Ediswan Type R.1280 PSU - Faulty

[af024]

Dear all,

I have recently acquired this monster PSU.

So far, I have found the following:-

Either MR2 and/or MR5 are faulty. They pull loads of current. Temporarily I have replaced these with a single silicon bridge rectifier. Would this be ok?
V4 had lost its vacuum – it was a CV140. I have replaced this with a known good 6D2.
12AT7s are fitted instead of 12AX7s. Not sure whether this is ok.
Some evidence of electrolytic capacitor replacement.
Neither of the variable 300Vd.c. HT1 or HT2 supplies provides any output volts.
The fixed 200Vd.c. output sits at approx 100Vd.c.
All heater supplies are ok.
If I remove V6, V7, V9 and V10, the 200Vd.c. stays at approx 100Vd.c, and the HT1 and HT2 supplies rise to approx 300Vd.c.

With all the valves reinserted, the voltage across MD1 is approx 150Vd.c. The voltage across V5 is approx 75Vd.c.

V1 measurements:-
Pin 3 400V
Pin 4 400V
Pin 5 90V
Pin 7/8 100V

V3 measurements:-
Pin 1 90V
Pin 2 80V
Pin 3 80V
Pin 6 100V
Pin 9 80V

Any ideas what might be going on please folks? Could the Metrosil be causing the problem perhaps? What might I replace it with?

Looking forward to your comments.

Thank you,


Andy

[af024]

Some photos ...

[Sean Williams]

Well, the rectifiers are bound to be shot!

I would also suggest that the 85a2 might be worth changing - I had this problem on my bench supply, once swapped it cured a lot of problems.

The 12at7 has a lower GM than a 12AX7, so it might be worthwhile subbing these for the right valves as well (ECC83)

HTH
Sean

[af024]

Thanks for your comments Sean,

Regrettably I don't have a replacement 85A2, I'll need to track one down from somewhere. I think I'm also going to struggle for a ruck of ECC83s, but I may be able to find at least one (to replace V3). Is an 85A2 something like a small version of a VR150? What sort of voltage would I expect to see across the 85A2?

Regards,


Andy

[wave solder]

An 85A2 is a low current 85V voltage stabiliser, the ignition voltage is down as 125 V.

[ronbryan]

Have you tried removing C8 (0.1uF 350V between V1 cathode and V3 grid pin 2) temporarily? It could be leaky, causing V3, the error amplifier to conduct too heavily on the left hand triode, causing the grid volts on V1 to be too low. This could explain why the output of the fixed supply is stuck at 100V.

Ron

[Ed_Dinning]

Hi Andy, good PSU's these if somewhat hernia inducing. If the volts at the anodes of the seies pass valves are ok then the transformers are ok and anything else will be cheap to fix. The usual faults result in the error amps causing the series pass to shut down and no/ reduced O/P volts.
PM me if you need any 85A2's.


Ed

[af024]

Thank you for everyone's comments (and for the kind offer of an 85A2 Ed!). Yes the thing would make an efective paper weight!

I lifted C8. It was indeed leaky. I replaced it with a new cap. The 200Vd.c. rail was still at 100Vd.c.

Finally, having gone round the area yet again, I found that R16 was very high in value (I could have sworn that I had been here before, never mind eh). I replaced that and hey presto, 200Vd.c. What was even more amazing was that it was EXACTLY 200Vd.c.

New readings are:-

V1 measurements:-
Pin 3 400V
Pin 4 400V
Pin 5 195V
Pin 7/8 200V

V3 measurements:-
Pin 1 195V
Pin 2 80V
Pin 3 80V
Pin 6 200V
Pin 9 80V

Regrettably I still don’t have HT1 or HT2 (doh).

I also found R42 and R59 to be o/c (which I have replaced)

Measurements around V2 or V11 (with respect to the common BK rail in the centre of the diagram) reveal:-
TC 400V
Pin 4 75V
Pin 5 -50V (ish) difficult to measure (loading)
Pin 8 approx 0V

Measurements around V6 (with respect to 0V of the 200Vd.c. rail) are:-

Pin 1 105V
Pin 2/6 115V
Pin 5 115V
Pin 7 200V

(Sorry, I probably would have been better off doing that with respect to the common BK rail).

Strange how both 300Vd.c. supplies are down and out.

Andy

[ronbryan]

Andy

It looks to me that the second supply from the top (120V winding to rectifier and C7) has packed up.

This supplies the positive supply that sits on the 0/300V 150mA output and feeds the error amp buffer V6 anode (via diode V4A and R28) and drives V2 and V11 control grids. R28 V6 anode load is a 4M7 resistor which could also be high or o/c.

Ron

[af024]

Hello Ron,

That seems logical, yet I have something like 150Vd.c. sitting across the last section of C7, so I guess it's working.

Andy

[af024]

I forgot to say that the 4M7 is slightly high (but only slightly).

Andy

[ronbryan]

Andy

Can you confirm that there is about 350V on the V4A diode anode and cathode with your meter referenced to the -ve leg of the 200V supply?

Ron

[af024]

Sorry for the delay in replying Ron, it would seem as though in order to get a connection, I have to pick my dial-up moment with my ISP these days!

Yes I can confirm that there is 370V on pin 2 and pin 5 of V4A.

Better measurements (digital meter) around V6 are:-

Pin 1 127V
Pin 2 133V
Pin 5 134V
Pin 6 133V
Pin 7 200V

Andy

[ronbryan]

Andy

V6 is turned hard on with only 6V between anode and cathode - but why?

Can you measure the both grid voltages on V7 - it would appear that the left hand triode is not conducting. With normal circuit operation, both grid voltages should be equal.

If there is an imbalance in the grid voltages, check the potential dividing resistors. Maybe the 150k HS resistor on the V7 left hand triode grid is o/c. This would affect both 300V supplies.

Ron

[af024]

Hello Ron,

Thanks for your note. Voltages around V7 are as follows:-

Pin 1 169V
Pin 2 3V
Pin 3 31V
Pin 6 201V
Pin 7 28V (adjustable of course)

So the grids are not at the same potential.

I shall check the 150k again (just in case).

Thanks.

Andy

[ronbryan]

Andy

The 150k HS / 33k HS resistors feeding V7 pin 2 via the 47k series resistor should have about 36V at their junction if their resistances are the correct values.

Ron

[af024]

R36 was indeed o/c. Using a passive measurement (ohms) in circuit showed it to be about the correct value! I know that in circuit passive measurement isn’t the best, but the result was uncanny.

I replaced that and I nearly got 300Vd.c. on each HT1 and HT2. As well as C8, I found C10 and C13 to be leaky.

I replaced R42 and R37 as 150k instead of 120k (I don’t have any 120k and I didn’t want a series or parallel combination. Seems ok. I used standard carbon types (nothing fancy).

I don’t know what ‘HS’ means on the circuit, unless it’s ‘High Stability’. What’s really strange is that all the faulty ones were this type (Dubiliner with some sort of sleeving). So much for ‘HS’ then I think!

The left hand meter also sticks. I stripped it down and made some adjustments. It’s better but still not right. Then it stopped working altogether. Yes you’ve guessed it, yet another ‘HS’ resistor had failed – R61. Again I don’t have a 300k, so I propose 330k. I better change R60 as well (only a matter of time methinks before that meter also packs up).

So, the total damage was:-

C8, R16, R15, C10, R36, R37, R44, R42, C13, R39, R37, MR2/MR5 (replaced with silicon bridge rectifier), V4, R60, R61, front panel lamp 6.3V 0.06A (can’t find one), smashed lamp lens cover (not sure what to do here), sticky meter (oh well, never mind), numerous binding posts damaged (might live with this).

Interesting all other metal rectifiers are performing well (how long for is another matter I suppose).

So was it worth it? Well probably not, but at least I know which resistor types I should suspect going forwards and I do at least have a working PSU now.

It offers:-

• 3.15-CT-3.15 x 2
• Fixed -200Vd.c.
• Variable 300Vd.c. x2 (HT1 and HT2)
• The fixed output can be used in conjunction with either HT1 or HT2 to provide 200-500Vd.c. if required
• Integral meters (to measure HT1 and HT2 volts or current)
• Hernia

As soon as I can, I’ll pop a photo of the offending resistors (and the TCC caps) on the forum just for your interest. I’d recommend that you home straight in on those resistor types.

Many thanks for everyone’s help and kind offer of bits and pieces – much appreciated.

Best regards,


Andy

[af024]

Some of the offending components....

Has anyone found these to be particularly troublesome - especially the resistors?

Cheers,

Andy

[Brian R Pateman]

Well the capacitors look very familiar - the usual waxy suspects among them.

The resistors look as though they have been running hot at some point. I'm pretty sure I have a load of NOS resistors of similar type, together with some nice black Welwyn precision ones lurking around somewhere, let me know if you would like some.

Also if you post a picture of the broken lens I might have something suitable.

Regards,

[af024]

Hello Brian,

Many thanks for your reply. Yes they do indeed look at if they have been running hot. The ones I replaced them with don’t, so I’m not sure what the original fault was that started it all off.

Many thanks too for your kind offer of some resistors. Certainly if you have them and are willing to part with them, then, I’d really appreciate the 120k and 300k ones, The 33k, 270k and 150k that I’ve already replaced with standard carbons are hopefully ok. I used gold tolerance 2W carbon types.

As regards the lamp holder/lens, I’ve taken a close-up of the front (sorry about the shadow) and one from the rear (showing all the dirt and the HS resistors associated with the meters). There was a tiny fragment of the lens left in the holder (looks like it screwed in). It was transparent.

Just for fun, I’ve also posted a photo of the front panel (not cleaned up yet) just so you can see what it looks like.

Thanks again.

Regards,


Andy
PS The earlier photo is deceiving, the caps have metal casings, but they are TCC type. Do they have wax innards?