are you sure that the amp as set up for your mains voltage. Does the transformer still get hot without the rectifier tube, or without any tubes. If so (and you are going to replace it anyway) disconnect all secondaries and try it again. If it still gets hot with no load, and it is wired for 230 (or whatever you have) volts, then I am afraid that it is toast.

I brought this up because your voltage readings are very lopsided. Usually a transformer that has this many turns shorted will blow the mains fuse immediately, or (if the fuse has been uprated) smoke quickly.

If the transformer is toast, I would suspect the disconnected cap. Someone disconnected it for a reason. It may have caused a few blown fuses glowing rectifier tubes, and started the demise of the transformer, before it was disconnected.

I am not familiar with this amp, but the 900 volt center tapped guess by Kevin is too high for a conventional 300B amp. It could be reasonable if the amp used choke input and cathode bias, or a circuit similar to the "monkey amp" or the "free lunch" circuit. A schematic or some wire tracing may be needed to specify a new transformer.

[Patrick Dixon]

Originally posted by tubelab.com
Before condemning the mains transformer check to see if the center tap is actually grounded (standby switch?). Are you sure that the amp as set up for your mains voltage. Does the transformer still get hot without the rectifier tube, or without any tubes.

I'm pretty sure the amp is set for UK mains. I believe that it was sold by Kevin Scott (Definitive Audio) to the guy who owns it now, in the days when DA represented BP. I think BP were also UK based in those days. I'd guess it's 10-15 years old.

The transformer does have three taps wired to ground wire and a mains earth - all connected to the chassis. I'm not sure how its connected internally though. Aside from the heater secondaries, there's also an HT (?) winding that feeds the BZ81 which I'll check voltage on.

I made the above measurements without tubes, and although the transformer doesn't get as hot as quickly as it did previously, It feels very slightly warm to the touch - after switching off.

Originally posted by tubelab.com If the transformer is toast, I would suspect the disconnected cap. Someone disconnected it for a reason. It may have caused a few blown fuses glowing rectifier tubes, and started the demise of the transformer, before it was disconnected.

The photo isn't clear but it looks like the cap wasn't so much disconnected (I removed the tag) as just never soldered in the first place. The wire was wound through the tag making a mechanical connection, but there is no evidence of there having been any solder applied. It's possible that the connection has either been intermittent or has failed at some point.

Originally posted by tubelab.com I am not familiar with this amp, but the 900 volt center tapped guess by Kevin is too high for a conventional 300B amp. It could be reasonable if the amp used choke input and cathode bias, or a circuit similar to the "monkey amp" or the "free lunch" circuit. A schematic or some wire tracing may be needed to specify a new transformer.

The rectifier feeds the 6.5H marked choke, the output of which feeds the 300Bs and output transformers and has //ed 470uF 400V smoothing caps and a 10R load to chassis ground.

I'll work on a schematic.

Thanks for taking the time to help

[schiller]

Originally posted by kevinkr
Hi Patrick,


Seems like this transformer might have been a 900VCT type which incidentally is very high for a 300B application. You might want to double check to make sure that the primary voltage was correct. (Not 220V on a 250V mains for example.)

No it is not at all high. It is a choke input supply, with about 400V DC. The bleeder loads the supply permanently, so even if the tubes do not conduct, the voltage doesn't climb to 450*1,4.
By the way, the 400v rating of the supply cap is really marginal.

A suitable transformer sould provide: 2x450v/250mA, 5v/2A, 5v/2A (for the 2 300B's), whatever the rectifier requires (5V/3A if memory serves), and depending on the configuration of the driver stages 6,3v/2A if all the driver tubes are anode followers or twice 6,3V/1A if there is a SRPP or a cathode follower in the circuit.
It easy to see if the filaments of all driver tubes are fed from one winding or 2. I don't have a clue for the second HT supply, but looking at the voltage rating of the caps can help.

The bleeder - resistor to ground between the 2 output transformers- cannot be 10 Ohm, in this case 40A would pass throught it. 1kOhm seems more reasonable.

Regards
Konstantinos

[Patrick Dixon]

The bleeder is 10K - 10R was a typo.

I think the second HT supply is tapped from the main HT supply - 800VCT. The caps on that are 385V, so they are probably marginal too.

The driver tubes (E182CC & ECC82) are fed from a common heater supply, the EZ81 and EZ80 are each fed from separate heater supplies. The EZ80 looks like it supplies a bias voltage for the output valves, whilst the EZ81 is the HT rectifier for the input stage valves.

The 300Bs have DC heaters, series fed through a 0.33R resistor.

Apparently mains transformer failure is common in these prototype amps - BP subsequently changed suppliers and solved the problem. I'm waiting to hear on a supplier/cost for a replacement

[Patrick Dixon]

Schematics:-

sch1
sch1

[max29]

Patrick Dixon

Am fixing an older Border Patrol psu.
Can i ask you to send the schematics.


Max