Everyone has their own way of doing things. I'm not claiming my method is better, in fact it probably isn't since other people have restored more sets than I ever will. But the method works for me.
The following assumes you are aware of the inherent dangers dangers of medling with old high voltage circuitry and take appropriate precautions. Follow my method at your own risk !
I prefer to perform as much corrective action as is required without actually powering up a set. I'm also resisting the temptation to just dive in and replace all the wax and electrolytic capacitors. To illustrate the method I use, I will use my first chassis restoration, a Peto Scott H52, as an example. This set has a fairly typical 5-valve AC-mains circuit that is typical of a mid to late 1940's.
Once I start on a set I prefer to finish it in one go. Therfore, before investing too much time in a set, I like to do the following checks. These identify any long lead time items that need to be obtained, or might even reveal that perhaps this may be a scrap set (though I've yet to scrap a set).
|1)||Quick visual inspection, really looking
only for obvious damage such as burnt, melted or missing
components. Electrolytics that have bulged or are
obviously leaking their contents also noted.
In the case of the H52, the only thing noted was that a resistor in the first RF stage had obviously been replaced. This was noted for later.
|2)||Check that the fitted valves are the
correct type, or are an equivalent.
In this case, all the valves matched and the glass bulbs were secure in their holder. However, in anticipation of having to tilt the chassis to various angles during testing I prefer to remove all the valves at this stage so as to avoid possible damage to them.
Although I didn't do this on the H52, given that I'm trying to identify parts that may need to be ordered, I would check valves for heater-cathode shorts and also check that the heater is not open circuit.
|3)||Check the main wound components, namely the mains transformer, the audio output transformer and the HT smoothing choke (though some sets replace this with a resistor).|
|This "Trader" service sheets often give the appropriate DC resistances for the various windings on these parts. The values for the H52 are shown opposite.|
|However, in reality, we can only really test for open circuits, since a shorted turn may well have little effect upon the DC resistance. However, I also check transformers for primary-to-secondary short circuits ; I've no idea how likely to occur these will be, but since you're waving a resistance meter around anyway then it only takes another few seconds to do the check.|
Always treat any signs of previous work as suspicious. As previously noted, a resistor (of value 47K) had clearly been replaced in the front end. Checking its connections showed that it was in fact R5, whoops ! I guess the person who fitted it in the first place read that the "anode load" resistor was 47K, but didn't click that V1 has two anodes !?
I replaced this with a 1K 1W resistor. The power rating is way over-the-top since the anode current is only 3mA (power = 3mA squared times 1K = 3mW!) but a modern 1W resistor seemed to match the typical size of the other original resistors.
General note : A modern ¼W resistor typically may only have a
maximum element voltage of 200-250v, a point worth bearing in
mind with valve circuitry.
Next on the list should be to check each individual resistor value. I didn't actually do this on the H52 but have done so with all subsequent sets. This doesn't take long and if nothing else helps familiarise yourself with the layout of the components
The primary purpose of this step is to reform the main electrolytics. However, it is also a good time to do a few checks.
I apply H.T. from a variable-voltage power supply via a limiting resistor (I used a 47K 2W resistor but only 'cos I had a suitable one lying around .. can you guess where it came from ;-). Under fault conditions the resistor will limit current such that burning or explosions should not occur. WARNING : Don't assume that this makes the circuit electrically safe ... there may still be capacitors in the circuit charged up to several hundred volts !!!
The H.T. voltage is gradually raised, all the time monitoring the HT current. In most sets there should be no direct D.C. path from H.T. to ground (assuming valves not present) and thus all the H.T. current is due to leakage through capacitors. However, as can be seen in the above front end circuit, the H52 has a potential divider chain biasing the hexode section of V1. This is quite unusual, most sets omitting the lower resistor. In the case of the H52, however, this extra H.T. current needs to be accounted for when calculating the overal H.T. leakage current.
After a while I had the variable power supply at a voltage that was producing around 100v on the HT line. Time to check some of the other components. The first point to measure was the previously mentioned potential divider. The juction was at approximately 50v, so it was assumed that the resistors were OK
Other places checked were all decoupled supplies. An example of such a supply is where R5 is decoupled to ground via a capacitor (see previous diagram). Both sides of R5 should be at the same potential, if it is not then the capacitor must be leaky. Note, however that where high value resistors are used (R5 was only 1K) that you need to take account of the impeadence of your test meter ; check by measuring the voltage ocross the resistor, adding the measured voltage across the capacitor and see if the total is equal to the HT voltage.
On the H52, the junction of R9 and C15
was found to be at only 40v, so I assumed that C15, a
dreaded wax capacitor, must be leaking like a sieve so I
replaced it by a 0.1uF 600v capacitor. And the voltage
was still 40v ! R9 was checked and found to be high
impeadance so was replaced by a 100K 2W part (the service
sheet indicates the resistor passes < 2mA .. thats
0.4mW disipated by R9, however a 2W resistor matched the
size of the original. With subsequent radios I now check
all resistors before applying H.T.
All other decoupled supplies seemed acceptable, so time to check "that" capacitor (C26 on the H52). This is a nasty little capacitor that can cause untold damage if it leaks by causing V4 to turn hard on, burning the output transfromer, frying R23 and damaging HT chokes and even the output and HT rectifier valves. In the H52, this junction of C26 and R20 was at 5 volts. Clearly, "that" capacitor was duff and was replaced. Some people replace this capacitor as a matter of course, and I have to admit that in every set I've tested so far I have had to replace this capacitor.
Next, R20 and R21 were checked since if they were open circuit
the output valve would again pass excessive current (as I've
mentioned before, I now check all resistors).
Well, almost. First, some sets have a supression capacitor across the mains supply (not the case with the H52). I must admit I would replace such a capacitor without thinking on the grounds of safety.
Second, do you trust the rubber insulation in a 50 year old mains cable ? No, neither do I, so at this stage I temporarilly replace it with some shiney new PVC cable ... I'll replace the P.V.C. with something more appropriate later.
Next, check that the correct voltage has been selected.
Up to this point, it has made no difference whether we were dealing with an AC-only or an AC/DC set. However from this point on I am specifically dealing with AC-only sets.
At this point, the valves are still omitted from the chassis. In addition, if the set utilised a metal rectifier for the HT then I guess I would temporarilly disconnect it. Mains is then gradually applied to the primary. With around 25v on the mains side, its time to wizz round the secondaries and check we have some voltages. The mains is then gradually wound up to 100%, carefully checking for smoke or any burning smell. I leave 100% mains applied for at least 5 minutes. The voltages of all secondaries are then checked against the data in the service sheet. Note that you should expect slightly higher voltage readings since as yet we have not applied any load on any of the secondaries. However, one particular point to check is that each half of the center-tapped HT winding has a roughly equal voltage.
Yep. One at a time, starting with the output valve. Mains is applied so as to provide power for the output valve's heater, but H.T. is still supplied from the variable voltage power supply, this time without the 47K series resistor used previously. The H.T. voltage is gradually increased from zero, all the time monitoring the H.T. current.
Most service sheets give the
expected voltages and currents for each valve. In the case of the
H52, the output valve takes 45mA anode + 7.5mA screen ie 52.3mA
from the H.T. line. This particular H52 took approximately 45mA,
perhaps a trifle low. Checking the resistors R22 and R23 (refer
previous circuit block) confirmed them to be OK, so this probably
indicates a slightly under performing (but acceptable) output
At this stage I cant resist sticking my finger on the grid of the output valve (via a resistor)(yes, I am a chicken) just to hear a buzz from the speaker !
Each remaining valve is added in turn (except the rectifier) and the change in supply current checked. I hope I don't need to mention I switch the set off and wait 30 seconds before inserting each valve !
With V1 fitted, a makeshift length of wire bunged in the aerial socket and around 100v H.T. a whooshing sound could be heard in the speaker which dissapeared at around 120v, after which silence but for the odd crackle. With normal H.T. applied, I turned the tuning dial and wey-hey ! A station ! (I guess a professional would have pre-tuned the set to a local radio station, but I only knew where the stations where on V.H.F. ;-).
Now for the biggie. The Variable D.C. supply is removed and the rectifier valve refitted. With a volt meter connected to the H.T. line, the mains is gradually applied via a variac, all the time checking for smoke or excessive brightness of the rectifier valve. All went smoothly and at around 60% or 70% of nominal mains I started to hear the radio station again. With the mains at 100%, it was time to listen to the radio for a while, watching for any sudden dipping in the HT voltage (a slight drop is normal as the valves slowly get to their final operating temperature) and looking out / smelling for any smoke.
After 20 minutes, all seemed well. It was, however, 20 minutes of hell. Not because I was expecting trouble, it was just that BRMB don't half play some crap music at times!
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15th September 2001