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Old 21st Oct 2018, 10:02 am   #1
phykell
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Question TVM Sound PK200

Hi,

Does anyone have any circuit diagrams or any other information regarding a TVM Sound PK200? It’s a 4-channel amplifier running 2x MPD 6550A, 1x ECC82 and 1x ECC83. From what I’ve seen on Google, it looks like it should e running 2x KT88 valves so I assume they’ve been swapped for the 6550As for some reason.

The unit currently has no sound output but I’m hoping it’s worth some effort to get it working again.

Thanks
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Old 21st Oct 2018, 1:13 pm   #2
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Default Re: TVM Sound PK200

I do not know this unit but on the subject of 6550 valves (They come in various iterations eg 6550A and 6550C but treat them as the same) they are electrically identical to the KT88 and will likely have been used as replacements when the original KT88 needed replaced. They may have a different internal construction but that need not concern you, the result is more or less the same and indeed I have some Russian made KT88 and 6550A and apart from a different shape of glass envelope the internal electrode structure looks identical on a visual examination. Make of that what you will!
I am happily using a quad of GE (USA) 6550 in my KT88 power amplifier as replacements for the original Svetlana Russian made ones that a couple of went gassy on me.
A hi res picture of the amplifier with a view of the wiring underneath will be of great help as it should be possible to reverse engineer the circuit if its hard wired.
You say its a 4 channel amplifier? Sorry but with only 2 power output valves it is either a mono push-pull amplifier (most likely) or possibly a stereo (2 channel) single ended amplifier. Or do you mean it has 4 inputs? Again pictures will be useful. The big clue to this will be the number of transformers on the chassis.
Andy.

Last edited by bikerhifinut; 21st Oct 2018 at 1:17 pm. Reason: A bit extra
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Old 21st Oct 2018, 1:33 pm   #3
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Default Re: TVM Sound PK200

Ah got it! googled it and its a 100W PA amp. Thats quite possible from 2x KT88/6550 but they will have been run very hard indeed to get that out. I havent found a circuit diagram but it will certainly be what is called a fixed bias design to get the most out of the valves.
The first question is, have you any experience with working on high voltage gear? its likely to have in the region of 500volts or more on the power supply.
If you aren't au fait and the nature of your query suggests this, I would advise you get some knowledgable assistance and the correct toolsif you intend to DIY this, otherwise its find someone to do it for you. There may be someone on this forum able/willing to help in that respect, at a price of course.
A.
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Old 22nd Oct 2018, 1:17 am   #4
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Default Re: TVM Sound PK200

Thanks for your reply - much appreciated

I’ve just found a couple of links myself which agree with what you say and it’s great that the KT88 is probably a suitable replacement should the 6550A units be faulty. I’ve attached an image which shows the layout underneath the amplifier:

https://1drv.ms/u/s!AkB_A0SO1G6dkiA2nKQoTVVfDOdV

https://1drv.ms/u/s!AkB_A0SO1G6dkh-BU5R0zavIe9Wj

I’ve worked on some valve equipment in the past but I have very little test gear at the moment - I fully intend to get some though
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Old 22nd Oct 2018, 9:42 pm   #5
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Default Re: TVM Sound PK200

Some quick testing shows that the green capacitor (an assumption) to the bridge rectifier appears to have failed. Unfortunately, I haven’t been able to find a circuit diagram so I can’t identify what it might be. I saw the very same amp mentioned elsewhere and it appears that the same thing had happened to that one as well. Sure enough, there appears to be some heating marks on the chassis and the device has bubbled a bit so I can’t see any identifying marks on it and it appears to be open circuit - this would tend to explain why there’s no HT on the valve anodes or across the filter caps. I’m getting virtually no voltage out of the bridge rectifier but the diodes test fine.

Does anyone have a suggestion on what to replace the green device with? I was thinking of taking it off and seeing if I can pick it apart to see what it is. The standby light works fine and the fuse is intact.
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Old 23rd Oct 2018, 11:34 am   #6
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Default Re: TVM Sound PK200

This circuit (the one from the GEC amplifier design book of 1957)

http://www.jacmusic.com/KT88/SCHEMATIC5.gif

Would be a good bet for the basics of this amplifier give or take the PSU and the ECC8x for the octal triodes shown.
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Old 23rd Oct 2018, 6:40 pm   #7
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Default Re: TVM Sound PK200

Quote:
Originally Posted by phykell View Post
Does anyone have a suggestion on what to replace the green device with? I was thinking of taking it off and seeing if I can pick it apart to see what it is. The standby light works fine and the fuse is intact.
The only green things I can see in your under-chassis photo are vitreous wirewound resistors. I suspect it is the surge limiting resistor burnt out because of a fault elsewhere.
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Old 23rd Oct 2018, 7:19 pm   #8
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Default Re: TVM Sound PK200

Quote:
The only green things I can see in your under-chassis photo are vitreous wirewound resistors. I suspect it is the surge limiting resistor burnt out because of a fault elsewhere.
Thanks - the green device I meant was the one by the diodes - there's some white oxidation on the case above (below) it. I've noted that the HT fuse, though intact, is a 3.15A instead of 1A (marked by the fuse holder) so I thought that it could mean that the original blew and was replaced by a higher value which then caused the green device to blow.

I'm trying to map out the components so that I can create a workable circuit diagram and I've ordered some replacement capacitors which I'll do as a matter of course. Based on what I've read online, it would probably be a good idea to have a Variac to power it up once I'm ready!

Last edited by phykell; 23rd Oct 2018 at 7:20 pm. Reason: Corrections
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Old 23rd Oct 2018, 8:46 pm   #9
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Thumbs down Re: TVM Sound PK200

I would think that a 1 Amp fuse would be sufficient to cope with the surge charging the capacitors and plenty for normal running. I will leave it for someone with more high-power amplifier experience to suggest a suitable resistance value to replace the O/C one.
Possible reasons for it burning out could include duff electrolytics, S/C diode(s) and excess anode current(s) in the output pair (probably due to the 0.1uF caps leaking).
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Old 24th Oct 2018, 12:14 pm   #10
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Default Re: TVM Sound PK200

If it's any use, a first attempt at reverse engineering part of the circuit to a schematic from online photo's so far, there might be errors etc, I've only shown component values where I was reasonably sure of ID.

If I can, I'll do a bit more later on.

Lawrence.
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Old 24th Oct 2018, 12:46 pm   #11
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Default Re: TVM Sound PK200

The attached picture is the circuit I reverse engineered of an unmolested one a few years back. Hope it helps. Yours looks correct so far!
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Old 24th Oct 2018, 1:05 pm   #12
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Default Re: TVM Sound PK200

Cheers, I just finished another bit as I saw your post, I'll post that part schematic up anyways.

Lawrence.
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Old 24th Oct 2018, 1:11 pm   #13
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Default Re: TVM Sound PK200

The other bit.

EDIT: Whoops, forgot the valve pin numbers, will post again.

EDIT: Revised schematic with valve pin numbers now shown.

Lawrence.
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Old 24th Oct 2018, 2:38 pm   #14
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Default Re: TVM Sound PK200

Bit different from the GEC version, then. I wonder why they did the phase splitter differently?
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Old 24th Oct 2018, 8:04 pm   #15
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Thumbs up Re: TVM Sound PK200

Thanks so much for all the effort - amazing!

I’ll compare with what I have but these diagrams are going to be extremely helpful so thanks again.

It looks like the resistor that’s failed is a 15 ohm 10w unit from the diagram then and it should be easy enough to source a new part. I’ll make sure I get that fitted at the same time as the new filter caps and the correct fuse(!).
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Old 24th Oct 2018, 8:17 pm   #16
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Default Re: TVM Sound PK200

Watch out for high voltages in this amp. main HT could be nasty.

Lawrence.
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Old 25th Oct 2018, 3:59 pm   #17
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Default Re: TVM Sound PK200

I make that about 800mA max for the 15 Ohm 10 Watt resistor.
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Old 3rd Nov 2018, 11:09 am   #18
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Default Re: TVM Sound PK200

Thanks everyone - the amplifier is working!

I replaced the old wax/paper capacitors, presuming that they are leaking DC by now, despite the capacitane being within specification; and I also installed the correct HT fuse. The body of the 15 ohm 10w resistor was badly burned so it’s lucky Omegaman was able to provide the value for me - the faulty device was open circuit so it was definitely responsible for the lack of HT.

The only remaining items are that the standby light is very dim so I probably should replace that. Also, the 10uF 450v electrolytics look to me as though they may have swelled up for some reason but they’re pretty much all the same so I wondered if it’s the normal appearance for this component - radials are much easier to identify as having swelled. They also look as though they were replacements at some point but could still be a decade old.

https://1drv.ms/u/s!AkB_A0SO1G6dlGPNhhaP8ZoT5iwy

Lastly, there are a couple of remaining capacitors which I haven’t replaced and don’t know whether to bother other or not. The schematic doesn’t show one of them and it’s hard to tell what value or type it is:

https://1drv.ms/u/s!AkB_A0SO1G6dlGVISQglYf1n_34d

I’m nearly finished on the schematic so will post that for reference when I can.
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Old 3rd Nov 2018, 11:34 am   #19
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Default Re: TVM Sound PK200

The little 470pF Wima axial cap is probably fine - personally never had any trouble with them. Not sure I would leave those Radiospares caps if doing a full restoration.
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Old 15th Nov 2018, 11:04 pm   #20
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Default Re: TVM Sound PK200

Still working on the circuit diagram but the recap is done apart from the 300uF 300V smoothing electrolytics in the power supply:

https://1drv.ms/u/s!AkB_A0SO1G6dlGn7-HpXBpmUhPpV

However, I now have some hum to deal with:

https://1drv.ms/u/s!AkB_A0SO1G6dlGfcVdneC31qpDnQ

I'm hoping now that it's just the smoothing capacitors that may be causing this but I now have the additional difficulty of finding suitable components that will fit the mounting bracket - the current ones are 40mm diameter but modern ones are significantly smaller. I'm guessing that I should be able to replace the old ones with higher value, e.g. 330uF, 390uF or even 470uF - does that sound OK?

I've seen a 450v 470uF in a 40mm diameter so that might be the ideal choice but is there such a thing as a reducer or is there some other method of replacing these older, larger capacitors? Also, I can only find capacitors with "snap in" connections - are there any with tags still around or are the snap-in connections OK for soldering a couple of leads/components to?

Last edited by phykell; 15th Nov 2018 at 11:27 pm.
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